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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/94779] New: Bad optimization of simple switch
Date: Mon, 27 Apr 2020 00:40:24 +0000
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X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 10.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94779

            Bug ID: 94779
           Summary: Bad optimization of simple switch
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

int f1(unsigned x)
{
    switch (x)
    {
        case 0:
            return 1;
        case 1:
            return 2;
    }
}

gcc fails to optimize this to `return x + 1`, instead opting for some rather
weird code generation (involving `sbb` on x86). However, adding this :

     if (x >=3D 2)
        __builtin_unreachable();

at the beginning of the function makes it be optimized properly. Maybe this=
 is
a sign of the `x >=3D 2` condition being always false (due to it leading to=
 UB)
being found too late ?=