GCC Skill: Оптимизация и отладка C/C++ проектов

--- name: gcc description: GCC compiler skill for C/C++ projects. Use when selecting optimization levels, warning flags, debug builds, LTO, sanitizer instrumentation, or diagnosing compilation errors with GCC. Covers flag selection for debug vs release, ABI concerns, preprocessor macros, profile-guided optimization, and integration with build systems. Activates on queries about gcc flags, compilation errors, performance tuning, warning suppression, or cross-standard compilation. --- # 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 | Goal | Recommended flags | |------|-------------------| | Debug | `-g -O0 -Wall -Wextra` | | Debug + debuggable optimisation | `-g -Og -Wall -Wextra` | | Release | `-O2 -DNDEBUG -Wall` | | Release (max perf, native only) | `-O3 -march=native -DNDEBUG` | | Release (min size) | `-Os -DNDEBUG` | | Sanitizer (dev) | `-g -O1 -fsanitize=address,undefined` | 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: ```c #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 ```text Need max throughput on a fixed machine? yes -> -O3 -march=native -flto no -> profiling available? yes -> -O2 -fprofile-use no -> -O2 Size-constrained (embedded, shared lib)? yes -> -Os (or -Oz with clang) ``` `-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) ```bash # Compile gcc -O2 -flto -c foo.c -o foo.o gcc -O2 -flto -c bar.c -o bar.o # Link (must pass -flto again) gcc -O2 -flto foo.o bar.o -o prog ``` 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](references/flags.md) for full flag reference. See `skills/binaries/linkers-lto` for linker-level LTO configuration. ### 6. Profile-guided optimisation (PGO) ```bash # Step 1: instrument gcc -O2 -fprofile-generate prog.c -o prog_inst # Step 2: run with representative workload ./prog_inst < workload.input # Step 3: optimise with profile gcc -O2 -fprofile-use -fprofile-correction prog.c -o prog ``` `-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 | Symptom | Likely cause | Fix | |---------|-------------|-----| | `undefined reference to 'foo'` | Missing `-lfoo` or wrong link order | Add `-lfoo`; move `-l` flags after object files | | `multiple definition of 'x'` | Variable defined (not just declared) in a header | Add `extern` in header, define in one `.c` | | `implicit declaration of function` | Missing `#include` | Add the header | | `warning: incompatible pointer types` | Wrong cast or missing prototype | Fix the type; check headers | | ABI errors with C++ | Mixed `-std=` or different `libstdc++` | Unify `-std=` across all TUs | | `relocation truncated` | Overflow on a 32-bit relative relocation | Use `-mcmodel=large` or restructure code | For sanitizer reports, use `skills/runtimes/sanitizers`. ### 9. Useful one-liners ```bash # Show all flags enabled at -O2 gcc -Q --help=optimizers -O2 | grep enabled # Preprocess only (check includes/macros) gcc -E -dD src.c -o src.i # Assembly output (Intel syntax) gcc -S -masm=intel -O2 foo.c -o foo.s # Show include search path gcc -v -E - < /dev/null 2>&1 | grep -A20 '#include <...>' # Check if a flag is supported gcc -Q --help=target | grep march ``` For a complete flag cheatsheet, see [references/flags.md](references/flags.md). For common error patterns and examples, see [references/examples.md](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