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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/101642] [12 Regression] ice in decompose, at
 wide-int.h:984
Date: Tue, 27 Jul 2021 14:11:48 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101642

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I think better testcase would be
int x;

unsigned short
foo (void)
{
  return __builtin_bswap16 (x) ? : x;
}
because otherwise you're asking for it to be DCEd.

The problem is that for __builtin_bswap16 like for many other builtins with=
 8
or 16-bit argument(s) the operand is promoted to int.
So, many of the match.pd rules just don't trigger for it at all, like:
 (simplify
  (bswap (bswap @0))
  @0)
 (simplify
  (bswap (bit_not (bswap @0)))
  (bit_not @0))
 (for bitop (bit_xor bit_ior bit_and)
  (simplify
   (bswap (bitop:c (bswap @0) @1))
   (bitop @0 (bswap @1))))
because there are casts it doesn't take into account for bswap16, others are
just incorrect, like:
 (for cmp (eq ne)
  (simplify
   (cmp (bswap @0) (bswap @1))
   (cmp @0 @1))
- it needs to convert @0 and @1 to the type of one of the bswaps, otherwise=
 it
could be comparing even the upper bits, and others are both incorrect and c=
ause
ICE, like
  (simplify
    (cmp (bswap @0) INTEGER_CST@1)
   (cmp @0 (bswap @1))))
Let me deal with just those now and the rest that just don't match can be d=
ealt
with later on.=