[Bug tree-optimization/83354] New: Missed optimization in math expression: pow(cbrt(x), y) == pow(x, y / 3)

["zamazan4ik at tut dot by" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83354

            Bug ID: 83354
           Summary: Missed optimization in math expression: pow(cbrt(x),
                    y) == pow(x, y / 3)
           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 x, double y)
{
    return pow(cbrt(x),y);
}

generates this assembly:


test(double, double):
        sub     rsp, 24
        vmovsd  QWORD PTR [rsp+8], xmm1
        call    cbrt
        vmovsd  xmm1, QWORD PTR [rsp+8]
        add     rsp, 24
        jmp     __pow_finite


As you can see, we can simplify it by calling pow(x, y/3). It should be faster.