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 -- flags, diagnostics, PGO, LTO, error triage
• Clang specifics: clang.md -- diagnostics, optimization remarks, clang-tidy, macOS
• Sanitizers: sanitizers.md -- ASan, UBSan, TSan, MSan, LSan decision tree and reports
• Static analysis: static-analysis.md -- clang-tidy, cppcheck, scan-build, CI integration
• Modern C/C++: 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-<flag> 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 < workload.input
gcc -O2 -fprofile-use -fprofile-correction prog.c -o prog

Clang (LLVM instrumentation):

clang -O2 -fprofile-instr-generate prog.c -o prog_inst
./prog_inst < workload.input
llvm-profdata merge -output=prog.profdata default.profraw
clang -O2 -fprofile-instr-use=prog.profdata prog.c -o prog

Sanitizer quick reference

Bug class	Sanitizer	Flag
Heap/stack/global OOB, use-after-free, double-free	ASan	-fsanitize=address
Signed overflow, null deref, bad shift, misaligned access	UBSan	-fsanitize=undefined
Data races	TSan	-fsanitize=thread
Uninitialised reads (Clang only, all-instrumented build)	MSan	-fsanitize=memory
Memory leaks	LSan	via ASan (detect_leaks=1) or standalone

Common combo: -fsanitize=address,undefined -fno-sanitize-recover=all -fno-omit-frame-pointer -g -O1

TSan and MSan are mutually exclusive with ASan. See sanitizers.md for report interpretation.

Static analysis (quick start)

# Generate compilation database
cmake -S . -B build -DCMAKE_EXPORT_COMPILE_COMMANDS=ON

# Run clang-tidy (recommended checks)
clang-tidy -checks='bugprone-*,clang-analyzer-*,performance-*,modernize-*' \
  -p build src/foo.cpp

# Run cppcheck
cppcheck --enable=warning,performance,portability --error-exitcode=1 src/

See static-analysis.md for .clang-tidy config, CI integration, and suppression patterns.

Common error triage

Symptom	Likely cause	Fix
undefined reference to 'foo'	Missing -lfoo or wrong link order	Libraries after objects: gcc main.o -lfoo
multiple definition of 'x'	Defined in header without static/inline	extern in header, define in one .c
implicit declaration of function	Missing #include or wrong standard	Add the header; check -std=
incompatible pointer types	Wrong cast or missing prototype	Fix type; enable -Wall
ABI errors in C++	Mixed -std= or different libstdc++	Unify standard across all TUs
relocation truncated	32-bit relocation overflow	-mcmodel=large or restructure

CMake integration

cmake_minimum_required(VERSION 3.28)
project(myproject LANGUAGES C CXX)

set(CMAKE_C_STANDARD 23)
set(CMAKE_CXX_STANDARD 23)
set(CMAKE_EXPORT_COMPILE_COMMANDS ON)

option(SANITIZE "Build with ASan+UBSan" OFF)
if(SANITIZE)
    set(san_flags -fsanitize=address,undefined -fno-sanitize-recover=all
                  -fno-omit-frame-pointer -g -O1)
    add_compile_options(${san_flags})
    add_link_options(${san_flags})
endif()

Useful one-liners

# Show all optimizations enabled at -O2 (GCC)
gcc -Q --help=optimizers -O2 | grep enabled

# Assembly output (Intel syntax)
gcc -S -masm=intel -O2 foo.c -o foo.s

# Preprocess and dump macros
gcc -dM -E - < /dev/null

# Clang optimization remarks (missed vectorization)
clang -O2 -Rpass-missed=loop-vectorize src.c

# Clang save all remarks to YAML
clang -O2 -fsave-optimization-record src.c

# Show include search path
gcc -v -E - < /dev/null 2>&1 | grep -A20 '#include <...>'

Compiler-specific details

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