>> "CPP optimizes even further. It remembers when a header file has a
>> wrapper `#ifndef'.  If a subsequent `#include' specifies that
>> header, and the macro in the `#ifndef' is still defined, it does not
>> bother to rescan the file at all."
> I thought about that, since the preprocessor is the obvious place for
> an optimisation, and ISTR C++ does something similar with the #import
> statement.  But I wasn't sure whether cutting corners like this would
> go down well with ANSI C.  If it does, all the better.

Yes, this comes under the "as if" rule.  The semantics are as if the
file were reread; gcc just remembers when the file is such that it can
be short-circuited this way without changing the semantics.

/~\ The ASCII				der Mouse
\ / Ribbon Campaign
 X  Against HTML	       mouse@rodents.montreal.qc.ca
/ \ Email!	     7D C8 61 52 5D E7 2D 39  4E F1 31 3E E8 B3 27 4B