On Tue, 1 Oct 2024, Jennifer Schmitz wrote:
> This patch implements 4 rules for logarithmic identities in match.pd
> under -funsafe-math-optimizations:
> 1) logN(1.0/a) -> -logN(a). This avoids the division instruction.
> 2) logN(C/a) -> logN(C) - logN(a), where C is a real constant. Same as 1).
> 3) logN(a) + logN(b) -> logN(a*b). This reduces the number of calls to
> log function.
> 4) logN(a) - logN(b) -> logN(a/b). Same as 4).
> Tests were added for float, double, and long double.
>
> The patch was bootstrapped and regtested on aarch64-linux-gnu and
> x86_64-linux-gnu, no regression.
> Additionally, SPEC 2017 fprate was run. While the transform does not seem
> to be triggered, we also see no non-noise impact on performance.
> OK for mainline?
Since log can set errno we have the builtins affect global memory and
thus have VDEFs, this posses issues for match.pd which does not assign
new VDEFs upon materializing the result, esp. for the case where
you duplicate a call. There's a similar issue for -frounding-math
where intermediate FP status changes can be lost. match.pd simply
follows the SSA use-def chains without regarding memory side-effects.
The transforms are guarded by flag_unsafe_math_optimizations but here
I think we need !HONOR_SIGN_DEPENDENT_ROUNDING, !flag_trapping_math
(exception state might be different for logN(a) - logN(b) -> logN(a/b),
at least WRT INEXACT?), and !flag_errno_math (because of the VDEFs).
+ /* Simplify logN(C/a) into logN(C)-logN(a). */
+ (simplify
+ (logs (rdiv:s REAL_CST@0 @1))
+ (minus (logs @0) (logs @1)))
I think you want
(minus (logs! @0) (logs @1))
here to make sure we constant-fold.
+ (simplify
+ (minus (logs:s @0) (logs:s @1))
+ (logs (rdiv @0 @1))))
I think that's somewhat dangerous for @1 == 0 given log for
zero arg results in -HUGE_VAL but a FP division by gives a NaN.
I'm not exactly sure whether !HONOR_INFINITIES && !HONOR_NANS
is good enough here.
Your testcases probably all trigger during GENERIC folding,
bypassing the VDEF issue - you might want to try assigning
the comparison operands to tempoaries to run into the actual
issues.
Thanks,
Richard.
> Signed-off-by: Jennifer Schmitz <jschm...@nvidia.com>
>
> gcc/
> * match.pd: Fold logN(1.0/a) -> -logN(a),
> logN(C/a) -> logN(C) - logN(a), logN(a) + logN(b) -> logN(a*b),
> and logN(a) - logN(b) -> logN(a/b).
>
> gcc/testsuite/
> * gcc.dg/tree-ssa/log_ident.c: New test.
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
