Re: [PATCH] A == 0 ? A : -A same as -A (when A is 0.0)

[Jeff Law via Gcc-patches]

On 8/29/2022 2:44 PM, Aldy Hernandez via Gcc-patches wrote:

The upcoming work for frange triggers a regression in
gcc.dg/tree-ssa/phi-opt-24.c.

For -O2 -fno-signed-zeros, we fail to transform the following into -A:

float f0(float A)
{
   //     A == 0? A : -A    same as -A
   if (A == 0)  return A;
   return -A;
}

This is because the abs/negative match.pd pattern here:

/* abs/negative simplifications moved from fold_cond_expr_with_comparison,
    Need to handle (A - B) case as fold_cond_expr_with_comparison does.
    Need to handle UN* comparisons.
    ...
    ...

Sees IL that has the 0.0 propagated.

Instead of:

   <bb 2> [local count: 1073741824]:
   if (A_2(D) == 0.0)
     goto <bb 4>; [34.00%]
   else
     goto <bb 3>; [66.00%]

   <bb 3> [local count: 708669601]:
   _3 = -A_2(D);

   <bb 4> [local count: 1073741824]:
   # _1 = PHI <A_2(D)(2), _3(3)>

It now sees:

   <bb 4> [local count: 1073741824]:
   # _1 = PHI <0.0(2), _3(3)>

which it leaves untouched, causing the if conditional to survive.

Adding a variant to the pattern that for real_zerop fixes the problem.

I am a match.pd weenie, and am avoiding touching more patterns that
may also benefit from handling real constants.  So please use simple
words if what I'm doing isn't correct ;-).

I did not include a testcase, as it's just phi-opt-24.c which will get
triggered when I commit the frange with endpoints work.

Tested on x86-64 & ppc64le Linux.

OK?

gcc/ChangeLog:

        * match.pd ((cmp @0 zerop) real_zerop (negate@1 @0)): Add variant
        for real zero.

OK
jeff