Re: [PATCH] install: Delete scripts/output-format.sed and support lld

["Fāng-ruì Sòng" <maskray@google.com>]

On Sun, May 3, 2020 at 2:11 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Sun, May 3, 2020 at 1:54 PM Fangrui Song via Libc-alpha
> <libc-alpha@sourceware.org> wrote:
> >
> > On 2020-05-03, Florian Weimer wrote:
> > >* Fangrui Song via Libc-alpha:
> > >
> > >> GNU ld and gold can skip a script with incompatible OUTPUT_FORMAT and
> > >> find the next script in the search path. So libc.so linked with lld
> > >> cannot be skipped automatically. However, this is considered a minor
> > >> functionality loss by lld maintainers.
> > >
> > >That completely depends on how typical installations set up their
> > >linker search paths.  Figuring that out is likely more work than
> > >providing the correct OUTPUT_FORMAT information in the linker script,
> > >so I'm not sure if the patch goes in the right direction.
> >
> > For typical installations, OUTPUT_FORMAT is not useful.
> > Even in multilib installations, OUTPUT_FORMAT is not useful unless, for
> > example, /usr/lib is in both -m32 and -m64's library search paths (this
> > is likely a misconfiguration).
> >
> > `OUTPUT_FORMAT(elf64-x86-64)` can provide some protection for GNU ld and
> > gold.
> >
> > >But as I said, it's probably better to find an alternative way to
> > >generate the right OUTPUT_FORMAT directive for the linker script.
> >
> > We (LLD developers) consider OUTPUT_FORMAT with GNU ld/gold semantic has
> > very little value. See https://bugs.llvm.org/show_bug.cgi?id=37432 and
> > https://bugs.llvm.org/show_bug.cgi?id=43740
> >
> > >In any case, the choice of linker for building glibc should result in
> > >as little semantic difference in the produced artifacts, to avoid
> > >inflating the compatibility matrix.  If OUTPUT_FORMAT is indeed
> > >unnecessary (based on the evaluation I mentioned), it should be
> > >removed unconditionally.
> >
> > If there is concern about the aforementioned value, then we probably
> > should not drop OUTPUT_FORMAT for now for GNU ld/gold. If people think
> > we can remove OUTPUT_FORMAT as well, I will be happy to create a
> > follow-up patch.
>
> Historical usage of OUTPUT_FORMAT was used to link object files of
> non-Linux/x86 formats and generate Linux/x86 output.  It was very useful
> during the days when many binary only libraries weren't available for
> Linux/x86.  It is still somewhat useful to link ELF object files for a different
> OS.  In both cases, the format of the first input file isn't the format of the
> output.  That is where OUTPUT_FORMAT comes in.
>
>
> --
> H.J.

LLD will not support --print-output-format.

OUTPUT_FORMAT might be useful at some point and is still needed for
unified i386/x86-64 directory trees, but not when distributions use
different directory trees for multiarch.

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