On Fri, Jan 31, 2020 at 6:01 PM Ulrich Weigand <uweig...@de.ibm.com> wrote:
>
> Hello,
>
> we've noticed some inconsistencies in how the component flags of -ffast-math
> are handled, see the discussion on the GCC list starting here:
> https://gcc.gnu.org/ml/gcc/2020-01/msg00365.html
>
> This patch fixes those inconsistencies. Specifically, there are the
> following changes:
>
> 1. Some component flags for -ffast-math are *set* with -ffast-math
> (changing the default), but are not reset with -fno-fast-math,
> causing the latter to have surprising results. (Note that since
> "-ffast-math -fno-fast-math" is short-cut by the driver, you'll
> only see the surprising results with "-Ofast -fno-fast-math".)
> This is fixed here by both setting and resetting the flags.
>
> This affects the following flags
> -fcx-limited-range
> -fexcess-precision=
>
> 2. Some component flags for -ffast-math are changed from their default,
> but are *not* included in the fast_math_flags_set_p test, causing
> __FAST_MATH__ to remain predefined even when the full set of fast
> math options is not actually in effect. This is fixed here by
> adding those flags into the fast_math_flags_set_p test.
>
> This affects the following flags:
> -fcx-limited-range
> -fassociative-math
> -freciprocal-math
>
> 3. For some math flags, set_fast_math_flags has code that sets their
> values only to what is already their default. The overall effect
> of this code is a complete no-op. This patch removes that dead code.
>
> This affects the following flags:
> -frounding-math
> -fsignaling-nans
>
>
> The overall effect of this patch is that now all component flags of
> -ffast-math are treated exactly equivalently:
> * they are set (changed from their default) with -ffast-math
> * they are reset to their default with -fno-fast-math
> * __FAST_MATH__ is only defined if the value of the flag matches
> what -ffast-math would have set it to
The last part is not obviously correct to me since it doesn't match
documentation which says
@item -ffast-math
@opindex ffast-math
Sets the options @option{-fno-math-errno}, @option{-funsafe-math-optimizations},
@option{-ffinite-math-only}, @option{-fno-rounding-math},
@option{-fno-signaling-nans}, @option{-fcx-limited-range} and
@option{-fexcess-precision=fast}.
This option causes the preprocessor macro @code{__FAST_MATH__} to be defined.
to me this reads as -ffast-math -fexcess-precision=standard defines
__FAST_MATH__.
The only relevant part to define __FAST_MATH__ is specifying -ffast-math, other
options are not relevant (which of course is contradicted by
implementation - where
I didn't actually follow its history in that respect). So can you
adjust documentation
as to when exactly __FAST_MATH__ is defined?
Also...
> Tested on s390x-ibm-linux.
>
> OK for mainline?
>
> Bye,
> Ulrich
>
> ChangeLog:
>
> * opts.c (set_fast_math_flags): In the !set case, also reset
> x_flag_cx_limited_range and x_flag_excess_precision. Remove dead
> code resetting x_flag_signaling_nans and x_flag_rounding_math.
> (fast_math_flags_set_p): Also test x_flag_cx_limited_range,
> x_flag_associative_math, and x_flag_reciprocal_math.
>
> diff --git a/gcc/opts.c b/gcc/opts.c
> index 7affeb4..4452793 100644
> --- a/gcc/opts.c
> +++ b/gcc/opts.c
> @@ -2850,18 +2850,14 @@ set_fast_math_flags (struct gcc_options *opts, int
> set)
> opts->x_flag_finite_math_only = set;
> if (!opts->frontend_set_flag_errno_math)
> opts->x_flag_errno_math = !set;
> - if (set)
> - {
> - if (opts->frontend_set_flag_excess_precision ==
> EXCESS_PRECISION_DEFAULT)
> - opts->x_flag_excess_precision
> - = set ? EXCESS_PRECISION_FAST : EXCESS_PRECISION_DEFAULT;
> - if (!opts->frontend_set_flag_signaling_nans)
> - opts->x_flag_signaling_nans = 0;
> - if (!opts->frontend_set_flag_rounding_math)
> - opts->x_flag_rounding_math = 0;
> - if (!opts->frontend_set_flag_cx_limited_range)
> - opts->x_flag_cx_limited_range = 1;
> - }
> + if (!opts->frontend_set_flag_cx_limited_range)
> + opts->x_flag_cx_limited_range = set;
> + if (!opts->frontend_set_flag_excess_precision)
> + opts->x_flag_excess_precision
> + = set ? EXCESS_PRECISION_FAST : EXCESS_PRECISION_DEFAULT;
> +
> + // -ffast-math should also reset -fsignaling-nans and -frounding-math,
> + // but since those are off by default, there's nothing to do for now.
...
but what happens to -fsignalling-nans -ffast-math then? Better leave those
in I'd say.
Note frontends come into play with what is considered -ffast-math
and -fno-fast-math but below flags are tested irrespectively of that
interpretation.
Note there's -fcx-fortran-rules similar to -fcx-limited-range but not tested
above. The canonical middle-end "flag" to look at is flag_complex_method.
Somehow -fcx-fortran-rules doesn't come into play at all above but it
affects -fcx-limited-range in another inconsistent way in that
-fcx-limited-range -fcx-fortran-rules and -fcx-fortran-rules -fcx-limited-range
behave the same (-fcx-fortran-rules takes precedence...). I guess
-fcomplex-method=ENUM should be exposed and -fcx-* made
appropriate aliases here.
You're tapping into a mine-field ;)
Richard.
> }
>
> /* When -funsafe-math-optimizations is set the following
> @@ -2884,10 +2880,13 @@ bool
> fast_math_flags_set_p (const struct gcc_options *opts)
> {
> return (!opts->x_flag_trapping_math
> + && !opts->x_flag_signed_zeros
> + && opts->x_flag_associative_math
> + && opts->x_flag_reciprocal_math
> && opts->x_flag_unsafe_math_optimizations
> && opts->x_flag_finite_math_only
> - && !opts->x_flag_signed_zeros
> && !opts->x_flag_errno_math
> + && opts->x_flag_cx_limited_range
> && opts->x_flag_excess_precision == EXCESS_PRECISION_FAST);
> }
>
> --
> Dr. Ulrich Weigand
> GNU/Linux compilers and toolchain
> ulrich.weig...@de.ibm.com
>
