[llvm-bugs] [Bug 35742] New: Missed optimization in math expression: log of pow or exp of log

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=35742

            Bug ID: 35742
           Summary: Missed optimization in math expression: log of pow or
                    exp of log
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: zheltonozhskiy at gmail.com
                CC: llvm-bugs at lists.llvm.org

#include <cmath>

double f(double a)
{
    return exp(log(a)) ;
}


double f2(double a, double b)
{
    return log(pow(a,b)) ;
}

clang(trunk) -g0 -O3 -march=bdver4 -ffast-math generates following assembly:

f(double): # @f(double)
  pushq %rax
  callq log
  popq %rax
  jmp exp # TAILCALL
f2(double, double): # @f2(double, double)
  pushq %rax
  callq pow
  popq %rax
  jmp log # TAILCALL 

While gcc generates

f(double):
  ret
f2(double, double):
  subq $24, %rsp
  vmovsd %xmm1, 8(%rsp)
  call __log_finite
  vmovsd 8(%rsp), %xmm1
  addq $24, %rsp
  vmulsd %xmm0, %xmm1, %xmm0
  ret

replacing exponent of natural logarithm with the number itself and ln(a^b) with
b*ln(a) (same for log10/log2)

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