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performancemathgccfloating-pointfast-math

What does gcc's ffast-math actually do?

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.

Solution

  • 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 *= x;
x *= x;

    Because floating-point arithmetic is not associative, the ordering and factoring of the operations will affect results due to round-off. Therefore, this optimization is not done under strict FP behavior.

    I haven't actually checked to see if GCC actually does this particular optimization. But the idea is the same.