[Bug tree-optimization/94779] New: Bad optimization of simple switch

gabravier at gmail dot com gcc-bugzilla@gcc.gnu.org
Mon Apr 27 00:40:24 GMT 2020


https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94779

            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 >= 2)
        __builtin_unreachable();

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


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