Problem with PFE approach [Was: Faster compilation speed]

[Devang Patel]

On Monday, August 19, 2002, at 06:26  AM, jepler@unpythonic.net wrote:

The following set of files will compile a program with or without PFE, but
using a PFE that contains both a.h and b.h, the behavior will change.


This is not implementation problem or PFE model problem.
If you are including a.h and b.h in PFE means what you're asking compiler to do 
is to compile following source


/// m.c
#include "a.h"
#include "b.h"
int main(void) {
#ifdef DEFB
return 1;
#else
return 0;
#endif;
}

And, no doubt, it can have different behavior then following original source

// m.c
#include "a.h"
int main(void) {
#ifdef DEFB
return 1;
#else
return 0;
#endif;
}

-Devang

So
the suggestion that files should be checked that they compile without PFE
is not enough to ensure that there aren't unintended changes in program
meaning in the presence of PFE.

// a.h
#define DEFA

// b.h
#define DEFB

// m.c
#include "a.h"
int main(void) {
#ifdef DEFB
return 1;
#else
return 0;
#endif;
}