Complilation Speedup

Setting up target_precompile_headers and ccache with CMake

1. How to Set Them Up

Precompiled Headers with target_precompile_headers

In your CMakeLists.txt, use the target_precompile_headers command to specify which headers to precompile. This can significantly speed up compilation by avoiding the need to re-parse common headers for each source file.

cmake_minimum_required(VERSION 3.16) # Requires at least CMake 3.16
project(YourProjectName)

# Define your executable or library
add_executable(my_executable main.cpp)

# Specify the headers to precompile
target_precompile_headers(my_executable PRIVATE
    <vector>
    <string>
    "my_header.h"
)

Using ccache

ccache is a compiler cache that speeds up recompilation by caching previous compilations and detecting when the same compilation is being done again. To use ccache with CMake, you can set it as a compiler launcher.

• Install ccache on your system if it's not already installed.
• Modify your CMakeLists.txt or set the CMAKE_CXX_COMPILER_LAUNCHER cache variable to use ccache.

# Set ccache as a compiler launcher, if found
find_program(CCACHE_PROGRAM ccache)
if(CCACHE_PROGRAM)
    set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE "${CCACHE_PROGRAM}")
endif()

Or, you can pass it as a command-line argument when invoking CMake:

cmake -DCMAKE_CXX_COMPILER_LAUNCHER=ccache ..

2. Disadvantages or Dangers

Precompiled Headers:

• Overhead for Small Projects: For very small projects, the overhead of creating a precompiled header might not be worth the compilation time saved.
• Maintenance: You need to keep the precompiled header list up to date, which can add a bit of maintenance overhead.
• Portability: Different compilers handle precompiled headers differently, which might introduce issues in cross-platform projects.

ccache:

• Disk Space: ccache uses disk space to store its cache. While it manages its size automatically, a large cache can consume significant disk space.
• Initial Slowdown: The first compilation will not be faster, as ccache needs to build its cache. The speed improvements are seen in subsequent compilations.
• Configuration Errors: Misconfiguration can lead to subtle bugs if ccache serves outdated or incorrect cached objects.

3. Compatibility with g++ and clang

Both target_precompile_headers and ccache are compatible with g++ and clang. The way they are set up in CMake does not depend on the choice between these compilers, as CMake abstracts the compiler-specific details. However, the effectiveness and behavior of precompiled headers can vary between compilers due to differences in how they implement and manage precompiled headers.

• ccache works transparently with both g++ and clang. It intercepts calls to the compiler and decides whether to compile or retrieve the result from its cache based on the input files' checksums.
• Precompiled headers might require specific attention when switching compilers or moving between different platforms, as the precompiled header format is not standardized across compilers.

In summary, using target_precompile_headers and ccache with CMake can significantly improve your C++ project's build times, especially for large projects or projects with many dependencies. While there are some considerations to keep in mind, these tools are generally beneficial and work well with both g++ and clang.