[Bug rtl-optimization/80197] pgo dramatically pessimizes scimark2 MonteCarlo benchmark

["ubizjak at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80197

Uroš Bizjak <ubizjak at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2017-03-27
                 CC|                            |jakub at gcc dot gnu.org,
                   |                            |rguenth at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Uroš Bizjak <ubizjak at gmail dot com> ---
For some reason, recently fixed if-conversion (PR79389) does not trigger in PGO
case. There is still a jump with -O2:

        mulsd   %xmm0, %xmm5
        mulsd   %xmm2, %xmm2
        addsd   %xmm2, %xmm5
        ucomisd %xmm5, %xmm4
        jb      .L17
.L16:
        addl    $1, %ebp
.L17:
        addl    $1, %edi
        cmpl    %edi, %ebx
        je      .L5

Since this asm corresponds to random operands, the jump can't be predicted:

        for (count=0; count<Num_samples; count++)
        {
            double x= Random_nextDouble(R);
            double y= Random_nextDouble(R);

            if ( x*x + y*y <= 1.0)
                 under_curve ++;

        }

Based on the discussion in PR79389, and the fact that -O2 and -O3 both compile
to a jump, I suspect that loop splitting cost model should be fine tuned to
also handle PGO case. Note that

Adding some CCs.