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

[Peter Smith via llvm-commits]

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