[llvm-bugs] [Bug 35604] New: Missed optimization in math expression: sqrt(a) * sqrt(b) == sqrt(a*b)

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https://bugs.llvm.org/show_bug.cgi?id=35604

            Bug ID: 35604
           Summary: Missed optimization in math expression: sqrt(a) *
                    sqrt(b) == sqrt(a*b)
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Common Code Generator Code
          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 a, double b)
{
    return sqrt(a) * sqrt(b);
}


generates this assembly:


test(double, double, double): # @test(double, double, double)
  vsqrtsd xmm0, xmm0, xmm0
  vsqrtsd xmm1, xmm1, xmm1
  vmulsd xmm0, xmm1, xmm0
  ret


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


test(double, double, double):
        vmulsd  xmm0, xmm0, xmm1
        vsqrtsd xmm0, xmm0, xmm0
        ret


Also if you will increase count of multiplication on sqrt(varible_name), gcc
continues generate more optimal code:


#include <cmath>

double test(double a, double b, double c)
{
    return sqrt(a) * sqrt(b) * sqrt(c);
}


clang:


test(double, double, double): # @test(double, double, double)
  vsqrtsd xmm0, xmm0, xmm0
  vsqrtsd xmm1, xmm1, xmm1
  vmulsd xmm0, xmm1, xmm0
  vsqrtsd xmm1, xmm2, xmm2
  vmulsd xmm0, xmm0, xmm1
  ret



gcc:


test(double, double, double):
        vmulsd  xmm2, xmm1, xmm2
        vmulsd  xmm0, xmm2, xmm0
        vsqrtsd xmm0, xmm0, xmm0
        ret

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