llvm dev - May 2015 - [LLVMdev] New EH representation for MSVC compatibility

After a long tale of sorrow and woe, my colleagues and I stand here before
you defeated. The Itanium EH representation is not amenable to implementing
MSVC-compatible exceptions. We need a new representation that preserves
information about how try-catch blocks are nested.

WinEH background
-------------------------------

Skip this if you already know a lot about Windows exceptions. On Windows,
every exceptional action that you can imagine is a function call. Throwing
an exception is a call. Destructor cleanups and finally blocks are calls to
outlined functions that run the cleanup code. Even catching an exception is
implemented as an outlined catch handler function which returns the address
of the basic block at which normal execution should continue.

This is *not* how Itanium landingpads work, where cleanups and catches are
executed after unwinding and clearing old function frames off the stack.
The transition to a landingpad is *not* like a function call, and this is
the only special control transfer used for Itanium EH. In retrospect,
having exactly one kind of control transfer turns out to be a great design
simplification. Go Itanium!

Instead, all MSVC EH personality functions (x86, x64, ARM) cross (C++, SEH)
are implemented with interval tables that express the nesting levels of
various source constructs like destructors, try ranges, catch ranges, etc.
When you rinse your program through LLVM IR today, this structure is what
gets lost.

New information
-------------------------

Recently, we have discovered that the tables for __CxxFrameHandler3 have
the additional constraint that the EH states assigned to a catch body must
immediately follow the state numbers assigned to the try body. The natural
scoping rules of C++ make it so that doing this numbering at the source
level is trivial, but once we go to LLVM IR CFG soup, scopes are gone. If
you want to know exactly what corner cases break down, search the bug
database and mailing lists. The explanations are too long for this RFC.


New representation
------------------------------

I propose adding the following new instructions, all of which (except for
resume) are glued to the top of their basic blocks, just like landingpads.
They all have an optional ‘unwind’ label operand, which provides the IR
with a tree-like structure of what EH action to take after this EH action
completes. The unwind label only participates in the formation of the CFG
when used in a catch block, and in other blocks it is considered opaque,
personality-specific information. If the unwind label is missing, then
control leaves the function after the EH action is completed. If a function
is inlined, EH blocks with missing unwind labels are wired up to the unwind
label used by the inlined call site.

The new representation is designed to be able to represent Itanium EH in
case we want to converge on a single EH representation in LLVM and Clang.
An IR pass can convert these actions to landingpads, typeid selector
comparisons, and branches, which means we can phase this representation in
on Windows at first and experiment with it slowly on other platforms. Over
time, we can move the landingpad conversion lower and lower in the stack
until it’s moved into DwarfEHPrepare. We’ll need to support landingpads at
least until LLVM 4.0, but we may want to keep them because they are the
natural representation for Itanium-style EH, and have a relatively low
support burden.

resume
-------------

; Old form still works, still means control is leaving the function.
resume <valty> %val
; New form overloaded for intra-frame unwinding or resuming normal execution
resume <valty> %val, label %nextaction
; New form for EH personalities that produce no value
resume void

Now resume takes an optional label operand which is the next EH action to
run. The label must point to a block starting with an EH action. The
various EH action blocks impose personality-specific rules about what the
targets of the resume can be.

catchblock
---------------

%val = catchblock <valty> [i8* @typeid.int, i32 7, i32* %e.addr]
    to label %catch.int unwind label %nextaction

The catchblock is a terminator that conditionally selects which block to
execute based on the opaque operands interpreted by the personality
function. If the exception is caught, the ‘to’ block is executed. If
unwinding should continue, the ‘unwind’ block is executed. Because the
catchblock is a terminator, no instructions can be inserted into a
catchblock. The MSVC personality function requires more than just a pointer
to RTTI data, so a variable list of operands is accepted. For an Itanium
personality, only one RTTI operand is needed. The ‘unwind’ label of a
catchblock must point to a catchend.

catchendblock
----------------

catchend unwind label %nextaction

The catchend is a terminator that unconditionally unwinds to the next
action. It is merely a placeholder to help reconstruct which invokes were
part of the catch blocks of a try. Invokes that are reached after a
catchblock without following any unwind edges must transitively unwind to
the first catchend block that the catchblock unwinds to. Executing such an
invoke that does not transitively unwind to the correct catchend block has
undefined behavior.

cleanupblock
--------------------

%val = cleanupblock <valty> unwind label %nextaction

This is not a terminator, and control is expected to flow into a resume
instruction which indicates which EH block runs next. If the resume
instruction and the unwind label disagree, behavior is undefined.

terminateblock
----------------------

; for noexcept
terminateblock [void ()* @std.terminate] unwind label %nextaction
; for exception specifications, throw(int)
terminateblock [void ()* @__cxa_unexpected, @typeid.int, ...] unwind label
%nextaction

This is a terminator, and the unwind label is where execution will continue
if the program continues execution. It also has an opaque,
personality-specific list of constant operands interpreted by the backend
of LLVM. The convention is that the first operand is the function to call
to end the program, and the rest determine if the program should end.

sehfilterblock?
------------------

One big hole in the new representation is SEH filter expressions. They
present a major complication because they do not follow a stack discipline.
Any EH action is reachable after an SEH filter runs. Because the CFG is so
useless for optimization purposes, it’s better to outline the filter in the
frontend and assume the filter can run during any potentially throwing
function call.

MSVC EH implementation strategy
----------------------------------------------

Skim this if you just need the semantics of the representation above, and
not the implementation details.

The new EH block representation allows WinEHPrepare to get a lot simpler.
EH blocks should now look a lot more familiar, they are single entry,
multi-exit regions of code. This is exactly equivalent to a function, and
we can call them funclets. The plan is to generate code for the parent
function first, skipping all exceptional blocks, and then generate separate
MachineFunctions for each subfunction in turn. I repeat, we can stop doing
outlining in IR. This was just a mistake, because I was afraid of grappling
with CodeGen.

WinEHPrepare will have two jobs now:
1. Mark down which basic blocks are reachable from which handler. Duplicate
any blocks that are reachable from two handlers until each block belongs to
exactly one funclet, pruning as many unreachable CFG edges as possible.
2. Demote SSA values that are defined in a funclet and used in another
funclet.

The instruction selection pass is the pass that builds MachineFunctions
from IR Functions. This is the pass that will be responsible for the split.
It will maintain information about the offsets of static allocas in
FunctionLoweringInfo, and will throw it away when all funclets have been
generated for this function. This means we don’t need to insert
framerecover calls anymore.

