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
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.
}
/* 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
