llvm dev - Oct 2016 - RFC: Absolute or "fixed address" symbols as immediate operands

Hi all,

I wanted to summarise some discussion on llvm-commits [0,1] as an RFC, as I
felt it demanded wider circulation.

Our support for references to absolute symbols is not very good. The symbol
will be resolved accurately in non-PIC code, but suboptimally: the symbol
reference cannot currently appear as the immediate operand of an
instruction, and the code generator cannot make any assumptions about the
value of the symbol (so for example, it could not use a R_X86_64_8
relocation if the value is known to be in the range 0..255).

In PIC mode, if the reference is not known to be DSO-local, the value is
loaded from the GOT (or a synthetic GOT entry), which again means
suboptimal code. If the reference is known to be DSO-local, the symbol will
be referenced with a PC relative relocation and therefore cannot be
resolved properly to an absolute value (c.f. https://reviews.llvm.org/D19844).
The latter case in particular would seem to indicate that a
representational change is required for correctness to distinguish
references to absolute symbols from references to regular symbols.

The specific change I have in mind is to allow !range metadata on
GlobalObjects. This would
be similar to existing !range metadata, but it would apply to the
"address"
of the attached GlobalObject, rather than any value loaded from it. Its
presence on a GlobalObject would also imply that the address of the
GlobalObject is "fixed" at link time. Alongside !range we could
potentially
use other sources of information, such as the relocation model, code model
and visibility, to identify "fixed" globals, although that can be done
separately.

I have been experimenting with a number of approaches to representation in
SDAG, and I have found one that seems to work best, and would be the least
intrusive (unfortunately most approaches to this problem are somewhat
intrusive).

Specifically, I want to:
1) move most of the body of ConstantSDNode to a new class,
ConstantIntSDNode, which would derives from ConstantSDNode. ConstantSDNode
would act as the base class for immediates-post-static-linking. Change most
references to ConstantSDNode in C++ code to refer to ConstantIntSDNode.
However, "imm" in tblgen code would continue to match ConstantSDNode.
2) introduce a new derived class of ConstantSDNode for references to
globals with !range metadata, and teach SDAG to use this new derived class
for fixed address references

I will shortly be sending out a patch that implements 1.

Thanks,
-- 
-- 
Peter

[0]
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160926/394194.html
[1]
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20161003/394983.html
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> On Oct 10, 2016, at 8:12 PM, Peter Collingbourne via llvm-dev <llvm-dev
at lists.llvm.org> wrote:
> 
> Hi all,
> 
> I wanted to summarise some discussion on llvm-commits [0,1] as an RFC, as I
felt it demanded wider circulation.
> 
> Our support for references to absolute symbols is not very good. The symbol
will be resolved accurately in non-PIC code, but suboptimally: the symbol
reference cannot currently appear as the immediate operand of an instruction,
and the code generator cannot make any assumptions about the value of the symbol
(so for example, it could not use a R_X86_64_8 relocation if the value is known
to be in the range 0..255).
> 
> In PIC mode, if the reference is not known to be DSO-local, the value is
loaded from the GOT (or a synthetic GOT entry), which again means suboptimal
code. If the reference is known to be DSO-local, the symbol will be referenced
with a PC relative relocation and therefore cannot be resolved properly to an
absolute value (c.f. https://reviews.llvm.org/D19844
<https://reviews.llvm.org/D19844>). The latter case in particular would
seem to indicate that a representational change is required for correctness to
distinguish references to absolute symbols from references to regular symbols.
> 
> The specific change I have in mind is to allow !range metadata on
GlobalObjects. This would
> be similar to existing !range metadata, but it would apply to the
"address" of the attached GlobalObject, rather than any value loaded
from it. Its presence on a GlobalObject would also imply that the address of the
GlobalObject is "fixed" at link time. Alongside !range we could
potentially use other sources of information, such as the relocation model, code
model and visibility, to identify "fixed" globals, although that can
be done separately.
Ok, I think I understand the use-case.
> I have been experimenting with a number of approaches to representation in
SDAG, and I have found one that seems to work best, and would be the least
intrusive (unfortunately most approaches to this problem are somewhat
intrusive).
> 
> Specifically, I want to:
> 1) move most of the body of ConstantSDNode to a new class,
ConstantIntSDNode, which would derives from ConstantSDNode. ConstantSDNode would
act as the base class for immediates-post-static-linking. Change most references
to ConstantSDNode in C++ code to refer to ConstantIntSDNode. However,
"imm" in tblgen code would continue to match ConstantSDNode.
> 2) introduce a new derived class of ConstantSDNode for references to
globals with !range metadata, and teach SDAG to use this new derived class for
fixed address references
ConstantSDNode is poorly named, and renaming it to ConstantIntSDNode is probably
the right thing to do independently of the other changes.