Generating EH state numbers for the TryBlockMap and StateUnwindTable is a
matter of building a tree of EH blocks and invokes. Every unwind edge from
an invoke or an EH block represents that the instruction is a child of the
target block. If the unwind edge is empty, it is a child of the parent
function, which is the root node of the tree. State numbers can be assigned
by doing a DFS traversal where invokes are visited before EH blocks, and EH
blocks can be visited in an arbitrary-but-deterministic order that vaguely
corresponds to source order. Invokes are immediately assigned the current
state number. Upon visiting an EH block, the state number is recorded as
the “low” state of the block. All invokes are assigned this state number.
The state number is incremented, and each child EH block is visited,
passing in the state number and producing a new state number. The final
state number is returned to the parent node.

Example IR from Clang
----------------------------------------

The C++:

struct Obj { ~Obj(); };
void f(int);
void foo() noexcept {
  try {
    f(1);
    Obj o;
    f(2);
  } catch (int e) {
    f(3);
    try {
      f(4);
    } catch (...) {
      f(5);
    }
  }
}

The IR for __CxxFrameHandler3:

define void @foo() personality i32 (...)* @__CxxFrameHandler3 {
  %e.addr = alloca i32
  invoke void @f(i32 1)
    to label %cont1 unwind label %maycatch.int
cont1:
  invoke void @f(i32 2)
    to label %cont2 unwind label %cleanup.Obj
cont2:
  call void @~Obj()
  br label %return
return:
  ret void

cleanup.Obj:
  cleanupblock unwind label %maycatch.int
  call void @~Obj()
  resume label %maycatch.int

maycatch.int:
  catchblock void [i8* @typeid.int, i32 7, i32* %e.addr]
    to label %catch.int unwind label %catchend1
catch.int:
  invoke void @f(i32 3)
    to label %cont3 unwind label %catchend1
cont3:
  invoke void @f(i32 4)
    to label %cont4 unwind label %maycatch.all
cont4:
  resume label %return

maycatch.all:
  catchblock void [i8* null, i32 0, i8* null]
    to label %catch.all unwind label %catchend2
catch.all:
  invoke void @f(i32 5)
    to label %cont5 unwind label %catchend2
cont5:
  resume label %cont4

catchend2:
  catchendblock unwind label %catchend1

catchend1:
  catchendblock unwind label %callterminate

callterminate:
  terminateblock [void ()* @std.terminate]
}
>From this IR, we can recover the original scoped nesting form that the
table formation requires.

I think that covers it. Feedback welcome. :)
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I like the way this sorts out with regard to funclet code generation.  It feels
very natural for Windows EH, though obviously not as natural for non-Windows
targets and I think it is likely to block some optimizations that are currently
possible with those targets.

> If the unwind label is missing, then control leaves the function after the
EH action is completed. If a function is inlined, EH blocks with missing unwind
labels are wired up to the unwind label used by the inlined call site.
Is this saying that a “missing” unwind label corresponds to telling the runtime
to continue the search at the next frame?

Your example looks wrong in this regard, unless I’m misunderstanding it.  It
looks like any exceptions that aren’t caught in that function will lead to a
terminate call.

> Invokes that are reached after a catchblock without following any unwind
edges must transitively unwind to the first catchend block that the catchblock
unwinds to.
I’m not sure I understand this correctly.  In particular, I’m confused about the
roles of resume and catchend.

> %val = cleanupblock <valty> unwind label %nextaction
Why isn’t this a terminator?  It seems like it performs the same sort of role as
catchblock, except presumably it is always entered.  I suppose that’s probably
the answer to my question, but it strikes me as an ambiguity in the scheme.  The
catchblock instruction is more or less a conditional branch whereas the
cleanupblock is more like a label with a hint as to an unconditional branch that
will happen later.  And I guess that’s another thing that bothers me -- a resume
instruction at the end of a catch implementation means something subtly
different than a resume instruction at the end of a cleanup implementation.




From: Reid Kleckner [mailto:rnk at google.com]
Sent: Friday, May 15, 2015 3:38 PM
To: LLVM Developers Mailing List; Bill Wendling; Nick Lewycky; Kaylor, Andrew
Subject: RFC: New EH representation for MSVC compatibility

After a long tale of sorrow and woe, my colleagues and I stand here before you
defeated. The Itanium EH representation is not amenable to implementing
MSVC-compatible exceptions. We need a new representation that preserves
information about how try-catch blocks are nested.

WinEH background
-------------------------------

Skip this if you already know a lot about Windows exceptions. On Windows, every
exceptional action that you can imagine is a function call. Throwing an
exception is a call. Destructor cleanups and finally blocks are calls to
outlined functions that run the cleanup code. Even catching an exception is
implemented as an outlined catch handler function which returns the address of
the basic block at which normal execution should continue.

This is *not* how Itanium landingpads work, where cleanups and catches are
executed after unwinding and clearing old function frames off the stack. The
transition to a landingpad is *not* like a function call, and this is the only
special control transfer used for Itanium EH. In retrospect, having exactly one
kind of control transfer turns out to be a great design simplification. Go
Itanium!

Instead, all MSVC EH personality functions (x86, x64, ARM) cross (C++, SEH) are
implemented with interval tables that express the nesting levels of various
source constructs like destructors, try ranges, catch ranges, etc. When you
rinse your program through LLVM IR today, this structure is what gets lost.

New information
-------------------------

Recently, we have discovered that the tables for __CxxFrameHandler3 have the
additional constraint that the EH states assigned to a catch body must
immediately follow the state numbers assigned to the try body. The natural
scoping rules of C++ make it so that doing this numbering at the source level is
trivial, but once we go to LLVM IR CFG soup, scopes are gone. If you want to
know exactly what corner cases break down, search the bug database and mailing
lists. The explanations are too long for this RFC.


New representation
------------------------------

I propose adding the following new instructions, all of which (except for
resume) are glued to the top of their basic blocks, just like landingpads. They
all have an optional ‘unwind’ label operand, which provides the IR with a
tree-like structure of what EH action to take after this EH action completes.
The unwind label only participates in the formation of the CFG when used in a
catch block, and in other blocks it is considered opaque, personality-specific
information. If the unwind label is missing, then control leaves the function
after the EH action is completed. If a function is inlined, EH blocks with
missing unwind labels are wired up to the unwind label used by the inlined call
site.

