C++ - [Compilation 5] Compilation Speed Ups

Header-only Library Hurts Compilation Time

My halo library takes ~2-3min to compile. I have ~20 executables, each executable does target_include_files(70_header_files).

Why It’s Slow

• Every translational unit is compiled once. The library src files are compiled first, then the user executables.
• Each executable compiles with its own #include halo/xxx.hpp. If halo/xxx.hpp includes many other header files, they will be dragged in as well.
  • Only compiled object code in .cpp goes files into the archive libhalo.a. Header files are not compiled. This is why header only hurts
  • So the time cost is size_of_include_graph * number of affected TU * single_recompilation_time.
• Templates make it worse
  • Template declaration and definition must be in include files. Whenever a translation unit (a .cpp file) uses (ODR-uses) MyTemplate<T>, the compiler implicitly instantiates that specialization in that TU, and each instantiation costs compile time.
  • If we just need one instance for all source files, we can declare the instance in one cpp file, then mark it as extern in the header file.

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      // In MyTemplate.hpp
      template<typename T>
      struct MyTemplate { void foo(); /*…*/ };
      extern template struct MyTemplate<int>;    // “don’t instantiate here”
    
      // In MyTemplate.cpp
      #include "MyTemplate.hpp"
      template struct MyTemplate<int>;           // instantiate once
    

What Can Be Done?

• Move heavy include, implementations into cpp.
• Pimpl (Cheshire Cat) idiom.
  • Strip implementation details out of public headers entirely: public API headers contain only a forward-declared struct Impl; pointer.
  • Changes in private implementation no longer force recompilation of dependents.
• [Advanced] Unity (Jumbo) builds. CMake has the UNITY_BUILD target property. By grouping multiple small .cpp into a single “unity” TU, you amortize include overhead.

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    set_target_properties(halo PROPERTIES UNITY_BUILD ON)
  

• [Advanced] PreCompiled HEaders (PCH), where pch.hpp includes the heaviest, most-stable headers.

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   target_precompile_headers(halo
   PRIVATE
       <Eigen/Dense>
       "$<$<COMPILE_LANGUAGE:CXX>:${CMAKE_CURRENT_SOURCE_DIR}/include/halo/pch.hpp>"
   )
  

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