llvm dev - Jan 2018 - [RFC] Improving compact x86-64 compact unwind descriptors

Here is our proposal to extend/enhance the x86-64 compact unwind
descriptors to fully describe the prologue/epilogue for asynchronous
unwinding.  I believe there are missing/lacking CFI directives as well,
but I'll save that for another thread.


Asynchronous Compact Unwind Descriptors
Ron Brender, VMS Software, Inc.
Revised January 25, 2018

1  Introduction
This document proposes means to extend so-called compact unwind
descriptors to support fully
asynchronous exception handling. This will make extended compact unwind
descriptors an
alternative to DWARF CFI (call frame information) for achieving
asynchronous exception
support.

Compact unwind descriptors can and have been used in both 64- and 32-bit
environments.
However, this proposal addresses only 64-bit environments. While the
ideas presented here can
be readily adapted for use in a 32-bit environment, for simplicity this
document makes no
attempt to do so.

There are generally three kinds of information that together form the
heart of modern software
exception handling systems:

 1. Information that is used to divide the remaining unwind information
into groups that are
    specific to particular regions of memory (often, but not
necessarily, associated with a
    single function) as well as provide a way to efficiently search for
and identify the
    grouping that is associated with a particular address in memory.

 2. Information that can be used to virtually or actually unwind from
the call frame of an
    executing function to the call frame of its caller (at the point of
the call).

 3. Identification of an associated personality routine that is invoked
by the general exception
    handling mechanization to guide how processing should proceed for a
given function, as
    well as additional “language specific data” needed for the
personality routine to do its
    job. Note that the personality routine and its related data are
specified as an adjunct to
    compiled code and totally opaque to the general mechanism (other
than the specified
    interface).

Note in particular that C++ exception handling is built on top of a
personality routine and
language specific data area ABI that itself can be implemented using
either DWARE CFI or
extended compact unwind information as described here. The choice
between the two is
transparent to C++.


2  Compact Unwind Overview
This section provides a brief overview of key features of the LLVM
compact unwind design. It
does not attempt a comprehensive re-statement of all aspects of the
design except to the extent
necessary to motivate and understand later proposed changes and
enhancements.

2.1  Compact Unwind Group Description
A compact unwind group consist of five fields, as follows:

| 63                             32 31                            0 |
|-------------------------------------------------------------------|
| STARTING-ADDRESS                                                  |
| LENGTH                           | COMPACT-UNWIND-DESCRIPTION     |
| PERSONALITY-FUNCTION-POINTER                                      |
| LANGUAGE-SPECIFIC-DATA-ADDRESS                                    |
|-------------------------------------------------------------------|

STARTING-ADDRESS (64-bits) is the lowest address of a region of memory
occupied by some
code, typically the entry point of a function.

LENGTH (32-bits) is the number of bytes included in this group,
typically including all and only
the code of a function.

COMPACT-UNWIND-DESCRIPTION (32-bits) is a description of the fully
formed frame of a
function and how to unwind it. This is described further following.

PERSONALITY-FUNCTION-POINTER (64-bit) is a pointer to the personality
routine.

LANGUAGE-SPECIFIC-DATA-ADDRESS (64-bits, sometimes abbreviated LSDA) is a
pointer to some data to be passed to the personality routine when it is
called.

A key observation is that the starting address plus length way of
describing a group means that
the set of groups for a compilation unit need not describe all of the
code in that unit. In
particular, it appears to be expected that no unwind information need be
generated for leaf
functions.

On the other hand, it is reasonable to expect that the groups that are
emitted are ordered by the
starting address. This means that a simple and fast binary search can be
used to map an address
to the group that applies to that address, if any.

It is useful to note that the run-time representation of unwind
information can vary from little
more than a simple concatenation of the compile-time information to a
substantial rewriting of
unwind information by the linker. The proposal favors simple
concatenation while maintaining
the same ordering of groups as their associated code.

2.2  Compact Unwind Frame Description
A compact unwind frame description describes a frame in sufficient
detail to be able to unwind
that frame to the frame of its caller.
 
| 31    28 27    24 23                                            0 |
|-------------------------------------------------------------------|
| FLAGS   | MODE   |                                                |
|-------------------------------------------------------------------|

At the top most level, there are four bits that are not of further
interest here. Interpretation of
these bits is neither used nor changed.

Also at the top-level is a 4-bit mode field. This is the tag of a
discriminated (tagged, variant)
union that selects the interpretation of the remaining 24 bits.

Of the 16 possible modes, only 5 are defined :

Code    Meaning      Description
0       Old          “Old” is presumed to refer to some historical
design that is no longer of interest.
                     It is treated here as Reserved.
1       RBP-based    The frame uses the RBP register as a frame pointer.
The size of the frame can
        frame        vary during execution.
2       RSP-based    The frame uses RSP as the frame pointer. The size
of the frame is fixed (at
        frame        compilation time).
3       Large RSP-   The frame uses RSP as the frame pointer, The size
of the frame is fixed (at
        based frame  compilation time); however, that size is too large
to express within this 32-bit
                     descriptor encoding.
4       DWARF        The frame, for whatever reason, cannot be
adequately described using the
        escape       compact unwind frame description. The remaining
24-bits are an index into
                     what the DWARF standard calls the .debug_frame
section (__eh_frame in
                     LLVM).

2.2.1  RBP-based Frame (MODE=1)
For a RBP-based frame, the remaining 24 bits are encoded as follows:

| 23        16 | 15 | 14                                          0 |
|-------------------------------------------------------------------|
| OFFSET       | 0  | REGS                                          |
|-------------------------------------------------------------------|

In a RBP-based frame the RBP register is pushed on the stack immediately
after the return
address, then RSP is moved to RBP. To unwind, RSP is restored with the
current RPB value,
then RBP is restored by popping off the stack, and the return is done by
popping the stack once
more into the instruction pointer.

All preserved registers are saved in a small range in the stack that
starts at RBP-8 to RBP-2040. 
The offset/8 relative to RBP is encoded in the 8-bit OFFSET field. The
registers saved are
encoded in the 15-bit REGS field as five 3-bit entries.

2.2.2  RSP-Based Frame (MODE=2)
For a RSP-based frame, the remaining 24 bits are encoded as follows:

|23        16 | 15      13 | 12      10 | 9                       0 |
|-------------------------------------------------------------------|
| SIZE        |            | CNT        | REG_PERM                  |
|-------------------------------------------------------------------|

In a RSP-based frame the stack pointer serves directly as the frame
pointer and RBP is available
for use as a general register. Upon entry, the stack pointer is
decremented by 8*SIZE bytes (the
maximum stack allocation is thus 2040 bytes). To unwind, the stack size
is added to the stack
pointer, and completed by popping the stack once more into the
instruction pointer.

All preserved registers are saved on the stack immediately after the
return address.  The number
of registers saved (up to 6) is encoded in the 3-bit CNT field. The
11-bit REG_PERM field
encodes which registers were saved and in what order.

2.2.3  Large RSP-Based Frame (MODE=3)
For a large RSP-based frame, the remaining 24 bits are encoded as follows:

|23        16 | 15      13 | 12      10 | 9                       0 |
|-------------------------------------------------------------------|
| SIZE        | ADJ        | CNT        | REG_PERM                  |
|-------------------------------------------------------------------|

This case is like the previous, except the stack size is too large to
encode in the compact unwind
encoding.  Instead, the function must include a "subq $nnnnnnnn, RSP"
instruction in its
prologue to allocate the stack.  The offset from the entry point of the
function to the nnnnnnnn
value in the function is given in the SIZE field.

Depending on the exact instructions used to save registers (PUSH versus
MOV), the nnnnnnnn
value in the instruction stream may not be quite the full stack size.
ADJ * 8 is the additional
adjustment needed to get the actual size.

2.2.4  DWARF Escape (MODE=4)
The frame, for whatever reason, cannot be adequately described using a
compact unwind frame
description. The remaining 24-bits are an index into what the DWARF
standard calls the
.debug_frame section (called __eh_frame in LLVM).


3  Asynchronous Changes and Enhancements
It is immediately obvious that omission of unwind information for leaf
functions (with any kind
of frame) precludes handling an exception that might occur during its
execution. It follows that
unwind information must cover all of the code of a module (with one
exception discussed
below). But if successive unwind groups are ordered (as previously
assumed) and also leave no
gaps, then the LENGTH field is redundant and can be omitted. The
beginning address of a
following group is always one byte past the end of the predecessor
group. There remains only the
question of how to identify the last group of a set.

It should also be clear that the unwind representation described in the
prior section is not
sufficient to unwind from an asynchronous exception that might occur in
either the prologue or
epilogue of a function. To see this consider what would happen if an
exception occurred at either
the entry point or the return instruction of either a RBP- or RSP-frame
function. To be able to
handle asynchronous exceptions at any point during function execution,
it is necessary to add
additional information to each unwind group.

These two considerations can be combined. The result is simply to
repurpose the LENGTH field
to encode prologue and epilogue information.

3.1  Extended MODEs
To preserve backward compatibility and to allow intermixing of
traditional and extended
compact unwind groups, new MODEs are defined as follows:

Code        Meaning                   Description
----------------------------------------------------------------------------------------------
8           Null frame                See below.
9           Extended RBP-based frame  Like mode 1 but combined with
extended prologue/epilogue
                                      information in place of a LENGTH field
10          Extended RSP-based frame  Like mode 2 but combined with
extended prologue/epilogue
                                      information in place of a LENGTH field
11          Extended Large RSP-based  Like mode 3 but combined with
extended prologue/epilogue
            frame                     information in place of a LENGTH field
12          Extended parameter(s)     See below.
 
A null frame (MODE = 8) is the simplest possible frame, with no
allocated stack of either kind
(hence no saved registers).  A null frame might be considered just a
special case of a RSP-based
frame with a stack size of zero. But unlike other frames, its frame
pointer is usually not 16-byte
aligned.
 
It appears technically feasible for a null frame function to have a
personality routine. However,
the utility of such a capability seems too meager to justify allowing
this. We propose to not
support this.

There remains the question of whether it is necessary or at least
desirable to strictly apply the
requirement that “all” code be covered by an unwind group. Based on
successful experience in
OpenVMS on the Itanium architecture (as well as Windows systems we
think), this alternative is
proposed:

    If the first attempt to lookup an unwind group for an exception
address fails, then it is
    (tentatively) assumed to have occurred within a null frame function
or in a part of a function
    that is adequately described by a null frame. The presumed return
address is (virtually or
    actually) popped from the top of stack and looked up. This second
attempted lookup must
    succeed, in which case processing continues normally. A failure is a
fatal error.

This concept of a “default null frame group” is especially convenient
for dealing with
disconnected code sequences such as trampolines, PLTs, and the like.

The extended RBP-based and RSP-based frame modes (MODEs = 9 and 10) are
simple
supersets of their more traditional predecessors.

The large RSP-based frame mode (MODE = 11) is also a superset of mode 3
except that instead
of finding the stack size in the instruction stream, it is found in a
preceding extended parameter
unwind group (MODE = 12). This difference is essential for support of
execute-only code.

To understand the extended parameter group, suppose that two groups
occur one after the other
but have the same STARTING-ADDRESS value. A binary search using the
STARTING-
ADDRESS field will ignore the first of the pair because the address
being sought can never be at
least as large as the first but less than the second. Such a group can
be used as an escape
convention that allows inserting additional information into the
sequence of groups that
otherwise cannot be easily included.

The extended modes are described more fully in the following sections.

3.2  Function Parts
Functions are generally considered to consist of three distinct parts:

1. A prologue, which allocates local storage, saves registers that must
be preserved for the
   benefit of callers, and possibly other housekeeping (such as
initializing any local state
   necessary for the correct management of the function’s execution).

2. A body, which performs the main work of the function.

3. An epilogue, which makes the result available to the caller, and
undoes the effects of the
   prologue (restores preserved registers, deallocates local storage,
and so on).

Here a word of caution is in order. This vocabulary tends to be applied
at multiple levels in
software architecture and each level has its own set of issues to
consider and resulting
requirements to be observed.

Note that a null frame function has no distinct prologue, body or
epilogue. Every instruction can
be viewed as simultaneously in all three parts, or in none of them.

This leads to the following proposal for the upper part of the extended
compact unwind
quadword for use in combination with MODE = 8 in the lower part.

| 63                            48 | 47                          32 |
|-------------------------------------------------------------------|
| RESERVED                         | 0            ...             0 |
|-------------------------------------------------------------------|


| 31                            16 | 15                           0 |
|-------------------------------------------------------------------|
| 0 0 0 0 | 1 0 0 0 | 0   ...    0 | 0            ...             0 |
|-------------------------------------------------------------------|



3.2.1  Function Prologue
For the purposes of exception handling, the key steps are:

 1. Allocate space on the stack in which to save preserved registers and
for other purposes
 2. Save the registers that must be preserved.

The DWARF call frame information (CFI) provides a description that is
precise and accurate at
each and every instruction in a function (not limited to just the
prologue). But experience shows
that this representation is bulky in space and expensive in time to
decode and interpret. Indeed,
compact unwind descriptors as described in Section 2 were created to
avoid these issues.

Experience with OpenVMS on the Alpha and Itanium architectures shows
that a key constraint
can greatly simplify the unwind description: require that no preserved
register can be changed
until all of them have been saved. The last such save ends the prologue
and the next instruction
begins the body. An unwind does not need (must not) restore any
preserved registers because
they are still valid. A simple length from the entry point suffices to
describe this boundary.
[The body does not necessarily need to begin until at least one of the
preserved registers is
actually changed.]
 