The new representation is designed to be able to represent Itanium EH in case we
want to converge on a single EH representation in LLVM and Clang. An IR pass can
convert these actions to landingpads, typeid selector comparisons, and branches,
which means we can phase this representation in on Windows at first and
experiment with it slowly on other platforms. Over time, we can move the
landingpad conversion lower and lower in the stack until it’s moved into
DwarfEHPrepare. We’ll need to support landingpads at least until LLVM 4.0, but
we may want to keep them because they are the natural representation for
Itanium-style EH, and have a relatively low support burden.

resume
-------------

; Old form still works, still means control is leaving the function.
resume <valty> %val
; New form overloaded for intra-frame unwinding or resuming normal execution
resume <valty> %val, label %nextaction
; New form for EH personalities that produce no value
resume void

Now resume takes an optional label operand which is the next EH action to run.
The label must point to a block starting with an EH action. The various EH
action blocks impose personality-specific rules about what the targets of the
resume can be.

catchblock
---------------

%val = catchblock <valty> [i8* @typeid.int<http://typeid.int>, i32
7, i32* %e.addr]
    to label %catch.int<http://catch.int> unwind label %nextaction

The catchblock is a terminator that conditionally selects which block to execute
based on the opaque operands interpreted by the personality function. If the
exception is caught, the ‘to’ block is executed. If unwinding should continue,
the ‘unwind’ block is executed. Because the catchblock is a terminator, no
instructions can be inserted into a catchblock. The MSVC personality function
requires more than just a pointer to RTTI data, so a variable list of operands
is accepted. For an Itanium personality, only one RTTI operand is needed. The
‘unwind’ label of a catchblock must point to a catchend.

catchendblock
----------------

catchend unwind label %nextaction

The catchend is a terminator that unconditionally unwinds to the next action. It
is merely a placeholder to help reconstruct which invokes were part of the catch
blocks of a try. Invokes that are reached after a catchblock without following
any unwind edges must transitively unwind to the first catchend block that the
catchblock unwinds to. Executing such an invoke that does not transitively
unwind to the correct catchend block has undefined behavior.

cleanupblock
--------------------

%val = cleanupblock <valty> unwind label %nextaction

This is not a terminator, and control is expected to flow into a resume
instruction which indicates which EH block runs next. If the resume instruction
and the unwind label disagree, behavior is undefined.

terminateblock
----------------------

; for noexcept
terminateblock [void ()* @std.terminate] unwind label %nextaction
; for exception specifications, throw(int)
terminateblock [void ()* @__cxa_unexpected,
@typeid.int<http://typeid.int>, ...] unwind label %nextaction

This is a terminator, and the unwind label is where execution will continue if
the program continues execution. It also has an opaque, personality-specific
list of constant operands interpreted by the backend of LLVM. The convention is
that the first operand is the function to call to end the program, and the rest
determine if the program should end.

sehfilterblock?
------------------

One big hole in the new representation is SEH filter expressions. They present a
major complication because they do not follow a stack discipline. Any EH action
is reachable after an SEH filter runs. Because the CFG is so useless for
optimization purposes, it’s better to outline the filter in the frontend and
assume the filter can run during any potentially throwing function call.

MSVC EH implementation strategy
----------------------------------------------

Skim this if you just need the semantics of the representation above, and not
the implementation details.

The new EH block representation allows WinEHPrepare to get a lot simpler. EH
blocks should now look a lot more familiar, they are single entry, multi-exit
regions of code. This is exactly equivalent to a function, and we can call them
funclets. The plan is to generate code for the parent function first, skipping
all exceptional blocks, and then generate separate MachineFunctions for each
subfunction in turn. I repeat, we can stop doing outlining in IR. This was just
a mistake, because I was afraid of grappling with CodeGen.

WinEHPrepare will have two jobs now:
1. Mark down which basic blocks are reachable from which handler. Duplicate any
blocks that are reachable from two handlers until each block belongs to exactly
one funclet, pruning as many unreachable CFG edges as possible.
2. Demote SSA values that are defined in a funclet and used in another funclet.

The instruction selection pass is the pass that builds MachineFunctions from IR
Functions. This is the pass that will be responsible for the split. It will
maintain information about the offsets of static allocas in
FunctionLoweringInfo, and will throw it away when all funclets have been
generated for this function. This means we don’t need to insert framerecover
calls anymore.

Generating EH state numbers for the TryBlockMap and StateUnwindTable is a matter
of building a tree of EH blocks and invokes. Every unwind edge from an invoke or
an EH block represents that the instruction is a child of the target block. If
the unwind edge is empty, it is a child of the parent function, which is the
root node of the tree. State numbers can be assigned by doing a DFS traversal
where invokes are visited before EH blocks, and EH blocks can be visited in an
arbitrary-but-deterministic order that vaguely corresponds to source order.
Invokes are immediately assigned the current state number. Upon visiting an EH
block, the state number is recorded as the “low” state of the block. All invokes
are assigned this state number. The state number is incremented, and each child
EH block is visited, passing in the state number and producing a new state
number. The final state number is returned to the parent node.

Example IR from Clang
----------------------------------------

The C++:

struct Obj { ~Obj(); };
void f(int);
void foo() noexcept {
  try {
    f(1);
    Obj o;
    f(2);
  } catch (int e) {
    f(3);
    try {
      f(4);
    } catch (...) {
      f(5);
    }
  }
}

The IR for __CxxFrameHandler3:

define void @foo() personality i32 (...)* @__CxxFrameHandler3 {
  %e.addr = alloca i32
  invoke void @f(i32 1)
    to label %cont1 unwind label %maycatch.int<http://maycatch.int>
cont1:
  invoke void @f(i32 2)
    to label %cont2 unwind label %cleanup.Obj
cont2:
  call void @~Obj()
  br label %return
return:
  ret void

cleanup.Obj:
  cleanupblock unwind label %maycatch.int<http://maycatch.int>
  call void @~Obj()
  resume label %maycatch.int<http://maycatch.int>

maycatch.int<http://maycatch.int>:
  catchblock void [i8* @typeid.int<http://typeid.int>, i32 7, i32*
%e.addr]
    to label %catch.int<http://catch.int> unwind label %catchend1
catch.int<http://catch.int>:
  invoke void @f(i32 3)
    to label %cont3 unwind label %catchend1
cont3:
  invoke void @f(i32 4)
    to label %cont4 unwind label %maycatch.all
cont4:
  resume label %return

maycatch.all:
  catchblock void [i8* null, i32 0, i8* null]
    to label %catch.all unwind label %catchend2
catch.all:
  invoke void @f(i32 5)
    to label %cont5 unwind label %catchend2
cont5:
  resume label %cont4

catchend2:
  catchendblock unwind label %catchend1

catchend1:
  catchendblock unwind label %callterminate

callterminate:
  terminateblock [void ()* @std.terminate]
}

From this IR, we can recover the original scoped nesting form that the table
formation requires.

I think that covers it. Feedback welcome. :)
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On 2015-05-15 18:37:58 -0400, Reid Kleckner said:
> After a long tale of sorrow and woe, my colleagues and I stand here 
> before you defeated. The Itanium EH representation is not amenable to 
> implementing MSVC-compatible exceptions. We need a new representation 
> that preserves information about how try-catch blocks are nested.
Hi,

Newbie here. This must be a dumb question, but it's not something I can 
understand from reading the design documents and RFCs.

