[cfe-dev] Borland-style PCHs in clang

[Douglas Gregor]

On Sep 29, 2010, at 12:12 AM, dawn at burble.org wrote:

> Sounds great.  What's your "preamble support"??

It's an optimization that we use when we end up repeatedly parsing the same file (e.g., during code completion), which builds a precompiled header up to (and including) the last preprocessor directive at the top of the main file, e.g., for

  #include "MyClass.h"
  #include <vector>
  #include <map>

  void foo() { }

we would build a "precompiled preamble" containing everything up to and including the #include <map>. When we then use that precompiled preamble, we load the precompiled header and then instruct the lexer to start processing at the end of the preamble.

> May be you can point me to some code in clang that I can poke around
> in??

Lexer::ComputePreamble figures out where the preamble ends; it could be taught to recognize #pragma hdrstop.

Preprocessor::setSkipMainFilePreamble() tells the preprocessor to skip some number of bytes at the beginning of the main file, so that it can resume at the proper place after loading a precompiled preamble.

For most of the rest of the system, a precompiled preamble is just a normal precompiled header. Nothing fancy here.

Set up for the precompiled preamble is a little messy. See lib/Frontend/ASTUnit.cpp, wherever it talks about preambles. 

	- Doug

> -Dawn
> 
> On Tue, Sep 28, 2010 at 08:18:33PM -0700, Sebastian Redl wrote:
>> 
>> On Sep 28, 2010, at 6:35 PM, dawn at burble.org wrote:
>> 
>>> 
>>> I'm trying to figure out the best way to support Borland style PCHs in
>>> clang.  Suppose we have headers x.h and y.h and file x.c
>>> as follows:
>>> 
>>>   #include "x.h"
>>>   #include "y.h"
>>>   #pragma hdrstop // everything above this line goes into the PCH
>>> 
>>> What's the best way to add this support to clang?
>> 
>> I think your best bet is to piggy-back on our preamble support. The main difference between that and #pragma hdrstop is that it finds the end of the includes automatically.
>> 
>> Sebastian
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