llvm dev - Dec 2013 - [LLVMdev] [cfe-dev] [Proposal] function attribute to reduce emission of vzeroupper instructions

On Thu, Dec 19, 2013 at 12:14 PM, Rafael Espíndola <
rafael.espindola at gmail.com> wrote:
> On 19 December 2013 14:31, Gao, Yunzhong
> <yunzhong_gao at playstation.sony.com> wrote:
> > Hi all,
> >
> >
> >
> > I would like to find out whether anyone will find it useful to add an
> x86-
> >
> > specific calling convention for reducing emission of vzeroupper
> > instructions.
> >
> >
> >
> > Current implementation:
> >
> > vzeroupper is inserted to any functions that use AVX instructions. The
> >
> > insertion points are:
> >
> > 1) before a call instruction;
> >
> > 2) before a return instruction;
> >
> >
> >
> > Background:
> >
> > vzeroupper is an AVX instruction; it is inserted to avoid performance
> > penalty
> >
> > when transitioning between x86 AVX mode and legacy SSE mode, e.g.,
when
> an
> >
> > AVX function calls a SSE function. However, vzeroupper is a slow
> > instruction; it
> >
> > adds to register pressure and hurts performance for AVX-to-AVX calls.
> >
> >
> >
> > My proposal:
> >
> > 1) (LLVM part) Add an x86-specific calling convention to the LLVM IR
> which
> >
> > specifies that an external function will be compiled with AVX support
and
> > its
> >
> > function definition does not use any legacy SSE instructions, e.g.,
> >
> >   declare x86_avxcc i32 @foo()
>
> I would suggest using metadata instead. The reasons are:
>
> * It could be applied to functions with different calling conventions.
> For example, on windows we would probably want to do this to thiscall
> (methods) too.
> * It the metadata is dropped, we would just produced slower but still
> correct code (calls vzeroupper).
>
Maybe a target-specific attribute instead?  It would still apply to all
CCs, but would never be dropped.
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> Maybe a target-specific attribute instead?  It would still apply to all
CCs,
> but would never be dropped.
That would work too, yes. I proposed metadata because it looks like it
can be dropped, but that is not a big issue. I would be OK with an
attribute too if that is more convenient or we want to make sure it is
kept.

Cheers,
Rafael

Hi Rafael and Reid,

To clarify,

With a target-specific attribute, the LLVM IR representation will be something
like this:
  declare i32 @foo() "x86_avx"="true"

With a target-specific metadata, the IR will be something like this:
   declare i32 @foo() !1
   ...
   !1 = metadata !{metadata !"x86_avx"}

If a backend does not understand this attribute or this metadata, it will have
no effect on code generation.

If this is what you mean, then I believe either approach will work for me.

Do you have any opinion on the clang part of the proposal?

I plan to take the next two weeks off from work, so I probably will respond to
emails only
sporadically. I hope you have a happy holiday there too,

- Gao.

_______________________________________
From: Rafael Espíndola [rafael.espindola at gmail.com]
Sent: Thursday, December 19, 2013 12:55 PM
To: Reid Kleckner
Cc: Gao, Yunzhong; cfe-dev at cs.uiuc.edu Developers (cfe-dev at cs.uiuc.edu);
LLVM Developers Mailing List (llvmdev at cs.uiuc.edu)
Subject: Re: [cfe-dev] [LLVMdev] [Proposal] function attribute to reduce
emission of vzeroupper instructions
> Maybe a target-specific attribute instead?  It would still apply to all
CCs,
> but would never be dropped.
That would work too, yes. I proposed metadata because it looks like it
can be dropped, but that is not a big issue. I would be OK with an
attribute too if that is more convenient or we want to make sure it is
kept.

Cheers,
Rafael