3.2.1.1  RSP-Based Frame (MODE = 10)
For a RSP-based frame, it is also necessary to know which instruction
adjusts the stack pointer:
before this instruction an unwind must only (virtually or actually) pop
the return address; and
after this instruction, the stack pointer must first be incremented to
deallocate the stack frame,
then the instruction pointer restored.

This leads to the following proposal for the upper part of the extended
compact unwind
quadword for use in combination with MODE = 10 in the lower part.

| 63                       48 47 46             40 39            32 |
|-------------------------------------------------------------------|
| RESERVED                   |  |  PROLOGUE-SIZE2 |  PROLOGUE-SIZE1 |
|-------------------------------------------------------------------|


Except for the MODE, the lower part is exactly like it would be for MODE
= 2.

PROLOGUE-SIZE1 is the length in bytes of the first part of the prologue
relative to the
STARTING-ADDRESS, up to and including the instruction that allocates the
stack.

PROLOGUE-SIZE2 is the length in bytes of the second part of the prologue
relative to the end
of the first part, up to a point after the last preserved register is
saved and before the first
preserved register is changed. (Recall, this point may not be unique.)

PROLOGUE-SIZE1 + PROLOGUE-SIZE2 gives the total size of the prologue.

The maximum prologue size allowed here is much greater than will be
typical. This is deliberate
to allow compilers freedom to use shrink-wrap type optimizations in
which safe operations that
need no body state are moved from the body to the beginning of the
prologue. This avoids the
cost of stack frame setup and teardown for simple special case handling
that often leads to an
early exit.

3.2.1.2  Large RSP-based Frame (MODE = 11)
This case is like the previous, except the stack size is too large to
encode in the compact unwind
encoding.  The SIZE field is interpreted as an offset relative to the
beginning of the containing
unwind group to (what is necessarily) an earlier extended parameter group.

3.2.1.3  RBP-Based Frame (MODE = 9)
The RBP-based frame is the same as for a RSP-based frame except that
there are two instructions
that mark the transition between the two parts of the prologue: the push
of the prior RBP value
onto the stack followed by the copy of RSP to RBP to establish the new
frame pointer.

There appears to be no reason to not make this simplifying requirement:
the push and copy
always occur together.

This leads to the following proposal for the upper part of the extended
compact unwind
quadword for use in combination with MODE = 9 in the lower part.

| 63                       48 47 46             40 39            32 |
|-------------------------------------------------------------------|
| RESERVED                   |  |  PROLOGUE-SIZE2 |  PROLOGUE-SIZE1 |
|-------------------------------------------------------------------|

Except for the MODE, the lower part is exactly like it would be for MODE
= 1.

PROLOGUE-SIZE1 is the length in bytes of the first part of the prologue
relative to the
STARTING-ADDRESS, up to and including the instruction that pushes the
prior RBP contents.

PROLOGUE-SIZE2 is the length in bytes of the second part of the prologue
relative to the end
of the first part, up to a point after the last preserved register is
saved and before the first
preserved register is changed. (Recall, this point may not be unique.)

PROLOGUE-SIZE1 + PROLOGUE-SIZE2 gives the total size of the prologue.

3.2.2  Function Epilogue
For the purposes of exception handling, the key steps in an epilogue are:

 1. Restore the registers that were preserved
 2. Deallocate space on the stack that was used to preserve registers
and for other purposes

A key observation is that restoring registers is idempotent. That is, if
in the midst of restoring
registers when an exception occurs, it will do no harm if all of the
preserved registers are
restored again when unwinding. In effect, restoration of registers need
not be distinguished from
the body of a function. It is not until reaching the first instruction
that deallocates stack is
executed that the “real” epilogue begins.

3.2.2.1  RSP-Based Frame (MODE = 10 or 11)
For a RSP-based frame, whether small or large, the only unwind action
remaining after stack
deallocation is to pop the return address into the instruction pointer
(RIP). In the vast majority of
cases this means that the epilogue of the code described by an unwind
group is just a 1-byte RET
instruction that occurs at the end of the unwind group. There are other
possibilities but they are
rare and con be dealt with separately (see below).

It suffices to include a single 1-bit field, E, in the upper part of the
extended compact unwind
description:

| 63                       48 47 46             40 39            32 |
|-------------------------------------------------------------------|
| RESERVED                  | E |  PROLOGUE-SIZE2 |  PROLOGUE-SIZE1 |
|-------------------------------------------------------------------|


The E flag indicates that the unwind group ends with a standard 1-byte
RET instruction.

3.2.2.2  RBP-Based Frame (MODE = 9)
The RBP-based frame is much the same as for a RSP-based frame except
that there are two
instructions that mark the transition between the two parts of the
prologue: the copy of RBP to
RSP to cut back the stack, followed by the pop of RBP to restore what
may be the callers frame
pointer. Thus the epilogue, for the purposes of unwinding, begins
immediately after the copy of
RBP to RSP. In the vast majority of cases, the immediately following two
instructions will
consist of POP RBP and RET. The POP is not idempotent because it changes
the stack pointer,
but the unwind processing code can distinguish whether the stack has
been popped or not based
on the address (3 versus 1 byte less than the highest address of the group).

It suffices to include a single 1-bit field, E, in the upper part of the
extended compact unwind
description:

| 63                       48 47 46             40 39            32 |
|-------------------------------------------------------------------|
| RESERVED                  | E |  PROLOGUE-SIZE2 |  PROLOGUE-SIZE1 |
|-------------------------------------------------------------------|

The E flag indicates that the unwind group ends with a standard
3-instruction return sequence.

3.3  Special Issues
Up until this point, it might appear that each unwind group corresponds
one-for-one to the code
for a single function. While common, this is not required. Important
variations are illustrated
here.

Issue/Problem                                        Possible Resolution
-------------------------------------------------------------------------------------------------
The first part of the prologue is too large to be    Use two unwind groups:
described by the 8-bit RxP-FRAME-SIZE1 field.        1) a null frame
group of any appropriate size,
                                                     2) a suitable
RxP-based frame.

The second part of the prologue is too large to be   Use three unwind
groups:
described by the 8-bit RxP-FRAME-SIZE2 field.        1) a RSP-frame
group to describe the first part of
                                                     the prologue only
(PROLOGUE-SIZE1 = the
                                                     length of the
entire group and PROLOGUE-SIZE2
                                                     = 0,
                                                     2) a RSP-frame
group to describe the second part
                                                     of the prologue
(PROLOGUE-SIZE1 & 2 both = 0
                                                     and no registers
saved),
                                                     3) a suitable
RxP-based frame group (with
                                                     PROLOGUE-SIZE1 & 2
both = 0).

There is more than one return location (without a    Generally use one
unwind group for each sequence
shared epilogue sequence).                           of code ending with
a RET. A group following a
                                                     RET will have no
prologue (PROLOGUE-SIZE1
                                                     & 2 both = 0).

There is more than one entry point to a function     Generally use one
unwind group for each sequence
(for example, a FORTRAN multiple entry point         of code beginning
at an entry point. Each group
subroutine or analogous assembly language            may have a normal
prologue and all but the last
equivalents)                                         might have NO
epilogue (E flag clear).
 

These examples are not exhaustive, of course. But they illustrate the
flexibility of the
mechanism, which should be suitable for the vast majority of cases in
practice.

3.4  Number of Unwind Groups
A simple concatenation of unwind groups by the linker that combines
unwind group sections
from multiple modules does not, of itself, provide direct information to
the unwind software
regarding how many unwind groups are present. However, the image
information (ELF
segments) should suffice to determine this number based on the size of
the segment and the
known size of each group. Alternatively, the linker might create special
symbols that can be used
to help determine the number of groups.

This proposal leaves this detail to target-specific linker, image loader
and exception handling
system to specify.

3.5  Interaction with Code Generation
The relatively simple and fixed size nature of the extended compact
unwind information
proposed here depends on observing certain restrictions on optimization
that might affect code in
prologues and epilogues. These restrictions were noted where relevant
throughout this section.
Here is a summary of those requirements:

 1. Use of any preserved register must be delayed until all of the
preserved registers have
    been saved.
 2. In the prologue of a function with a RBP-based frame, the two
instructions that save the
    prior RBP contents and copy the stack pointer to RBP must be kept
adjacent. Similarly,
    in the epilogue of a function with a RBP-based frame, the two
instructions that cut back
    the stack and restore the prior RBP contents must be kept adjacent.
[This adjacency
    requirement could be relaxed by introducing an additional MODE that
corresponds to having
    the RBP saved in memory but not yet updated from RSP. This does not
seem worthwhile.]
 3. For shrink wrap optimization in the presence of a
handler/personality routine, care must
    be taken to not move instructions into the prologue that might cause
an exception.
    Floating point exceptions and access violations are of particular
concern in this regard.

4  System Specific Extensions for OpenVMS
Omitted since nobody other than OpenVMS should care but it describes
adding an additional
four byte extension to the extended compact unwind group to pass along
some information from
the VAX Macro-32 compiler about VAX register usage.



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Hi John & Ron, I read through the proposal and had a couple of quick
observations.

1. The proposed encoding assumes that the epilogue instructions always come at
the end of the function -- or rather, just before the next function.  If there
is a stack protector __stack_chk_fail sequence, or there is NOP padding between
functions, then the epilogue cannot be expressed.  The proposed encoding allows
for instructions scheduled before the prologue, as long as they're
guaranteed to never throw an exception or spill registers, but there's no
similar affordance for after the epilogue.

2. In section 3.1, "A null frame (MODE = 8) is the simplest possible frame,
with no allocated stack of either kind (hence no saved registers). A null frame
might be considered just a special case of a RSP-based frame with a stack size
of zero. But unlike other frames, its frame pointer is usually not 16-byte
aligned."  Did you meant stack pointer here?  The x86_64 SysV abi requires
16-byte alignment of the stack pointer before a CALL instruction, but I
don't know of any alignment requirements on the frame pointer.

3. In the same section, you propose an alternative "default null frame
group" be defined,

"If the first attempt to lookup an unwind group for an exception address
fails, then it is (tentatively) assumed to have occurred within a null frame
function or in a part of a function that is adequately described by a null
frame. The presumed return address is (virtually or actually) popped from the
top of stack and looked up. This second attempted lookup must succeed, in which
case processing continues normally. A failure is a fatal error."

I suspect I missed a part of the proposal covering this, but if entries are
described by only start address, is there a problem of the previous non-null
function's range extending & picking up these functions if they
don't have entries?

4. For frameless functions -- small-stack RSP functions -- you're assuming
that the compiler saves and restores spilled registers with MOV instructions
instead of PUSH/POP instructions.  I think this is the normal behavior of the
compiler, something like

  subq	$0x98, %rsp
  movq	%rsi, 0x10(%rsp)
  movq	%rdi, 0x18(%rsp)

or whatever, but the proposed encoding can only allow for one change in stack
size.  The current encoding requires this for large-stack RSP functions (we get
the offset to the RSP instruction where we read the amount being decremented in
the compact unwind encoding), but for small stack frameless functions the
compiler can today express in compact unwind a combination of sub and push
instructions as long as it's a fixed amount within the range that the
small-stack RSP encoding can express.



