Re: Linux 2.6.0-test6

[Jamie Lokier]

Linus Torvalds wrote:
> On Sun, 28 Sep 2003, Sam Ravnborg wrote:
> > Would it help to require all major[1] header files to include all the
> > header files needed for them to compile?
> 
> It causes tons of extra work for the compiler if the compiler doesn't 
> optimize away redundant header files (same header file being included from 
> a lot of different sources).
>
> I did the pruning in sparse, and I think at least gcc-3 does it too, but 
> I'm not sure.

GCC does optimise away multiple header file inclusion, and has done
for a very long time, oh a decade or so :)

GCC will not reparse a header file if these conditions are met:

	1. The file has already been parsed at least once.

	2. Apart from comments, the entire file is surrounded by
	   "#ifndef symbol ... #endif" or "#if !defined (symbol) ... #endif".

	3. "symbol" is defined.

	4. The file names are the same after removal of "." and ".." components
	   and other path simplifications.

> If so, then sure, we could just require that the header files compile 
> cleanly, and for extra points verify that the end result is an empty 
> object file (ie no bad declarations anywhere..).

You can also use the "-H" option and check for a "Multiple include
guards may be useful for:" message, to check those #ifndefs.

-- Jamie
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