Secure gcc optimization options for numerics

Question

Which gcc compiler options may be safely used for numerical programming?

The easy way to turn on optimizations for gcc is to add -0# to the compiler options. It is tempting to say -O3. However I know that -O3 includes optimization which are non-save in the sense that results of numerical computations may differ once this option is included. Small changes in the result may be insignificant if the algorithm is stable. On the other hand, precision can be an issue for certain math operations, so math optimization can have significant impact.

I find it inconvenient to take compiler dependent issues into account in the process of debugging. I.e. I don't want to wonder whether minor changes in the code will lead to strongly different behavior because the compiler changed its optimizations internally.

Which options are safe to add if I want deterministic--and hence controllable--behavior in my code? Which are almost safe, that is, which options induce only minor uncertainties compared to performance benefits?

I think of options like: -finline -finline-limit=2000 which inlines functions even if they are long.

Answer 1

It is not true that -O3 includes numerically unsafe optimizations. According to the manual, -O3 includes the following optimization passes in comparison to -O2:

-finline-functions, -funswitch-loops, -fpredictive-commoning, -fgcse-after-reload, -ftree-vectorize and -fipa-cp-clone

You might be referring to -ffast-math, turned on by default with -Ofast, but not with -O3:

-ffast-math Sets -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans and -fcx-limited-range. This option causes the preprocessor macro __FAST_MATH__ to be defined.

This option is not turned on by any -O option besides -Ofast since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.

In other words, all of -O, -O2, and -O3 are safe for numeric programming.