> On Jan 26, 2018, at 7:54 AM, John Reagan <john.reagan at
vmssoftware.com> wrote:
> 
> Here is our proposal to extend/enhance the x86-64 compact unwind
> descriptors to fully describe the prologue/epilogue for asynchronous
> unwinding.  I believe there are missing/lacking CFI directives as well,
> but I'll save that for another thread.
> 
> 
> Asynchronous Compact Unwind Descriptors
> Ron Brender, VMS Software, Inc.
> Revised January 25, 2018
> 
> 1  Introduction
> This document proposes means to extend so-called compact unwind
> descriptors to support fully
> asynchronous exception handling. This will make extended compact unwind
> descriptors an
> alternative to DWARF CFI (call frame information) for achieving
> asynchronous exception
> support.
> 
> Compact unwind descriptors can and have been used in both 64- and 32-bit
> environments.
> However, this proposal addresses only 64-bit environments. While the
> ideas presented here can
> be readily adapted for use in a 32-bit environment, for simplicity this
> document makes no
> attempt to do so.
> 
> There are generally three kinds of information that together form the
> heart of modern software
> exception handling systems:
> 
> 1. Information that is used to divide the remaining unwind information
> into groups that are
>    specific to particular regions of memory (often, but not
> necessarily, associated with a
>    single function) as well as provide a way to efficiently search for
> and identify the
>    grouping that is associated with a particular address in memory.
> 
> 2. Information that can be used to virtually or actually unwind from
> the call frame of an
>    executing function to the call frame of its caller (at the point of
> the call).
> 
> 3. Identification of an associated personality routine that is invoked
> by the general exception
>    handling mechanization to guide how processing should proceed for a
> given function, as
>    well as additional “language specific data” needed for the
> personality routine to do its
>    job. Note that the personality routine and its related data are
> specified as an adjunct to
>    compiled code and totally opaque to the general mechanism (other
> than the specified
>    interface).
> 
> Note in particular that C++ exception handling is built on top of a
> personality routine and
> language specific data area ABI that itself can be implemented using
> either DWARE CFI or
> extended compact unwind information as described here. The choice
> between the two is
> transparent to C++.
> 
> 
> 2  Compact Unwind Overview
> This section provides a brief overview of key features of the LLVM
> compact unwind design. It
> does not attempt a comprehensive re-statement of all aspects of the
> design except to the extent
> necessary to motivate and understand later proposed changes and
> enhancements.
> 
> 2.1  Compact Unwind Group Description
> A compact unwind group consist of five fields, as follows:
> 
> | 63                             32 31                            0 |
> |-------------------------------------------------------------------|
> | STARTING-ADDRESS                                                  |
> | LENGTH                           | COMPACT-UNWIND-DESCRIPTION     |
> | PERSONALITY-FUNCTION-POINTER                                      |
> | LANGUAGE-SPECIFIC-DATA-ADDRESS                                    |
> |-------------------------------------------------------------------|
> 
> STARTING-ADDRESS (64-bits) is the lowest address of a region of memory
> occupied by some
> code, typically the entry point of a function.
> 
> LENGTH (32-bits) is the number of bytes included in this group,
> typically including all and only
> the code of a function.
> 
> COMPACT-UNWIND-DESCRIPTION (32-bits) is a description of the fully
> formed frame of a
> function and how to unwind it. This is described further following.
> 
> PERSONALITY-FUNCTION-POINTER (64-bit) is a pointer to the personality
> routine.
> 
> LANGUAGE-SPECIFIC-DATA-ADDRESS (64-bits, sometimes abbreviated LSDA) is a
> pointer to some data to be passed to the personality routine when it is
> called.
> 
> A key observation is that the starting address plus length way of
> describing a group means that
> the set of groups for a compilation unit need not describe all of the
> code in that unit. In
> particular, it appears to be expected that no unwind information need be
> generated for leaf
> functions.
> 
> On the other hand, it is reasonable to expect that the groups that are
> emitted are ordered by the
> starting address. This means that a simple and fast binary search can be
> used to map an address
> to the group that applies to that address, if any.
> 
> It is useful to note that the run-time representation of unwind
> information can vary from little
> more than a simple concatenation of the compile-time information to a
> substantial rewriting of
> unwind information by the linker. The proposal favors simple
> concatenation while maintaining
> the same ordering of groups as their associated code.
> 
> 2.2  Compact Unwind Frame Description
> A compact unwind frame description describes a frame in sufficient
> detail to be able to unwind
> that frame to the frame of its caller.
> 
> | 31    28 27    24 23                                            0 |
> |-------------------------------------------------------------------|
> | FLAGS   | MODE   |                                                |
> |-------------------------------------------------------------------|
> 
> At the top most level, there are four bits that are not of further
> interest here. Interpretation of
> these bits is neither used nor changed.
> 
> Also at the top-level is a 4-bit mode field. This is the tag of a
> discriminated (tagged, variant)
> union that selects the interpretation of the remaining 24 bits.
> 
> Of the 16 possible modes, only 5 are defined :
> 
> Code    Meaning      Description
> 0       Old          “Old” is presumed to refer to some historical
> design that is no longer of interest.
>                     It is treated here as Reserved.
> 1       RBP-based    The frame uses the RBP register as a frame pointer.
> The size of the frame can
>        frame        vary during execution.
> 2       RSP-based    The frame uses RSP as the frame pointer. The size
> of the frame is fixed (at
>        frame        compilation time).
> 3       Large RSP-   The frame uses RSP as the frame pointer, The size
> of the frame is fixed (at
>        based frame  compilation time); however, that size is too large
> to express within this 32-bit
>                     descriptor encoding.
> 4       DWARF        The frame, for whatever reason, cannot be
> adequately described using the
>        escape       compact unwind frame description. The remaining
> 24-bits are an index into
>                     what the DWARF standard calls the .debug_frame
> section (__eh_frame in
>                     LLVM).
> 
> 2.2.1  RBP-based Frame (MODE=1)
> For a RBP-based frame, the remaining 24 bits are encoded as follows:
> 
> | 23        16 | 15 | 14                                          0 |
> |-------------------------------------------------------------------|
> | OFFSET       | 0  | REGS                                          |
> |-------------------------------------------------------------------|
> 
> In a RBP-based frame the RBP register is pushed on the stack immediately
> after the return
> address, then RSP is moved to RBP. To unwind, RSP is restored with the
> current RPB value,
> then RBP is restored by popping off the stack, and the return is done by
> popping the stack once
> more into the instruction pointer.
> 
> All preserved registers are saved in a small range in the stack that
> starts at RBP-8 to RBP-2040. 
> The offset/8 relative to RBP is encoded in the 8-bit OFFSET field. The
> registers saved are
> encoded in the 15-bit REGS field as five 3-bit entries.
> 
> 2.2.2  RSP-Based Frame (MODE=2)
> For a RSP-based frame, the remaining 24 bits are encoded as follows:
> 
> |23        16 | 15      13 | 12      10 | 9                       0 |
> |-------------------------------------------------------------------|
> | SIZE        |            | CNT        | REG_PERM                  |
> |-------------------------------------------------------------------|
> 
> In a RSP-based frame the stack pointer serves directly as the frame
> pointer and RBP is available
> for use as a general register. Upon entry, the stack pointer is
> decremented by 8*SIZE bytes (the
> maximum stack allocation is thus 2040 bytes). To unwind, the stack size
> is added to the stack
> pointer, and completed by popping the stack once more into the
> instruction pointer.
> 
> All preserved registers are saved on the stack immediately after the
> return address.  The number
> of registers saved (up to 6) is encoded in the 3-bit CNT field. The
> 11-bit REG_PERM field
> encodes which registers were saved and in what order.
> 
> 2.2.3  Large RSP-Based Frame (MODE=3)
> For a large RSP-based frame, the remaining 24 bits are encoded as follows:
> 
> |23        16 | 15      13 | 12      10 | 9                       0 |
> |-------------------------------------------------------------------|
> | SIZE        | ADJ        | CNT        | REG_PERM                  |
> |-------------------------------------------------------------------|
> 
> This case is like the previous, except the stack size is too large to
> encode in the compact unwind
> encoding.  Instead, the function must include a "subq $nnnnnnnn,
RSP"
> instruction in its
> prologue to allocate the stack.  The offset from the entry point of the
> function to the nnnnnnnn
> value in the function is given in the SIZE field.
> 
> Depending on the exact instructions used to save registers (PUSH versus
> MOV), the nnnnnnnn
> value in the instruction stream may not be quite the full stack size.
> ADJ * 8 is the additional
> adjustment needed to get the actual size.
> 
> 2.2.4  DWARF Escape (MODE=4)
> The frame, for whatever reason, cannot be adequately described using a
> compact unwind frame
> description. The remaining 24-bits are an index into what the DWARF
> standard calls the
> .debug_frame section (called __eh_frame in LLVM).
> 
> 
> 3  Asynchronous Changes and Enhancements
> It is immediately obvious that omission of unwind information for leaf
> functions (with any kind
> of frame) precludes handling an exception that might occur during its
> execution. It follows that
> unwind information must cover all of the code of a module (with one
> exception discussed
> below). But if successive unwind groups are ordered (as previously
> assumed) and also leave no
> gaps, then the LENGTH field is redundant and can be omitted. The
> beginning address of a
> following group is always one byte past the end of the predecessor
> group. There remains only the
> question of how to identify the last group of a set.
> 
> It should also be clear that the unwind representation described in the
> prior section is not
> sufficient to unwind from an asynchronous exception that might occur in
> either the prologue or
> epilogue of a function. To see this consider what would happen if an
> exception occurred at either
> the entry point or the return instruction of either a RBP- or RSP-frame
> function. To be able to
> handle asynchronous exceptions at any point during function execution,
> it is necessary to add
> additional information to each unwind group.
> 
> These two considerations can be combined. The result is simply to
> repurpose the LENGTH field
> to encode prologue and epilogue information.
> 
> 3.1  Extended MODEs
> To preserve backward compatibility and to allow intermixing of
> traditional and extended
> compact unwind groups, new MODEs are defined as follows:
> 
> Code        Meaning                   Description
>
----------------------------------------------------------------------------------------------
> 8           Null frame                See below.
> 9           Extended RBP-based frame  Like mode 1 but combined with
> extended prologue/epilogue
>                                      information in place of a LENGTH field
> 10          Extended RSP-based frame  Like mode 2 but combined with
> extended prologue/epilogue
>                                      information in place of a LENGTH field
> 11          Extended Large RSP-based  Like mode 3 but combined with
> extended prologue/epilogue
>            frame                     information in place of a LENGTH field
> 12          Extended parameter(s)     See below.
> 
> A null frame (MODE = 8) is the simplest possible frame, with no
> allocated stack of either kind
> (hence no saved registers).  A null frame might be considered just a
> special case of a RSP-based
> frame with a stack size of zero. But unlike other frames, its frame
> pointer is usually not 16-byte
> aligned.
> 
> It appears technically feasible for a null frame function to have a
> personality routine. However,
> the utility of such a capability seems too meager to justify allowing
> this. We propose to not
> support this.
> 
> There remains the question of whether it is necessary or at least
> desirable to strictly apply the
> requirement that “all” code be covered by an unwind group. Based on
> successful experience in
> OpenVMS on the Itanium architecture (as well as Windows systems we
> think), this alternative is
> proposed:
> 
>    If the first attempt to lookup an unwind group for an exception
> address fails, then it is
>    (tentatively) assumed to have occurred within a null frame function
> or in a part of a function
>    that is adequately described by a null frame. The presumed return
> address is (virtually or
>    actually) popped from the top of stack and looked up. This second
> attempted lookup must
>    succeed, in which case processing continues normally. A failure is a
> fatal error.
> 
> This concept of a “default null frame group” is especially convenient
> for dealing with
> disconnected code sequences such as trampolines, PLTs, and the like.
> 
> The extended RBP-based and RSP-based frame modes (MODEs = 9 and 10) are
> simple
> supersets of their more traditional predecessors.
> 
> The large RSP-based frame mode (MODE = 11) is also a superset of mode 3
> except that instead
> of finding the stack size in the instruction stream, it is found in a
> preceding extended parameter
> unwind group (MODE = 12). This difference is essential for support of
> execute-only code.
> 
> To understand the extended parameter group, suppose that two groups
> occur one after the other
> but have the same STARTING-ADDRESS value. A binary search using the
> STARTING-
> ADDRESS field will ignore the first of the pair because the address
> being sought can never be at
> least as large as the first but less than the second. Such a group can
> be used as an escape
> convention that allows inserting additional information into the
> sequence of groups that
> otherwise cannot be easily included.
> 
> The extended modes are described more fully in the following sections.
> 
> 3.2  Function Parts
> Functions are generally considered to consist of three distinct parts:
> 
> 1. A prologue, which allocates local storage, saves registers that must
> be preserved for the
>   benefit of callers, and possibly other housekeeping (such as
> initializing any local state
>   necessary for the correct management of the function’s execution).
> 
> 2. A body, which performs the main work of the function.
> 
> 3. An epilogue, which makes the result available to the caller, and
> undoes the effects of the
>   prologue (restores preserved registers, deallocates local storage,
> and so on).
> 
> Here a word of caution is in order. This vocabulary tends to be applied
> at multiple levels in
> software architecture and each level has its own set of issues to
> consider and resulting
> requirements to be observed.
> 
> Note that a null frame function has no distinct prologue, body or
> epilogue. Every instruction can
> be viewed as simultaneously in all three parts, or in none of them.
> 
> This leads to the following proposal for the upper part of the extended
> compact unwind
> quadword for use in combination with MODE = 8 in the lower part.
> 
> | 63                            48 | 47                          32 |
> |-------------------------------------------------------------------|
> | RESERVED                         | 0            ...             0 |
> |-------------------------------------------------------------------|
> 
> 
> | 31                            16 | 15                           0 |
> |-------------------------------------------------------------------|
> | 0 0 0 0 | 1 0 0 0 | 0   ...    0 | 0            ...             0 |
> |-------------------------------------------------------------------|
> 
> 
> 
> 3.2.1  Function Prologue
> For the purposes of exception handling, the key steps are:
> 
> 1. Allocate space on the stack in which to save preserved registers and
> for other purposes
> 2. Save the registers that must be preserved.
> 
> The DWARF call frame information (CFI) provides a description that is
> precise and accurate at
> each and every instruction in a function (not limited to just the
> prologue). But experience shows
> that this representation is bulky in space and expensive in time to
> decode and interpret. Indeed,
> compact unwind descriptors as described in Section 2 were created to
> avoid these issues.
> 
> Experience with OpenVMS on the Alpha and Itanium architectures shows
> that a key constraint
> can greatly simplify the unwind description: require that no preserved
> register can be changed
> until all of them have been saved. The last such save ends the prologue
> and the next instruction
> begins the body. An unwind does not need (must not) restore any
> preserved registers because
> they are still valid. A simple length from the entry point suffices to
> describe this boundary.
> [The body does not necessarily need to begin until at least one of the
> preserved registers is
> actually changed.]
> 
> 3.2.1.1  RSP-Based Frame (MODE = 10)
> For a RSP-based frame, it is also necessary to know which instruction
> adjusts the stack pointer:
> before this instruction an unwind must only (virtually or actually) pop
> the return address; and
> after this instruction, the stack pointer must first be incremented to
> deallocate the stack frame,
> then the instruction pointer restored.
> 
> This leads to the following proposal for the upper part of the extended
> compact unwind
> quadword for use in combination with MODE = 10 in the lower part.
> 
> | 63                       48 47 46             40 39            32 |
> |-------------------------------------------------------------------|
> | RESERVED                   |  |  PROLOGUE-SIZE2 |  PROLOGUE-SIZE1 |
> |-------------------------------------------------------------------|
> 
> 
> Except for the MODE, the lower part is exactly like it would be for MODE
> = 2.
> 
> PROLOGUE-SIZE1 is the length in bytes of the first part of the prologue
> relative to the
> STARTING-ADDRESS, up to and including the instruction that allocates the
> stack.
> 
> PROLOGUE-SIZE2 is the length in bytes of the second part of the prologue
> relative to the end
> of the first part, up to a point after the last preserved register is
> saved and before the first
> preserved register is changed. (Recall, this point may not be unique.)
> 
> PROLOGUE-SIZE1 + PROLOGUE-SIZE2 gives the total size of the prologue.
> 
> The maximum prologue size allowed here is much greater than will be
> typical. This is deliberate
> to allow compilers freedom to use shrink-wrap type optimizations in
> which safe operations that
> need no body state are moved from the body to the beginning of the
> prologue. This avoids the
> cost of stack frame setup and teardown for simple special case handling
> that often leads to an
> early exit.
> 
> 3.2.1.2  Large RSP-based Frame (MODE = 11)
> This case is like the previous, except the stack size is too large to
> encode in the compact unwind
> encoding.  The SIZE field is interpreted as an offset relative to the
> beginning of the containing
> unwind group to (what is necessarily) an earlier extended parameter group.
> 
> 3.2.1.3  RBP-Based Frame (MODE = 9)
> The RBP-based frame is the same as for a RSP-based frame except that
> there are two instructions
> that mark the transition between the two parts of the prologue: the push
> of the prior RBP value
> onto the stack followed by the copy of RSP to RBP to establish the new
> frame pointer.
> 
> There appears to be no reason to not make this simplifying requirement:
> the push and copy
> always occur together.
> 
> This leads to the following proposal for the upper part of the extended
> compact unwind
> quadword for use in combination with MODE = 9 in the lower part.
> 
> | 63                       48 47 46             40 39            32 |
> |-------------------------------------------------------------------|
> | RESERVED                   |  |  PROLOGUE-SIZE2 |  PROLOGUE-SIZE1 |
> |-------------------------------------------------------------------|
> 
> Except for the MODE, the lower part is exactly like it would be for MODE
> = 1.
> 
> PROLOGUE-SIZE1 is the length in bytes of the first part of the prologue
> relative to the
> STARTING-ADDRESS, up to and including the instruction that pushes the
> prior RBP contents.
> 
> PROLOGUE-SIZE2 is the length in bytes of the second part of the prologue
> relative to the end
> of the first part, up to a point after the last preserved register is
> saved and before the first
> preserved register is changed. (Recall, this point may not be unique.)
> 
> PROLOGUE-SIZE1 + PROLOGUE-SIZE2 gives the total size of the prologue.
> 
> 3.2.2  Function Epilogue
> For the purposes of exception handling, the key steps in an epilogue are:
> 
> 1. Restore the registers that were preserved
> 2. Deallocate space on the stack that was used to preserve registers
> and for other purposes
> 
> A key observation is that restoring registers is idempotent. That is, if
> in the midst of restoring
> registers when an exception occurs, it will do no harm if all of the
> preserved registers are
> restored again when unwinding. In effect, restoration of registers need
> not be distinguished from
> the body of a function. It is not until reaching the first instruction
> that deallocates stack is
> executed that the “real” epilogue begins.
> 
> 3.2.2.1  RSP-Based Frame (MODE = 10 or 11)
> For a RSP-based frame, whether small or large, the only unwind action
> remaining after stack
> deallocation is to pop the return address into the instruction pointer
> (RIP). In the vast majority of
> cases this means that the epilogue of the code described by an unwind
> group is just a 1-byte RET
> instruction that occurs at the end of the unwind group. There are other
> possibilities but they are
> rare and con be dealt with separately (see below).
> 
> It suffices to include a single 1-bit field, E, in the upper part of the
> extended compact unwind
> description:
> 
> | 63                       48 47 46             40 39            32 |
> |-------------------------------------------------------------------|
> | RESERVED                  | E |  PROLOGUE-SIZE2 |  PROLOGUE-SIZE1 |
> |-------------------------------------------------------------------|
> 
> 
> The E flag indicates that the unwind group ends with a standard 1-byte
> RET instruction.
> 
> 3.2.2.2  RBP-Based Frame (MODE = 9)
> The RBP-based frame is much the same as for a RSP-based frame except
> that there are two
> instructions that mark the transition between the two parts of the
> prologue: the copy of RBP to
> RSP to cut back the stack, followed by the pop of RBP to restore what
> may be the callers frame
> pointer. Thus the epilogue, for the purposes of unwinding, begins
> immediately after the copy of
> RBP to RSP. In the vast majority of cases, the immediately following two
> instructions will
> consist of POP RBP and RET. The POP is not idempotent because it changes
> the stack pointer,
> but the unwind processing code can distinguish whether the stack has
> been popped or not based
> on the address (3 versus 1 byte less than the highest address of the
group).
> 
> It suffices to include a single 1-bit field, E, in the upper part of the
> extended compact unwind
> description:
> 
> | 63                       48 47 46             40 39            32 |
> |-------------------------------------------------------------------|
> | RESERVED                  | E |  PROLOGUE-SIZE2 |  PROLOGUE-SIZE1 |
> |-------------------------------------------------------------------|
> 
> The E flag indicates that the unwind group ends with a standard
> 3-instruction return sequence.
> 
> 3.3  Special Issues
> Up until this point, it might appear that each unwind group corresponds
> one-for-one to the code
> for a single function. While common, this is not required. Important
> variations are illustrated
> here.
> 
> Issue/Problem                                        Possible Resolution
>
-------------------------------------------------------------------------------------------------
> The first part of the prologue is too large to be    Use two unwind groups:
> described by the 8-bit RxP-FRAME-SIZE1 field.        1) a null frame
> group of any appropriate size,
>                                                     2) a suitable
> RxP-based frame.
> 
> The second part of the prologue is too large to be   Use three unwind
> groups:
> described by the 8-bit RxP-FRAME-SIZE2 field.        1) a RSP-frame
> group to describe the first part of
>                                                     the prologue only
> (PROLOGUE-SIZE1 = the
>                                                     length of the
> entire group and PROLOGUE-SIZE2
>                                                     = 0,
>                                                     2) a RSP-frame
> group to describe the second part
>                                                     of the prologue
> (PROLOGUE-SIZE1 & 2 both = 0
>                                                     and no registers
> saved),
>                                                     3) a suitable
> RxP-based frame group (with
>                                                     PROLOGUE-SIZE1 & 2
> both = 0).
> 
> There is more than one return location (without a    Generally use one
> unwind group for each sequence
> shared epilogue sequence).                           of code ending with
> a RET. A group following a
>                                                     RET will have no
> prologue (PROLOGUE-SIZE1
>                                                     & 2 both = 0).
> 
> There is more than one entry point to a function     Generally use one
> unwind group for each sequence
> (for example, a FORTRAN multiple entry point         of code beginning
> at an entry point. Each group
> subroutine or analogous assembly language            may have a normal
> prologue and all but the last
> equivalents)                                         might have NO
> epilogue (E flag clear).
> 
> 
> These examples are not exhaustive, of course. But they illustrate the
> flexibility of the
> mechanism, which should be suitable for the vast majority of cases in
> practice.
> 
> 3.4  Number of Unwind Groups
> A simple concatenation of unwind groups by the linker that combines
> unwind group sections
> from multiple modules does not, of itself, provide direct information to
> the unwind software
> regarding how many unwind groups are present. However, the image
> information (ELF
> segments) should suffice to determine this number based on the size of
> the segment and the
> known size of each group. Alternatively, the linker might create special
> symbols that can be used
> to help determine the number of groups.
> 
> This proposal leaves this detail to target-specific linker, image loader
> and exception handling
> system to specify.
> 
> 3.5  Interaction with Code Generation
> The relatively simple and fixed size nature of the extended compact
> unwind information
> proposed here depends on observing certain restrictions on optimization
> that might affect code in
> prologues and epilogues. These restrictions were noted where relevant
> throughout this section.
> Here is a summary of those requirements:
> 
> 1. Use of any preserved register must be delayed until all of the
> preserved registers have
>    been saved.
> 2. In the prologue of a function with a RBP-based frame, the two
> instructions that save the
>    prior RBP contents and copy the stack pointer to RBP must be kept
> adjacent. Similarly,
>    in the epilogue of a function with a RBP-based frame, the two
> instructions that cut back
>    the stack and restore the prior RBP contents must be kept adjacent.
> [This adjacency
>    requirement could be relaxed by introducing an additional MODE that
> corresponds to having
>    the RBP saved in memory but not yet updated from RSP. This does not
> seem worthwhile.]
> 3. For shrink wrap optimization in the presence of a
> handler/personality routine, care must
>    be taken to not move instructions into the prologue that might cause
> an exception.
>    Floating point exceptions and access violations are of particular
> concern in this regard.
> 
> 4  System Specific Extensions for OpenVMS
> Omitted since nobody other than OpenVMS should care but it describes
> adding an additional
> four byte extension to the extended compact unwind group to pass along
> some information from
> the VAX Macro-32 compiler about VAX register usage.
> 
> 
>

