GCC

1. GCC manual

   Index of the GNU Compiler Collection manual.

2. Invoking GCC

   Compilation stages and the grouped options.

3. Optimization options

   -O levels, LTO and inlining flags.

4. Warning options

   -Wall, -Wextra and specific warnings.

5. Debugging options

   -g, -ggdb and DWARF output.

6. Instrumentation options

   -fsanitize, -pg and coverage flags.

7. Link options

   -l, -L, -static, -shared and -Wl pass-through.

8. Function attributes

   noreturn, always_inline, unused, constructor and visibility attributes.

9. Extended asm

   Inline assembly with operands and clobbers.

10. C extensions

    Statement expressions, typeof, designated initializers and builtins.

11. Preprocessor options

    -D, -I, -E and dependency generation with -MMD.

12. GCC internals

    GIMPLE, RTL, pass manager and machine descriptions.

Each section links to the official page. Titles and summaries by Extra Credit; the docs belong to their authors.