Why don't we write and use our own personality function, and then we 
avoid all these restrictions on the interval tables? On Windows, we 
would still have to catch exceptions with SEH, of course. But SEH 
should be language-independent, in the sense that it concerns only 
unwinding for the "low level" parts of the ABI, like restoring 
non-volatile registers. It doesn't seem to make sense that LLVM, being 
a language-independent IR, should concern itself with personality 
functions specific to Microsoft's C++ run-time library.

I understand we want to link compatibility with object code from Visual 
Studio, but I didn't think that the personality-specific unwind tables 
were actually an ABI "artifact". I mean, let's say you compile a 
function in Visual Studio, the result is a function with some mangled 
symbol that other object code can make references through the linker. 
But the other object code never incestuously refers to the unwind 
tables of the function it calls, right?

I'm speaking from the point of view of an implementor of a new 
programming language who wants to interoperate with C++. I've already 
got code that can decode the Microsoft RTTI info coming from C++ 
exceptions, and the pointers to those can be extracted with SEH 
GetExceptionPointers, etc. To support catching exceptions in my 
language, or at least calling cleanups while unwinding, I really don't 
care about the C++ personality function. After all my language might 
not even have the concept of (nested) try/catch blocks. The custom 
personality function does have to be supplied as part of the run-time, 
but as a frontend implementor I'm prepared to have to write a run-time 
anyway.

Steve

On Sat, May 16, 2015 at 7:29 AM, Steve Cheng <steve.ckp at gmail.com>
wrote:
> On 2015-05-15 18:37:58 -0400, Reid Kleckner said:
>
>  After a long tale of sorrow and woe, my colleagues and I stand here
>> before you defeated. The Itanium EH representation is not amenable to
>> implementing MSVC-compatible exceptions. We need a new representation
that
>> preserves information about how try-catch blocks are nested.
>>
>
> Hi,
>
> Newbie here. This must be a dumb question, but it's not something I can
> understand from reading the design documents and RFCs.
>
> Why don't we write and use our own personality function, and then we
avoid
> all these restrictions on the interval tables? On Windows, we would still
> have to catch exceptions with SEH, of course. But SEH should be
> language-independent, in the sense that it concerns only unwinding for the
> "low level" parts of the ABI, like restoring non-volatile
registers. It
> doesn't seem to make sense that LLVM, being a language-independent IR,
> should concern itself with personality functions specific to
Microsoft's
> C++ run-time library.
>
> I understand we want to link compatibility with object code from Visual
> Studio, but I didn't think that the personality-specific unwind tables
were
> actually an ABI "artifact". I mean, let's say you compile a
function in
> Visual Studio, the result is a function with some mangled symbol that other
> object code can make references through the linker. But the other object
> code never incestuously refers to the unwind tables of the function it
> calls, right?
>
> I'm speaking from the point of view of an implementor of a new
programming
> language who wants to interoperate with C++. I've already got code that
can
> decode the Microsoft RTTI info coming from C++ exceptions, and the pointers
> to those can be extracted with SEH GetExceptionPointers, etc. To support
> catching exceptions in my language, or at least calling cleanups while
> unwinding, I really don't care about the C++ personality function.
After
> all my language might not even have the concept of (nested) try/catch
> blocks. The custom personality function does have to be supplied as part of
> the run-time, but as a frontend implementor I'm prepared to have to
write a
> run-time anyway.
>
Right, doing our own personality function is possible, but still has half
the challenge of using __CxxFrameHandler3, and introduces a new runtime
dependency that isn't there currently. Given that it won't save that
much
work, I'd rather not introduce a dependency that wasn't there before.

The reason it's still hard is that you have to split the main function up
into more than one subfunction. The exception object is allocated in the
frame of the function calling __CxxThrow, and it has to stay alive until
control leaves the catch block receiving the exception. This is different
from Itanium, where the exception object is constructed in heap memory and
the pointer is saved in TLS. If this were not the case, we'd use the
__gxx_personaltity_v0-style landingpad approach and make a new personality
variant that understands MS RTTI.

We could try to do all this outlining in Clang, but that blocks a lot of
LLVM optimizations. Any object with a destructor (std::string) is now
escaped into the funclet that calls the destructor, and simple
transformations (SROA) require interprocedural analysis. This affects the
code on the normal code path and not just the exceptional path. While EH
constructs like try / catch are fairly rare in C++, destructor cleanups are
very, very common, and I'd rather not pessimize so much code.
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We already have something like what you describe in the form of mingw support. 
It runs on Windows and handles exceptions using a DWARF-style personality
function and (I think) an LLVM-provided implementation of the libc++abi library.

What this doesn't do is provide interoperability with MSVC-compiled objects.
For instance, you can't throw an exception from MSVC-compiled code and catch
it with clang/LLVM-compiled code or vice versa.  With the (too fragile)
implementation we have in place right now you can do that (at least in cases
that don't break for other reasons), and we want to be able to continue that
capability with a new, more robust, solution.

-Andy

-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Steve Cheng
Sent: Saturday, May 16, 2015 7:30 AM
To: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] RFC: New EH representation for MSVC compatibility

On 2015-05-15 18:37:58 -0400, Reid Kleckner said:
> After a long tale of sorrow and woe, my colleagues and I stand here 
> before you defeated. The Itanium EH representation is not amenable to 
> implementing MSVC-compatible exceptions. We need a new representation 
> that preserves information about how try-catch blocks are nested.
Hi,

Newbie here. This must be a dumb question, but it's not something I can
understand from reading the design documents and RFCs.

Why don't we write and use our own personality function, and then we avoid
all these restrictions on the interval tables? On Windows, we would still have
to catch exceptions with SEH, of course. But SEH should be language-independent,
in the sense that it concerns only unwinding for the "low level" parts
of the ABI, like restoring non-volatile registers. It doesn't seem to make
sense that LLVM, being a language-independent IR, should concern itself with
personality functions specific to Microsoft's C++ run-time library.

I understand we want to link compatibility with object code from Visual Studio,
but I didn't think that the personality-specific unwind tables were actually
an ABI "artifact". I mean, let's say you compile a function in
Visual Studio, the result is a function with some mangled symbol that other
object code can make references through the linker.
But the other object code never incestuously refers to the unwind tables of the
function it calls, right?

I'm speaking from the point of view of an implementor of a new programming
language who wants to interoperate with C++. I've already got code that can
decode the Microsoft RTTI info coming from C++ exceptions, and the pointers to
those can be extracted with SEH GetExceptionPointers, etc. To support catching
exceptions in my language, or at least calling cleanups while unwinding, I
really don't care about the C++ personality function. After all my language
might not even have the concept of (nested) try/catch blocks. The custom
personality function does have to be supplied as part of the run-time, but as a
frontend implementor I'm prepared to have to write a run-time anyway.

