[PATCH, v3] PR 35954: rebuild of precompiled headers

[Ralf Wildenhues <Ralf dot Wildenhues at gmx dot de>]

On my i686-unknown-linux-gnu system, configured with
  .../configure -C --enable-languages=c,c++

each make invocation regenerates the precompiled headers anew, and
breaks builds with read-only source trees.

These are the relevant Makefile bits:

host_builddir = ./${host_alias}/bits
pch1_output_anchor = ${host_builddir}/stdc++.h

host_headers = \
[...]
        ${glibcxx_srcdir}/include/precompiled/stdc++.h \
        ${glibcxx_srcdir}/include/precompiled/stdtr1c++.h \
        ${glibcxx_srcdir}/include/precompiled/extc++.h

allstamped = \
[...]
        stamp-tr1 stamp-tr1-impl stamp-debug stamp-parallel stamp-host

${pch1a_output}: ${allstamped} ${host_builddir}/c++config.h ${pch1_source}
	if [ ! -d "${pch1_output_builddir}" ]; then \
	  mkdir -p ${pch1_output_builddir}; \
	fi; \
	$(CXX) $(PCHFLAGS) $(AM_CPPFLAGS) -O0 -g ${pch1_source} -o $@
	touch ${pch1_output_anchor}

stamp-host: ${host_headers} ${host_headers_noinst} stamp-${host_alias}
	@if [ ! -f stamp-host ]; then \
	  (cd ${host_builddir} ;\
	  $(LN_S) ${host_headers} . || true ;\
	  $(LN_S) ${glibcxx_srcdir}/$(BASIC_FILE_H) basic_file.h || true ;\
[...]
	  $(LN_S) ${glibcxx_srcdir}/$(CTIME_H) time_members.h || true);\
	fi ;\
	$(STAMP) stamp-host

but i686-unknown-linux-gnu/bits/stdc++.h is a symlink to
${glibcxx_srcdir}/include/precompiled/stdc++.h

so via the symlink there is a circular dependency between the stamp-host
and the ${pch1a_output} rule, it's just that make does not see that.

The 'touch' was added by
<http://gcc.gnu.org/ml/libstdc++/2006-07/msg00080.html>.  IMVHO it is
superfluous: the header file is not newer just because a new precompiled
header exists for it.  The precompiled header should not generate
rebuilds of depending files.  I can only assume this touch was added
because of the bug fixed in
<http://gcc.gnu.org/ml/gcc-patches/2008-03/msg01103.html> which hasn't
been reviewed yet.  But maybe there's another reason I'm not aware of?

While we're at it, let's inline the 'mkdir -p' here, too, as it saves a
fork during the first build, and doesn't increase the number of forks on
subsequent builds.  (And would fix potential parallel build errors for
non-GNU make, not sure if that's relevant for libstdc++.)

OK for trunk and 4.3?  I've had this patch for a while, but I'll be
bootstrapping it again just in case.

Thanks,
Ralf

libstdc++-v3/ChangeLog:
2008-04-18  Ralf Wildenhues  <Ralf.Wildenhues@gmx.de>

        PR libstdc++/35954
	* include/Makefile.am (pch*_output): Do not touch pch*_output_anchor.
	Call 'mkdir -p' unconditionally, but ignore its return value.
	* include/Makefile.in: Regenerate.

diff --git a/libstdc++-v3/include/Makefile.am b/libstdc++-v3/include/Makefile.am
index 68e8e35..1769a5e 100644
--- a/libstdc++-v3/include/Makefile.am
+++ b/libstdc++-v3/include/Makefile.am
@@ -1123,34 +1123,22 @@ ${host_builddir}/gthr-default.h: ${toplevel_srcdir}/gcc/${glibcxx_thread_h} \
 
 # Build two precompiled C++ includes, stdc++.h.gch/*.gch
 ${pch1a_output}: ${allstamped} ${host_builddir}/c++config.h ${pch1_source}
-	if [ ! -d "${pch1_output_builddir}" ]; then \
-	  mkdir -p ${pch1_output_builddir}; \
-	fi; \
+	-mkdir -p ${pch1_output_builddir}
 	$(CXX) $(PCHFLAGS) $(AM_CPPFLAGS) -O0 -g ${pch1_source} -o $@
-	touch ${pch1_output_anchor}
 
 ${pch1b_output}: ${allstamped} ${host_builddir}/c++config.h ${pch1_source}
-	if [ ! -d "${pch1_output_builddir}" ]; then \
-	  mkdir -p ${pch1_output_builddir}; \
-	fi; \
+	-mkdir -p ${pch1_output_builddir}
 	$(CXX) $(PCHFLAGS) $(AM_CPPFLAGS) -O2 -g ${pch1_source} -o $@
-	touch ${pch1_output_anchor}
 
 # Build a precompiled TR1 include, stdtr1c++.h.gch/O2.gch
 ${pch2_output}: ${pch2_source} ${pch1_output}
-	if [ ! -d "${pch2_output_builddir}" ]; then \
-	  mkdir -p ${pch2_output_builddir}; \
-	fi; \
+	-mkdir -p ${pch2_output_builddir}
 	$(CXX) $(PCHFLAGS) $(AM_CPPFLAGS) -O2 -g ${pch2_source} -o $@
-	touch ${pch2_output_anchor}
 
 # Build a precompiled extension include, extc++.h.gch/O2.gch
 ${pch3_output}: ${pch3_source} ${pch2_output}
-	if [ ! -d "${pch3_output_builddir}" ]; then \
-	  mkdir -p ${pch3_output_builddir}; \
-	fi; \
+	-mkdir -p ${pch3_output_builddir}
 	$(CXX) $(PCHFLAGS) $(AM_CPPFLAGS) -O2 -g ${pch3_source} -o $@
-	touch ${pch3_output_anchor}
 
 # For robustness sake (in light of junk files or in-source
 # configuration), copy from the build or source tree to the install