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<p class="MsoNormal">Hi All.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">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></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">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></p>
<p class="MsoNormal">In order to encode the new registers of 16-31 and the additional instructions, a new encoding prefix called EVEX<span style="color:#1F497D">,</span> which extends the existing VEX encoding<span style="color:#1F497D">,</span> was introduced
as shown below:<o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas;color:black">The EVEX encoding format:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:9.5pt;font-family:Consolas"> EVEX Opcode ModR/M [SIB] [Disp32] / [Disp8*N] [Immediate]</span><o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"># of bytes: 4 1 1 1 4 / 1 1<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The existing VEX encoding format:<o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"> [VEX] OPCODE ModR/M [SIB] [DISP] [IMM]<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas"># of bytes: 0,2,3 1 1 0,1 0,1,2,4 0,1<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-size:9.5pt;font-family:Consolas;color:green"><o:p> </o:p></span></p>
<p class="MsoNormal">Note that the EVEX prefix requires 4 bytes whereas the VEX prefix can take only up to 3 bytes.<o:p></o:p></p>
<p class="MsoNormal">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></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">For example: “vmovss %xmm0, 32(%rsp,%rax,4)“, has the following 2 possible encodings:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">EVEX encoding (8 bytes long):<o:p></o:p></p>
<p class="MsoNormal"> 62 f1 7e 08 11 44 84 08 vmovss %xmm0, 32(%rsp,%rax,4)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">VEX encoding (6 bytes long):<o:p></o:p></p>
<p class="MsoNormal"> c5 fa 11 44 84 20 vmovss %xmm0, 32(%rsp,%rax,4)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">See reported Bugzilla bugs about this proposed optimization:<span style="color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><a href="https://llvm.org/bugs/show_bug.cgi?id=23376">https://llvm.org/bugs/show_bug.cgi?id=23376</a><o:p></o:p></span></p>
<p class="MsoNormal"><a href="https://llvm.org/bugs/show_bug.cgi?id=29162">https://llvm.org/bugs/show_bug.cgi?id=29162</a><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">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></p>
<p class="MsoNormal">A new pass is to be added at the pre-emit stage<span style="color:#1F497D">.<o:p></o:p></span></p>
<p class="MsoNormal">No need for special Opt flags, as it is always better to use the reduced VEX encoding when possible.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thank you for any comments or questions that you may have.<o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal">Sincerely, <o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal">Gadi. <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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