Steve


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On Fri, May 15, 2015 at 5:27 PM, Kaylor, Andrew <andrew.kaylor at
intel.com>
wrote:
>  I like the way this sorts out with regard to funclet code generation.
> It feels very natural for Windows EH, though obviously not as natural for
> non-Windows targets and I think it is likely to block some optimizations
> that are currently possible with those targets.
>
Right, it will block some of today's optimizations by default. I'm OK
with
this because we can add those optimizations back by checking if the
personality is Itanium-family (sjlj, arm, or dwarf), and optimizing EH
codepaths is not usually performance critical.

> > If the unwind label is missing, then control leaves the function after
> the EH action is completed. If a function is inlined, EH blocks with
> missing unwind labels are wired up to the unwind label used by the inlined
> call site.
>
>
>
> Is this saying that a “missing” unwind label corresponds to telling the
> runtime to continue the search at the next frame?
>
Yep. For the C++ data structure it would simply be a missing or null
operand.

> Your example looks wrong in this regard, unless I’m misunderstanding it.
> It looks like any exceptions that aren’t caught in that function will lead
> to a terminate call.
>
Well, those are the intended semantics of noexcept, unless I'm mistaken.
And the inliner *should* wire up the unwind edge of the terminateblock to
the unwind edge of the inlined invoke instruction, because it's natural to
lower terminateblock to a catch-all plus termination call block. I wanted
to express that as data, though, so that in the common case that the
noexcept function is not inlined, we can simply flip the "noexcept"
bit in
the EH info. There's a similar optimization we can do for Itanium that we
miss today.
> Invokes that are reached after a catchblock without following any unwind
> edges must transitively unwind to the first catchend block that the
> catchblock unwinds to.
>
>
>
> I’m not sure I understand this correctly.  In particular, I’m confused
> about the roles of resume and catchend.
>
catchendblock is really there to support figuring out which calls were
inside the catch scope. resume has two roles: moving to the next EH action
after a cleanup, and transitioning from the catch block back to normal
control flow. Some of my coworkers said it should be split into two
instructions for each purpose, and I could go either way.

> > %val = cleanupblock <valty> unwind label %nextaction
>
>
>
> Why isn’t this a terminator?  It seems like it performs the same sort of
> role as catchblock, except presumably it is always entered.  I suppose
> that’s probably the answer to my question, but it strikes me as an
> ambiguity in the scheme.  The catchblock instruction is more or less a
> conditional branch whereas the cleanupblock is more like a label with a
> hint as to an unconditional branch that will happen later.  And I guess
> that’s another thing that bothers me -- a resume instruction at the end of
> a catch implementation means something subtly different than a resume
> instruction at the end of a cleanup implementation.
>
Yeah, reusing the resume instruction for both these things might not be
good. I liked not having to add more terminator instructions, though. I
think most optimizations will not care about the differences between the
two kinds of resume. For CFG formation purposes, it either has one
successor or none, and that's enough for most users.

I felt that cleanupblock should not be a terminator because it keeps the IR
more concise. The smaller an IR construct is, the more people seem to
understand it, so I tried to go with that.
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Hi,

Thanks for sending this out.  We're looking forward to seeing this come
about, since we need funclet separation for LLILC as well (and I have cycles to
spend on it, if that would be helpful).

Some questions about the new proposal:

- Do the new forms of resume have any implied read/write side-effects, or do
they work just like a branch?  In particular, I'm wondering what prevents
reordering a call across a resume.  Is this just something that code motion
optimizations are expected to check for explicitly to avoid introducing UB per
the "Executing such an invoke [or call] that does not transitively unwind
to the correct catchend block has undefined behavior" rule?

- Does LLVM already have other examples of terminators that are glued to the top
of their basic blocks, or will these be the first?  I ask because it's a
somewhat nonstandard thing (a block in the CFG that can't have instructions
added to it) that any code placement algorithms (PRE, PGO probe insertion, Phi
elimination, RA spill/copy placement, etc.) may need to be adjusted for.  The
adjustments aren't terrible (conceptually it's no worse than having
unsplittable edges from each of the block's preds to each of its succs), but
it's something to be aware of.

- Since this will require auditing any code with special processing of resume
instructions to make sure it handles the new resume forms correctly, I wonder if
it might be helpful to give resume (or the new forms of it) a different name,
since then it would be immediately clear which code has/hasn't been updated
to the new model.

- Is the idea that a resume (of the sort that resumes normal execution) ends
only one catch/cleanup, or that it can end any number of them?  Did you consider
having it end a single one, and giving it a source that references (in a
non-flow-edge-inducing way) the related catchend?  If you did that, then:
+ The code to find a funclet region could terminate with confidence when it
reaches this sort of resume, and
+ Resumes which exit different catches would have different sources and thus
couldn't be merged, reducing the need to undo tail-merging with code
duplication in EH preparation (by blocking the tail-merging in the first place)

- What is the plan for cleanup/__finally code that may be executed on either
normal paths or EH paths?  One could imagine a number of options here:
+ require the IR producer to duplicate code for EH vs non-EH paths
+ duplicate code for EH vs non-EH paths during EH preparation
+ use resume to exit these even on the non-EH paths; code doesn't need to be
duplicated (but could and often would be as an optimization for hot/non-EH
paths), and normal paths could call the funclet at the end of the day
and it isn't clear to me which you're suggesting.  Requiring duplication
can worst-case quadratically expand the code (in that if you have n levels of
cleanup-inside-cleanup-inside-cleanup-…, and each cleanup has k code bytes
outside the next-inner cleanup, after duplication you'll have k*n + k*(n-1)
+ … or O(k*n^2) bytes total [compared to k*n before duplication]), which I'd
think could potentially be a problem in pathological inputs.

Thanks
-Joseph

From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Reid Kleckner
Sent: Friday, May 15, 2015 6:38 PM
To: LLVM Developers Mailing List; Bill Wendling; Nick Lewycky; Kaylor, Andrew
Subject: [LLVMdev] RFC: New EH representation for MSVC compatibility

After a long tale of sorrow and woe, my colleagues and I stand here before you
defeated. The Itanium EH representation is not amenable to implementing
MSVC-compatible exceptions. We need a new representation that preserves
information about how try-catch blocks are nested.

WinEH background
-------------------------------

Skip this if you already know a lot about Windows exceptions. On Windows, every
exceptional action that you can imagine is a function call. Throwing an
exception is a call. Destructor cleanups and finally blocks are calls to
outlined functions that run the cleanup code. Even catching an exception is
implemented as an outlined catch handler function which returns the address of
the basic block at which normal execution should continue.

This is *not* how Itanium landingpads work, where cleanups and catches are
executed after unwinding and clearing old function frames off the stack. The
transition to a landingpad is *not* like a function call, and this is the only
special control transfer used for Itanium EH. In retrospect, having exactly one
kind of control transfer turns out to be a great design simplification. Go
Itanium!

