Localize g++ compile options within code

Question

I'm looking for a simple way to localize certain g++ (g++-4.9 to be specific) compile options to certain lines of code or at least targeted functions. I'm interested generally speaking, but also specifically to the -fast-math, -ffinite-math-only and -fno-signed-zeros options.

I presume that localization at the *.cpp file level is possible with make utility, but I'm hoping there is a way to enable it in the code itself, through #pragma or __attribute__ or something. I want to do this not only to minimize dependencies to external files (i.e. risk of incorrect makefile) but also to hopefully hyperlocalize certain FP behavior to specific equations within a function.

Alternatively, if localization of FP behavior by inline directives is NOT possible, what can I do to at least trigger a compile time error if desired compiler directive is NOT enabled in project build (e.g. makefile is lost or inappropriately modified).

I would presume that such inline optimization might be compiler specific, g++ in this case, but that is a compromise I'm willing to take.

Answer 1

In gcc you can use function attribute optimize:

void f () __attribute__ ((optimize("fast-math"), optimize("finite-math-only"), optimize("no-signed-zeros")));

Answer 2

You might be able to turn on some bits of this with the fpmath option in a function attribute, but this was not clear to me from the docs. In light of that, I will focus on detection instead:

-fast-math already turns on -ffinite-math-only, so you don't need to worry about that. The docs for -fast-math say:

This option causes the preprocessor macro FAST_MATH to be defined.

Which means it can be detected via

#ifndef __FAST_MATH__
#error "The -fast-math compiler option is required"
#endif

I have not yet found a compile-time way to detect the presence of -fno-signed-zeros

Answer 3

^{I'm not sure that you are using the "localize" word correctly. Localization is related to adapting software to users of different human languages (French, Russian, Chinese...)}

Perhaps you want to ask the compiler to optimize some functions with other optimization flags.

This is possible using #pragma GCC optimize etc... or using some function attributes