[llvm-dev] RFC: code size reduction in X86 by replacing EVEX with VEX encoding

Wed Nov 23 23:27:56 PST 2016

Thanx. This makes sense.
Note that there are many tests, mostly under test/CodeGen/X86, that are affected by this optimization and I had to modify them as they include a check of the generated encoding.
If we add such a disabling opt flag, should we now keep two sets of tests? One for the optimization on and one when it is disabled?

Thanx!

Gadi.

From: Craig Topper [mailto:craig.topper at gmail.com]
Sent: Wednesday, November 23, 2016 18:13
To: Haber, Gadi <gadi.haber at intel.com>; Hal Finkel <hfinkel at anl.gov>
Cc: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] RFC: code size reduction in X86 by replacing EVEX with VEX encoding

I would like a command line option to disable this optimization. That way tests can still verify that EVEX instructions came out of isel by using -show-mc-encoding.

On Wed, Nov 23, 2016 at 5:01 AM Hal Finkel via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

________________________________
From: "Gadi via llvm-dev Haber" <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
To: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
Sent: Wednesday, November 23, 2016 5:50:42 AM
Subject: [llvm-dev] RFC: code size reduction in X86 by replacing EVEX with        VEX encoding

Hi All.

This is an RFC for a proposed target specific X86 optimization for reducing code size in the encoding of AVX-512 instructions when possible.

When the AVX512F instruction set was introduced in X86 it included additional 32 registers of 512bit size each ZMM0 - ZMM31, as well as additional 16 XMM registers XMM16-XMM31 and 16 YMM registers YMM16-YMM31.
In order to encode the new registers of 16-31 and the additional instructions, a new encoding prefix called EVEX, which extends the existing VEX encoding, was introduced as shown below:

The EVEX encoding format:
            EVEX Opcode ModR/M [SIB] [Disp32] / [Disp8*N] [Immediate]
# of bytes: 4    1      1      1      4       / 1         1

The existing VEX encoding format:
            [VEX]   OPCODE ModR/M [SIB] [DISP]   [IMM]
# of bytes: 0,2,3   1      1      0,1   0,1,2,4  0,1

Note that the EVEX prefix requires 4 bytes whereas the VEX prefix can take only up to 3 bytes.
Consequently, for the SKX architecture, many instructions that use only the lower registers of XMM0-XMM15 or YMM0-YMM15, can be encoded by either the EVEX or the VEX format. For such cases, using the VEX encoding results in a code size reduction of ~2 bytes even though it is compiled with the AVX512F/AVX512VL features enabled.

For example: “vmovss  %xmm0, 32(%rsp,%rax,4)“, has the following 2 possible encodings:

EVEX encoding (8 bytes long):
            62 f1 7e 08 11 44 84 08         vmovss  %xmm0, 32(%rsp,%rax,4)

VEX encoding (6 bytes long):
           c5 fa 11 44 84 20                      vmovss  %xmm0, 32(%rsp,%rax,4)

See reported Bugzilla bugs about this proposed optimization:
https://llvm.org/bugs/show_bug.cgi?id=23376
https://llvm.org/bugs/show_bug.cgi?id=29162

The proposed optimization implementation is to add a table of all EVEX opcodes that can be encoded via VEX in a new header file placed under lib/Target/X86.
A new pass is to be added at the pre-emit stage.
It might be better to have TableGen generate the mapping table for you instead of manually making a table yourself. TableGen has a feature that is specifically designed to make mapping tables like this. For examples, grep for InstrMapping in:

lib/Target/Hexagon/Hexagon.td
lib/Target/Mips/MipsDSPInstrFormats.td
lib/Target/Mips/MipsInstrFormats.td
lib/Target/Mips/Mips32r6InstrFormats.td
lib/Target/PowerPC/PPC.td
lib/Target/AMDGPU/SIInstrInfo.td
lib/Target/AMDGPU/R600Instructions.td
lib/Target/SystemZ/SystemZInstrFormats.td
lib/Target/Lanai/LanaiInstrInfo.td

I've used this feature a few times in the PowerPC backend, and it's quite convenient.

 -Hal

No need for special Opt flags, as it is always better to use the reduced VEX encoding when possible.

Thank you for any comments or questions that you may have.

Sincerely,

Gadi.

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Hal Finkel
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Leadership Computing Facility
Argonne National Laboratory
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