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

[Andrew MacLeod <amacleod@redhat.com>]

On 3/17/22 19:27, Jeff Law via Gcc-patches wrote:
>
> On 3/15/2022 2:03 AM, Roger Sayle wrote:
>>> -----Original Message-----
>>> From: Richard Biener <richard.guenther@gmail.com>
>>> Sent: 15 March 2022 07:29
>>> To: Roger Sayle <roger@nextmovesoftware.com>
>>> Cc: GCC Patches <gcc-patches@gcc.gnu.org>
>>> Subject: Re: [PATCH] Ignore (possible) signed zeros in operands of FP
>>> comparisons.
>>>
>>> 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.

I'm going to hazard a guess that this can be handled in the upcoming 
floating point range support?  there was a start of a conversation in 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=24021 about a month ago.

I know *zero* about floating point, but It seems like when we track 
floating point ranges, we will naturally be able to integrate

   _2 = _1 * 0.0;
   _3 = _2 < y_5(D);

that _2 evaluates to +/- 0.0  and when we do look at  (_2 < y_5)   that 
VRP  can simplify that to 0.0 < y?  (or any patch which uses 
simplification and ranges).    It seems like it should be 
straightforward anyway.

Andrew