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From: "kristerw at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/106513] New: bswap is incorrectly generated
Date: Wed, 03 Aug 2022 11:56:49 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106513

            Bug ID: 106513
           Summary: bswap is incorrectly generated
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kristerw at gcc dot gnu.org
  Target Milestone: ---

GCC may incorrectly generate bswap instructions for code not doing a correct
swap. This can be seen by running the function from testsuite/gcc.dg/pr4050=
1.c
as

typedef long int int64_t;

__attribute__((noinline)) int64_t
swap64 (int64_t n)
{
  return (((n & (((int64_t) 0xff) )) << 56) |
          ((n & (((int64_t) 0xff) << 8)) << 40) |
          ((n & (((int64_t) 0xff) << 16)) << 24) |
          ((n & (((int64_t) 0xff) << 24)) << 8) |
          ((n & (((int64_t) 0xff) << 32)) >> 8) |
          ((n & (((int64_t) 0xff) << 40)) >> 24) |
          ((n & (((int64_t) 0xff) << 48)) >> 40) |
          ((n & (((int64_t) 0xff) << 56)) >> 56));
}

int main (void)
{
  volatile int64_t n =3D 0x8000000000000000l;

  if (swap64(n) !=3D 0xffffffffffffff80l)
    __builtin_abort ();

  return 0;
}

This fails at -Os and higher optimization levels.=