Re: [PATCH 2/2] libstdc++: build the source directories in parallel

On Tue, 11 Aug 2026 at 10:58, <[email protected]> wrote:
>
> From: Kyrylo Tkachov <[email protected]>
>
> 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.
>
> 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%.


Splendid!

>
>
> Bootstrapped on aarch64-none-linux-gnu.  Ok for trunk?

Could you please add a comment to src/experimental/Makefile.am that
says to keep the list of convenience libs in sync with the
prerequisites of all-in-experimental in src/Makefile.am, something
similar to the existing "Keep this list sync'd" comment on line 45 of
src/Makefile.am

That will ensure that when we add libc++29convenience.la and link that
into libstdc++exp.a, we don't forget to update the prereqs of
all-in-experimental.

OK with that change, thanks.

>
> 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.
>
> Signed-off-by: Kyrylo Tkachov <[email protected]>
> ---
>  libstdc++-v3/src/Makefile.am | 20 ++++++++++++++++++++
>  libstdc++-v3/src/Makefile.in | 20 ++++++++++++++++++++
>  2 files changed, 40 insertions(+)
>
> diff --git a/libstdc++-v3/src/Makefile.am b/libstdc++-v3/src/Makefile.am
> index 35a497a1588..17fd6ed6064 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 811d0a5ce4a..104033fd679 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 $@
> --
> 2.50.1 (Apple Git-155)
>