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

["H.J. Lu" <hjl.tools@gmail.com>]

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.