fast-math

Does anyone know why GCC/Clang will not optimist function test1 in the below code sample to simply use just the RCPPS instruction when using the fast-math option? Is there another compiler flag that would generate this code? typedef float float4 __attribute__((vector_size(16))); float4 test1(float4 v) { return 1.0f / v; } You can see the compiled output here: https://goo.gl/jXsqat Because the precision of RCPPS is a lot lower than float division. An option to enable that optimization would not be appropriate as part of -ffast-math . The x86 target options of the gcc manual says there in fact

I understand gcc's --ffast-math flag can greatly increase speed for float ops, and goes outside of IEEE standards, but I can't seem to find information on what is really happening when it's on. Can anyone please explain some of the details and maybe give a clear example of how something would change if the flag was on or off? I did try digging through S.O. for similar questions but couldn't find anything explaining the workings of ffast-math. Mysticial As you mentioned, it allows optimizations that do not preserve strict IEEE compliance. An example is this: x = x*x*x*x*x*x*x*x; to x *= x; x *=