g++：如果在-O2或-O3之前指定-fno-omit-frame-pointer是否有效？

Question

We have a complex hierarchical make file system where the CXXFLAGS is appended in several places (several separate makefiles of individual libraries).

There's one master file that seems to be getting included in every other make file. So I specified the -fno-omit-frame-pointer flag there.

When I compile, I see the above flag and after that I see -O2 . My question is, if the -O2 flag sets -fomit-frame-pointer , will the latest setting take effect?

Answer 1

With gcc/g++ you can use the -Q --help=optimizers flags to find out the exact set of optimizations that are enabled.

With my version of gcc I get:

$ gcc -Q --help=optimizers -O2 | fgrep omit-frame-pointer

-fomit-frame-pointer                [enabled]

and

$ gcc -Q --help=optimizers -fno-omit-frame-pointer -O2 | fgrep omit-frame-pointer

-fomit-frame-pointer                [disabled]

So -O2 doesn't seem to overwrite the previous -fno-omit-frame-pointer .

Anyway check your environment.