GCC online documentation

https://gcc.gnu.org/onlinedocs/

1. GCC 5.5 manuals

https://gcc.gnu.org/onlinedocs/5.5.0/

GCC 5.5 Manual (also in PDF or PostScript or an HTML tarball)
Using the GNU Compiler Collection - For GCC version 5.5.0

postscript ['pəʊs(t)skrɪpt]：n. 附言，又及

2. Chapter 3: GCC Command Options

3.9 Options for Debugging Your Program or GCC
3.10 Options That Control Optimization