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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/98865] New: Missed transform of (a >> 63) * b
Date: Thu, 28 Jan 2021 13:38:30 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98865

            Bug ID: 98865
           Summary: Missed transform of (a >> 63) * b
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

unsigned long foo (unsigned long a, unsigned long b)
{
  return (a >> 63) * b;
}

generates

foo:
.LFB0:
        .cfi_startproc
        shrq    $63, %rdi
        movq    %rdi, %rax
        imulq   %rsi, %rax
        ret

but we can do (like llvm):

foo:                                    # @foo
        .cfi_startproc
# %bb.0:
        movq    %rdi, %rax
        sarq    $63, %rax
        andq    %rsi, %rax
        retq=