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From: "amonakov at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/105504] New: Fails to break dependency for vcvtss2sd
 xmm, xmm, mem
Date: Fri, 06 May 2022 09:12:40 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105504

            Bug ID: 105504
           Summary: Fails to break dependency for vcvtss2sd xmm, xmm, mem
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: amonakov at gcc dot gnu.org
  Target Milestone: ---

Created attachment 52933
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D52933&action=3Dedit
testcase

Hit by core-math team at
https://gcc.gnu.org/pipermail/gcc-help/2022-May/141480.html

Compile the attached testcase with -O2 -march=3Dhaswell (other AVX-capable =
Intel
families except Alderlake are affected too) and observe that the big basic
block begins with

.L6:
        vcvtss2sd       xmm1, xmm1, DWORD PTR [rsp-4]

This creates a false dependency on the previous assignment into xmm1, resul=
ting
in wildly varying (and suboptimal) throughput figures depending on how long=
 the
CPU stalls waiting for the previous assignment to complete.

GCC has code to emit such instructions in a manner that avoids false
dependencies (see e.g. PR89071), but here it doesn't seem to work.


Also there's a potentially related issue that GCC copies the initial xmm0 v=
alue
to eax via stack in the beginning of the function:

cr_exp10f:
        vmovss  DWORD PTR [rsp-4], xmm0
        mov     eax, DWORD PTR [rsp-4]

This seems wrong since xmm-reg moves on Haswell are 1 cycle afaict.=