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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/96226] New: Failure to optimize shift+not to
 rotate
Date: Thu, 16 Jul 2020 21:06:16 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 11.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96226

            Bug ID: 96226
           Summary: Failure to optimize shift+not to rotate
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

int32_t f(int32_t x)
{
    return ~(1 << (x & 0x1F));
}

This can be transformed to doing a rotate of -2 and x. This transformation =
is
done by LLVM, but not by GCC.

PS: GCC seems capable of doing this optimization, but only if `x & 0x1F` is
replaced with `x`, which either means that GCC is underoptimizing this or t=
hat
LLVM is somehow wrong.=