10.2 Precompiled headers

Precompiled headers PCH, are a Clang feature designed to improve Clang’s frontend performance. The basic idea is to create an AST (Abstract Syntax Tree) for a header file and reuse this AST during compilation for sources that include the header file.

Generating a precompiled header file is simple [5]. Suppose you have the following header file, header.h :

1 #pragma once 
 
2  
 
3 void foo() { 
 
4 }

Figure 10.1: Header file to be compiled to PCH

You can generate a PCH for it with the following command:

$ <...>/llvm-project/install/bin/clang -cc1 -emit-pch        \
                                        -x c++-header header.h \
  ...