Instead, all MSVC EH personality functions (x86, x64, ARM) cross (C++, SEH) are
implemented with interval tables that express the nesting levels of various
source constructs like destructors, try ranges, catch ranges, etc. When you
rinse your program through LLVM IR today, this structure is what gets lost.

New information
-------------------------

Recently, we have discovered that the tables for __CxxFrameHandler3 have the
additional constraint that the EH states assigned to a catch body must
immediately follow the state numbers assigned to the try body. The natural
scoping rules of C++ make it so that doing this numbering at the source level is
trivial, but once we go to LLVM IR CFG soup, scopes are gone. If you want to
know exactly what corner cases break down, search the bug database and mailing
lists. The explanations are too long for this RFC.


New representation
------------------------------

I propose adding the following new instructions, all of which (except for
resume) are glued to the top of their basic blocks, just like landingpads. They
all have an optional ‘unwind’ label operand, which provides the IR with a
tree-like structure of what EH action to take after this EH action completes.
The unwind label only participates in the formation of the CFG when used in a
catch block, and in other blocks it is considered opaque, personality-specific
information. If the unwind label is missing, then control leaves the function
after the EH action is completed. If a function is inlined, EH blocks with
missing unwind labels are wired up to the unwind label used by the inlined call
site.

The new representation is designed to be able to represent Itanium EH in case we
want to converge on a single EH representation in LLVM and Clang. An IR pass can
convert these actions to landingpads, typeid selector comparisons, and branches,
which means we can phase this representation in on Windows at first and
experiment with it slowly on other platforms. Over time, we can move the
landingpad conversion lower and lower in the stack until it’s moved into
DwarfEHPrepare. We’ll need to support landingpads at least until LLVM 4.0, but
we may want to keep them because they are the natural representation for
Itanium-style EH, and have a relatively low support burden.

resume
-------------

; Old form still works, still means control is leaving the function.
resume <valty> %val
; New form overloaded for intra-frame unwinding or resuming normal execution
resume <valty> %val, label %nextaction
; New form for EH personalities that produce no value
resume void

Now resume takes an optional label operand which is the next EH action to run.
The label must point to a block starting with an EH action. The various EH
action blocks impose personality-specific rules about what the targets of the
resume can be.

catchblock
---------------

%val = catchblock <valty> [i8* @typeid.int<http://typeid.int>, i32
7, i32* %e.addr]
    to label %catch.int<http://catch.int> unwind label %nextaction

The catchblock is a terminator that conditionally selects which block to execute
based on the opaque operands interpreted by the personality function. If the
exception is caught, the ‘to’ block is executed. If unwinding should continue,
the ‘unwind’ block is executed. Because the catchblock is a terminator, no
instructions can be inserted into a catchblock. The MSVC personality function
requires more than just a pointer to RTTI data, so a variable list of operands
is accepted. For an Itanium personality, only one RTTI operand is needed. The
‘unwind’ label of a catchblock must point to a catchend.

catchendblock
----------------

catchend unwind label %nextaction

The catchend is a terminator that unconditionally unwinds to the next action. It
is merely a placeholder to help reconstruct which invokes were part of the catch
blocks of a try. Invokes that are reached after a catchblock without following
any unwind edges must transitively unwind to the first catchend block that the
catchblock unwinds to. Executing such an invoke that does not transitively
unwind to the correct catchend block has undefined behavior.

cleanupblock
--------------------

%val = cleanupblock <valty> unwind label %nextaction

This is not a terminator, and control is expected to flow into a resume
instruction which indicates which EH block runs next. If the resume instruction
and the unwind label disagree, behavior is undefined.

terminateblock
----------------------

; for noexcept
terminateblock [void ()* @std.terminate] unwind label %nextaction
; for exception specifications, throw(int)
terminateblock [void ()* @__cxa_unexpected,
@typeid.int<http://typeid.int>, ...] unwind label %nextaction

This is a terminator, and the unwind label is where execution will continue if
the program continues execution. It also has an opaque, personality-specific
list of constant operands interpreted by the backend of LLVM. The convention is
that the first operand is the function to call to end the program, and the rest
determine if the program should end.

sehfilterblock?
------------------

One big hole in the new representation is SEH filter expressions. They present a
major complication because they do not follow a stack discipline. Any EH action
is reachable after an SEH filter runs. Because the CFG is so useless for
optimization purposes, it’s better to outline the filter in the frontend and
assume the filter can run during any potentially throwing function call.

MSVC EH implementation strategy
----------------------------------------------

Skim this if you just need the semantics of the representation above, and not
the implementation details.

The new EH block representation allows WinEHPrepare to get a lot simpler. EH
blocks should now look a lot more familiar, they are single entry, multi-exit
regions of code. This is exactly equivalent to a function, and we can call them
funclets. The plan is to generate code for the parent function first, skipping
all exceptional blocks, and then generate separate MachineFunctions for each
subfunction in turn. I repeat, we can stop doing outlining in IR. This was just
a mistake, because I was afraid of grappling with CodeGen.

WinEHPrepare will have two jobs now:
1. Mark down which basic blocks are reachable from which handler. Duplicate any
blocks that are reachable from two handlers until each block belongs to exactly
one funclet, pruning as many unreachable CFG edges as possible.
2. Demote SSA values that are defined in a funclet and used in another funclet.

The instruction selection pass is the pass that builds MachineFunctions from IR
Functions. This is the pass that will be responsible for the split. It will
maintain information about the offsets of static allocas in
FunctionLoweringInfo, and will throw it away when all funclets have been
generated for this function. This means we don’t need to insert framerecover
calls anymore.

Generating EH state numbers for the TryBlockMap and StateUnwindTable is a matter
of building a tree of EH blocks and invokes. Every unwind edge from an invoke or
an EH block represents that the instruction is a child of the target block. If
the unwind edge is empty, it is a child of the parent function, which is the
root node of the tree. State numbers can be assigned by doing a DFS traversal
where invokes are visited before EH blocks, and EH blocks can be visited in an
arbitrary-but-deterministic order that vaguely corresponds to source order.
Invokes are immediately assigned the current state number. Upon visiting an EH
block, the state number is recorded as the “low” state of the block. All invokes
are assigned this state number. The state number is incremented, and each child
EH block is visited, passing in the state number and producing a new state
number. The final state number is returned to the parent node.

