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From: "llvm at rifkin dot dev" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/109044] New: Missed fold for (n - 1) / 2 when
 n is odd
Date: Mon, 06 Mar 2023 18:18:59 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 12.2.1
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: llvm at rifkin dot dev
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109044

            Bug ID: 109044
           Summary: Missed fold for (n - 1) / 2 when n is odd
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: llvm at rifkin dot dev
  Target Milestone: ---

int foo(unsigned n) {
    if(n % 2 =3D=3D 0) __builtin_unreachable();
    return (n - 1) / 2;
}

int bar(unsigned n) {
    return n >> 1;
}



foo(unsigned int):
        lea     eax, [rdi-1]
        shr     eax
        ret
bar(unsigned int):
        mov     eax, edi
        shr     eax
        ret


https://godbolt.org/z/3G7enYdnM

https://alive2.llvm.org/ce/z/m3qbdN=