clang_compile_time_cookbook.md

Clang Compile Time Cookbook

In this cookbook I will discuss several techniques to reduce the compile time of the llvm frontend clang. I will put the focus on heavy template and constexpr code.

Clang Tools Pipeline

First it is important to understand the internals of clang. Clang is a collection of tools which are pipelined.

Preprocessor

• Performs the actions of the C preprocessor
• The command line option -E stops clang after this stage

Parsing

• Parses the source code and generates the AST
• The command line option -precompile stops clang after this stage

IR generation

• Converts the AST into an optimizer-specific intermediate representation (IR)
  • Source
  • InstantiateClass / InstantiateFunction
  • PerformPendingInstantiations
  • CodeGen Function
  • PerfFunctionPasses
  • PerfModulePasses / OptModule
• The command line option -emit-llvm stops clang after this stage

Compiler backend

• Converts the IR into target-specific assembly code
  • CodeGenPasses
• The command line option -S stops clang after this stage

Assembler

• Converts target-specific assembly code into target-specific machine code object files
• The command line option -c stops clang after this stage

Linker

• Combines multiple object files into a single image

Analyse Clang Compile Time

Since clang 9 you can analyse compile time very detailed with the cxx flag -ftime-trace. This flag generates a json file which can be loaded into the chrome trace analyser chrome://tracing/ or an online tool

Once you have generated the json file and opened it in chrome you will notice the different stages of the clang pipeline e.g. source, frontend, and backend.

• It can be hard to read through the trace if you use anonymous functions (lambdas) since they will be listed as (anonymous class)::operator(). For better readability you should use templated functions or Functors.