John and Ron,

I developed the original compact unwind implementation for macOS 10.6 back in
2009.  I tried to leave space in the design to support finer grain exception
handling such as for asynchronous or for the shrink wrap optimization.   The
idea I had at the time was  instead of having just one 32-bit compact unwind
info per function, there could be an array of them each covering a different
range. All but the first would have the UNWIND_IS_NOT_FUNCTION_START bit set.

On macOS, the linker takes the 32-byte “compact unwind group description”
records and folds them down into a complete different (read only) runtime table
which basically maps an offset in the DSO into a 32-bit compact info for that
address.

It seems like from your proposal that your linker keeps the “compact unwind
group description” records as is and just concatenates them, and the runtime
would need to understand the prolog/epilog encoding.  I would think that just
widening the group record (as you suggest in 5.1) would be much simpler (like
SHT_REL was widened to SHT_RELA).

-Nick


> On Jan 26, 2018, at 7:54 AM, John Reagan via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> Here is our proposal to extend/enhance the x86-64 compact unwind
> descriptors to fully describe the prologue/epilogue for asynchronous
> unwinding.  I believe there are missing/lacking CFI directives as well,
> but I'll save that for another thread.
> 
> 
> Asynchronous Compact Unwind Descriptors
> Ron Brender, VMS Software, Inc.
> Revised January 25, 2018
> 
> 1  Introduction
> This document proposes means to extend so-called compact unwind
> descriptors to support fully
> asynchronous exception handling. This will make extended compact unwind
> descriptors an
> alternative to DWARF CFI (call frame information) for achieving
> asynchronous exception
> support.
> 
> Compact unwind descriptors can and have been used in both 64- and 32-bit
> environments.
> However, this proposal addresses only 64-bit environments. While the
> ideas presented here can
> be readily adapted for use in a 32-bit environment, for simplicity this
> document makes no
> attempt to do so.
> 
> There are generally three kinds of information that together form the
> heart of modern software
> exception handling systems:
> 
>  1. Information that is used to divide the remaining unwind information
> into groups that are
>     specific to particular regions of memory (often, but not
> necessarily, associated with a
>     single function) as well as provide a way to efficiently search for
> and identify the
>     grouping that is associated with a particular address in memory.
> 
>  2. Information that can be used to virtually or actually unwind from
> the call frame of an
>     executing function to the call frame of its caller (at the point of
> the call).
> 
>  3. Identification of an associated personality routine that is invoked
> by the general exception
>     handling mechanization to guide how processing should proceed for a
> given function, as
>     well as additional “language specific data” needed for the
> personality routine to do its
>     job. Note that the personality routine and its related data are
> specified as an adjunct to
>     compiled code and totally opaque to the general mechanism (other
> than the specified
>     interface).
> 
> Note in particular that C++ exception handling is built on top of a
> personality routine and
> language specific data area ABI that itself can be implemented using
> either DWARE CFI or
> extended compact unwind information as described here. The choice
> between the two is
> transparent to C++.
> 
> 
> 2  Compact Unwind Overview
> This section provides a brief overview of key features of the LLVM
> compact unwind design. It
> does not attempt a comprehensive re-statement of all aspects of the
> design except to the extent
> necessary to motivate and understand later proposed changes and
> enhancements.
> 
> 2.1  Compact Unwind Group Description
> A compact unwind group consist of five fields, as follows:
> 
> | 63                             32 31                            0 |
> |-------------------------------------------------------------------|
> | STARTING-ADDRESS                                                  |
> | LENGTH                           | COMPACT-UNWIND-DESCRIPTION     |
> | PERSONALITY-FUNCTION-POINTER                                      |
> | LANGUAGE-SPECIFIC-DATA-ADDRESS                                    |
> |-------------------------------------------------------------------|
> 
> STARTING-ADDRESS (64-bits) is the lowest address of a region of memory
> occupied by some
> code, typically the entry point of a function.
> 
> LENGTH (32-bits) is the number of bytes included in this group,
> typically including all and only
> the code of a function.
> 
> COMPACT-UNWIND-DESCRIPTION (32-bits) is a description of the fully
> formed frame of a
> function and how to unwind it. This is described further following.
> 
> PERSONALITY-FUNCTION-POINTER (64-bit) is a pointer to the personality
> routine.
> 
> LANGUAGE-SPECIFIC-DATA-ADDRESS (64-bits, sometimes abbreviated LSDA) is a
> pointer to some data to be passed to the personality routine when it is
> called.
> 
> A key observation is that the starting address plus length way of
> describing a group means that
> the set of groups for a compilation unit need not describe all of the
> code in that unit. In
> particular, it appears to be expected that no unwind information need be
> generated for leaf
> functions.
> 
> On the other hand, it is reasonable to expect that the groups that are
> emitted are ordered by the
> starting address. This means that a simple and fast binary search can be
> used to map an address
> to the group that applies to that address, if any.
> 
> It is useful to note that the run-time representation of unwind
> information can vary from little
> more than a simple concatenation of the compile-time information to a
> substantial rewriting of
> unwind information by the linker. The proposal favors simple
> concatenation while maintaining
> the same ordering of groups as their associated code.
> 
> 2.2  Compact Unwind Frame Description
> A compact unwind frame description describes a frame in sufficient
> detail to be able to unwind
> that frame to the frame of its caller.
>  
> | 31    28 27    24 23                                            0 |
> |-------------------------------------------------------------------|
> | FLAGS   | MODE   |                                                |
> |-------------------------------------------------------------------|
> 
> At the top most level, there are four bits that are not of further
> interest here. Interpretation of
> these bits is neither used nor changed.
> 
> Also at the top-level is a 4-bit mode field. This is the tag of a
> discriminated (tagged, variant)
> union that selects the interpretation of the remaining 24 bits.
> 
> Of the 16 possible modes, only 5 are defined :
> 
> Code    Meaning      Description
> 0       Old          “Old” is presumed to refer to some historical
> design that is no longer of interest.
>                      It is treated here as Reserved.
> 1       RBP-based    The frame uses the RBP register as a frame pointer.
> The size of the frame can
>         frame        vary during execution.
> 2       RSP-based    The frame uses RSP as the frame pointer. The size
> of the frame is fixed (at
>         frame        compilation time).
> 3       Large RSP-   The frame uses RSP as the frame pointer, The size
> of the frame is fixed (at
>         based frame  compilation time); however, that size is too large
> to express within this 32-bit
>                      descriptor encoding.
> 4       DWARF        The frame, for whatever reason, cannot be
> adequately described using the
>         escape       compact unwind frame description. The remaining
> 24-bits are an index into
>                      what the DWARF standard calls the .debug_frame
> section (__eh_frame in
>                      LLVM).
> 
> 2.2.1  RBP-based Frame (MODE=1)
> For a RBP-based frame, the remaining 24 bits are encoded as follows:
> 
> | 23        16 | 15 | 14                                          0 |
> |-------------------------------------------------------------------|
> | OFFSET       | 0  | REGS                                          |
> |-------------------------------------------------------------------|
> 
> In a RBP-based frame the RBP register is pushed on the stack immediately
> after the return
> address, then RSP is moved to RBP. To unwind, RSP is restored with the
> current RPB value,
> then RBP is restored by popping off the stack, and the return is done by
> popping the stack once
> more into the instruction pointer.
> 
> All preserved registers are saved in a small range in the stack that
> starts at RBP-8 to RBP-2040. 
> The offset/8 relative to RBP is encoded in the 8-bit OFFSET field. The
> registers saved are
> encoded in the 15-bit REGS field as five 3-bit entries.
> 
> 2.2.2  RSP-Based Frame (MODE=2)
> For a RSP-based frame, the remaining 24 bits are encoded as follows:
> 
> |23        16 | 15      13 | 12      10 | 9                       0 |
> |-------------------------------------------------------------------|
> | SIZE        |            | CNT        | REG_PERM                  |
> |-------------------------------------------------------------------|
> 
> In a RSP-based frame the stack pointer serves directly as the frame
> pointer and RBP is available
> for use as a general register. Upon entry, the stack pointer is
> decremented by 8*SIZE bytes (the
> maximum stack allocation is thus 2040 bytes). To unwind, the stack size
> is added to the stack
> pointer, and completed by popping the stack once more into the
> instruction pointer.
> 
> All preserved registers are saved on the stack immediately after the
> return address.  The number
> of registers saved (up to 6) is encoded in the 3-bit CNT field. The
> 11-bit REG_PERM field
> encodes which registers were saved and in what order.
> 
> 2.2.3  Large RSP-Based Frame (MODE=3)
> For a large RSP-based frame, the remaining 24 bits are encoded as follows:
> 
> |23        16 | 15      13 | 12      10 | 9                       0 |
> |-------------------------------------------------------------------|
> | SIZE        | ADJ        | CNT        | REG_PERM                  |
> |-------------------------------------------------------------------|
> 
> This case is like the previous, except the stack size is too large to
> encode in the compact unwind
> encoding.  Instead, the function must include a "subq $nnnnnnnn,
RSP"
> instruction in its
> prologue to allocate the stack.  The offset from the entry point of the
> function to the nnnnnnnn
> value in the function is given in the SIZE field.
> 
> Depending on the exact instructions used to save registers (PUSH versus
> MOV), the nnnnnnnn
> value in the instruction stream may not be quite the full stack size.
> ADJ * 8 is the additional
> adjustment needed to get the actual size.
> 
> 2.2.4  DWARF Escape (MODE=4)
> The frame, for whatever reason, cannot be adequately described using a
> compact unwind frame
> description. The remaining 24-bits are an index into what the DWARF
> standard calls the
> .debug_frame section (called __eh_frame in LLVM).
> 
> 
> 3  Asynchronous Changes and Enhancements
> It is immediately obvious that omission of unwind information for leaf
> functions (with any kind
> of frame) precludes handling an exception that might occur during its
> execution. It follows that
> unwind information must cover all of the code of a module (with one
> exception discussed
> below). But if successive unwind groups are ordered (as previously
> assumed) and also leave no
> gaps, then the LENGTH field is redundant and can be omitted. The
> beginning address of a
> following group is always one byte past the end of the predecessor
> group. There remains only the
> question of how to identify the last group of a set.
> 
> It should also be clear that the unwind representation described in the
> prior section is not
> sufficient to unwind from an asynchronous exception that might occur in
> either the prologue or
> epilogue of a function. To see this consider what would happen if an
> exception occurred at either
> the entry point or the return instruction of either a RBP- or RSP-frame
> function. To be able to
> handle asynchronous exceptions at any point during function execution,
> it is necessary to add
> additional information to each unwind group.
> 
> These two considerations can be combined. The result is simply to
> repurpose the LENGTH field
> to encode prologue and epilogue information.
> 
> 3.1  Extended MODEs
> To preserve backward compatibility and to allow intermixing of
> traditional and extended
> compact unwind groups, new MODEs are defined as follows:
> 
> Code        Meaning                   Description
>
----------------------------------------------------------------------------------------------
> 8           Null frame                See below.
> 9           Extended RBP-based frame  Like mode 1 but combined with
> extended prologue/epilogue
>                                       information in place of a LENGTH
field
> 10          Extended RSP-based frame  Like mode 2 but combined with
> extended prologue/epilogue
>                                       information in place of a LENGTH
field
> 11          Extended Large RSP-based  Like mode 3 but combined with
> extended prologue/epilogue
>             frame                     information in place of a LENGTH
field
> 12          Extended parameter(s)     See below.
>  
> A null frame (MODE = 8) is the simplest possible frame, with no
> allocated stack of either kind
> (hence no saved registers).  A null frame might be considered just a
> special case of a RSP-based
> frame with a stack size of zero. But unlike other frames, its frame
> pointer is usually not 16-byte
> aligned.
>  
> It appears technically feasible for a null frame function to have a
> personality routine. However,
> the utility of such a capability seems too meager to justify allowing
> this. We propose to not
> support this.
> 
> There remains the question of whether it is necessary or at least
> desirable to strictly apply the
> requirement that “all” code be covered by an unwind group. Based on
> successful experience in
> OpenVMS on the Itanium architecture (as well as Windows systems we
> think), this alternative is
> proposed:
> 
>     If the first attempt to lookup an unwind group for an exception
> address fails, then it is
>     (tentatively) assumed to have occurred within a null frame function
> or in a part of a function
>     that is adequately described by a null frame. The presumed return
> address is (virtually or
>     actually) popped from the top of stack and looked up. This second
> attempted lookup must
>     succeed, in which case processing continues normally. A failure is a
> fatal error.
> 
> This concept of a “default null frame group” is especially convenient
> for dealing with
> disconnected code sequences such as trampolines, PLTs, and the like.
> 
> The extended RBP-based and RSP-based frame modes (MODEs = 9 and 10) are
> simple
> supersets of their more traditional predecessors.
> 
> The large RSP-based frame mode (MODE = 11) is also a superset of mode 3
> except that instead
