Re: [PATCH] Ignore (possible) signed zeros in operands of FP comparisons.

[Richard Sandiford <richard.sandiford@arm.com>]

Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> On Mon, Mar 14, 2022 at 8:26 PM Roger Sayle <roger@nextmovesoftware.com> wrote:
>> I've been wondering about the possible performance/missed-optimization
>> impact of my patch for PR middle-end/98420 and similar IEEE correctness
>> fixes that disable constant folding optimizations when worrying about -0.0.
>> In the common situation where the floating point result is used by a
>> FP comparison, there's no distinction between +0.0 and -0.0, so some
>> HONOR_SIGNED_ZEROS optimizations that we'd usually disable, are safe.
>>
>> Consider the following interesting example:
>>
>> int foo(int x, double y) {
>>     return (x * 0.0) < y;
>> }
>>
>> Although we know that x (when converted to double) can't be NaN or Inf,
>> we still worry that for negative values of x that (x * 0.0) may be -0.0
>> and so perform the multiplication at run-time.  But in this case, the
>> result of the comparison (-0.0 < y) will be exactly the same as (+0.0 < y)
>> for any y, hence the above may be safely constant folded to "0.0 < y"
>> avoiding the multiplication at run-time.
>>
>> This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
>> and make -k check with no new failures, and allows GCC to continue to
>> optimize cases that we optimized in GCC 11 (without regard to correctness).
>> Ok for mainline?
>
> Isn't that something that gimple-ssa-backprop.c is designed to handle?  I wonder
> if you can see whether the signed zero speciality can be retrofitted there?
> It currently tracks "sign does not matter", so possibly another state,
> "sign of zero
> does not matter" could be introduced there.

I agree that would be a nice thing to have FWIW.  gimple-ssa-backprop.c
was added to avoid regressions in one specific fold-const -> match.pd
move, but it was supposed to be suitable for other similar things
in future.

Thanks,
Richard