https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121595
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
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Keywords| |easyhack
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Matteo Nicoli from comment #2)
> This optimization must still be available when using the -ffast-math option,
> right?
No, just -fno-trapping-math is enough to be able to optimize away the `+0.0`.
Basically `a+0.0` can cause a trap and currently GCC's default is
trapping-math.
The reason why `fabs(a+0.0)` can be optimized to just `fabs(a)` is because
while `a+0.0` might change the sign; it is ignored with fabs as fabs will
always zero out the sign bit.
Note also `copy_sign(a+0.0, b)` can also be optimized to just `copy_sign(a, b)`
for similar reasons (that is the sign bit of `a+0.0` is ignored so the `+0.0`
part can be dropped).
