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. 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.
