[llvm-bugs] [Bug 35605] New: Missed optimization in math expression: pow(cbrt(x), y) == pow(x, y / 3)

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=35605

            Bug ID: 35605
           Summary: Missed optimization in math expression: pow(cbrt(x),
                    y) == pow(x, y / 3)
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: zamazan4ik at tut.by
                CC: llvm-bugs at lists.llvm.org

clang(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): # @test(double, double)
  push rax
  vmovsd qword ptr [rsp], xmm1 # 8-byte Spill
  call cbrt
  vmovsd xmm1, qword ptr [rsp] # 8-byte Reload
  pop rax
  jmp pow # TAILCALL


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

-- 
You are receiving this mail because:
You are on the CC list for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-bugs/attachments/20171210/3c77f0c2/attachment.html>