> of finding the stack size in the instruction stream, it is found in a
> preceding extended parameter
> unwind group (MODE = 12). This difference is essential for support of
> execute-only code.
> 
> To understand the extended parameter group, suppose that two groups
> occur one after the other
> but have the same STARTING-ADDRESS value. A binary search using the
> STARTING-
> ADDRESS field will ignore the first of the pair because the address
> being sought can never be at
> least as large as the first but less than the second. Such a group can
> be used as an escape
> convention that allows inserting additional information into the
> sequence of groups that
> otherwise cannot be easily included.
> 
> The extended modes are described more fully in the following sections.
> 
> 3.2  Function Parts
> Functions are generally considered to consist of three distinct parts:
> 
> 1. A prologue, which allocates local storage, saves registers that must
> be preserved for the
>    benefit of callers, and possibly other housekeeping (such as
> initializing any local state
>    necessary for the correct management of the function’s execution).
> 
> 2. A body, which performs the main work of the function.
> 
> 3. An epilogue, which makes the result available to the caller, and
> undoes the effects of the
>    prologue (restores preserved registers, deallocates local storage,
> and so on).
> 
> Here a word of caution is in order. This vocabulary tends to be applied
> at multiple levels in
> software architecture and each level has its own set of issues to
> consider and resulting
> requirements to be observed.
> 
> Note that a null frame function has no distinct prologue, body or
> epilogue. Every instruction can
> be viewed as simultaneously in all three parts, or in none of them.
> 
> This leads to the following proposal for the upper part of the extended
> compact unwind
> quadword for use in combination with MODE = 8 in the lower part.
> 
> | 63                            48 | 47                          32 |
> |-------------------------------------------------------------------|
> | RESERVED                         | 0            ...             0 |
> |-------------------------------------------------------------------|
> 
> 
> | 31                            16 | 15                           0 |
> |-------------------------------------------------------------------|
> | 0 0 0 0 | 1 0 0 0 | 0   ...    0 | 0            ...             0 |
> |-------------------------------------------------------------------|
> 
> 
> 
> 3.2.1  Function Prologue
> For the purposes of exception handling, the key steps are:
> 
>  1. Allocate space on the stack in which to save preserved registers and
> for other purposes
>  2. Save the registers that must be preserved.
> 
> The DWARF call frame information (CFI) provides a description that is
> precise and accurate at
> each and every instruction in a function (not limited to just the
> prologue). But experience shows
> that this representation is bulky in space and expensive in time to
> decode and interpret. Indeed,
> compact unwind descriptors as described in Section 2 were created to
> avoid these issues.
> 
> Experience with OpenVMS on the Alpha and Itanium architectures shows
> that a key constraint
> can greatly simplify the unwind description: require that no preserved
> register can be changed
> until all of them have been saved. The last such save ends the prologue
> and the next instruction
> begins the body. An unwind does not need (must not) restore any
> preserved registers because
> they are still valid. A simple length from the entry point suffices to
> describe this boundary.
> [The body does not necessarily need to begin until at least one of the
> preserved registers is
> actually changed.]
>  
> 3.2.1.1  RSP-Based Frame (MODE = 10)
> For a RSP-based frame, it is also necessary to know which instruction
> adjusts the stack pointer:
> before this instruction an unwind must only (virtually or actually) pop
> the return address; and
> after this instruction, the stack pointer must first be incremented to
> deallocate the stack frame,
> then the instruction pointer restored.
> 
> This leads to the following proposal for the upper part of the extended
> compact unwind
> quadword for use in combination with MODE = 10 in the lower part.
> 
> | 63                       48 47 46             40 39            32 |
> |-------------------------------------------------------------------|
> | RESERVED                   |  |  PROLOGUE-SIZE2 |  PROLOGUE-SIZE1 |
> |-------------------------------------------------------------------|
> 
> 
> Except for the MODE, the lower part is exactly like it would be for MODE
> = 2.
> 
> PROLOGUE-SIZE1 is the length in bytes of the first part of the prologue
> relative to the
> STARTING-ADDRESS, up to and including the instruction that allocates the
> stack.
> 
> PROLOGUE-SIZE2 is the length in bytes of the second part of the prologue
> relative to the end
> of the first part, up to a point after the last preserved register is
> saved and before the first
> preserved register is changed. (Recall, this point may not be unique.)
> 
> PROLOGUE-SIZE1 + PROLOGUE-SIZE2 gives the total size of the prologue.
> 
> The maximum prologue size allowed here is much greater than will be
> typical. This is deliberate
> to allow compilers freedom to use shrink-wrap type optimizations in
> which safe operations that
> need no body state are moved from the body to the beginning of the
> prologue. This avoids the
> cost of stack frame setup and teardown for simple special case handling
> that often leads to an
> early exit.
> 
> 3.2.1.2  Large RSP-based Frame (MODE = 11)
> This case is like the previous, except the stack size is too large to
> encode in the compact unwind
> encoding.  The SIZE field is interpreted as an offset relative to the
> beginning of the containing
> unwind group to (what is necessarily) an earlier extended parameter group.
> 
> 3.2.1.3  RBP-Based Frame (MODE = 9)
> The RBP-based frame is the same as for a RSP-based frame except that
> there are two instructions
> that mark the transition between the two parts of the prologue: the push
> of the prior RBP value
> onto the stack followed by the copy of RSP to RBP to establish the new
> frame pointer.
> 
> There appears to be no reason to not make this simplifying requirement:
> the push and copy
> always occur together.
> 
> This leads to the following proposal for the upper part of the extended
> compact unwind
> quadword for use in combination with MODE = 9 in the lower part.
> 
> | 63                       48 47 46             40 39            32 |
> |-------------------------------------------------------------------|
> | RESERVED                   |  |  PROLOGUE-SIZE2 |  PROLOGUE-SIZE1 |
> |-------------------------------------------------------------------|
> 
> Except for the MODE, the lower part is exactly like it would be for MODE
> = 1.
> 
> PROLOGUE-SIZE1 is the length in bytes of the first part of the prologue
> relative to the
> STARTING-ADDRESS, up to and including the instruction that pushes the
> prior RBP contents.
> 
> PROLOGUE-SIZE2 is the length in bytes of the second part of the prologue
> relative to the end
> of the first part, up to a point after the last preserved register is
> saved and before the first
> preserved register is changed. (Recall, this point may not be unique.)
> 
> PROLOGUE-SIZE1 + PROLOGUE-SIZE2 gives the total size of the prologue.
> 
> 3.2.2  Function Epilogue
> For the purposes of exception handling, the key steps in an epilogue are:
> 
>  1. Restore the registers that were preserved
>  2. Deallocate space on the stack that was used to preserve registers
> and for other purposes
> 
> A key observation is that restoring registers is idempotent. That is, if
> in the midst of restoring
> registers when an exception occurs, it will do no harm if all of the
> preserved registers are
> restored again when unwinding. In effect, restoration of registers need
> not be distinguished from
> the body of a function. It is not until reaching the first instruction
> that deallocates stack is
> executed that the “real” epilogue begins.
> 
> 3.2.2.1  RSP-Based Frame (MODE = 10 or 11)
> For a RSP-based frame, whether small or large, the only unwind action
> remaining after stack
> deallocation is to pop the return address into the instruction pointer
> (RIP). In the vast majority of
> cases this means that the epilogue of the code described by an unwind
> group is just a 1-byte RET
> instruction that occurs at the end of the unwind group. There are other
> possibilities but they are
> rare and con be dealt with separately (see below).
> 
> It suffices to include a single 1-bit field, E, in the upper part of the
> extended compact unwind
> description:
> 
> | 63                       48 47 46             40 39            32 |
> |-------------------------------------------------------------------|
> | RESERVED                  | E |  PROLOGUE-SIZE2 |  PROLOGUE-SIZE1 |
> |-------------------------------------------------------------------|
> 
> 
> The E flag indicates that the unwind group ends with a standard 1-byte
> RET instruction.
> 
> 3.2.2.2  RBP-Based Frame (MODE = 9)
> The RBP-based frame is much the same as for a RSP-based frame except
> that there are two
> instructions that mark the transition between the two parts of the
> prologue: the copy of RBP to
> RSP to cut back the stack, followed by the pop of RBP to restore what
> may be the callers frame
> pointer. Thus the epilogue, for the purposes of unwinding, begins
> immediately after the copy of
> RBP to RSP. In the vast majority of cases, the immediately following two
> instructions will
> consist of POP RBP and RET. The POP is not idempotent because it changes
> the stack pointer,
> but the unwind processing code can distinguish whether the stack has
> been popped or not based
> on the address (3 versus 1 byte less than the highest address of the
group).
> 
> It suffices to include a single 1-bit field, E, in the upper part of the
> extended compact unwind
> description:
> 
> | 63                       48 47 46             40 39            32 |
> |-------------------------------------------------------------------|
> | RESERVED                  | E |  PROLOGUE-SIZE2 |  PROLOGUE-SIZE1 |
> |-------------------------------------------------------------------|
> 
> The E flag indicates that the unwind group ends with a standard
> 3-instruction return sequence.
> 
> 3.3  Special Issues
> Up until this point, it might appear that each unwind group corresponds
> one-for-one to the code
> for a single function. While common, this is not required. Important
> variations are illustrated
> here.
> 
> Issue/Problem                                        Possible Resolution
>
-------------------------------------------------------------------------------------------------
> The first part of the prologue is too large to be    Use two unwind groups:
> described by the 8-bit RxP-FRAME-SIZE1 field.        1) a null frame
> group of any appropriate size,
>                                                      2) a suitable
> RxP-based frame.
> 
> The second part of the prologue is too large to be   Use three unwind
> groups:
> described by the 8-bit RxP-FRAME-SIZE2 field.        1) a RSP-frame
> group to describe the first part of
>                                                      the prologue only
> (PROLOGUE-SIZE1 = the
>                                                      length of the
> entire group and PROLOGUE-SIZE2
>                                                      = 0,
>                                                      2) a RSP-frame
> group to describe the second part
>                                                      of the prologue
> (PROLOGUE-SIZE1 & 2 both = 0
>                                                      and no registers
> saved),
>                                                      3) a suitable
> RxP-based frame group (with
>                                                      PROLOGUE-SIZE1 & 2
> both = 0).
> 
> There is more than one return location (without a    Generally use one
> unwind group for each sequence
> shared epilogue sequence).                           of code ending with
> a RET. A group following a
>                                                      RET will have no
> prologue (PROLOGUE-SIZE1
>                                                      & 2 both = 0).
> 
> There is more than one entry point to a function     Generally use one
> unwind group for each sequence
> (for example, a FORTRAN multiple entry point         of code beginning
> at an entry point. Each group
> subroutine or analogous assembly language            may have a normal
> prologue and all but the last
> equivalents)                                         might have NO
> epilogue (E flag clear).
>  
> 
> These examples are not exhaustive, of course. But they illustrate the
> flexibility of the
> mechanism, which should be suitable for the vast majority of cases in
> practice.
> 
> 3.4  Number of Unwind Groups
> A simple concatenation of unwind groups by the linker that combines
> unwind group sections
> from multiple modules does not, of itself, provide direct information to
> the unwind software
> regarding how many unwind groups are present. However, the image
> information (ELF
> segments) should suffice to determine this number based on the size of
> the segment and the
> known size of each group. Alternatively, the linker might create special
> symbols that can be used
> to help determine the number of groups.
> 
> This proposal leaves this detail to target-specific linker, image loader
> and exception handling
> system to specify.
> 
> 3.5  Interaction with Code Generation
> The relatively simple and fixed size nature of the extended compact
> unwind information
> proposed here depends on observing certain restrictions on optimization
> that might affect code in
> prologues and epilogues. These restrictions were noted where relevant
> throughout this section.
> Here is a summary of those requirements:
> 
>  1. Use of any preserved register must be delayed until all of the
> preserved registers have
>     been saved.
>  2. In the prologue of a function with a RBP-based frame, the two
> instructions that save the
>     prior RBP contents and copy the stack pointer to RBP must be kept
> adjacent. Similarly,
>     in the epilogue of a function with a RBP-based frame, the two
> instructions that cut back
>     the stack and restore the prior RBP contents must be kept adjacent.
> [This adjacency
>     requirement could be relaxed by introducing an additional MODE that
> corresponds to having
>     the RBP saved in memory but not yet updated from RSP. This does not
> seem worthwhile.]
>  3. For shrink wrap optimization in the presence of a
> handler/personality routine, care must
>     be taken to not move instructions into the prologue that might cause
> an exception.
>     Floating point exceptions and access violations are of particular
> concern in this regard.
> 
> 4  System Specific Extensions for OpenVMS
> Omitted since nobody other than OpenVMS should care but it describes
> adding an additional
> four byte extension to the extended compact unwind group to pass along
> some information from
> the VAX Macro-32 compiler about VAX register usage.
> 
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

