build-acceleration

What this skill does

# 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

```bash
# 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

```bash
# 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):

```cmake
# 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:

```ini
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++)

```bash
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
```

### 4. Precompiled headers (PCH)

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

```cmake
# CMake ≥3.16 native PCH support
target_precompile_headers(mylib PRIVATE
    <vector>
    <string>
    <unordered_map>
    "myproject/common.h"
)

# Share PCH across targets (avoids recompilation)
target_precompile_headers(myapp REUSE_FROM mylib)
```

```c
// 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
# CMake ≥3.16 unity build
set_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:

```cpp
// 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.

```bash
# GCC / Clang
gcc -g -gsplit-dwarf -o prog main.c

# CMake global
add_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:

```bash
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

```bash
# Install on all machines
apt-get install distcc

# Start daemon on worker machines
distccd --daemon --allow 192.168.1.0/24 --jobs 8

# Client: set DISTCC_HOSTS
export DISTCC_HOSTS="localhost/4 worker1/8 worker2/8"
make -j20 CC="distcc gcc"

# CMake integration
set(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

```bash
# Install
apt-get install iwyu

# Run via CMake
cmake -S . -B build -DCMAKE_CXX_INCLUDE_WHAT_YOU_USE=iwyu
cmake --build build 2>&1 | tee iwyu.log

# Apply fixes automatically
fix_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](references/ccache-config.md).

## 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