The GCC manual: http://gcc.gnu.org/onlinedocs/
"Code Optimization Using the GNU C Compiler," by Rahul U Joshi: http://www.linuxgazette.com/issue71/joshi.html
This article describes some of the code optimization techniques used by the GNU C Compiler, in order to give the reader a feel of what code optimization is and how it can increase the efficiency of the generated object code.
Using and Porting GNU CC for Version 2.8, by Richard Stallman (Free Software Foundation). Also available online from http://www.delorie.com/gnu/docs/gcc/gcc_toc.html and many other locations.
Chapter 6 of the online book Securing and Optimizing Linux, RedHat Edition: A Hands on Guide talks extensively about compiler flags. It is located at http://www.linuxdoc.org/LDP/solrhe/Securing-Optimizing-Linux-RH-Edition-v1.3/gen-optim.html. The whole book (available in different formats) can be found at http://www.linuxdoc.org/guides.html#securing_linux.
More Apache and platform-specific performance-tuning notes can be found at http://httpd.apache.org/docs/misc/perf-tuning.html.