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 buildtime 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

# Installapt-get install ccache   # Ubuntu/Debianbrew install ccache      # macOS
# Check hit rateccache -s
# Configure cache size (default 5GB)ccache -M 20G
# Invalidate cache if neededccache -C

CMake integration (recommended over prefix hacks):

# CMakeLists.txtfind_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 = 20Gcompression = truecompression_level = 6# For CI: share cache across jobscache_dir = /shared/ccache

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

cargo install sccache# Or: brew install sccache
# Set as compiler launcherexport RUSTC_WRAPPER=sccache          # for Rustexport CMAKE_C_COMPILER_LAUNCHER=sccache    # for CMake
# With S3 backendexport SCCACHE_BUCKET=my-build-cacheexport SCCACHE_REGION=us-east-1sccache --start-server
sccache --show-stats

4. Precompiled headers (PCH)

PCH compiles a large header once and reuses the binary form.

# CMake ≥3.16 native PCH supporttarget_precompile_headers(mylib PRIVATE    <vector>    <string>    <unordered_map>    "myproject/common.h")
# Share PCH across targets (avoids recompilation)target_precompile_headers(myapp REUSE_FROM mylib)

// Traditional: stdafx.h / pch.h approach// All TUs include pch.h as the very first include// pch.h includes heavy system headers#pragma once#include <stdio.h>#include <stdlib.h>#include <string.h>

PCH is most effective when headers are large and stable (STL, Boost, Qt). Avoid PCH for frequently-changing project headers.

5. Unity / jumbo builds

Combine multiple .cpp files into one TU to reduce header parsing overhead and improve inlining.

# CMake ≥3.16 unity buildset_target_properties(mylib PROPERTIES UNITY_BUILD ON)# Control batch size (default 8 files per unity TU)set_target_properties(mylib PROPERTIES UNITY_BUILD_BATCH_SIZE 16)
# Exclude specific files from unity (e.g., if they have ODR issues)set_source_files_properties(problem.cpp PROPERTIES SKIP_UNITY_BUILD_INCLUSION ON)

Manual unity file:

// unity_build.cpp#include "module_a.cpp"#include "module_b.cpp"#include "module_c.cpp"

Watch out for: anonymous namespaces (each TU has its own), using namespace in headers, duplicate static variables.

6. split-DWARF — reduce link time

Split DWARF puts debug info in .dwo sidecar files, dramatically reducing what the linker must process.

# GCC / Clanggcc -g -gsplit-dwarf -o prog main.c
# CMake globaladd_compile_options(-gsplit-dwarf)
# Combine .dwo files for distribution (optional)dwp -o prog.dwp prog  # GNU dwp tool

Pair with --gdb-index for faster GDB startup:

gcc -g -gsplit-dwarf -Wl,--gdb-index -o prog main.c

Link time comparison (large project, typical): -g full DWARF ~4×–6× longer link vs -gsplit-dwarf.

7. distcc — distributed compilation

# Install on all machinesapt-get install distcc
# Start daemon on worker machinesdistccd --daemon --allow 192.168.1.0/24 --jobs 8
# Client: set DISTCC_HOSTSexport DISTCC_HOSTS="localhost/4 worker1/8 worker2/8"make -j20 CC="distcc gcc"
# CMake integrationset(CMAKE_C_COMPILER_LAUNCHER distcc)set(CMAKE_CXX_COMPILER_LAUNCHER distcc)

Stack with ccache: CC="ccache distcc gcc" — ccache checks local cache first, falls back to distcc.

8. Include pruning with IWYU

# Installapt-get install iwyu
# Run via CMakecmake -S . -B build -DCMAKE_CXX_INCLUDE_WHAT_YOU_USE=iwyucmake --build build 2>&1 | tee iwyu.log
# Apply fixes automaticallyfix_include < iwyu.log --nosafe_headers

See skills/build-systems/include-what-you-use for full IWYU workflow.

For ccache configuration options, see references/ccache-config.md [blocked].

Related skills

• Use skills/build-systems/cmake for CMake project structure
• Use skills/build-systems/include-what-you-use for IWYU header pruning
• Use skills/rust/rust-build-times for Rust-specific build acceleration
• Use skills/debuggers/dwarf-debug-format for split-DWARF internals