[PATCH] D26029: [LLD][ARM] ARM IRELATIVE relocations are in rel.dyn

[Rui Ueyama via llvm-commits]

The conversion of the synthetic section is now ongoing, so it might be a
practical option to just wait for it to complete. If you cannot wait for
it, I think I'm okay with this patch with TODO to remove it later. What do
you think?

On Tue, Nov 1, 2016 at 6:58 AM, Peter Smith <peter.smith at linaro.org> wrote:

> I've done a bit more research. It looks like even though the IRELATIVE
> relocations are in the .rel.plt the ifunc resolver is always called at
> startup and is not resolved at runtime. I don't know why this is the
> case, it may be that the number of IRELATIVE calls is sufficiently
> small that there isn't much overhead to just do it.
>
> From some communication with the person that did the initial
> implementation for ARM it seems like there were two models of ifunc at
> the time, x86 which used .got.plt and PPC which used .got. ARM decided
> to follow the PPC model and AArch64 followed the x86 model. From
> memory the reasons for choosing the ppc model were:
>
> - Alan Modra's post on the PPC implementation:
> https://sourceware.org/ml/binutils/2009-07/msg00351.html
> - Conceptually it artifically divides the IRELATIVE uses for the
> .rel.plt from the IRELATIVE relocations in .rel.dyn for function
> pointers for local ifuncs that don't need PLT entries.
> - On some architectures such as Mips the .got.plt takes are a precious
> resource.
>
> One thing I'll need to check is if we handle function pointers
> correctly on x86 and aarch64.
>
> Peter
>
> On 31 October 2016 at 20:43, Rafael Espíndola
> <rafael.espindola at gmail.com> wrote:
> > On 31 October 2016 at 14:14, Peter Smith <peter.smith at linaro.org> wrote:
> >> peter.smith added a comment.
> >>
> >> On reflection I think that it could be possible to clean this up if the
> rel.dyn and rel.plt OutputSections are rewritten in terms of InputSections.
> The linker in Relocation.cpp would create the R_<TARGET>_IRELATIVE
> relocations in an InputSection called .rel.iplt or .rela.iplt. On targets
> such as AArch64 and X86 rela.iplt would be allocated to the .rela.plt
> OutputSection, on ARM and PPC (if the PPC lld Target supported it) would go
> into the .rel.dyn OutputSection.
> >
> > I like this. It also solves the problem of putting the IRELATIVE
> > relocations after the other ones.
> >
> > As for why other targets try to use .rela.plt, it is probably because
> > .rela.plt is processed lazily and .rela.dyn is not. Do you know if
> > there  is some ARM/PPC restriction that would force IRELATIVE to be
> > processed eagerly?
> >
> > Cheers,
> > Rafael
>
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