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From: "aldyh at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/103559] Can't optimize away < 0 check on sqrt
Date: Wed, 29 Mar 2023 17:46:09 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103559

Aldy Hernandez <aldyh at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |DUPLICATE
                 CC|                            |aldyh at gcc dot gnu.org

--- Comment #5 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
(In reply to Jeremy R. from comment #0)
> For a simple invocation of sqrt, gcc inserts a < 0 check to set math errno
> if needed. E.g.
>=20
> float f(float x) {
>     return sqrt(x);
> }
>=20
> Is generated as
>=20
> f(float):
>         vxorps  xmm1, xmm1, xmm1
>         vucomiss        xmm1, xmm0
>         ja      .L10
>         vsqrtss xmm0, xmm0, xmm0
>         ret
> .L10:
>         jmp     sqrtf
>=20
>=20
> Unfortunately, this check is still present when the GCC is able to prove
> that x is non-negative:
>=20
> float f(float x) {
>     if(x < 0) [[unlikely]] {
>         __builtin_unreachable();
>     } else {
>         return sqrt(x);
>     }
> }

x could also be a NAN which I *think* the hardware sqrt won't handle, so a
better test would be:

if (x < 0.0 || __builtin_isnan(x)) [[unlikely]]
  __builtin_unreachable ();

or perhaps:

if (!__builtin_isgreater(x, 0.0))

Either way, this is a subset of PR91645 so I'm closing it as a duplicate.

*** This bug has been marked as a duplicate of bug 91645 ***=