[gcc r17-3333] libstdc++: build the source directories in parallel

https://gcc.gnu.org/g:a323ee5162f35540785dad1aa36761f3f349e0ed

commit r17-3333-ga323ee5162f35540785dad1aa36761f3f349e0ed
Author: Kyrylo Tkachov <[email protected]>
Date:   Tue Aug 11 10:01:58 2026 +0200

    libstdc++: build the source directories in parallel
    
    libstdc++-v3/src is built by walking SUBDIRS, and automake implements that
    walk as a shell loop.  The per-standard directories are therefore built one
    after another no matter how large -j is, and most of them hold only a handful
    of files.  Sampling the compiler processes during a bootstrap shows a mean of
    between 2 and 4 concurrent compiles against -j64, with exactly one source
    directory active at any moment, and long stretches at a concurrency of two.
    
    libstdc++ is the last thing to finish in every stage, so for most of that time
    nothing else in the build is running and the machine is idle.
    
    The directories are independent of each other, apart from src/experimental,
    which links the convenience archives of c++23, c++26, filesystem and
    libbacktrace.  Give make an explicit target per directory, with that one
    dependency written down, and let it overlap them.  automake's own loop still
    runs afterwards, finds everything built and costs nothing, so the recursion
    for install, clean and the other targets is untouched, and note in
    src/experimental/Makefile.am that its lists of archives and the prerequisites
    here have to stay together.
    
    On aarch64 a clean rebuild of libstdc++-v3/src with the final stage compiler
    falls by 62%, and a release checking bootstrap by 12.8%.
    
    Bootstrapped on aarch64-none-linux-gnu.
    
    libstdc++-v3/ChangeLog:
    
            * src/Makefile.am (all-recursive): Depend on all-parallel-subdirs.
            (all-parallel-subdirs, all-in-%): New targets.
            (all-in-experimental): Depend on the directories whose convenience
            archives it links.
            * src/Makefile.in: Regenerate.
            * src/experimental/Makefile.am: Add comment about keeping the
            convenience library lists in sync with src/Makefile.am.
    
    Signed-off-by: Kyrylo Tkachov <[email protected]>

Diff:
---
 libstdc++-v3/src/Makefile.am              | 20 ++++++++++++++++++++
 libstdc++-v3/src/Makefile.in              | 20 ++++++++++++++++++++
 libstdc++-v3/src/experimental/Makefile.am |  2 ++
 3 files changed, 42 insertions(+)

diff --git a/libstdc++-v3/src/Makefile.am b/libstdc++-v3/src/Makefile.am
index 35a497a15886..17fd6ed60645 100644
--- a/libstdc++-v3/src/Makefile.am
+++ b/libstdc++-v3/src/Makefile.am
@@ -46,6 +46,26 @@ endif
 SUBDIRS = c++98 c++11 c++17 c++20 c++23 c++26 \
 	$(filesystem_dir) $(backtrace_dir) $(experimental_dir)
 
+# automake walks SUBDIRS with a shell loop, so the source directories are
+# built strictly one after another however large -j is.  They are independent
+# of each other apart from the convenience archives that src/experimental
+# links, so build them from make instead and let it overlap them.  automake's
+# own loop still runs afterwards, where it finds everything already built and
+# costs nothing, and the recursion for install, clean and the rest is
+# untouched.
+all-recursive: all-parallel-subdirs
+
+all-parallel-subdirs: $(SUBDIRS:%=all-in-%)
+
+all-in-%:
+	$(MAKE) $(AM_MAKEFLAGS) -C $* all
+
+# src/experimental links the convenience archives of these directories.
+all-in-experimental: all-in-c++23 all-in-c++26 \
+	$(filesystem_dir:%=all-in-%) $(backtrace_dir:%=all-in-%)
+
+.PHONY: all-parallel-subdirs
+
 # Cross compiler support.
 if VTV_CYGMIN
 toolexeclib_LTLIBRARIES = libvtv.la libstdc++.la
diff --git a/libstdc++-v3/src/Makefile.in b/libstdc++-v3/src/Makefile.in
index 811d0a5ce4a8..104033fd6796 100644
--- a/libstdc++-v3/src/Makefile.in
+++ b/libstdc++-v3/src/Makefile.in
@@ -1020,6 +1020,26 @@ uninstall-am: uninstall-toolexeclibLTLIBRARIES
 .PRECIOUS: Makefile
 
 
+# automake walks SUBDIRS with a shell loop, so the source directories are
+# built strictly one after another however large -j is.  They are independent
+# of each other apart from the convenience archives that src/experimental
+# links, so build them from make instead and let it overlap them.  automake's
+# own loop still runs afterwards, where it finds everything already built and
+# costs nothing, and the recursion for install, clean and the rest is
+# untouched.
+all-recursive: all-parallel-subdirs
+
+all-parallel-subdirs: $(SUBDIRS:%=all-in-%)
+
+all-in-%:
+	$(MAKE) $(AM_MAKEFLAGS) -C $* all
+
+# src/experimental links the convenience archives of these directories.
+all-in-experimental: all-in-c++23 all-in-c++26 \
+	$(filesystem_dir:%=all-in-%) $(backtrace_dir:%=all-in-%)
+
+.PHONY: all-parallel-subdirs
+
 @VTV_CYGMIN_TRUE@vtv_stubs.cc:
 @VTV_CYGMIN_TRUE@	rm -f $@
 @VTV_CYGMIN_TRUE@	$(LN_S) $(toplevel_srcdir)/libstdc++-v3/libsupc++/vtv_stubs.cc $@
diff --git a/libstdc++-v3/src/experimental/Makefile.am b/libstdc++-v3/src/experimental/Makefile.am
index bad10b40e21d..0af0aa6377fe 100644
--- a/libstdc++-v3/src/experimental/Makefile.am
+++ b/libstdc++-v3/src/experimental/Makefile.am
@@ -51,6 +51,8 @@ contract26.o: contract26.cc
 
 libstdc__exp_la_SOURCES = $(sources)
 
+## Keep these lists sync'd with src/Makefile.am:all-in-experimental, which
+## builds those directories before this one links their archives.
 libstdc__exp_la_LIBADD = \
 	$(top_builddir)/src/c++23/libc++23convenience.la \
 	$(top_builddir)/src/c++26/libc++26convenience.la \