[Bug c++/54780] G++ does not find precompiled headers in some cases

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54780

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-10-02 13:53:13 UTC ---
(In reply to comment #0)
> Next the script copies common.h to the main directory and compiles the sources
> again using the exact same compiler switches. What happens is that g++ does not
> find the precompiled header, probably because it first looks in the source dir
> and finds common.h but not the corresponding pch and just stops looking. This
> makes the compilation very slow.

This is by design.  A header included as #include "common.h" looks in the
current dir first, so the compiler looks for ./common.h.gch, then for
./common.h, then stops because it's found.  The alternative would be to
continue searching *every* other directory looking for common.h.gch, which
would be *much* slower (there might be no PCH anywhere, so for projects that
don't use PCH it would mean checking every single directory for every single
header)

Maybe the solution you want is to use #include <common.h> and add -I. to the
compilation commands, ensuring that -Ipchdir comes before -I. because that will
not start with the current dir, and will look for pchdir/common.h.gch first and
will stop when found. Doing that makes all three steps in your script very
fast.