How to write and use chained PCHs

[Eli Friedman]

On Tue, Aug 13, 2013 at 6:19 AM, Yao SHEN <yaoshen at andrew.cmu.edu> wrote:

> 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?
>
>
You might want to consider modifying your source code to use modules; see
http://clang.llvm.org/docs/Modules.html .  Alternatively, instead of
compiling individual headers using PCH, you could precompile one big header
which includes all the commonly used headers in your project.

This is a hard problem; if it were easy, clang would do it for you already.
:)

-Eli
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