C Cpp Compilers

C/C++ Compilers

Guidance for compiling, analyzing, and optimizing C and C++ code with GCC and Clang in 2026.

Reference files

• GCC specifics: [gcc.md](reference/gcc.md) -- flags, diagnostics, PGO, LTO, error triage
• Clang specifics: [clang.md](reference/clang.md) -- diagnostics, optimization remarks, clang-tidy, macOS
• Sanitizers: [sanitizers.md](reference/sanitizers.md) -- ASan, UBSan, TSan, MSan, LSan decision tree and reports
• Static analysis: [static-analysis.md](reference/static-analysis.md) -- clang-tidy, cppcheck, scan-build, CI integration
• Modern C/C++: [modern-cpp.md](reference/modern-cpp.md) -- C++20 modules, C++23/26 features, C23, migration

Standards baseline (2026)

| Language | Preferred standard | GCC support | Clang support | |----------|--------------------|-------------|---------------| | C | -std=c23 (or -std=c17 for broad compat) | GCC 15+ | Clang 18+ | | C++ | -std=c++23 (or -std=c++20 minimum) | GCC 14+ | Clang 18+ |

Always pass the standard flag explicitly. Never rely on compiler defaults.

Build modes

| Goal | Flags | |------|-------| | Debug | -g -O0 -Wall -Wextra -Wpedantic | | Debug (GDB-friendly optimized) | -g -Og -Wall -Wextra | | Release | -O2 -DNDEBUG -Wall | | Release (max throughput, native) | -O3 -march=native -DNDEBUG -flto | | Release (min binary size) | -Os -DNDEBUG (Clang: -Oz) | | Sanitizer build | -g -O1 -fsanitize=address,undefined -fno-omit-frame-pointer |

Warning discipline

Start with -Wall -Wextra -Wpedantic. Add -Werror in CI.

Suppress narrow scopes only:

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-parameter"
void callback(int ctx, int unused) { (void)ctx; }
#pragma GCC diagnostic pop

Clang equivalent works the same. For project-wide suppression, prefer .clang-tidy config or -Wno- in build system, not in source.

Optimization decision tree

Need max throughput on known hardware?
  yes -> -O3 -march=native -flto
  no  -> Have profiling data?
           yes -> -O2 -fprofile-use (GCC) / -fprofile-instr-use (Clang)
           no  -> -O2

Size-constrained (embedded, shared lib)?
  yes -> -Os (GCC/Clang) or -Oz (Clang only)

Numerical code that tolerates IEEE relaxation?
  yes -> -Ofast (enables -ffast-math; breaks NaN/inf handling)
  no  -> stay with -O2/-O3

-O3 vs -O2: -O3 adds aggressive loop transforms and wider inlining. Benchmark before committing -- i-cache pressure can cause regressions.

LTO

# GCC
gcc -O2 -flto=auto -c foo.c bar.c
gcc -O2 -flto=auto foo.o bar.o -o prog

# Clang (ThinLTO preferred for large projects)
clang -O2 -flto=thin -fuse-ld=lld -c foo.c bar.c
clang -O2 -flto=thin -fuse-ld=lld foo.o bar.o -o prog

Use gcc-ar / gcc-ranlib for GCC LTO archives. Clang ThinLTO links 5-10x faster than full LTO with comparable code quality.

PGO (profile-guided optimization)

GCC:

gcc -O2 -fprofile-generate prog.c -o prog_inst
./prog_inst &1 | grep -A20 '#include '

Compiler-specific details

For GCC-specific flags, PGO nuances, and error patterns, see [gcc.md](reference/gcc.md). For Clang diagnostics, optimization remarks, macOS toolchain, and clang-tidy, see [clang.md](reference/clang.md). For C++20 modules, C++23/26, and C23 features, see [modern-cpp.md](reference/modern-cpp.md).

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: kaynetik
• Source: kaynetik/skills
• License: MIT

Install and usage instructions live in the source repository linked above.