[Bug tree-optimization/83349] New: Missed optimization in math expression: aggressive optimization with std::pow

[zamazan4ik at tut dot by]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83349

            Bug ID: 83349
           Summary: Missed optimization in math expression: aggressive
                    optimization with std::pow
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zamazan4ik at tut dot by
  Target Milestone: ---

gcc(trunk) with '--std=c++17 -O3 -march=native -ffast-math' flags for this
code:

#include <cmath>

double test(double a, double x)
{
    return pow(a, x) * a * a * a * a;
}

generates this assembly:


test(double, double):
        sub     rsp, 24
        vmovsd  QWORD PTR [rsp+8], xmm0
        call    __pow_finite
        vmovsd  xmm2, QWORD PTR [rsp+8]
        vmulsd  xmm2, xmm2, xmm2
        vmulsd  xmm2, xmm2, xmm2
        vmulsd  xmm0, xmm2, xmm0
        add     rsp, 24
        ret


As you can see, me can simplify it by adding 4 to 'x' variable and after call
std::pow.