> -----Original Message-----
> From: Joseph Myers <[email protected]>
> Sent: 11 October 2025 01:41
> To: Matthew Malcomson <[email protected]>
> Cc: Prathamesh Kulkarni <[email protected]>; Jonathan Wakely
> <[email protected]>; [email protected]; Thomas Schwinge
> <[email protected]>; Sam James <[email protected]>
> Subject: Re: libstdc++ DT_NEEDED on libatomic after PR81358
>
> External email: Use caution opening links or attachments
>
>
> On Fri, 10 Oct 2025, Matthew Malcomson wrote:
>
> > Questions for others:
> > 1) We'd like to double-check that this sounds like a problem before
> > trying to change it.
> > 2) Does this trigger thoughts of anything else to look out for?
> > 3) Would the best solution be to add some `--as-needed` push/pop
> state
> > arguments into the link line?
>
> Yes, I'd expect libstdc++ to be linked (DT_NEEDED) with libatomic only
> if it actually has undefined references that libatomic resolves.
Hi Joseph,
Apologies for getting back late on this.
>From what I see, the issue with pseudo dependency on libatomic.so (DT_NEEDED)
>probably comes from libatomic.so
being placed in gcc subdir, which is where toplevel libtool.m4 seems to pick it
up from:
for p in `eval "$output_verbose_link_cmd"`; do
case $p in
-L* | -R* | -l*)
# Some compilers place space between "-{L,R}" and the path.
# Remove the space.
if test $p = "-L" ||
test $p = "-R"; then
prev=$p
continue
else
prev=
fi
if test "$pre_test_object_deps_done" = no; then
case $p in
-L* | -R*)
# Internal compiler library paths should come after those
# provided the user. The postdeps already come after the
# user supplied libs so there is no need to process them.
if test -z "$_LT_TAGVAR(compiler_lib_search_path, $1)"; then
_LT_TAGVAR(compiler_lib_search_path, $1)="${prev}${p}"
else
_LT_TAGVAR(compiler_lib_search_path,
$1)="${_LT_TAGVAR(compiler_lib_search_path, $1)} ${prev}${p}"
fi
;;
# The "-l" case would never come before the object being
# linked, so don't bother handling this case.
esac
else
if test -z "$_LT_TAGVAR(postdeps, $1)"; then
_LT_TAGVAR(postdeps, $1)="${prev}${p}"
else
_LT_TAGVAR(postdeps, $1)="${_LT_TAGVAR(postdeps, $1)} ${prev}${p}"
fi
fi
;;
For eg, the generated libtool for libstdc++v3 shows:
# The directories searched by this compiler when creating a shared library.
compiler_lib_search_dirs="/home/prathameshk/gcc/stage1-build/aarch64-linux-gnu/libstdc++-v3/src
/home/prathameshk/gcc/stage1-build/aarch64-linux-gnu/libstdc++-v3/src/.libs
/home/prathameshk/gcc/stage1-build/aarch64-linux-gnu/libstdc++-v3/libsupc++/.libs
/home/prathameshk/gcc/stage1-build/./gcc /lib/aarch64-linux-gnu
/usr/lib/aarch64-linux-gnu /lib /usr/lib"
# Dependencies to place before and after the objects being linked to
# create a shared library.
predep_objects="/lib/aarch64-linux-gnu/crti.o
/home/prathameshk/gcc/stage1-build/./gcc/crtbeginS.o"
postdep_objects="/home/prathameshk/gcc/stage1-build/./gcc/crtendS.o
/lib/aarch64-linux-gnu/crtn.o"
predeps=""
postdeps=" -lgcc_s -latomic -lc -lgcc_s "
I was wondering if we could modify libtool.m4 to wrap the options around
--as-needed/--no-as-needed
but I assume that'd only work for GNU ld (or a linker that supports these
options), so it may perhaps not
be the the right approach ?
The following workaround seems to work to remove dependency of libatomic.so on
libstdc++.so:
diff --git a/libstdc++-v3/configure.ac b/libstdc++-v3/configure.ac
index 339e7bb17a4..d5a5ef53e69 100644
--- a/libstdc++-v3/configure.ac
+++ b/libstdc++-v3/configure.ac
@@ -157,6 +157,9 @@ pic_mode='default'
# Eliminate -lstdc++ addition to postdeps for cross compiles.
postdeps_CXX=`echo " $postdeps_CXX " | sed 's, -lstdc++ ,,g'`
+# Eliminate -latomic addition to postdeps since the library doesn't depend on
libatomic
+postdeps_CXX=`echo " $postdeps_CXX " | sed 's, -latomic , ,g'`
+
# Possibly disable most of the library.
## TODO: Consider skipping unncessary tests altogether in this case, rather
## than just ignoring the results. Faster /and/ more correct, win win.
With the patch, readelf --dynamic libstdc++.so no longer shows libatomic.so as
dependency:
0x0000000000000001 (NEEDED) Shared library: [libm.so.6]
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x0000000000000001 (NEEDED) Shared library: [libgcc_s.so.1]
0x000000000000000e (SONAME) Library soname: [libstdc++.so.6]
But I am not sure if this approach is entirely correct ?
If we choose to proceed with it, I will post a patch to similarly filter out
-latomic for other target libraries that don't depend on libatomic.so but have
the dependency introduced.
If this doesn't look correct, could you please suggest how to proceed ?
Signed-off-by: Prathamesh Kulkarni <[email protected]>
Thanks,
Prathamesh
>
> --
> Joseph S. Myers
> [email protected]
