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

[Rafael Espíndola]

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).

Cheers,
Rafael