Currently, in order for the dependencies to be tracked and stored in a .gch
file to be subsequently used with -fpch-deps, one must manually specify -MD
switch when compiling the precompiled header. Otherwise, the dependency
information is not recorded in the precompiled header, and subsequent
compilation of a source file with -fpch-deps will erroneously lists no
dependencies. For example:
caladan:~ >g++ -x c++header test.h
caladan:~ >g++ test.cpp -MD -MP -MF deps.txt -o test.x
will give
caladan:~ >cat deps.txt
test.x: test.cpp
while
caladan:~ >g++ -MD -MF /dev/null -x c++header test.h
caladan:~ >g++ -fpch-deps test.cpp -MD -MP -MF deps.txt -o test.x
produces the full list of dependencies in deps.txt. This is unfortunately both
counter-intuitive and undocumented, leaving one in the dark as to why the
seemingly correct code above does not produce the expected result.
Attached is the proposed short patch which would by default activate dependency
tracking (-MD) for .pch compilations. It has been tested on Linux (RHEL4) with
GCC 4.1.1 and SVN HEAD, with no adverse sideeffects.
--
Summary: Dependency tracking should be on by default when
copiling .gchs
Product: gcc
Version: 4.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: pch
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: mjuric at astro dot princeton dot edu
GCC build triplet: i686-pc-linux-gnu
GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28483
