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

Simon Atanasyan via llvm-dev llvm-dev at lists.llvm.org
Tue Jun 21 22:40:22 PDT 2016

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

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

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