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

Hi !

During linking LLD and other linkers builds a static hash table for dynamic
symbols,
so that in runtime dynamic linker can use this table and find symbols faster.

--hash-style=style linker option is used to select the hash type:
"Set the type of linker's hash table(s). style can be either
"sysv" for classic ELF ".hash" section,
"gnu" for new style GNU ".gnu.hash" section or
"both" for both the classic ELF ".hash" and new
style GNU ".gnu.hash" hash tables. The default is
"sysv"."

GNU_HASH format is a better version of hash table
(https://sourceware.org/ml/binutils/2006-10/msg00377.html).

We have --hash-style=sysv currently set by default in LLD. 
Though recently (see https://bugs.llvm.org//show_bug.cgi?id=34712) GNU linkers
switched to use
".gnu.hash" section format in addition to normally used classic
".hash".
So they defaults --hash-style to "both" and this looks to be released
with binutils 2.30.

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

> 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

>> 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.

I don't think there is much value in being different from the gnu
linkers in here since the common case is to have the compiler driver
pass an explicit option.

So +1 to changing the default to "both".

Cheers,
Rafael

George Rimar <grimar at accesssoftek.com> writes:
> Hi !
>
> During linking LLD and other linkers builds a static hash table for dynamic
symbols,
> so that in runtime dynamic linker can use this table and find symbols
faster.
>
> --hash-style=style linker option is used to select the hash type:
> "Set the type of linker's hash table(s). style can be either
"sysv" for classic ELF ".hash" section,
> "gnu" for new style GNU ".gnu.hash" section or
"both" for both the classic ELF ".hash" and new
> style GNU ".gnu.hash" hash tables. The default is
"sysv"."
>
> GNU_HASH format is a better version of hash table
(https://sourceware.org/ml/binutils/2006-10/msg00377.html).
>
> We have --hash-style=sysv currently set by default in LLD. 
> Though recently (see https://bugs.llvm.org//show_bug.cgi?id=34712) GNU
linkers switched to use
> ".gnu.hash" section format in addition to normally used classic
".hash".
> So they defaults --hash-style to "both" and this looks to be
released with binutils 2.30.
>
> 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

On Mon, Oct 2, 2017 at 4:15 PM, Rafael Avila de Espindola <
rafael.espindola at gmail.com> wrote:
> I don't think there is much value in being different from the gnu
> linkers in here since the common case is to have the compiler driver
> pass an explicit option.
>
> So +1 to changing the default to "both".
>
... except for MIPS, as the way how we need to sort .gnu.hash section
contents is not compatible with a MIPS ABI requirement.

Cheers,
> Rafael
>
> George Rimar <grimar at accesssoftek.com> writes:
>
> > Hi !
> >
> > During linking LLD and other linkers builds a static hash table for
> dynamic symbols,
> > so that in runtime dynamic linker can use this table and find symbols
> faster.
> >
> > --hash-style=style linker option is used to select the hash type:
> > "Set the type of linker's hash table(s). style can be either
"sysv" for
> classic ELF ".hash" section,
> > "gnu" for new style GNU ".gnu.hash" section or
"both" for both the
> classic ELF ".hash" and new
> > style GNU ".gnu.hash" hash tables. The default is
"sysv"."
> >
> > GNU_HASH format is a better version of hash table (
> https://sourceware.org/ml/binutils/2006-10/msg00377.html).
> >
> > We have --hash-style=sysv currently set by default in LLD.
> > Though recently (see https://bugs.llvm.org//show_bug.cgi?id=34712) GNU
> linkers switched to use
> > ".gnu.hash" section format in addition to normally used
classic ".hash".
> > So they defaults --hash-style to "both" and this looks to be
released
> with binutils 2.30.
> >
> > 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
>
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