[llvm-bugs] [Bug 29222] New: Combining MMX with AVX suboptimal

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=29222

            Bug ID: 29222
           Summary: Combining MMX with AVX suboptimal
           Product: clang
           Version: 3.8
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: kobalicek.petr at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

The following code:

#include <mmintrin.h>
#include <immintrin.h>

int fn(int x) {
  __m64 mm = _mm_set1_pi32(x);
  mm = _mm_packs_pi16(mm, mm);

  __m128i xmm = _mm_movpi64_epi64(mm);
  xmm = _mm_packs_epi16(xmm, xmm);
  return _mm_cvtsi128_si32(xmm);
}

Compiled with '-O2 -Wall -mavx2 -m32 -fomit-frame-pointer' produces:

fn(int):
  sub     esp, 20
  vbroadcastss    xmm0, dword ptr [esp + 24] # Cool idea, but not 
  vmovlps qword ptr [esp + 8], xmm0          # in our context.
  movq    mm0, qword ptr [esp + 8]           # !!!
  packsswb        mm0, mm0
  movq    qword ptr [esp], mm0               # These moves are
  vmovq   xmm0, qword ptr [esp]              # correct.
  vpacksswb       xmm0, xmm0, xmm0
  vmovd   eax, xmm0
  add     esp, 20
  ret


I know that MMX is not used anymore, but I wonder why clang prefers a code-path
that is one instruction longer and contains 2 memory accesses more than a more
straightforward 'punpckldq'.

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