[LLVMdev] pow operator on Windows

[David Given]

On 12/02/11 04:52, Michael Smith wrote:
[...]
> 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 x86 has got several different FPU instruction sets, which don't all
work at the same precision. In particular, using 387 instructions uses
80-bit floats when working with temporaries, while SSE instructions use
64-bit floats.

gcc's default choice is 387, so if clang is defaulting to SSE (which
most compiler software these days does because 387 instructions are
horrible), this might explain the results you get.

You might want to look at the generated machine code to see how they
differ. If this *is* the problem, you can tell gcc to use a particular
instruction set with -mfpmath=386 or -mfpmath=sse.

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