[cfe-dev] Clang not generating pow_finite with -ffast-math

[Murali, Sriram]

Hi,

I am trying to make clang generate code similar to gcc for the following function with –ffast-math option.
#include <cmath>
double foo(double val, double i) {
     double t = log(exp(val));
     return pow(t, i);
}

There are a few issues here.

·        Firstly, clang is not optimizing log(exp(val)) away into val.

·        Secondly, clang is calling an choosing an intrinsic of pow, instead of a library call to @__pow_finite

  %0 = tail call double @llvm.pow.f64(double %call1, double %i)

I am wondering if there is a performance benefit in using @llvm.pow.f64, because at the end of the day, llvm generates code for x86 as follows:
               .cfi_def_cfa_offset 16
               movsd   %xmm1, (%rsp)           # 8-byte Spill
               callq       __exp_finite
               callq       __log_finite
               movsd   (%rsp), %xmm1           # 8-byte Reload
               popq      %rax
               jmp        pow

I believe __pow_finite is  faster than pow. Please correct me if I am wrong.

Thanks
Ram
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