Example IR from Clang
----------------------------------------

The C++:

struct Obj { ~Obj(); };
void f(int);
void foo() noexcept {
  try {
    f(1);
    Obj o;
    f(2);
  } catch (int e) {
    f(3);
    try {
      f(4);
    } catch (...) {
      f(5);
    }
  }
}

The IR for __CxxFrameHandler3:

define void @foo() personality i32 (...)* @__CxxFrameHandler3 {
  %e.addr = alloca i32
  invoke void @f(i32 1)
    to label %cont1 unwind label %maycatch.int<http://maycatch.int>
cont1:
  invoke void @f(i32 2)
    to label %cont2 unwind label %cleanup.Obj
cont2:
  call void @~Obj()
  br label %return
return:
  ret void

cleanup.Obj:
  cleanupblock unwind label %maycatch.int<http://maycatch.int>
  call void @~Obj()
  resume label %maycatch.int<http://maycatch.int>

maycatch.int<http://maycatch.int>:
  catchblock void [i8* @typeid.int<http://typeid.int>, i32 7, i32*
%e.addr]
    to label %catch.int<http://catch.int> unwind label %catchend1
catch.int<http://catch.int>:
  invoke void @f(i32 3)
    to label %cont3 unwind label %catchend1
cont3:
  invoke void @f(i32 4)
    to label %cont4 unwind label %maycatch.all
cont4:
  resume label %return

maycatch.all:
  catchblock void [i8* null, i32 0, i8* null]
    to label %catch.all unwind label %catchend2
catch.all:
  invoke void @f(i32 5)
    to label %cont5 unwind label %catchend2
cont5:
  resume label %cont4

catchend2:
  catchendblock unwind label %catchend1

catchend1:
  catchendblock unwind label %callterminate

callterminate:
  terminateblock [void ()* @std.terminate]
}

From this IR, we can recover the original scoped nesting form that the table
formation requires.

I think that covers it. Feedback welcome. :)
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On Mon, May 18, 2015 at 12:03 PM, Joseph Tremoulet <jotrem at
microsoft.com>
wrote:
>  Hi,
>
>
>
> Thanks for sending this out.  We're looking forward to seeing this come
> about, since we need funclet separation for LLILC as well (and I have
> cycles to spend on it, if that would be helpful).
>
>
>
> Some questions about the new proposal:
>
>
>
> - Do the new forms of resume have any implied read/write side-effects, or
> do they work just like a branch?  In particular, I'm wondering what
> prevents reordering a call across a resume.  Is this just something that
> code motion optimizations are expected to check for explicitly to avoid
> introducing UB per the "Executing such an invoke [or call] that does
not
> transitively unwind to the correct catchend block has undefined
behavior"
> rule?
>
Yes, crossing a resume from a catchblock ends the lifetime of the exception
object, so I'd say that's a "writes escaped memory"
constraint. That said,
a resume after a cleanupblock doesn't, but I'm not sure it's worth
having
this kind of fine-grained analysis. I'm OK teaching SimplifyCFG to combine
cleanupblocks and leaving it at that.

> - Does LLVM already have other examples of terminators that are glued to
> the top of their basic blocks, or will these be the first?  I ask because
> it's a somewhat nonstandard thing (a block in the CFG that can't
have
> instructions added to it) that any code placement algorithms (PRE, PGO
> probe insertion, Phi elimination, RA spill/copy placement, etc.) may need
> to be adjusted for.  The adjustments aren't terrible (conceptually
it's no
> worse than having unsplittable edges from each of the block's preds to
each
> of its succs), but it's something to be aware of.
>
No, LLVM doesn't have anything like this yet. It does have unsplittable
critical edges, which can come from indirectbr and the unwind edge of an
invoke. I don't think it'll be too hard to teach transforms how to deal
with one more, but maybe that's unrealistic youthful optimism. :)

- Since this will require auditing any code with special processing
of
> resume instructions to make sure it handles the new resume forms correctly,
> I wonder if it might be helpful to give resume (or the new forms of it) a
> different name, since then it would be immediately clear which code
> has/hasn't been updated to the new model.
>
There aren't that many references to ResumeInst across LLVM, so I'm not
too
scared. I'm not married to reusing 'resume', other candidate names
include
'unwind' and 'continue', and I'd like more ideas.

> - Is the idea that a resume (of the sort that resumes normal execution)
> ends only one catch/cleanup, or that it can end any number of them?  Did
> you consider having it end a single one, and giving it a source that
> references (in a non-flow-edge-inducing way) the related catchend?  If you
> did that, then:
>
> + The code to find a funclet region could terminate with confidence when
> it reaches this sort of resume, and
>
> + Resumes which exit different catches would have different sources and
> thus couldn't be merged, reducing the need to undo tail-merging with
code
> duplication in EH preparation (by blocking the tail-merging in the first
> place)
>
We already have something like this for cleanupblocks because the resume
target and unwind label of the cleanupblock must match. It isn't as strong
as having a reference to the catchblock itself, because tail merging could
kick in like you mention. Undoing this would be and currently is the job of
WinEHPrepare. I guess I felt like the extra representational complexity
wasn't worth the confidence that it would buy us.

> - What is the plan for cleanup/__finally code that may be executed on
> either normal paths or EH paths?  One could imagine a number of options
> here:
>
> + require the IR producer to duplicate code for EH vs non-EH paths
>
> + duplicate code for EH vs non-EH paths during EH preparation
>
> + use resume to exit these even on the non-EH paths; code doesn't need
to
> be duplicated (but could and often would be as an optimization for
> hot/non-EH paths), and normal paths could call the funclet at the end of
> the day
>
> and it isn't clear to me which you're suggesting.  Requiring
duplication
> can worst-case quadratically expand the code (in that if you have n levels
> of cleanup-inside-cleanup-inside-cleanup-…, and each cleanup has k code
> bytes outside the next-inner cleanup, after duplication you'll have k*n
+
> k*(n-1) + … or O(k*n^2) bytes total [compared to k*n before duplication]),
> which I'd think could potentially be a problem in pathological inputs.
>
I want to have separate normal and exceptional codepaths, but at -O0 all
the cleanup work should be bundled up in a function that gets called from
both those paths.

Today, for C++ destructors, we emit two calls to the destructor: one on the
normal path and one on the EH path. For __finally, we outline the finally
body early in clang and emit two calls to it as before, but passing in the
frameaddress as an argument. I think this is a great place to be. It keeps
our -O0 code size small, simplifies the implementation, and allows us to
inline one or both call sites if we think it's profitable.
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> On May 15, 2015, at 3:37 PM, Reid Kleckner <rnk at google.com> wrote:
> After a long tale of sorrow and woe, my colleagues and I stand here before
you defeated. The Itanium EH representation is not amenable to implementing
MSVC-compatible exceptions. We need a new representation that preserves
information about how try-catch blocks are nested.
A couple quick apologies: this response is pretty late, and for the same reasons
I’ve only been able to skim the rest of the thread.

