On Fri, Aug 29, 2025 at 2:20 PM Matteo Nicoli
<matteo.nicoli...@gmail.com> wrote:
Here’s the patch with the modified comment before the rule in match.pd
OK. Do you have git access? Btw, the ChangeLog is currently wrong:
Subject: [PATCH] tree-optimization/121595 - the optimization fabs(a+0.0) ->
fabs(a) should apply only for non trapping case
* added a match _expression_
- gcc/match.pd
* added tests
- gcc/testsuite/gcc.dg/fabs-plus-zero-1.c
- gcc/testsuite/gcc.dg/fabs-plus-zero-2.c
there's a missing patch description and the changlog part should look like this:
PR tree-optimization/121595
* match.pd (fabs(a + 0.0) -> fabs (a)): New pattern.
* gcc.dg/fabs-plus-zero-1.c: New testcase.
* gcc.dg/fabs-plus-zero-2.c: Likewise.
On Aug 29, 2025, at 12:53 PM, Richard Biener <richard.guent...@gmail.com> wrote:
On Fri, Aug 29, 2025 at 10:56 AM Matteo Nicoli
<matteo.nicoli...@gmail.com> wrote:
Dear Richard,
It can trap with sNaN ± 0.0. ±Inf ± 0.0 = ±Inf, so it does not raise an FE_OVERFLOW (because there’s no overflow of a finite quantity), and qNaN does not raise an FE_INVALID because it’s quiet.
There’s already a check for sNaN in fold-const.cc
/* Don't allow the fold with -fsignaling-nans. */
if (arg ? tree_expr_maybe_signaling_nan_p (arg) : HONOR_SNANS (type))
return false;
As far as I know, the purpose of this bug fix was to suppress this specific optimization when the program is compiled with -ftrapping-math flag.
So we've discussed on IRC and the conclusion was that with default
exception handling !HONOR_SNANS would be sufficient
but alternate exception handling might trap on x + 0.0 when x is
denormal. Ideally we'd have a separate flag for
non-default exception handling, but as-is we don't.
Please add a comment like
/* With non-default exception handling denormal + 0.0 might trap.
Otherwise !HONOR_SNANS would be sufficient here. */
The patch is OK with that change.
Richard.
Best regards,
Matteo
On Aug 28, 2025, at 10:43 AM, Richard Biener <richard.guent...@gmail.com> wrote:
On Sat, Aug 23, 2025 at 11:56 PM Matteo Nicoli
<matteo.nicoli...@gmail.com> wrote:
Dear reviewers,
I attached a patch for bug 121595 (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121595). I signed it, and added `Reviewed-by: Andrew Pinski <andrew.pin...@oss.qualcomm.com>` (here in CC).
+/* fabs(x + 0.0) -> fabs(x), safe even with signed zeros when
-fno-trapping-math. */
+(for op (plus minus)
+ (simplify
+ (abs (op @0 real_zerop@1))
+ (if (!flag_trapping_math)
+ (abs @0))))
so forgive my ignorance, possibly IEEE abs() never raises FP exceptions
(unless operating on sNaN?)? But does Inf + 0.0 raise FE_OVERFLOW?
Does NaN + 0.0 raise FE_INVALID?
So what I wonder is whether !HONOR_SNANS (@0) would be enough to check?
I refrained from trusting AI on those questions ...
Richard.
Best regards,
Matteo
tree-optimization-121595.patch