[cfe-dev] Borland-style PCHs in clang

[dawn at burble.org]

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

In bcc, we would use the command:
    bcc -H -H=x.pch -c x.c

which would have the effect:
    first compile:
	x.pch is created from x.h and y.h
    subsequent compiles:
	x.pch is verified (compile options and defines checked,
	included files and order checked, etc.), then
	parsing begins after #pragma hdrstop

The easiest way to do this using clang would be to 
create and invoke the makefile:
    xtmp.h: x.h y.h
	#header file created from all lines of x.c up to #pragma hdrstop 
    xtmp.c: x.c
	#source file created from all lines after #pragma hdrstop 
    xtmp.pch: xtmp.h
	clang -cc1 xtmp.h -emit-pch -o xtmp.pch
    x.o: xy.pch
	clang -cc1 -include-pch xy.pch xtmp.c
But that has problems:
1. name of files in debug info are wrong.
2. if file y.c includes x.h but not y.h, y.c can't share the PCH info built into xy.pch.

To fix #2, we could use the makefile:
    xtmp.c: x.c
	#source file created from all lines after #pragma hdrstop 
    x.pch: x.h
	clang -cc1 x.h -emit-pch -o x.pch
    xy.pch: x.pch x.h
	clang -cc1 -include-pch x.pch -chained-pch y.h -emit-pch -o xy.pch 
    x.o: xy.pch
	clang -cc1 -include-pch xy.pch xtmp.c
But this doesn't allow for PCH verification of the first part of x.c (up to the
#pragma hdrstop - the part that had been built into xtmp.h).

... and both of these have the problem of requiring us to do some makefile magic on the fly.

What's the best way to add this support to clang?

Thanks,
-Dawn