>Hi John & Ron, I read through the proposal and had a couple of quick
observations.

    Hi Jason. Thanks for the interesting observations and questions.
>1. The proposed encoding assumes that the epilogue instructions always
come at the end of the  function -- or rather, just before the next
function.  If there is a stack protector __stack_chk_fail sequence, or
there is NOP padding between functions, then the epilogue cannot be
expressed.  The proposed encoding allows for instructions scheduled
before the prologue, as long as they're guaranteed to never throw an
exception or spill registers, but there's no similar affordance for
after the epilogue.

    There are at least two alternatives for this situation:
    a.  The main group can be ended at the RET instruction and
        a new group started to cover the following bytes.
        Interestingly, both NOPs and a __stack_chk_fail call
        sequence are not really unwindable instructions.
        Perhaps a new NOUNWIND mode should be added for such
        cases; this would protect against an asynchronous
        exception attempting what is likely a dangerous stack
        walk.
    b.  Starting a new group is admittedly a rather large hammer
        that is perhaps OK if rare but not if it is common. If
        common, an alternative might be to widen the E (epilogue)
        field to allow more idiomatic alternatives. 4 bits might
        be enough: 8 codes for a RET followed by 0 to 7 NOPs +
        1 code for RET + __stack_chk_fail call + 1 code for
        just a __stack_chk_fail call + 1 code for no RET and
        nothing following. For anything else, fall back to a.
    Personally I rather like the combination of a. + b.
>2. In section 3.1, "A null frame (MODE = 8) is the simplest possible
frame, with no allocated stack of either kind (hence no saved
registers). A null frame might be considered just a special case of a
RSP-based frame with a stack size of zero. But unlike other frames,
its frame pointer is usually not 16-byte aligned." Did you meant stack
pointer here?  The x86_64 SysV abi requires 16-byte alignment of the
stack pointer before a CALL instruction, but I don't know of any
alignment requirements on the frame pointer.

    Yes, it would be clearer to say the stack pointer is not
    16-byte aligned.

    Aside: in my vocabulary, the frame pointer for a RSP-based
    or null frame function is the stack pointer. (And I find it
    an abuse of the English language to refer to a RSP-based
    frame as "frameless".) So what I wrote strictly is correct,
    but it does tend to confuse. Sorry...

3. In the same section, you propose an alternative "default null frame
group" be defined,
>"If the first attempt to lookup an unwind group for an exception
address fails, then it is (tentatively) assumed to have occurred
within a null frame function or in a part of a function that is
adequately described by a null frame. The presumed return address is
(virtually or actually) popped from the top of stack and looked up.
This second attempted lookup must succeed, in which case processing
continues normally. A failure is a fatal error."

I suspect I missed a part of the proposal covering this, but if
entries are described by only start address, is there a problem of the
previous non-null function's range extending & picking up these
functions if they don't have entries?

    For normal entries, I admit I sorta hand waved in Section 3.4
    over how to determine the end address of the last group. There
    are lots of alternatives. Most involve some kind of dummy
    extended parameter group (MODE =12) where the STARTING-ADDRESS
    is one more than the last address of the last "real" group.
    There might also be linker assistance of some kind. In any
    case I think it a problem that need not be solved upfront but
    can wait for broader issues to settle.

    The default unwind group has no starting address, of course.
    It is defined by what is left over after all other normal
    groups have been considered.
>4. For frameless functions -- small-stack RSP functions -- you're
assuming that the compiler saves and restores spilled registers with
MOV instructions instead of PUSH/POP instructions.  I think this is
the normal behavior of the compiler, something like

  subq    $0x98, %rsp
  movq    %rsi, 0x10(%rsp)
  movq    %rdi, 0x18(%rsp)

or whatever, but the proposed encoding can only allow for one change
in stack size.  The current encoding requires this for large-stack RSP
functions (we get the offset to the RSP instruction where we read the
amount being decremented in the compact unwind encoding), but for
small stack frameless functions the compiler can today express in
compact unwind a combination of sub and push instructions as long as
it's a fixed amount within the range that the small-stack RSP encoding
can express.

    You are quite right, with the understanding that "the range
    that the small-stack RSP encoding can express" is in fact the
    body of the function and excludes the prologue and epilogue.
    Our proposal, of course, is trying to describe the prologue and
    epilogue as well.

    My understanding of modern x86 processors is that there is
    no meaningful performance difference between using pushes/pops
    versus moves. That is, using only moves in prologues and
    epilogues is a negligible price to pay in our quest for
    a simple and compact unwind representation.
    But I do take the point that this should be mentioned in
    Section 3.5 regarding code generation implications.

Thanks again,
Ron



On Fri, Jan 26, 2018 at 7:19 PM, Jason Molenda via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi John & Ron, I read through the proposal and had a couple of quick
> observations.
> ...
>
>
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Hi Nick,

It is a pleasure to be in contact with the creator of the compact unwind
approach!

I can see how an array of 32-bit unwind blocks could be used to describe
each distinct point within a function (within a prolog in particular). But
then you end up with six or seven or more such blocks for a large
percentage of functions, don't you? Seems like a lot of additional space
for something that is usually simple, compact (sic) and idiomatic and
dilutes the original benefit. We are seeking a summary description in a
small fixed size to supplement the base unwind block.

Since you mention it, why have the UNWIND_IS_NOT_FUNCTION_START flag? Seems
like you don't need it for unwinding purposes. Especially in a function
that has a code layout where the entry point is not the first/lowest
addressed instruction of the function (which I have seen in some GEM
produced code for Alpha). An aside, but just curious.

Not being a MAC guy the macOS two-level lookup scheme has always been
rather a mystery. What little I know is gleaned from the
compact_unwind_encoding.h file found around the net. But I have never seen
the .c/.cxx/.cpp that goes with it to better understand how the mapping
works. Always figured the was considered proprietary but maybe I just don't
know where/how to look. Is there public code and/or design note information
to look at?

You are correct that this paper tends toward trying to create a scheme with
little or no linker involvement. There are good reasons to ask more of the
linker for better space economy, so this is still under discussion.

You are also correct that the unwinder needs to interpret the additional
prolog/epilog information. But the rest of your comment has me confused:
the extra 2-bytes is just a widening as you suggest, right? And there is no
Section 5.1--did you mean 3.1?

