gcc

GCC

Purpose

Guide agents through GCC invocation: flag selection, build modes, warning triage, PGO, LTO, and common error patterns. Assume the project uses GNU Make, CMake, or a shell script.

Triggers

• "What flags should I use for a release build?"
• "GCC is giving me a warning/error I don't understand"
• "How do I enable LTO / PGO with GCC?"
• "How do I compile with -fsanitize?"
• "My binary is too large / too slow"
• Undefined reference errors, ABI mismatch, missing symbols

Workflow

1. Choose a build mode

Always pass -std=c11 / -std=c++17 (or the required standard) explicitly. Never rely on the implicit default.

2. Warning discipline

Start with -Wall -Wextra. For stricter standards compliance add -Wpedantic. To treat all warnings as errors in CI: -Werror.

Suppress a specific warning only in a narrow scope:

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-parameter"
// ...
#pragma GCC diagnostic pop

Do not pass -w (silences everything) except as a last resort for third-party headers.

3. Debug information

• -g — DWARF debug info, default level 2
• -g3 — includes macro definitions (useful with GDB macro expand)
• -ggdb — DWARF extensions optimal for GDB
• -gsplit-dwarf — splits .dwo files; reduces link time, needed for debuginfod

Pair -g with -Og (not -O0) when you need readable optimised code in GDB.

4. Optimisation decision tree

-O3 vs -O2: -O3 adds aggressive loop transformations (-funswitch-loops, -fpeel-loops, -floop-interchange) and more aggressive inlining. Use -O3 only after benchmarking; it occasionally regresses due to i-cache pressure.

-Ofast: enables -ffast-math which breaks IEEE 754 semantics (NaN handling, associativity). Avoid unless the numerical domain explicitly permits it.

5. Link-time optimisation (LTO)

Use gcc-ar / gcc-ranlib instead of ar / ranlib when archiving LTO objects into static libs.

For parallel LTO: -flto=auto (uses make-style jobserver) or -flto=N.

See references/flags.md for full flag reference. See skills/binaries/linkers-lto for linker-level LTO configuration.

6. Profile-guided optimisation (PGO)

-fprofile-correction handles profile data inconsistencies from multi-threaded runs.

7. Preprocessor and standards

• Inspect macro expansion: gcc -E file.c | less
• Dump predefined macros: gcc -dM -E - < /dev/null
• Force strict standard: -std=c11 -pedantic-errors
• Disable GNU extensions: -std=c11 (not -std=gnu11)

8. Common error triage

For sanitizer reports, use skills/runtimes/sanitizers.

9. Useful one-liners

For a complete flag cheatsheet, see references/flags.md. For common error patterns and examples, see references/examples.md.

Related skills

• Use skills/runtimes/sanitizers to add -fsanitize=* builds
• Use skills/compilers/clang when switching to clang/LLVM
• Use skills/binaries/linkers-lto for advanced LTO linker flags
• Use skills/debuggers/gdb for debugging GCC-built binaries