[llvm-dev] [LLD] thunk implementation correctness depends on order of input section.

[Peter Smith via llvm-dev]

Thank you both for the feedback.

I think that there will be a good deal of commonality between
architectures that need thunks. I agree that it will be easier to
generalise later when we need to.

ARM also has an optimisation of a thunk at the start of the section.
In effect change state and fall through to a function at the start of
the section. It is also quite a bit down the priority list.

Peter

On 22 June 2016 at 06:40, Simon Atanasyan <simon at atanasyan.com> wrote:
> First of all thanks for finding the bug.
>
> I agree with Rui that right now we can manage without general thunk
> infrastructure. Let's provide at least a few "thunk" implementation
> and after that reconsider necessity of common thunk framework. As to
> MIPS there is one more type of thunk (keyword is .MIPS.stubs) and one
> more optimization of current thunk (putting a thunk in the beginning
> of the section if the section contains the only function required the
> thunk). But I do not plan to implement these features in the near
> future.
>
> On Wed, Jun 22, 2016 at 5:29 AM, Rui Ueyama <ruiu at google.com> wrote:
>> The first think I'd want to say is to not try too hard to use the existing
>> "framework" in LLD. The current thunk mechanism was made just for MIPS
>> non-PIC/PIC function calls and are not proven to be generic or useful for
>> other purposes. We are cool with that because it just works for MIPS (except
>> the bug you found) and satisfies our needs. And the amount of code for MIPS
>> thunk is so small that we are not serious about reusing it. Probably the
>> name given to it ("thunk") is too generic -- maybe we should rename it
>> MipsLA25Thunk or something like that. I think you want to create a new type
>> of thunk for ARM.
>>
>> The bug that we sometimes create broken MIPS thunks seems to have introduced
>> in r265673 which Rafael made. Before that patch, we didn't assume that
>> section VAs are available in scanRelocs. I think we want to revert that
>> change (although it cannot simply be reverted because the patch was
>> submitted in April, and many changes has been made on it since then.)
>>
>> Rafael, can you take at that change?
>>
>> On Tue, Jun 21, 2016 at 9:38 PM, Peter Smith <peter.smith at linaro.org> wrote:
>>>
>>> I've been working on supporting ARM/Thumb interworking thunks in LLD
>>> and have encountered a limitation that I think it is worth bringing up
>>> in a wider context. This is all LLD specific, apologies if I've abused
>>> llvm-dev here.
>>>
>>> TL;DR summary:
>>> - Thunks in lld may not work if they are added to InputSections that
>>> have already been scanned.
>>> - There is a short term fix, but in the longer term I think that we
>>> will want to allow multiple passes of the relocations.
>>> - I'd like to know if there are any preferences on a solution.
>
> --
> Simon Atanasyan