That said, I don’t understand why you’d keep ConstantSDNode around and introduce
a new derived class of it.  This seems like something that a new “imm"
immediate matcher would handle: it would match constants in a certain range, or
a GlobalAddressSDNode known-to-be-small.

-Chris
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On Mon, Oct 10, 2016 at 8:31 PM, Chris Lattner <clattner at apple.com>
wrote:
>
> On Oct 10, 2016, at 8:12 PM, Peter Collingbourne via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hi all,
>
> I wanted to summarise some discussion on llvm-commits [0,1] as an RFC, as
> I felt it demanded wider circulation.
>
> Our support for references to absolute symbols is not very good. The
> symbol will be resolved accurately in non-PIC code, but suboptimally: the
> symbol reference cannot currently appear as the immediate operand of an
> instruction, and the code generator cannot make any assumptions about the
> value of the symbol (so for example, it could not use a R_X86_64_8
> relocation if the value is known to be in the range 0..255).
>
> In PIC mode, if the reference is not known to be DSO-local, the value is
> loaded from the GOT (or a synthetic GOT entry), which again means
> suboptimal code. If the reference is known to be DSO-local, the symbol will
> be referenced with a PC relative relocation and therefore cannot be
> resolved properly to an absolute value (c.f.
> https://reviews.llvm.org/D19844). The latter case in particular would
> seem to indicate that a representational change is required for correctness
> to distinguish references to absolute symbols from references to regular
> symbols.
>
> The specific change I have in mind is to allow !range metadata on
> GlobalObjects. This would
> be similar to existing !range metadata, but it would apply to the
> "address" of the attached GlobalObject, rather than any value
loaded from
> it. Its presence on a GlobalObject would also imply that the address of the
> GlobalObject is "fixed" at link time. Alongside !range we could
potentially
> use other sources of information, such as the relocation model, code model
> and visibility, to identify "fixed" globals, although that can be
done
> separately.
>
>
> Ok, I think I understand the use-case.
>
> I have been experimenting with a number of approaches to representation in
> SDAG, and I have found one that seems to work best, and would be the least
> intrusive (unfortunately most approaches to this problem are somewhat
> intrusive).
>
> Specifically, I want to:
> 1) move most of the body of ConstantSDNode to a new class,
> ConstantIntSDNode, which would derives from ConstantSDNode. ConstantSDNode
> would act as the base class for immediates-post-static-linking. Change
> most references to ConstantSDNode in C++ code to refer to
> ConstantIntSDNode. However, "imm" in tblgen code would continue
to match
> ConstantSDNode.
> 2) introduce a new derived class of ConstantSDNode for references to
> globals with !range metadata, and teach SDAG to use this new derived class
> for fixed address references
>
>
> ConstantSDNode is poorly named, and renaming it to ConstantIntSDNode is
> probably the right thing to do independently of the other changes.
>
> That said, I don’t understand why you’d keep ConstantSDNode around and
> introduce a new derived class of it.  This seems like something that a new
> “imm" immediate matcher would handle: it would match constants in a
certain
> range, or a GlobalAddressSDNode known-to-be-small.
>
To begin with: I'm not sure that GlobalAddressSDNode is the right node to
use for these types of immediates. It seems that we have two broad classes
of globals here: those with a fixed-at-link-time address (e.g. regular
non-PIC symbols, absolute symbols) and those where the address needs to be
computed (e.g. PC-relative addresses, TLS variables). To me it seems like
the first class is much more similar to immediates than to the second
class. That suggested to me that there ought to be two separate
representations for global variables, where the former are "morally"
immediates, and the latter are not (i.e. the existing GlobalAddressSDNode).

