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

[Bruce Hoult via llvm-dev]

Not exactly fall through :-)  It has to form the address of the other entry
point (+1 for Thumb) in a temp register and then BX to it. It just means
the constant added to the PC is very small...


On Wed, Jun 22, 2016 at 12:06 PM, Peter Smith via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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
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