I think the basic ideas in this proposal seem reasonable, although it seems they
may have evolved a bit over the course of the thread.  A few points do stand out
to me:
> Instead, all MSVC EH personality functions (x86, x64, ARM) cross (C++, SEH)
are implemented with interval tables that express the nesting levels of various
source constructs like destructors, try ranges, catch ranges, etc. When you
rinse your program through LLVM IR today, this structure is what gets lost.
Yes.  It seems to me that the main additional thing that your proposed IR
preserves is the ability to very easily reconstruct the tree of control flow. 
Correct?
> New information
> -------------------------
> 
> Recently, we have discovered that the tables for __CxxFrameHandler3 have
the additional constraint that the EH states assigned to a catch body must
immediately follow the state numbers assigned to the try body. The natural
scoping rules of C++ make it so that doing this numbering at the source level is
trivial, but once we go to LLVM IR CFG soup, scopes are gone. If you want to
know exactly what corner cases break down, search the bug database and mailing
lists. The explanations are too long for this RFC.
I don’t quite understand this constraint (aren’t the EH states within the catch
body in a different function and therefore numbered separately?), and I don’t
really understand why it makes anything about EH harder (aren’t you still going
to have exactly the same numbering problems with cleanups/catches being shared
between invoke sites?) as opposed to simply being something that you didn’t
design your current implementation around, but it doesn’t really matter.  If the
new representation is better, it’s better.
> New representation
> ------------------------------
> 
> I propose adding the following new instructions, all of which (except for
resume) are glued to the top of their basic blocks, just like landingpads.
The fact that most of these are terminators makes pinning to the absolutely
beginning really problematic.  An edge that can’t support arbitrary code is one
thing (although usually they’re at least splittable!), but at the very least, we
need to be able to drop phis and debug instructions in basic blocks, or you’re
going to completely wreck basic optimizability.

You should consider giving these a consistent prefix, like “eh_” or “eh.”, just
to clearly distinguish them.  You can rename “resume” and “landingpad”, too.
> They all have an optional ‘unwind’ label operand, which provides the IR
with a tree-like structure of what EH action to take after this EH action
completes. The unwind label only participates in the formation of the CFG when
used in a catch block, and in other blocks it is considered opaque,
personality-specific information. If the unwind label is missing, then control
leaves the function after the EH action is completed. If a function is inlined,
EH blocks with missing unwind labels are wired up to the unwind label used by
the inlined call site.
For the terminators, this unwind label makes sense.  For the non-terminators
(just cleanupblock, I think), you’re going to need to define what it means, its
strength of reference, etc.  For example, if I have a cleanup that doesn’t
terminate (imagine a destructor that calls abort()), the IR will contain no CFG
links from the basic block with the cleanupblock to the basic block with the
resume.  The basic block with the resume, and all the downstream EH blocks, may
even be unreachable; and so the natural tendency would be to remove them.  Does
a reference from a cleanupblock keep its target alive?
> The new representation is designed to be able to represent Itanium EH in
case we want to converge on a single EH representation in LLVM and Clang. An IR
pass can convert these actions to landingpads, typeid selector comparisons, and
branches, which means we can phase this representation in on Windows at first
and experiment with it slowly on other platforms. Over time, we can move the
landingpad conversion lower and lower in the stack until it’s moved into
DwarfEHPrepare. We’ll need to support landingpads at least until LLVM 4.0, but
we may want to keep them because they are the natural representation for
Itanium-style EH, and have a relatively low support burden.
I agree that we could migrate Itanium to this pattern fairly successfully, as
long as we’re agreed that we’re not setting of goal of eventually emitting
identical IR for different personalities.
> resume
> -------------
> 
> ; Old form still works, still means control is leaving the function.
> resume <valty> %val
> ; New form overloaded for intra-frame unwinding or resuming normal
execution
> resume <valty> %val, label %nextaction
> ; New form for EH personalities that produce no value
> resume void
> 
> Now resume takes an optional label operand which is the next EH action to
run. The label must point to a block starting with an EH action. The various EH
action blocks impose personality-specific rules about what the targets of the
resume can be.
I agree with the feedback elsewhere that you should separate these instructions.
> catchendblock
> ----------------
> 
> catchend unwind label %nextaction
> 
> The catchend is a terminator that unconditionally unwinds to the next
action. It is merely a placeholder to help reconstruct which invokes were part
of the catch blocks of a try. Invokes that are reached after a catchblock
without following any unwind edges must transitively unwind to the first
catchend block that the catchblock unwinds to. Executing such an invoke that
does not transitively unwind to the correct catchend block has undefined
behavior.
I think the rule you’re looking for here is that it’s undefined behavior if
control flow from a catchblock doesn’t eventually reach the corresponding
catchendblock (or reaches the catchblock again before that point).  It’s not
unwind-specific.
> cleanupblock
> --------------------
> 
> %val = cleanupblock <valty> unwind label %nextaction
> 
> This is not a terminator, and control is expected to flow into a resume
instruction which indicates which EH block runs next. If the resume instruction
and the unwind label disagree, behavior is undefined.
What’s the expectation here?  Is each cleanupblock conceptually an independent
cleanup, or is it a legal transformation to combine successive cleanupblocks? 
Is it okay for the code within a cleanupblock to be reachable from multiple
cleanupblock instructions?  Is there an expectation about how many resumes are
reachable?  Does the cleanupblock instruction itself prevent reordering in any
way?
> terminateblock
> ----------------------
> 
> ; for noexcept
> terminateblock [void ()* @std.terminate] unwind label %nextaction
> ; for exception specifications, throw(int)
> terminateblock [void ()* @__cxa_unexpected, @typeid.int
<http://typeid.int/>, ...] unwind label %nextaction
> 
> This is a terminator, and the unwind label is where execution will continue
if the program continues execution. It also has an opaque, personality-specific
list of constant operands interpreted by the backend of LLVM. The convention is
that the first operand is the function to call to end the program, and the rest
determine if the program should end.
It looks like you’re expecting this be useful for things like EH filters;
that's cool, but it suggests the name might not be appropriate.  Maybe the
instruction should be called “eh.filter” and it should take a string label to
tell the personality the kind of filtering to do?  And then the function pointer
is just another thing in the list of constant parameters.

Also, std::unexpected doesn’t necessarily terminate the program; it gets a
chance to remap the exception.  I don’t know that that changes anything — it
just means that control continues to the unwind label — but you should keep that
in mind in your examples.
> sehfilterblock?
> ------------------
> 
> One big hole in the new representation is SEH filter expressions. They
present a major complication because they do not follow a stack discipline. Any
EH action is reachable after an SEH filter runs. Because the CFG is so useless
for optimization purposes, it’s better to outline the filter in the frontend and
assume the filter can run during any potentially throwing function call.
It’s not that it doesn’t follow a stack discipline, it’s that it’s not quite
part of the same stack as other EH actions.  It’s similar in spirit to a catch
block in Smalltalk, which runs before the stack is unwound because it has the
ability to resume control at the throw point.  One way to model this would be to
have a list of such blocks handing off the invoke (or a landingpad-like
instruction that’s easily found from the invoke), where inlining would pull them
from outer invokes to inner calls.  But the control flow is inherently strange
because it can essentially loop within the invoke, and that’s really hard to
model.

John.
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