I went over a couple of approaches for representing "moral" immediates
in
my llvm-commits post. The first one seems to be more like what you're
suggesting:
> - Introduce a new opcode for absolute symbol constants. This intuitively
seemed like the least risky approach, as individual instructions could "opt
in" to the new absolute symbol references. However, this seems hard to fit
into the existing SDAG pattern matching engine, as the engine expects each
"variable" to have a specific opcode. I tried adding special support
for
"either of the two constant opcodes" to the matcher, but I could not
see a
good way to do it without making fundamental changes to how patterns are
matched.
>
> - Use the ISD::Constant opcode for absolute symbol constants, but
introduce a separate class for them. This also seemed problematic, as there
is a strong assumption (both in existing SDAG code and in generated code)
of a many-to-one mapping from opcodes to classes.

We can solve part of the problem with the second approach with a base class
for ISD::Constant. As I worked on that approach, I found that it did turn
out to be a good fit overall: in many cases we're already adhering to a
principle that an unrestricted immediate maps onto potentially relocatable
bytes in the output file. The X86 and ARM backends illustrate this quite
well: the X86 instruction set generally uses power-of-2 wide immediate
forms that neatly map onto instruction bytes, and ARM generally uses
compressed immediate forms (e.g. "mod_imm") which would naturally
match
only real constant integers. Using that principle, we can restrict (e.g.)
ImmLeaf to constant integers (see https://reviews.llvm.org/D25355). In
cases where this mapping isn't quite right, we can use more restrictive
matchers.

I'm still a little uneasy about the second approach, and would be
interested in my first approach, but I'm not sure if it would be practical.

Thanks,
-- 
-- 
Peter
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On Mon, Oct 10, 2016 at 8:12 PM, Peter Collingbourne <peter at pcc.me.uk>
wrote:
> The specific change I have in mind is to allow !range metadata on
> GlobalObjects. This would
> be similar to existing !range metadata, but it would apply to the
> "address" of the attached GlobalObject, rather than any value
loaded from
> it. Its presence on a GlobalObject would also imply that the address of the
> GlobalObject is "fixed" at link time.
>
Going back to IR-level representation: here is an alternative
representation based on a suggestion from Eli.

Introduce a new type of GlobalValue called GlobalConstant. GlobalConstant
would fit into the GlobalValue hierarchy like this:

   - GlobalValue
   - GlobalConstant
      - GlobalPointer
         - GlobalIndirectSymbol
            - GlobalAlias
            - GlobalIFunc
         - GlobalObject
            - Function
            - GlobalVariable

GlobalValue would no longer be assumed to be of pointer type. The getType()
overload that takes a PointerType, as well as getValueType() would be moved
down to GlobalPointer. (A nice side benefit of this is that it would help
flush out cases where we are unnecessarily depending on global pointee
types.)

A GlobalConstant can either be a definition or a declaration. A definition
would look like this:

@foo = globalconst i32 42

while a declaration would look like this:

@foo = external globalconst i32

GlobalConstant could also hold a linkage and visibility. Looking at the
other attributes that a GlobalValue can hold, many of them do not seem
appropriate for GlobalConstant and could potentially be moved to
GlobalPointer.

Thoughts?

