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From: "roger at nextmovesoftware dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/101403] [12 Regression] wrong code with
 __builtin_bswap16() at -O1 by r12-2137
Date: Sat, 10 Jul 2021 16:26:23 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101403

Roger Sayle <roger at nextmovesoftware dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |roger at nextmoveso=
ftware dot com

--- Comment #3 from Roger Sayle <roger at nextmovesoftware dot com> ---
Testing a fix. Sorry for the inconvenience.  An even more reduced test case=
 is:
unsigned int foo (unsigned int a)
{
  unsigned int u;
  /* b =3D=3D 0x8000 */
  unsigned short b =3D __builtin_bswap16 (a);
  /* result =3D 0x0008 */
  // This works: return b >> 12;
  // This doesn't:
  return b >> (u, 12);
}

int main (void)
{
  unsigned int x =3D foo (0x80);
  if (x !=3D 0x0008)
    __builtin_abort ();
  return 0;
}
The change in signedness of the right shift (from logical to arithmetic)
triggers an oversight in my recent folding optimization.=