Re: [PATCH] libstdc++, Darwin: Handle a linker warning [PR112397].

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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On Sun, 18 Feb 2024, 16:15 Iain Sandoe, <iains.gcc@gmail.com> wrote:

> Tested on i686-darwin9, x86_64-darwin14,17,19,21,23, x86_64-linux,
> aarch64-linux-gnu,
>
> OK for trunk?
> eventual back-ports?
>

Yup, ok for all.


thanks
> Iain
>
> --- 8< ---
>
> Darwin's linker warns when we make a direct branch to code that is
> in a weak definition (citing that if a different implementation of
> the weak function is chosen by the dynamic linker this would be an
> error).
>
> As the analysis in the PR shows, this can happen when we have hot/
> cold partitioning and there is an error path that is primarily cold
> but makes use of epilogue code in the hot section.  In this simple
> case, we can easily deduce that the code is in fact safe; however
> that is not something we can realistically implement in the linker.
>
> Since the user-replaceable allocators are implemented using weak
> definitions, this is a warning that is frequently flagged up in both
> the testsuite and end-user code.
>
> The chosen solution here is to suppress the hot/cold partitioning for
> these cases (it is unlikely to impact performance much c.f. the
> actual allocation).
>
>         PR target/112397
>
> libstdc++-v3/ChangeLog:
>
>         * configure: Regenerate.
>         * configure.ac: Detect if we are building for Darwin.
>         * libsupc++/Makefile.am: If we are building for Darwin, then
>         suppress hot/cold partitioning for the array allocators.
>         * libsupc++/Makefile.in: Regenerated.
>
> Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
> Co-authored-by: Jonathan Wakely <jwakely@redhat.com>
> ---
>  libstdc++-v3/configure             | 35 +++++++++++++++++++++++-------
>  libstdc++-v3/configure.ac          |  6 +++++
>  libstdc++-v3/libsupc++/Makefile.am |  8 +++++++
>  libstdc++-v3/libsupc++/Makefile.in |  6 +++++
>  4 files changed, 47 insertions(+), 8 deletions(-)
>
> diff --git a/libstdc++-v3/configure.ac b/libstdc++-v3/configure.ac
> index c68cac4f345..37396bd6ebb 100644
> --- a/libstdc++-v3/configure.ac
> +++ b/libstdc++-v3/configure.ac
> @@ -109,6 +109,12 @@ ACX_LT_HOST_FLAGS
>  AC_SUBST(enable_shared)
>  AC_SUBST(enable_static)
>  AM_CONDITIONAL([ENABLE_DARWIN_AT_RPATH], [test x$enable_darwin_at_rpath =
> xyes])
> +os_is_darwin=no
> +case ${host_os} in
> +  darwin*) os_is_darwin=yes ;;
> +  *) ;;
> +esac
> +AM_CONDITIONAL([OS_IS_DARWIN], [test x${os_is_darwin} = xyes])
>
>  if test "$enable_vtable_verify" = yes; then
>    predep_objects_CXX="${predep_objects_CXX}
> ${glibcxx_builddir}/../libgcc/vtv_start.o"
> diff --git a/libstdc++-v3/libsupc++/Makefile.am
> b/libstdc++-v3/libsupc++/Makefile.am
> index d0e1618507e..e151ce7a1fe 100644
> --- a/libstdc++-v3/libsupc++/Makefile.am
> +++ b/libstdc++-v3/libsupc++/Makefile.am
> @@ -132,6 +132,14 @@ atomicity_file =
> ${glibcxx_srcdir}/$(ATOMICITY_SRCDIR)/atomicity.h
>  atomicity.cc: ${atomicity_file}
>         $(LN_S) ${atomicity_file} ./atomicity.cc || true
>
> +if OS_IS_DARWIN
> +# See PR 112397
> +new_opvnt.lo: new_opvnt.cc
> +       $(LTCXXCOMPILE) -fno-reorder-blocks-and-partition -I. -c $<
> +new_opvnt.o: new_opvnt.cc
> +       $(CXXCOMPILE) -fno-reorder-blocks-and-partition -I. -c $<
> +endif
> +
>  # AM_CXXFLAGS needs to be in each subdirectory so that it can be
>  # modified in a per-library or per-sub-library way.  Need to manually
>  # set this option because CONFIG_CXXFLAGS has to be after
> --
> 2.39.2 (Apple Git-143)
>
>