Thanks again,
Ron

On Fri, Jan 26, 2018 at 9:54 PM, Nick Kledzik via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> John and Ron,
>
> I developed the original compact unwind implementation for macOS 10.6 back
> in 2009.  I tried to leave space in the design to support finer grain
> exception handling such as for asynchronous or for the shrink wrap
> optimization.   The idea I had at the time was  instead of having just one
> 32-bit compact unwind info per function, there could be an array of them
> each covering a different range. All but the first would have the
> UNWIND_IS_NOT_FUNCTION_START bit set.
>
> On macOS, the linker takes the 32-byte “compact unwind group description”
> records and folds them down into a complete different (read only) runtime
> table which basically maps an offset in the DSO into a 32-bit compact info
> for that address.
>
> It seems like from your proposal that your linker keeps the “compact
> unwind group description” records as is and just concatenates them, and the
> runtime would need to understand the prolog/epilog encoding.  I would think
> that just widening the group record (as you suggest in 5.1) would be much
> simpler (like SHT_REL was widened to SHT_RELA).
>
> -Nick
>
>
>
> > On Jan 26, 2018, at 7:54 AM, John Reagan via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> > Here is our proposal to extend/enhance the x86-64 compact unwind
> > descriptors to fully describe the prologue/epilogue for asynchronous
> > unwinding.  I believe there are missing/lacking CFI directives as
well,
> > but I'll save that for another thread.
> >
> >
> > Asynchronous Compact Unwind Descriptors
> > Ron Brender, VMS Software, Inc.
> > Revised January 25, 2018
> >
> > 1  Introduction
> > This document proposes means to extend so-called compact unwind
> > descriptors to support fully
> > asynchronous exception handling. This will make extended compact
unwind
> > descriptors an
> > alternative to DWARF CFI (call frame information) for achieving
> > asynchronous exception
> > support.
> >
> > Compact unwind descriptors can and have been used in both 64- and
32-bit
> > environments.
> > However, this proposal addresses only 64-bit environments. While the
> > ideas presented here can
> > be readily adapted for use in a 32-bit environment, for simplicity
this
> > document makes no
> > attempt to do so.
> >
> > There are generally three kinds of information that together form the
> > heart of modern software
> > exception handling systems:
> >
> >  1. Information that is used to divide the remaining unwind
information
> > into groups that are
> >     specific to particular regions of memory (often, but not
> > necessarily, associated with a
> >     single function) as well as provide a way to efficiently search
for
> > and identify the
> >     grouping that is associated with a particular address in memory.
> >
> >  2. Information that can be used to virtually or actually unwind from
> > the call frame of an
> >     executing function to the call frame of its caller (at the point
of
> > the call).
> >
> >  3. Identification of an associated personality routine that is
invoked
> > by the general exception
> >     handling mechanization to guide how processing should proceed for
a
> > given function, as
> >     well as additional “language specific data” needed for the
> > personality routine to do its
> >     job. Note that the personality routine and its related data are
> > specified as an adjunct to
> >     compiled code and totally opaque to the general mechanism (other
> > than the specified
> >     interface).
> >
> > Note in particular that C++ exception handling is built on top of a
> > personality routine and
> > language specific data area ABI that itself can be implemented using
> > either DWARE CFI or
> > extended compact unwind information as described here. The choice
> > between the two is
> > transparent to C++.
> >
> >
> > 2  Compact Unwind Overview
> > This section provides a brief overview of key features of the LLVM
> > compact unwind design. It
> > does not attempt a comprehensive re-statement of all aspects of the
> > design except to the extent
> > necessary to motivate and understand later proposed changes and
> > enhancements.
> >
> > 2.1  Compact Unwind Group Description
> > A compact unwind group consist of five fields, as follows:
> >
> > | 63                             32 31                            0 |
> > |-------------------------------------------------------------------|
> > | STARTING-ADDRESS                                                  |
> > | LENGTH                           | COMPACT-UNWIND-DESCRIPTION     |
> > | PERSONALITY-FUNCTION-POINTER                                      |
> > | LANGUAGE-SPECIFIC-DATA-ADDRESS                                    |
> > |-------------------------------------------------------------------|
> >
> > STARTING-ADDRESS (64-bits) is the lowest address of a region of memory
> > occupied by some
> > code, typically the entry point of a function.
> >
> > LENGTH (32-bits) is the number of bytes included in this group,
> > typically including all and only
> > the code of a function.
> >
> > COMPACT-UNWIND-DESCRIPTION (32-bits) is a description of the fully
> > formed frame of a
> > function and how to unwind it. This is described further following.
> >
> > PERSONALITY-FUNCTION-POINTER (64-bit) is a pointer to the personality
> > routine.
> >
> > LANGUAGE-SPECIFIC-DATA-ADDRESS (64-bits, sometimes abbreviated LSDA)
is a
> > pointer to some data to be passed to the personality routine when it
is
> > called.
> >
> > A key observation is that the starting address plus length way of
> > describing a group means that
> > the set of groups for a compilation unit need not describe all of the
> > code in that unit. In
> > particular, it appears to be expected that no unwind information need
be
> > generated for leaf
> > functions.
> >
> > On the other hand, it is reasonable to expect that the groups that are
> > emitted are ordered by the
> > starting address. This means that a simple and fast binary search can
be
> > used to map an address
> > to the group that applies to that address, if any.
> >
> > It is useful to note that the run-time representation of unwind
> > information can vary from little
> > more than a simple concatenation of the compile-time information to a
> > substantial rewriting of
> > unwind information by the linker. The proposal favors simple
> > concatenation while maintaining
> > the same ordering of groups as their associated code.
> >
> > 2.2  Compact Unwind Frame Description
> > A compact unwind frame description describes a frame in sufficient
> > detail to be able to unwind
> > that frame to the frame of its caller.
> >
> > | 31    28 27    24 23                                            0 |
> > |-------------------------------------------------------------------|
> > | FLAGS   | MODE   |                                                |
> > |-------------------------------------------------------------------|
> >
> > At the top most level, there are four bits that are not of further
> > interest here. Interpretation of
> > these bits is neither used nor changed.
> >
> > Also at the top-level is a 4-bit mode field. This is the tag of a
> > discriminated (tagged, variant)
> > union that selects the interpretation of the remaining 24 bits.
> >
> > Of the 16 possible modes, only 5 are defined :
> >
> > Code    Meaning      Description
> > 0       Old          “Old” is presumed to refer to some historical
> > design that is no longer of interest.
> >                      It is treated here as Reserved.
> > 1       RBP-based    The frame uses the RBP register as a frame
pointer.
> > The size of the frame can
> >         frame        vary during execution.
> > 2       RSP-based    The frame uses RSP as the frame pointer. The size
> > of the frame is fixed (at
> >         frame        compilation time).
> > 3       Large RSP-   The frame uses RSP as the frame pointer, The size
> > of the frame is fixed (at
> >         based frame  compilation time); however, that size is too
large
> > to express within this 32-bit
> >                      descriptor encoding.
> > 4       DWARF        The frame, for whatever reason, cannot be
> > adequately described using the
> >         escape       compact unwind frame description. The remaining
> > 24-bits are an index into
> >                      what the DWARF standard calls the .debug_frame
> > section (__eh_frame in
> >                      LLVM).
> >
> > 2.2.1  RBP-based Frame (MODE=1)
> > For a RBP-based frame, the remaining 24 bits are encoded as follows:
> >
> > | 23        16 | 15 | 14                                          0 |
> > |-------------------------------------------------------------------|
> > | OFFSET       | 0  | REGS                                          |
> > |-------------------------------------------------------------------|
> >
> > In a RBP-based frame the RBP register is pushed on the stack
immediately
> > after the return
> > address, then RSP is moved to RBP. To unwind, RSP is restored with the
> > current RPB value,
> > then RBP is restored by popping off the stack, and the return is done
by
> > popping the stack once
> > more into the instruction pointer.
> >
> > All preserved registers are saved in a small range in the stack that
> > starts at RBP-8 to RBP-2040.
> > The offset/8 relative to RBP is encoded in the 8-bit OFFSET field. The
> > registers saved are
> > encoded in the 15-bit REGS field as five 3-bit entries.
> >
> > 2.2.2  RSP-Based Frame (MODE=2)
> > For a RSP-based frame, the remaining 24 bits are encoded as follows:
> >
> > |23        16 | 15      13 | 12      10 | 9                       0 |
> > |-------------------------------------------------------------------|
> > | SIZE        |            | CNT        | REG_PERM                  |
> > |-------------------------------------------------------------------|
> >
> > In a RSP-based frame the stack pointer serves directly as the frame
> > pointer and RBP is available
> > for use as a general register. Upon entry, the stack pointer is
> > decremented by 8*SIZE bytes (the
> > maximum stack allocation is thus 2040 bytes). To unwind, the stack
size
> > is added to the stack
> > pointer, and completed by popping the stack once more into the
> > instruction pointer.
> >
> > All preserved registers are saved on the stack immediately after the
> > return address.  The number
> > of registers saved (up to 6) is encoded in the 3-bit CNT field. The
> > 11-bit REG_PERM field
> > encodes which registers were saved and in what order.
> >
> > 2.2.3  Large RSP-Based Frame (MODE=3)
> > For a large RSP-based frame, the remaining 24 bits are encoded as
> follows:
> >
> > |23        16 | 15      13 | 12      10 | 9                       0 |
> > |-------------------------------------------------------------------|
> > | SIZE        | ADJ        | CNT        | REG_PERM                  |
> > |-------------------------------------------------------------------|
> >
> > This case is like the previous, except the stack size is too large to
> > encode in the compact unwind
> > encoding.  Instead, the function must include a "subq $nnnnnnnn,
RSP"
> > instruction in its
> > prologue to allocate the stack.  The offset from the entry point of
the
> > function to the nnnnnnnn
> > value in the function is given in the SIZE field.
> >
> > Depending on the exact instructions used to save registers (PUSH
versus
> > MOV), the nnnnnnnn
> > value in the instruction stream may not be quite the full stack size.
> > ADJ * 8 is the additional
> > adjustment needed to get the actual size.
> >
> > 2.2.4  DWARF Escape (MODE=4)
> > The frame, for whatever reason, cannot be adequately described using a
> > compact unwind frame
> > description. The remaining 24-bits are an index into what the DWARF
> > standard calls the
> > .debug_frame section (called __eh_frame in LLVM).
> >
> >
> > 3  Asynchronous Changes and Enhancements
> > It is immediately obvious that omission of unwind information for leaf
> > functions (with any kind
> > of frame) precludes handling an exception that might occur during its
> > execution. It follows that
> > unwind information must cover all of the code of a module (with one
> > exception discussed
> > below). But if successive unwind groups are ordered (as previously
> > assumed) and also leave no
> > gaps, then the LENGTH field is redundant and can be omitted. The
> > beginning address of a
> > following group is always one byte past the end of the predecessor
> > group. There remains only the
> > question of how to identify the last group of a set.
> >
> > It should also be clear that the unwind representation described in
the
> > prior section is not
> > sufficient to unwind from an asynchronous exception that might occur
in
> > either the prologue or
> > epilogue of a function. To see this consider what would happen if an
> > exception occurred at either
> > the entry point or the return instruction of either a RBP- or
RSP-frame
> > function. To be able to
> > handle asynchronous exceptions at any point during function execution,
> > it is necessary to add
> > additional information to each unwind group.
> >
> > These two considerations can be combined. The result is simply to
> > repurpose the LENGTH field
> > to encode prologue and epilogue information.
> >
> > 3.1  Extended MODEs
> > To preserve backward compatibility and to allow intermixing of
> > traditional and extended
> > compact unwind groups, new MODEs are defined as follows:
> >
> > Code        Meaning                   Description
> > ------------------------------------------------------------
> ----------------------------------
> > 8           Null frame                See below.
> > 9           Extended RBP-based frame  Like mode 1 but combined with
> > extended prologue/epilogue
> >                                       information in place of a LENGTH
> field
> > 10          Extended RSP-based frame  Like mode 2 but combined with
> > extended prologue/epilogue
> >                                       information in place of a LENGTH
> field
> > 11          Extended Large RSP-based  Like mode 3 but combined with
> > extended prologue/epilogue
> >             frame                     information in place of a LENGTH
> field
> > 12          Extended parameter(s)     See below.
> >
> > A null frame (MODE = 8) is the simplest possible frame, with no
> > allocated stack of either kind
> > (hence no saved registers).  A null frame might be considered just a
> > special case of a RSP-based
> > frame with a stack size of zero. But unlike other frames, its frame
> > pointer is usually not 16-byte
> > aligned.
> >
> > It appears technically feasible for a null frame function to have a
> > personality routine. However,
> > the utility of such a capability seems too meager to justify allowing
> > this. We propose to not
> > support this.
> >
> > There remains the question of whether it is necessary or at least
> > desirable to strictly apply the
> > requirement that “all” code be covered by an unwind group. Based on
> > successful experience in
> > OpenVMS on the Itanium architecture (as well as Windows systems we
> > think), this alternative is
> > proposed:
> >
> >     If the first attempt to lookup an unwind group for an exception
> > address fails, then it is
> >     (tentatively) assumed to have occurred within a null frame
function
> > or in a part of a function
> >     that is adequately described by a null frame. The presumed return
> > address is (virtually or
> >     actually) popped from the top of stack and looked up. This second
> > attempted lookup must
> >     succeed, in which case processing continues normally. A failure is
a
> > fatal error.
> >
> > This concept of a “default null frame group” is especially convenient
> > for dealing with
> > disconnected code sequences such as trampolines, PLTs, and the like.
> >
> > The extended RBP-based and RSP-based frame modes (MODEs = 9 and 10)
are
> > simple
> > supersets of their more traditional predecessors.
> >
> > The large RSP-based frame mode (MODE = 11) is also a superset of mode
3
> > except that instead
