Optimization and flags for making a static library with g++

Question

I am just starting with g++ compiler on Linux and got some questions on the compiler flags. Here are they

Optimizations

I read about optimization flags -O1, -O2 and -O3 in the g++ manual page. I didn't understood when to use these flags. Usually what optimization level do you use? The g++ manual says the following for -O2.

Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. The compiler does not perform loop unrolling or function inlining when you specify -O2. As compared to -O, this option increases both compilation time and the performance of the generated code.

If it is not doing inlining and loop unrolling, how the said performance befits are achieved and is this option recommended?

Static Library

How do I create a static library using g++? In Visual Studio, I can choose a class library project and it will be compiled into "lib" file. What is the equivalent in g++?

Answer 1

The rule of thumb:

When you need to debug, use -O0 (and -g to generate debugging symbols.)

When you are preparing to ship it, use -O2.

When you use gentoo, use -O3...!

When you need to put it on an embedded system, use -Os (optimize for size, not for efficiency.)