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From: "goon.pri.low at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/112687] New: missed-optimization: switch
 statement does not simplify to it's expression
Date: Thu, 23 Nov 2023 21:43:03 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 13.2.1
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: goon.pri.low at gmail dot com
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112687

            Bug ID: 112687
           Summary: missed-optimization: switch statement does not
                    simplify to it's expression
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: goon.pri.low at gmail dot com
  Target Milestone: ---

This following code:
int unopt(int v) {
    switch (v & 3) {
        case 0: return 0;
        case 1: return 1;
        case 2: return 2;
        case 3: return 3;
        default: __builtin_unreachable();
    }
}

unopt:
        mov     eax, edi
        and     eax, 3
        lea     edx, [rax-1]
        cmp     edx, 3
        mov     edx, 0
        cmovnb  eax, edx
        ret

Ought to be optimized to:
int opt(int v) {
    return v & 3;
}

opt:
        mov     eax, edi
        and     eax, 3
        ret

Note: I decided to separate this problem from
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112645 since it was unrelate=
d to
the main bug, though Richard offers some initial insight into the problem.=