Hmm, I received zero feedback on this proposal, when it appeared
important for a number of users.
What's your take, Andreas, Tijl (your patch essentially with a bit
of an updated description), and toolchain?
Gerald
On Wed, 27 Feb 2019, Gerald Pfeifer wrote:
> Hi Tijl, hi everyone,
>
> and let me add Andreas who has been helping on the GCC side (both
> ports, viz. his work on arm and powerpc, and upstream) and toolchain@!
:
> On Sun, 24 Feb 2019, Tijl Coosemans wrote:
>> GCC_4.3.0 instead of GCC_3.3.0. The gcc commit that changed this
>> doesn't explain why this was done, but we'll have to make the same
>> change in FreeBSD ARM libgcc_s to be ABI compatible (since _Unwind* is
>> part of the ABI). This isn't a blocker for the patch.
>>
>> I emailed the patch to gerald on 2017-02-21. He responded in the usual
>> way that he prefers patches submitted upstream and because I thought the
>> patch would not be accepted upstream he proposed an alternative solution
>> where gcc would always add -rpath on FreeBSD so you didn't have to
>> specify it on the command line. I responded this wouldn't fix the case
>> where clang was used as a linker (e.g. to combine fortran and c++ code
>> in one program) and that the FAQ on the gcc website said it was a bad
>> idea for other reasons. I also said upstream might accept my patch if
>> it was a configure option but that the gcc configure scripts are
>> complicated and I didn't know where to add it exactly. Then silence.
>
> To move this forward, let me include an updated version of the patch
> Tijl shared on 2017-02-21 (which still was in my inbox/todo list) for
> consideration for our ports collection, initially for lang/gcc8 given
> that this is the default in the ports collection.
>
>
> (The lang/gcc* ports actually do carry local patches, e.g. for arm or
> powerpc or -fuse-ld=lld, but you are right that I usually try to get
> things upstream first, fixing things upstream myself when I can, or
> asking for help. The problem in this specific case was/is that I'm
> quite not enough into this area so cannot really assess and clearly
> stalling over that was not good.)
>
>
> Find patch-gfortran-libgcc attached which should simply plug into
> lang/gcc8/files and lang/gcc8-devel/files.
>
> Feedback very welcome!
>
> Gerald
GCC has two runtime libraries: The static library libgcc.a (-lgcc) and
the shared library libgcc_s.so (-lgcc_s). Both implement many of the
same functions but they also each have their unique functions. When
gcc links programs and libraries there are three possibilities:
1. gcc -static-libgcc or gcc -static: -lgcc
=> Just use libgcc.a.
2. gcc -shared-libgcc: -lgcc_s -lgcc
=> Link with libgcc_s first, so libgcc.a is only used for its unique
functions.
3. gcc: -lgcc -Wl,--as-needed -lgcc_s -Wl,--no-as-needed
=> Link with libgcc.a first so libgcc_s is only used for its unique
functions (_Unwind_* functions).
Approach 3 is the default for gcc and it's also what clang and clang++ use;
approach 2 is the default for gfortran, g++ and probably other front ends.
This patch make 3 the default for gfortran. It significantly reduces
the use of libgcc_s. The _Unwind_* functions are also available in the
old base system libgcc_s which means this reduces the need for
-rpath /usr/local/lib/gccN in ports that depend on libraries built with
gfortran. Consider a dependency tree like this:
prog -> libA -> libgcc_s (old base system libgcc_s is fine)
-> libB -> libgcc_s (libB built with gfortran, needs new libgcc_s)
Here prog needs to be linked with -rpath /usr/local/lib/gccN even if it's
a normal C program compiled with clang. Without -rpath it will fail to
start because it loads old libgcc_s first as a dependency of libA and then
it fails to load libB. With this patch libB works with old base system
libgcc_s or may not need libgcc_s at all, so prog does not need to be
linked with -rpath.
Upstream is unlikely accept a patch like this because libgfortran calls
some _Unwind_* functions and so always needs libgcc_s. Also because
every Fortran program and library links to libgfortran it makes sense
that option 2 above is the default. On FreeBSD where clang and GCC
compiled code can be mixed and where multiple libgcc_s may be installed,
option 3 is just a lot easier to deal with.
The bug that sparked this is PR 208120 (but note there's a lot of
misleading information in that bug. CMake is not actually doing
anything wrong.)
--- UTC
--- gcc/fortran/gfortranspec.c.orig 2015-06-26 17:47:23 UTC
+++ gcc/fortran/gfortranspec.c
@@ -404,7 +404,7 @@ For more information about these matters
}
}
-#ifdef ENABLE_SHARED_LIBGCC
+#if 0
if (library)
{
unsigned int i;
--- libgfortran/Makefile.in.orig 2019-02-22 14:22:13.000000000 +0000
+++ libgfortran/Makefile.in 2019-02-27 16:27:08.856408000 +0000
@@ -625,7 +625,7 @@
$(LTLDFLAGS) $(LIBQUADLIB) ../libbacktrace/libbacktrace.la \
$(HWCAP_LDFLAGS) \
-lm $(extra_ldflags_libgfortran) \
- $(version_arg) -Wc,-shared-libgcc
+ $(version_arg)
libgfortran_la_DEPENDENCIES = $(version_dep) libgfortran.spec $(LIBQUADLIB_DEP)
cafexeclib_LTLIBRARIES = libcaf_single.la
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