Re: [PATCH v2] Fix -ffast-math flags handling inconsistencies

[Segher Boessenkool <segher@kernel.crashing.org>]

Hi!

On Tue, Feb 11, 2020 at 07:43:31PM +0100, Ulrich Weigand wrote:
> 1. If a component flag of -ffast-math (or -funsafe-math-optimizations)
>    is explicitly set (or reset) on the command line, this should override
>    any implicit change due to -f(no-)fast-math, no matter in which order
>    the flags come on the command line.  This change affects all flags.

Ack.

> 2. Any component flag modified from its default by -ffast-math should
>    be reset to the default by -fno-fast-math.  This was previously
>    not done for the following flags:
>       -fcx-limited-range
>       -fexcess-precision=

Ack.

> 3. Once -ffinite-math-only is true, the -f(no-)signaling-nans flag has
>    no meaning (if we have no NaNs at all, it does not matter whether
>    there is a difference between quiet and signaling NaNs).  Therefore,
>    it does not make sense for -ffast-math to imply -fno-signaling-nans.
>    This is also a documentation change.

Ack.

> 4. -ffast-math is documented to imply -fno-rounding-math, however the
>    latter setting is the default anyway; therefore it does not make
>    sense to try to modify it from its default setting.

Ack.  It might be more robust to *do* set it, so that it still works if
the default ever changes.

> 5. The __FAST_MATH__ preprocessor macro should be defined if and only
>    if all the component flags of -ffast-math are set to the value that
>    is documented as the effect of -ffast-math.  The following flags
>    were currently *not* so tested:
>      -fcx-limited-range
>      -fassociative-math
>      -freciprocal-math
>      -frounding-math
>    (Note that we should still *test* for -fno-rounding-math here even
>    though it is not set as per 4.  -ffast-math -frounding-math should
>    not set the __FAST_MATH__ macro.)
>    This is also a documentation change.

Ack.

All looks good to me, but I'm not maintainer of this.

This should be mentioned in the release notes I think, it does change
behaviour (for the better, but any change is a change).

Thanks,


Segher