[llvm-bugs] [Bug 35706] New: Missed optimization in math expression: log10(pow(10.0, x)) == x

via llvm-bugs llvm-bugs at lists.llvm.org
Wed Dec 20 07:57:18 PST 2017


            Bug ID: 35706
           Summary: Missed optimization in math expression:
                    log10(pow(10.0,x)) == x
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: zamazan4ik at tut.by
                CC: llvm-bugs at lists.llvm.org

Clang (trunk) with '-O3 -std=c++17 -ffast-math' for this code:

#include <cmath>

double test10(double x)
    return log10(pow(10.0, x));

double test2(double x)
    return log2(pow(2.0, x));

generates this:

  .quad 4621819117588971520 # double 10
test10(double): # @test10(double)
  push rax
  movaps xmm1, xmm0
  movsd xmm0, qword ptr [rip + .LCPI0_0] # xmm0 = mem[0],zero
  call pow
  pop rax
  jmp log10 # TAILCALL
test2(double): # @test2(double)
  push rax
  call exp2
  pop rax
  jmp log2 # TAILCALL

But we can simplify it by logX(pow(X,Y)) == Y.

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