How to write and use chained PCHs

[Yao SHEN]

Hello,

This is Yao Shen. I got a problem recently and hope someone here can 
help me.

I am using llvm+clang to parse c/c++ codes. My objective is to do some 
instrumentations towards these codes by inserting instructions into the 
codes.

The problem is: the codes are somewhat large. They usually larger than 
50M. And there are a lot of header files shared by c/c++ files. All of 
these make the parsing process very slowly (> 30 minutes).

So I am think if I can use PCH. In the first step, I will compile all of 
the header files into PCHs. And then when parsing c/c++ files, I load 
corresponding PCH files in the PPCallback using ASTReader. It works when 
the header file is simple (not including other headers). However, if 
there is a chained including, it cannot work.

For example, if A.h includes B.h, and if I first convert B.h into 
B.h.pch, and A.h into A.h.pch, then when parsing main.c which includes 
A.h, the identifiers in B.h cannot be recognized. And if the main.c 
include both A.h and B.h, the parse will not pass successfully even if I 
use #ifdef... in B.h to avoid redefinitions.

Maybe I have to use chainedincludesSource. However, there is few guides 
I can get. I don't know how to use it.

Is there anyone who can help? Do you think using 
PPCallback::InclusionDirective is a right way in this situation?

Thank you in advance!
Yao Shen