build-acceleration

Build Acceleration

Purpose

Guide agents through reducing C/C++ build times using caching (ccache/sccache), distributed compilation (distcc), unity/jumbo builds, precompiled headers, split-DWARF for faster linking, and include pruning with IWYU.

Triggers

• "My C++ build is too slow — how do I speed it up?"
• "How do I set up ccache / sccache?"
• "How do precompiled headers work with CMake?"
• "How do I set up distributed compilation with distcc?"
• "How do I reduce link times with split-DWARF?"
• "How do I find which headers are slowing down compilation?"

Workflow

1. Diagnose the bottleneck first

# Time the full build
time cmake --build build -j$(nproc)

# Find the slowest TUs (CMake ≥3.16 with --profiling-output)
cmake -S . -B build -DCMAKE_CXX_FLAGS="-ftime-report"
cmake --build build 2>&1 | grep "Total" | sort -t: -k2 -rn | head -20

# Ninja build timings (use ninja -j1 for serial timing)
ninja -C build -j1 2>&1 | grep "^\[" | sort -t" " -k2 -rn | head -20

2. ccache — compiler cache

# Install
apt-get install ccache   # Ubuntu/Debian
brew install ccache      # macOS

# Check hit rate
ccache -s

# Configure cache size (default 5GB)
ccache -M 20G

# Invalidate cache if needed
ccache -C

CMake integration (recommended over prefix hacks):

# CMakeLists.txt
find_program(CCACHE_PROGRAM ccache)
if(CCACHE_PROGRAM)
    set(CMAKE_C_COMPILER_LAUNCHER   ${CCACHE_PROGRAM})
    set(CMAKE_CXX_COMPILER_LAUNCHER ${CCACHE_PROGRAM})
endif()

Key ~/.config/ccache/ccache.conf options:

max_size = 20G
compression = true
compression_level = 6
# For CI: share cache across jobs
cache_dir = /shared/ccache

3. sccache — cloud-compatible cache (Rust, C/C++)

cargo install sccache
# Or: brew install sccache

# Set as compiler launcher
export RUSTC_WRAPPER=sccache          # for Rust
export CMAKE_C_COMPILER_LAUNCHER=sccache    # for CMake

# With S3 backend
export SCCACHE_BUCKET=my-build-cache
export SCCACHE_REGION=us-east-1
sccache --start-server

sccache --show-stats