Consistant behaviour of float code with GCC

Question

I do some numerical computing, and I have often had problems with floating points computations when using GCC. For my current purpose, I don't care too much about the real precision of the results, but I want this firm property:

no matter WHERE the SAME code is in my program, when it is run on the SAME inputs, I want it to give the SAME outputs.

How can I force GCC to do this? And specifically, what is the behavior of --fast-math, and the different -O optimizations?

I've heard that GCC might try to be clever, and sometimes load floats in registers, and sometime read them directly from memory, and that this might change the precision of the floats, resulting in a different output. How can I avoid this?

Again, I want :

• my computations to be fast
• my computations to be reliable (ie. same input -> same result)
• I don't care that much about the precision for this particular code, so I can be fine with reduced precision if this brings reliability

could anyone tell me what is the way to go for this problem?

Answer 1

If your targets include x86 processors, using the switch that makes gcc use SSE2 instructions (instead of the historical stack-based ones) will make these run more like the others.

If your targets include PowerPC processors, using the switch that makes gcc not use the fmadd instruction (to replace a multiplication followed by an addition in the source code) will make these run more like the others.

Do not use --fast-math: this allows the compiler to take some shortcuts, and this will cause differences between architectures. Gcc is more standard-compliant, and therefore predictable, without this option.

Including your own math functions (exp, sin, ...) in your application instead of relying on those from the system's library can only help with predictability.

And lastly, even when the compiler does rigorously respect the standard (I mean C99 here), there may be some differences, because C99 allows intermediate results to be computed with a higher precision than required by the type of the expression. If you really want the program always to give the same results, write three-address code. Or, use only the maximum precision available for all computations, which would be double if you can avoid the historical x86 instructions. In any case do not use lower-precision floats in an attempt to improve predictability: the effect would be the opposite, as per the above clause in the standard.