Which free C compiler provides options for more optimization?
There are actually not many free compilers available. gcc is a "free compiler" and is probably one of the best when it comes to optimization, even when compared to proprietary compilers.
Some independent benchmarks are linked here: http://gcc.gnu.org/benchmarks/
a source to share
I believe Intel allows its Linux ICC compilers to be used for free for non-commercial development. ICC beats gcc and Visual Studio when it comes to code generation for x86 and x86-64 (i.e. it usually generates faster code and can do a decent job of automatic vectorization (SIMD) in some cases).
a source to share
This is a difficult question to answer since you did not tell us which platform you are using, neither hardware nor os ....
But joemoe is right, gcc tends to excel in this field.
(As a side note: there are commercial compilers on some platforms that are better, but since you get so much more than just the gcc compiler, it's hard to beat ...)
a source to share