[LLVMdev] pow operator on Windows

[Michael Smith]

I have a very simple test case on Windows that shows some surprising behavior. This doesn't seem to be a problem on Linux.

The example is:
#include <stdio.h>
#include <math.h>
double heat(double Pr) {
  return pow(Pr, 0.33);
}
int main(int argc, char **argv) {
  double Nu = heat(291.00606180486119);
  printf("%.20f\n", Nu);
}

I've tested with MinGW's gcc.exe 3.4.5 and the Visual Studio 2008 C++ compiler, and LLVM 2.7 and 2.8.
With "gcc test.c; ./a.exe", the printed result is 6.50260946378542390000
With "clang -emit-llvm -c test.c -o test.bc; lli test.bc", the printed result is 6.50260946378542480000

The difference in the last 2 digits is significant in the scientific software I'm working on. Does anyone have an explanation for why they differ, or how to make them behave the same?

Thanks,
Michael Smith