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From: "llvm at rifkin dot dev" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/103559] New: Can't optimize away < 0 check on
 sqrt
Date: Sat, 04 Dec 2021 20:58:21 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103559

            Bug ID: 103559
           Summary: Can't optimize away < 0 check on sqrt
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: llvm at rifkin dot dev
  Target Milestone: ---

For a simple invocation of sqrt, gcc inserts a < 0 check to set math errno =
if
needed. E.g.

float f(float x) {
    return sqrt(x);
}

Is generated as

f(float):
        vxorps  xmm1, xmm1, xmm1
        vucomiss        xmm1, xmm0
        ja      .L10
        vsqrtss xmm0, xmm0, xmm0
        ret
.L10:
        jmp     sqrtf


Unfortunately, this check is still present when the GCC is able to prove th=
at x
is non-negative:

float f(float x) {
    if(x < 0) [[unlikely]] {
        __builtin_unreachable();
    } else {
        return sqrt(x);
    }
}

LLVM suffers from the same problem, even with __builtin_assume().
https://godbolt.org/z/ddcoMj3oz

This is a very common pattern, and I'd imagine the argument for sqrt is oft=
en
able to be shown to be positive. This would be a helpful enhancement.=