https://llvm.org/bugs/show_bug.cgi?id=23376
Bug ID: 23376
Summary: AVX512: We don't always need EVEX for VEX-encodable
instructions.
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Backend: X86
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
This:
define float @t(float %a, float %b) {
%res = fadd float %a, %b
ret float %res
}
Generates the EVEX-prefixed:
62f17e0858c1 vaddss %xmm1, %xmm0, %xmm0
instead of using VEX:
c5fa58c1 vaddss %xmm1, %xmm0, %xmm0
AFAIK, there is no transition penalty between AVX512 and AVX, so I believe the
only reason to use EVEX in this case is to access more registers (xmm0-31).
That's not always useful, so we should recognize that, and relax EVEX->VEX when
possible.
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