Thanks,
-- 
-- 
Peter
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On 10/24/2016 1:07 PM, Peter Collingbourne via llvm-dev
wrote:
> On Mon, Oct 10, 2016 at 8:12 PM, Peter Collingbourne <peter at pcc.me.uk
> <mailto:peter at pcc.me.uk>> wrote:
>
>     The specific change I have in mind is to allow !range metadata on
>     GlobalObjects. This would
>     be similar to existing !range metadata, but it would apply to the
>     "address" of the attached GlobalObject, rather than any value
>     loaded from it. Its presence on a GlobalObject would also imply
>     that the address of the GlobalObject is "fixed" at link time.
>
> Going back to IR-level representation: here is an alternative 
> representation based on a suggestion from Eli.
>
> Introduce a new type of GlobalValue called GlobalConstant. 
> GlobalConstant would fit into the GlobalValue hierarchy like this:
>
>   * GlobalValue
>       o GlobalConstant
>       o GlobalPointer
>           + GlobalIndirectSymbol
>               # GlobalAlias
>               # GlobalIFunc
>           + GlobalObject
>               # Function
>               # GlobalVariable
>
> GlobalValue would no longer be assumed to be of pointer type. The 
> getType() overload that takes a PointerType, as well as getValueType() 
> would be moved down to GlobalPointer. (A nice side benefit of this is 
> that it would help flush out cases where we are unnecessarily 
> depending on global pointee types.)
>
> A GlobalConstant can either be a definition or a declaration. A 
> definition would look like this:
>
> @foo = globalconst i32 42
This is equivalent to writing "foo = 42" in assembly?
> while a declaration would look like this:
>
> @foo = external globalconst i32
>
> GlobalConstant could also hold a linkage and visibility. Looking at 
> the other attributes that a GlobalValue can hold, many of them do not 
> seem appropriate for GlobalConstant and could potentially be moved to 
> GlobalPointer.
>
> Thoughts?
>
How do you plan to use this?  The concept makes sense, but I've never 
actually seen anyone use symbols this way.

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux
Foundation Collaborative Project

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> On Oct 24, 2016, at 1:07 PM, Peter Collingbourne <peter at pcc.me.uk>
wrote:
> 
> On Mon, Oct 10, 2016 at 8:12 PM, Peter Collingbourne <peter at pcc.me.uk
<mailto:peter at pcc.me.uk>> wrote:
> The specific change I have in mind is to allow !range metadata on
GlobalObjects. This would
> be similar to existing !range metadata, but it would apply to the
"address" of the attached GlobalObject, rather than any value loaded
from it. Its presence on a GlobalObject would also imply that the address of the
GlobalObject is "fixed" at link time.
>  
> Going back to IR-level representation: here is an alternative
representation based on a suggestion from Eli.
> 
> Introduce a new type of GlobalValue called GlobalConstant. GlobalConstant
would fit into the GlobalValue hierarchy like this:
> GlobalValue
> GlobalConstant
> GlobalPointer
> GlobalIndirectSymbol
> GlobalAlias
> GlobalIFunc
> GlobalObject
> Function
> GlobalVariable
> GlobalValue would no longer be assumed to be of pointer type. The getType()
overload that takes a PointerType, as well as getValueType() would be moved down
to GlobalPointer. (A nice side benefit of this is that it would help flush out
cases where we are unnecessarily depending on global pointee types.)
Hi Peter,

I agree that it makes sense to introduce a new GlobalConstant IR node for this
sort of thing.  That said, have you considered a design where GlobalConstant is
still required to be a pointer type?  If you did this, you would end up with a
simpler and less invasive design of:
GlobalValue
GlobalConstant
GlobalIndirectSymbol
GlobalAlias
GlobalIFunc
GlobalObject
Function
GlobalVariable
I think that this would be better for (e.g.) the X86 backend anyway, since
global objects can be assigned to specific addresses with linker maps, and thus
have small addresses (and this is expressible with the range metadata).  This
means that GlobalConstant and other GlobalValues should all be usable in the
same places in principle.

-Chris
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