On Fri, 6 Sep 2019, Richard Biener wrote:
> On Thu, 5 Sep 2019, Barnaby Wilks wrote:
>
> > Hello,
> >
> > This patch changes a match.pd expression so that binary math expressions
> > will not be done in the precision of it's widest type if
> > -funsafe-math-optimizations is enabled.
> >
> > This patch has been extracted from
> > https://gcc.gnu.org/ml/gcc-patches/2019-09/msg00072.html
> > based on upstream comments.
> >
> > For example take the function:
> >
> > float f (float x, float y) {
> > double z = 1.0 / x;
> > return z * y;
> > }
> >
> > Without this patch this would generate the following (with -Ofast):
> >
> > _1 = (double) x_4(D);
> > z_5 = 1.0e+0 / _1;
> > _2 = (double) y_6(D);
> > _3 = _2 * z_5;
> > _7 = (float) _3;
> > return _7;
> >
> > This contains 3 unnecessary casts - since all outputs of the expression
> > are in single precision, the calculation itself can be done entirely in
> > single precision - instead of converting the operands to double,
> > doing the calculation and then converting the result back to
> > single precision again.
> >
> > With this patch (and -Ofast) the following GIMPLE is generated:
> >
> > _5 = 1.0e+0 / x_1(D);
> > _3 = y_2(D) * _5;
> > return _3;
> >
> > The benefits can then be seen in the generated code:
> >
> > Without this patch
> >
> > f:
> > fcvt d1, s1
> > fcvt d0, s0
> > fdiv d0, d1, d0
> > fcvt s0, d0
> > ret
> >
> >
> > With this patch
> >
> > f:
> > fdiv s0, s1, s0
> > ret
> >
> > Added tests to verify that no unnecessary casts are kept.
> >
> > Bootstrapped and regtested on aarch64 and x86_64 with no regressions.
> >
> > I don't have write access, so if OK for trunk then can someone commit on
> > my behalf?
>
> OK. I'll commit it after a round of testing.
So on x86_64 I see the testcase FAILing:
FAIL: gcc.dg/fold-binary-math-casts.c scan-tree-dump-not optimized
"\\\\(double\\\\)"
FAIL: gcc.dg/fold-binary-math-casts.c scan-tree-dump-not optimized
"\\\\(float\\\\)"
It's
float
k (float a)
{
return 1.0 / sqrt (a);
}
where we don't narrow the sqrt call itself (leftover from the
other pattern?)
Richard.
> Thanks,
> Richard.
>
> > Regards
> > Barney
> >
> > gcc/ChangeLog:
> >
> > 2019-09-05 Barnaby Wilks <barnaby.wi...@arm.com>
> >
> > * match.pd: Add flag_unsafe_math_optimizations check
> > before deciding on the widest type in a binary math operation.
> >
> > gcc/testsuite/ChangeLog:
> >
> > 2019-09-05 Barnaby Wilks <barnaby.wi...@arm.com>
> >
> > * gcc.dg/fold-binary-math-casts.c: New test.
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imendörffer; HRB 247165 (AG München)
