使用O3优化级别编译GSL

Question

My program heavily depends on the special functions from GSL and thus I would like to make it run faster, so I wish to compile GSL with higher optimization levels.

When I compile gsl, the default CFLAGS is "-g -O2" if I do nothing when I configure with "./configure". I am wondering why gsl is defaulting to an optimization level of O2 only since O3 is compliant to standards. I tried to compile and test with "./configure CFLAGS='-g -O3'", things worked. But I'm still not sure if everything would work.

Can anyone tell me why GSL is defaulting to O2 instead of O3? Would it be dangerous if I default to O3? Thanks!

Answer 1

The optimization level 3 is something that should only be used in case it is absolutely sure that it helps the library.

Since that level activates optimizations that may increase the size of the code a lot. This means in some cases it creates binaries that are even slower compared to a binary optimized with -O2 . How ever that happens rarely. More likely are effects like a massively increased time to compile it, along with a increased binary size and a barely measurable performance change.

That -O3 actually breaks something was pretty common some time back, but in the last couple of years I did not have a single case where -O3 actually optimized something that caused the binary to break.

In the end the optimization level is something you can just test. Since -O2 is the default, it is a pretty safe bet that this is the best settings for the compile operation in this case. If you feel like it you could try to compile it with a different setting to see if it makes any performance difference.

Interesting options are -O3 and even -Os . I had cases in the past were both options gave improved performance over -O2 .

So the real answer is: Try it and see what happens.