> > of finding the stack size in the instruction stream, it is found in a
> > preceding extended parameter
> > unwind group (MODE = 12). This difference is essential for support of
> > execute-only code.
> >
> > To understand the extended parameter group, suppose that two groups
> > occur one after the other
> > but have the same STARTING-ADDRESS value. A binary search using the
> > STARTING-
> > ADDRESS field will ignore the first of the pair because the address
> > being sought can never be at
> > least as large as the first but less than the second. Such a group can
> > be used as an escape
> > convention that allows inserting additional information into the
> > sequence of groups that
> > otherwise cannot be easily included.
> >
> > The extended modes are described more fully in the following sections.
> >
> > 3.2  Function Parts
> > Functions are generally considered to consist of three distinct parts:
> >
> > 1. A prologue, which allocates local storage, saves registers that
must
> > be preserved for the
> >    benefit of callers, and possibly other housekeeping (such as
> > initializing any local state
> >    necessary for the correct management of the function’s execution).
> >
> > 2. A body, which performs the main work of the function.
> >
> > 3. An epilogue, which makes the result available to the caller, and
> > undoes the effects of the
> >    prologue (restores preserved registers, deallocates local storage,
> > and so on).
> >
> > Here a word of caution is in order. This vocabulary tends to be
applied
> > at multiple levels in
> > software architecture and each level has its own set of issues to
> > consider and resulting
> > requirements to be observed.
> >
> > Note that a null frame function has no distinct prologue, body or
> > epilogue. Every instruction can
> > be viewed as simultaneously in all three parts, or in none of them.
> >
> > This leads to the following proposal for the upper part of the
extended
> > compact unwind
> > quadword for use in combination with MODE = 8 in the lower part.
> >
> > | 63                            48 | 47                          32 |
> > |-------------------------------------------------------------------|
> > | RESERVED                         | 0            ...             0 |
> > |-------------------------------------------------------------------|
> >
> >
> > | 31                            16 | 15                           0 |
> > |-------------------------------------------------------------------|
> > | 0 0 0 0 | 1 0 0 0 | 0   ...    0 | 0            ...             0 |
> > |-------------------------------------------------------------------|
> >
> >
> >
> > 3.2.1  Function Prologue
> > For the purposes of exception handling, the key steps are:
> >
> >  1. Allocate space on the stack in which to save preserved registers
and
> > for other purposes
> >  2. Save the registers that must be preserved.
> >
> > The DWARF call frame information (CFI) provides a description that is
> > precise and accurate at
> > each and every instruction in a function (not limited to just the
> > prologue). But experience shows
> > that this representation is bulky in space and expensive in time to
> > decode and interpret. Indeed,
> > compact unwind descriptors as described in Section 2 were created to
> > avoid these issues.
> >
> > Experience with OpenVMS on the Alpha and Itanium architectures shows
> > that a key constraint
> > can greatly simplify the unwind description: require that no preserved
> > register can be changed
> > until all of them have been saved. The last such save ends the
prologue
> > and the next instruction
> > begins the body. An unwind does not need (must not) restore any
> > preserved registers because
> > they are still valid. A simple length from the entry point suffices to
> > describe this boundary.
> > [The body does not necessarily need to begin until at least one of the
> > preserved registers is
> > actually changed.]
> >
> > 3.2.1.1  RSP-Based Frame (MODE = 10)
> > For a RSP-based frame, it is also necessary to know which instruction
> > adjusts the stack pointer:
> > before this instruction an unwind must only (virtually or actually)
pop
> > the return address; and
> > after this instruction, the stack pointer must first be incremented to
> > deallocate the stack frame,
> > then the instruction pointer restored.
> >
> > This leads to the following proposal for the upper part of the
extended
> > compact unwind
> > quadword for use in combination with MODE = 10 in the lower part.
> >
> > | 63                       48 47 46             40 39            32 |
> > |-------------------------------------------------------------------|
> > | RESERVED                   |  |  PROLOGUE-SIZE2 |  PROLOGUE-SIZE1 |
> > |-------------------------------------------------------------------|
> >
> >
> > Except for the MODE, the lower part is exactly like it would be for
MODE
> > = 2.
> >
> > PROLOGUE-SIZE1 is the length in bytes of the first part of the
prologue
> > relative to the
> > STARTING-ADDRESS, up to and including the instruction that allocates
the
> > stack.
> >
> > PROLOGUE-SIZE2 is the length in bytes of the second part of the
prologue
> > relative to the end
> > of the first part, up to a point after the last preserved register is
> > saved and before the first
> > preserved register is changed. (Recall, this point may not be unique.)
> >
> > PROLOGUE-SIZE1 + PROLOGUE-SIZE2 gives the total size of the prologue.
> >
> > The maximum prologue size allowed here is much greater than will be
> > typical. This is deliberate
> > to allow compilers freedom to use shrink-wrap type optimizations in
> > which safe operations that
> > need no body state are moved from the body to the beginning of the
> > prologue. This avoids the
> > cost of stack frame setup and teardown for simple special case
handling
> > that often leads to an
> > early exit.
> >
> > 3.2.1.2  Large RSP-based Frame (MODE = 11)
> > This case is like the previous, except the stack size is too large to
> > encode in the compact unwind
> > encoding.  The SIZE field is interpreted as an offset relative to the
> > beginning of the containing
> > unwind group to (what is necessarily) an earlier extended parameter
> group.
> >
> > 3.2.1.3  RBP-Based Frame (MODE = 9)
> > The RBP-based frame is the same as for a RSP-based frame except that
> > there are two instructions
> > that mark the transition between the two parts of the prologue: the
push
> > of the prior RBP value
> > onto the stack followed by the copy of RSP to RBP to establish the new
> > frame pointer.
> >
> > There appears to be no reason to not make this simplifying
requirement:
> > the push and copy
> > always occur together.
> >
> > This leads to the following proposal for the upper part of the
extended
> > compact unwind
> > quadword for use in combination with MODE = 9 in the lower part.
> >
> > | 63                       48 47 46             40 39            32 |
> > |-------------------------------------------------------------------|
> > | RESERVED                   |  |  PROLOGUE-SIZE2 |  PROLOGUE-SIZE1 |
> > |-------------------------------------------------------------------|
> >
> > Except for the MODE, the lower part is exactly like it would be for
MODE
> > = 1.
> >
> > PROLOGUE-SIZE1 is the length in bytes of the first part of the
prologue
> > relative to the
> > STARTING-ADDRESS, up to and including the instruction that pushes the
> > prior RBP contents.
> >
> > PROLOGUE-SIZE2 is the length in bytes of the second part of the
prologue
> > relative to the end
> > of the first part, up to a point after the last preserved register is
> > saved and before the first
> > preserved register is changed. (Recall, this point may not be unique.)
> >
> > PROLOGUE-SIZE1 + PROLOGUE-SIZE2 gives the total size of the prologue.
> >
> > 3.2.2  Function Epilogue
> > For the purposes of exception handling, the key steps in an epilogue
are:
> >
> >  1. Restore the registers that were preserved
> >  2. Deallocate space on the stack that was used to preserve registers
> > and for other purposes
> >
> > A key observation is that restoring registers is idempotent. That is,
if
> > in the midst of restoring
> > registers when an exception occurs, it will do no harm if all of the
> > preserved registers are
> > restored again when unwinding. In effect, restoration of registers
need
> > not be distinguished from
> > the body of a function. It is not until reaching the first instruction
> > that deallocates stack is
> > executed that the “real” epilogue begins.
> >
> > 3.2.2.1  RSP-Based Frame (MODE = 10 or 11)
> > For a RSP-based frame, whether small or large, the only unwind action
> > remaining after stack
> > deallocation is to pop the return address into the instruction pointer
> > (RIP). In the vast majority of
> > cases this means that the epilogue of the code described by an unwind
> > group is just a 1-byte RET
> > instruction that occurs at the end of the unwind group. There are
other
> > possibilities but they are
> > rare and con be dealt with separately (see below).
> >
> > It suffices to include a single 1-bit field, E, in the upper part of
the
> > extended compact unwind
> > description:
> >
> > | 63                       48 47 46             40 39            32 |
> > |-------------------------------------------------------------------|
> > | RESERVED                  | E |  PROLOGUE-SIZE2 |  PROLOGUE-SIZE1 |
> > |-------------------------------------------------------------------|
> >
> >
> > The E flag indicates that the unwind group ends with a standard 1-byte
> > RET instruction.
> >
> > 3.2.2.2  RBP-Based Frame (MODE = 9)
> > The RBP-based frame is much the same as for a RSP-based frame except
> > that there are two
> > instructions that mark the transition between the two parts of the
> > prologue: the copy of RBP to
> > RSP to cut back the stack, followed by the pop of RBP to restore what
> > may be the callers frame
> > pointer. Thus the epilogue, for the purposes of unwinding, begins
> > immediately after the copy of
> > RBP to RSP. In the vast majority of cases, the immediately following
two
> > instructions will
> > consist of POP RBP and RET. The POP is not idempotent because it
changes
> > the stack pointer,
> > but the unwind processing code can distinguish whether the stack has
> > been popped or not based
> > on the address (3 versus 1 byte less than the highest address of the
> group).
> >
> > It suffices to include a single 1-bit field, E, in the upper part of
the
> > extended compact unwind
> > description:
> >
> > | 63                       48 47 46             40 39            32 |
> > |-------------------------------------------------------------------|
> > | RESERVED                  | E |  PROLOGUE-SIZE2 |  PROLOGUE-SIZE1 |
> > |-------------------------------------------------------------------|
> >
> > The E flag indicates that the unwind group ends with a standard
> > 3-instruction return sequence.
> >
> > 3.3  Special Issues
> > Up until this point, it might appear that each unwind group
corresponds
> > one-for-one to the code
> > for a single function. While common, this is not required. Important
> > variations are illustrated
> > here.
> >
> > Issue/Problem                                        Possible
Resolution
> > ------------------------------------------------------------
> -------------------------------------
> > The first part of the prologue is too large to be    Use two unwind
> groups:
> > described by the 8-bit RxP-FRAME-SIZE1 field.        1) a null frame
> > group of any appropriate size,
> >                                                      2) a suitable
> > RxP-based frame.
> >
> > The second part of the prologue is too large to be   Use three unwind
> > groups:
> > described by the 8-bit RxP-FRAME-SIZE2 field.        1) a RSP-frame
> > group to describe the first part of
> >                                                      the prologue only
> > (PROLOGUE-SIZE1 = the
> >                                                      length of the
> > entire group and PROLOGUE-SIZE2
> >                                                      = 0,
> >                                                      2) a RSP-frame
> > group to describe the second part
> >                                                      of the prologue
> > (PROLOGUE-SIZE1 & 2 both = 0
> >                                                      and no registers
> > saved),
> >                                                      3) a suitable
> > RxP-based frame group (with
> >                                                      PROLOGUE-SIZE1
& 2
> > both = 0).
> >
> > There is more than one return location (without a    Generally use one
> > unwind group for each sequence
> > shared epilogue sequence).                           of code ending
with
> > a RET. A group following a
> >                                                      RET will have no
> > prologue (PROLOGUE-SIZE1
> >                                                      & 2 both =
0).
> >
> > There is more than one entry point to a function     Generally use one
> > unwind group for each sequence
> > (for example, a FORTRAN multiple entry point         of code beginning
> > at an entry point. Each group
> > subroutine or analogous assembly language            may have a normal
> > prologue and all but the last
> > equivalents)                                         might have NO
> > epilogue (E flag clear).
> >
> >
> > These examples are not exhaustive, of course. But they illustrate the
> > flexibility of the
> > mechanism, which should be suitable for the vast majority of cases in
> > practice.
> >
> > 3.4  Number of Unwind Groups
> > A simple concatenation of unwind groups by the linker that combines
> > unwind group sections
> > from multiple modules does not, of itself, provide direct information
to
> > the unwind software
> > regarding how many unwind groups are present. However, the image
> > information (ELF
> > segments) should suffice to determine this number based on the size of
> > the segment and the
> > known size of each group. Alternatively, the linker might create
special
> > symbols that can be used
> > to help determine the number of groups.
> >
> > This proposal leaves this detail to target-specific linker, image
loader
> > and exception handling
> > system to specify.
> >
> > 3.5  Interaction with Code Generation
> > The relatively simple and fixed size nature of the extended compact
> > unwind information
> > proposed here depends on observing certain restrictions on
optimization
> > that might affect code in
> > prologues and epilogues. These restrictions were noted where relevant
> > throughout this section.
> > Here is a summary of those requirements:
> >
> >  1. Use of any preserved register must be delayed until all of the
> > preserved registers have
> >     been saved.
> >  2. In the prologue of a function with a RBP-based frame, the two
> > instructions that save the
> >     prior RBP contents and copy the stack pointer to RBP must be kept
> > adjacent. Similarly,
> >     in the epilogue of a function with a RBP-based frame, the two
> > instructions that cut back
> >     the stack and restore the prior RBP contents must be kept
adjacent.
> > [This adjacency
> >     requirement could be relaxed by introducing an additional MODE
that
> > corresponds to having
> >     the RBP saved in memory but not yet updated from RSP. This does
not
> > seem worthwhile.]
> >  3. For shrink wrap optimization in the presence of a
> > handler/personality routine, care must
> >     be taken to not move instructions into the prologue that might
cause
> > an exception.
> >     Floating point exceptions and access violations are of particular
> > concern in this regard.
> >
> > 4  System Specific Extensions for OpenVMS
> > Omitted since nobody other than OpenVMS should care but it describes
> > adding an additional
> > four byte extension to the extended compact unwind group to pass along
> > some information from
> > the VAX Macro-32 compiler about VAX register usage.
> >
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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