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From: "gabravier at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/94834] New: Failure to optimize loop bswap
 pattern
Date: Tue, 28 Apr 2020 22:35:21 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94834

            Bug ID: 94834
           Summary: Failure to optimize loop bswap pattern
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

uint32_t load(const uint8_t* data)
{
    uint32_t val =3D 0;
    for (int i =3D 0; i < sizeof(val) * CHAR_BIT; i +=3D CHAR_BIT)
    {
        val |=3D *data++ << i;
    }
    return val;
}

This can be optimized to a single 32-bit load. LLVM does this transformatio=
n,
gcc just unrolls the loop and misses the transformation.

LLVM gives :

load(unsigned char const*): # @load(unsigned char const*)
  mov eax, dword ptr [rdi]
  ret

GCC gives :

load(unsigned char const*):
  movzx edx, BYTE PTR [rdi+1]
  movzx eax, BYTE PTR [rdi]
  sal edx, 8
  or edx, eax
  movzx eax, BYTE PTR [rdi+2]
  sal eax, 16
  or edx, eax
  movzx eax, BYTE PTR [rdi+3]
  sal eax, 24
  or eax, edx
  ret

See also https://godbolt.org/z/kmYTLZ=