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<p class="MsoNormal"><span style="color:#1F497D">Thanks for the tip.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Indeed, the EVEX opcodes in X86 have a convenient naming that help in this.<a name="_MailEndCompose"><o:p></o:p></a></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Sincerely, <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Gadi.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p></o:p></span></p>
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<p class="MsoNormal"><a name="_____replyseparator"></a><b>From:</b> Hal Finkel [mailto:hfinkel@anl.gov]
<br>
<b>Sent:</b> Wednesday, November 23, 2016 15:01<br>
<b>To:</b> Haber, Gadi <gadi.haber@intel.com><br>
<b>Cc:</b> llvm-dev@lists.llvm.org<br>
<b>Subject:</b> Re: [llvm-dev] RFC: code size reduction in X86 by replacing EVEX with VEX encoding<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<blockquote style="border:none;border-left:solid #1010FF 1.5pt;padding:0in 0in 0in 4.0pt;margin-left:3.75pt;margin-top:5.0pt;margin-bottom:5.0pt">
<p class="MsoNormal" style="margin-bottom:12.0pt"><b><span style="font-family:"Helvetica",sans-serif;color:black">From:
</span></b><span style="font-family:"Helvetica",sans-serif;color:black">"Gadi via llvm-dev Haber" <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>><br>
<b>To: </b><a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a><br>
<b>Sent: </b>Wednesday, November 23, 2016 5:50:42 AM<br>
<b>Subject: </b>[llvm-dev] RFC: code size reduction in X86 by replacing EVEX with VEX encoding</span><span style="font-size:12.0pt;font-family:"Helvetica",sans-serif;color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">Hi All.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"> <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">This is an RFC for a proposed target specific X86 optimization for reducing code size in the encoding of AVX-512 instructions when possible.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"> <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">In order to encode the new registers of 16-31 and the additional instructions, a new encoding prefix called EVEX</span><span style="color:#1F497D">,</span><span style="color:black"> which extends the existing VEX
encoding</span><span style="color:#1F497D">,</span><span style="color:black"> was introduced as shown below:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> </span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:9.5pt;font-family:Consolas;color:black">The EVEX encoding format:</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:9.5pt;font-family:Consolas;color:black"> EVEX Opcode ModR/M [SIB] [Disp32] / [Disp8*N] [Immediate]</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:9.5pt;font-family:Consolas;color:black"># of bytes: 4 1 1 1 4 / 1 1</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"> <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">The existing VEX encoding format:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:9.5pt;font-family:Consolas;color:black"> [VEX] OPCODE ModR/M [SIB] [DISP] [IMM]</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:9.5pt;font-family:Consolas;color:black"># of bytes: 0,2,3 1 1 0,1 0,1,2,4 0,1</span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:9.5pt;font-family:Consolas;color:green"> </span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">Note that the EVEX prefix requires 4 bytes whereas the VEX prefix can take only up to 3 bytes.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"> <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">For example: “vmovss %xmm0, 32(%rsp,%rax,4)“, has the following 2 possible encodings:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"> <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">EVEX encoding (8 bytes long):<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"> 62 f1 7e 08 11 44 84 08 vmovss %xmm0, 32(%rsp,%rax,4)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"> <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">VEX encoding (6 bytes long):<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"> c5 fa 11 44 84 20 vmovss %xmm0, 32(%rsp,%rax,4)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"> <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">See reported Bugzilla bugs about this proposed optimization:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><a href="https://llvm.org/bugs/show_bug.cgi?id=23376" target="_blank">https://llvm.org/bugs/show_bug.cgi?id=23376</a></span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"><a href="https://llvm.org/bugs/show_bug.cgi?id=29162" target="_blank">https://llvm.org/bugs/show_bug.cgi?id=29162</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"> <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">A new pass is to be added at the pre-emit stage</span><span style="color:#1F497D">.</span><span style="color:black"><o:p></o:p></span></p>
</blockquote>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:black">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:<br>
<br>
lib/Target/Hexagon/Hexagon.td<br>
lib/Target/Mips/MipsDSPInstrFormats.td<br>
lib/Target/Mips/MipsInstrFormats.td<br>
lib/Target/Mips/Mips32r6InstrFormats.td<br>
lib/Target/PowerPC/PPC.td<br>
lib/Target/AMDGPU/SIInstrInfo.td<br>
lib/Target/AMDGPU/R600Instructions.td<br>
lib/Target/SystemZ/SystemZInstrFormats.td<br>
lib/Target/Lanai/LanaiInstrInfo.td<br>
<br>
I've used this feature a few times in the PowerPC backend, and it's quite convenient.<br>
<br>
-Hal<o:p></o:p></span></p>
<blockquote style="border:none;border-left:solid #1010FF 1.5pt;padding:0in 0in 0in 4.0pt;margin-left:3.75pt;margin-top:5.0pt;margin-bottom:5.0pt">
<p class="MsoNormal"><span style="color:black">No need for special Opt flags, as it is always better to use the reduced VEX encoding when possible.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"> <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">Thank you for any comments or questions that you may have.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> </span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">Sincerely, <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> </span><span style="color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black">Gadi. <o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:black"> <o:p></o:p></span></p>
<p><span style="font-family:"Helvetica",sans-serif;color:black">---------------------------------------------------------------------<br>
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<br>
-- <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;color:black">Hal Finkel<br>
Lead, Compiler Technology and Programming Languages<br>
Leadership Computing Facility<br>
Argonne National Laboratory<o:p></o:p></span></p>
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