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Re: [PATCH] Change default to -fno-math-errno
On Mon, 30 Oct 2017, Wilco Dijkstra wrote:
> Richard Biener wrote:
> > Should we also get the __ieee764_ entries used if the compiler sets
> > __NO_MATH_ERRNO__? That is, if the librari advertises not setting errno
> > via math_errhandling is it still allowed to set/clobber errno anyways?
>
> That's a good question! I checked and the math wrappers currently use
> __FINITE_MATH_ONLY__ to decide whether to use the IEEE754 version
> or the wrapper which sets errno. However that's actually incorrect given
> that the only purpose of the wrappers is to set errno, not to add IEEE
> handling (which is done in the math implementation).
The semantics of __*_finite definitely include finite-math-only, as they
aren't all just disabling the wrappers (e.g. sysdeps/i386/i686/fpu/e_log.S
has __log_finite separate from __ieee754_log).
> Note -ffinite-math-only appears to imply -fno-math-errno, eventhough
> this is not clear from the documentation - is that expected? In principle
> these options are orthogonal, setting errno due to a range error is still
> valid even if you don't use infinities or NaNs.
Well, it's expected for glibc (as an empirical observation about what the
__*_finite entry points do). It may not necessarily be an implication for
GCC. And given appropriate documentation, it doesn't necessarily
contradict math_errhandling & MATH_ERRNO being nonzero (in that errno
setting on underflow is implementation-defined in that case - glibc
documents semantics involving setting errno on underflow to 0 but not
necessarily otherwise, but could document the effects of
-ffinite-math-only potentially disabling the errno setting on underflow to
0).
--
Joseph S. Myers
joseph@codesourcery.com