Extensions:
-
http://www.ibm.com/developerworks/library/l-gcc-hacks/
Good selection of useful features.
Internals:
-
https://gcc.gnu.org/onlinedocs/gccint/index.html
Generated from
gcc/doc/gccint.texi, so very official, yay! -
https://en.wikibooks.org/wiki/GNU_C_Compiler_Internals/GNU_C_Compiler_Architecture