lld status with powerpc64

[Bill Schmidt]

On 11/8/21 8:11 AM, Adhemerval Zanella wrote:
>
> On 08/11/2021 10:59, Bill Schmidt wrote:
>> On 11/8/21 7:54 AM, Adhemerval Zanella wrote:
>>> On 08/11/2021 10:26, Bill Schmidt wrote:
>>>> On 11/8/21 5:37 AM, Adhemerval Zanella wrote:
>>>>> On 07/11/2021 11:24, Bill Schmidt wrote:
>>>>>> Please coordinate with Alan Modra and Nemanja Ivanovic on this topic.  There are ongoing discussions about bfd and lld linker support around @notoc that will be resolved soon, and Tulio is on vacation, so I don't want the community to make steps they'll have to undo later, or for people to engage in duplicate work.
>>>>> For this specific issue I just sent a patch to fix it on glibc side [1].
>>>>> However I think it would be good if lld also implements the ld.bfd
>>>>> optimization to fallback to older stub generation if no pcrel relocation
>>>>> is found (although it is debatable that @notoc should implicit generate
>>>>> older ISA code depending of the resulting objects being linked against).
>>>>>
>>>>> [1] https://patchwork.sourceware.org/project/glibc/patch/20211108113316.8867-1-adhemerval.zanella@linaro.org/
>>>> Hi Adhemerval,
>>>>
>>>> Current course and speed is that @notoc will imply pcrel stubs on P10 and later,
>>>> but will not do so on P9 and earlier.  The relocation currently associated with
>>>> @notoc actually came with ELFv2 on P8 and, although no compilers ever generated 
>>>> @notoc, assembly routines using it prior to P10 are a valid case and should not be
>>>> punished.  This can be handled by generating a different reloc for @notoc in the
>>>> two cases.
>>>>
>>>> If this solution holds up, then changes to glibc should be unnecessary.
>>> The main problem is this imposes an extra burden for the linker, where it 
>>> need to implement the ld.bfd optimization to not generate the power10 stubs 
>>> if no pcrel is found.  And it seems that lld does not yet support this
>>> yes and I guess it has not been an issue because @notoc in assembly
>>> routines should be rare. 
>>>
>>> It also means that stubs generation are subject to a combination of
>>> relocation on different objects (@notoc on assembly does not necessary
>>> generate power10 stub with default linker option).  I think it should
>>> be ok, although I see this as really confusing since it took some time
>>> to figure out what ld.lfd was doing.
>> My only point is that the linker teams from both groups have been huddling about
>> this, so I wanted you to be aware the issue goes a little deeper than it appears
>> on the surface.  How glibc deals with this is up to glibc, of course. :)
>>
> Well, my patch had to disable notoc exactly because lld version does not support
> such optimization (so default builds always use power10 instruction).  It means
> that lld powerpc64le support is either incomplete or @notoc handling need to be
> clarified.
Precisely.