[PATCH v2] Fix -ffast-math flags handling inconsistencies
Jeff Law
law@redhat.com
Sat Nov 21 04:33:48 GMT 2020
On 2/11/20 11:43 AM, Ulrich Weigand wrote:
> Hello,
>
> this second version of the patch incorporates all changes discussed
> in the original mail thread starting here:
> https://gcc.gnu.org/ml/gcc-patches/2020-01/msg02097.html
>
> The patch now implements the following set of changes:
>
> 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.
>
> 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=
>
> 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.
>
> 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.
>
> 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.
>
>
> Tested on s390x-ibm-linux.
>
> OK for mainline?
>
> Bye,
> Ulrich
>
> gcc/ChangeLog:
>
> * doc/invoke.texi (-ffast-math): Remove mention of -fno-signaling-nans.
> Clarify conditions when __FAST_MATH__ preprocessor macro is defined.
>
> * opts.c (common_handle_option): Pass OPTS_SET to set_fast_math_flags
> and set_unsafe_math_optimizations_flags.
> (set_fast_math_flags): Add OPTS_SET argument, and use it to avoid
> setting flags already explicitly set on the command line. In the !set
> case, also reset x_flag_cx_limited_range and x_flag_excess_precision.
> Never reset x_flag_signaling_nans or x_flag_rounding_math.
> (set_unsafe_math_optimizations_flags): Add OPTS_SET argument, and use
> it to avoid setting flags already explicitly set on the command line.
> (fast_math_flags_set_p): Also test x_flag_cx_limited_range,
> x_flag_associative_math, x_flag_reciprocal_math, and
> x_flag_rounding_math.
It appears this was dropped on the floor. It looks reasonable to me.
Please retest and commit. Thanks!
Jeff
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