llvm dev - Oct 2017 - [EXT] Should we switch to --hash-style=both by default in LLD ?

+1 for both table formats. Then we're compatible by default no matter what.
If somebody cares about .o file size, they can explicitly specify their
--hash-style.

On Mon, Oct 2, 2017 at 5:35 AM, George Rimar via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> >> Le 2 oct. 2017 à 14:23, George Rimar <grimar at
accesssoftek.com> a écrit
> :
> >>
> >> I think we can switch LLD either to "both" or probably
to "gnu" by
> default as well.
> >> Initial version of patch that changes default to "both"
is here: D38407
> >>
> >> Any thoughts ?
> >>
> >> Best regards,
> >> George | Developer | Access Softek, Inc
> >
> >Hi,
> >
> >I initially wanted to switch the default to "gnu" as well in
GNU linkers.
> However it was argued that this would generate ELF fiels that do not follow
> the ELF ABI. See here >https://sourceware.org/ml/
> binutils/2017-07/msg00160.html Thus it has been chosen to use
"both"
> rather than "gnu".
> >
> >Cheers,
> >Romain
>
> Fair enough, argument that we should not emit ELF files that aren't ABI
> compliant by default looks reasonable.
> Use of "both" looks ideal for me then.
>
> George.
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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Changing default to "both" is probably fine, but what does that change
actually mean in practice? I believe clang already passes --hash-style=both
to the linker by default.

On Mon, Oct 2, 2017 at 9:28 AM, Reid Kleckner via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> +1 for both table formats. Then we're compatible by default no matter
> what. If somebody cares about .o file size, they can explicitly specify
> their --hash-style.
>
> On Mon, Oct 2, 2017 at 5:35 AM, George Rimar via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> >> Le 2 oct. 2017 à 14:23, George Rimar <grimar at
accesssoftek.com> a
>> écrit :
>> >>
>> >> I think we can switch LLD either to "both" or
probably to "gnu" by
>> default as well.
>> >> Initial version of patch that changes default to
"both" is here: D38407
>> >>
>> >> Any thoughts ?
>> >>
>> >> Best regards,
>> >> George | Developer | Access Softek, Inc
>> >
>> >Hi,
>> >
>> >I initially wanted to switch the default to "gnu" as well
in GNU
>> linkers. However it was argued that this would generate ELF fiels that
do
>> not follow the ELF ABI. See here >https://sourceware.org/ml/bin
>> utils/2017-07/msg00160.html Thus it has been chosen to use
"both" rather
>> than "gnu".
>> >
>> >Cheers,
>> >Romain
>>
>> Fair enough, argument that we should not emit ELF files that aren't
ABI
>> compliant by default looks reasonable.
>> Use of "both" looks ideal for me then.
>>
>> George.
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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Le 2 oct. 2017 à 18:37, Rui Ueyama <ruiu at google.com<mailto:ruiu at
google.com>> a écrit :

Changing default to "both" is probably fine, but what does that change
actually mean in practice? I believe clang already passes --hash-style=both to
the linker by default.

Not exactly.

The current Linux driver (I haven't checked other ELF-based systems) reads
this
(https://github.com/llvm-mirror/clang/blob/master/lib/Driver/ToolChains/Linux.cpp#L228)


  // Do not use 'gnu' hash style for Mips targets because .gnu.hash
  // and the MIPS ABI require .dynsym to be sorted in different ways.
  // .gnu.hash needs symbols to be grouped by hash code whereas the MIPS
  // ABI requires a mapping between the GOT and the symbol table.
  // Android loader does not support .gnu.hash.
  // Hexagon linker/loader does not support .gnu.hash
  if (!IsMips && !IsAndroid && !IsHexagon) {
    if (Distro.IsRedhat() || Distro.IsOpenSUSE() ||
        (Distro.IsUbuntu() && Distro >= Distro::UbuntuMaverick))
      ExtraOpts.push_back("--hash-style=gnu");

    if (Distro.IsDebian() || Distro.IsOpenSUSE() || Distro ==
Distro::UbuntuLucid ||
        Distro == Distro::UbuntuJaunty || Distro == Distro::UbuntuKarmic)
      ExtraOpts.push_back("--hash-style=both");
  }

Given that several libc implementation supports this in the Linux ecosystem
(glibc, musl, uClibc, most likely others) I think lld should provide good
defaults by default, instead of relying on what some distro maintainer have
hardcoded for their own distro in clang.

Cheers,
ROmain
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