Should we place the .plt before other text sections?

[Rafael Espíndola via llvm-commits]

It is odd, I cannot reproduce right now bfd and gold failing to create
the thunk. Currently we put PLT after .text. Given that it is mostly
incidental that putting it before helps to avoid thunks, I now think
this would make sense only as a short term hack.

Thanks for checking.

Cheers,
Rafael


On 28 September 2016 at 11:19, Peter Smith <peter.smith at linaro.org> wrote:
> There isn't any reason why R_ARM_THM_CALL shouldn't work with range
> extension thunks. I can make at least ld.bfd generate a
> __func_from_thumb thunk to a PLT entry, at least with some assembler.
> This was with binutils 2.25.
>
> There may be problems with crtbeginS.o as I notice that the only
> R_ARM_THM_CALL (at least in my crtbeginS.o) is via a weak reference
> __cxa_finalize which may introduce some problems if it is undefined.
> I'll try and see if there is anything I can reproduce as it might be a
> bug somewhere in the toolchain.
>
> It is true that placing the PLT closest to the .text section will
> minimise the need for long-range thunks, whether this is immediately
> before or immediately after shouldn't matter. In theory the best place
> is in the middle which would mean that we could go up to 32Mb of .text
> size on Thumb 2 without needing a range extension thunk. That would be
> a bit messy to implement though.
>
> I'll take a look at the prelink documentation
> http://people.redhat.com/jakub/prelink.pdf , when I read it many years
> ago it did have some advice on placing the linker created sections to
> make its life easier.
>
> To summarise, if placing the PLT early makes it more likely to be
> closer to the .text section I'm in favour.
>
> Peter
>
>
>
>
> On 28 September 2016 at 09:00, Rafael Espíndola
> <rafael.espindola at gmail.com> wrote:
>> I noticed a few interesting things while testing Peter's patch on ARM:
>>
>> * crtbeginS.o and other crt files use R_ARM_THM_CALL to functions
>> defined in shared libraries.
>> * We don't restrict the order of .plt sections.
>> * Gold and bfd seem to always put .plt at the start of the rx PT_LOAD.
>> * Gold and bfd assume that a .plt entry is in range when processing
>> R_ARM_THM_CALL, but not R_ARM_CALL.
>>
>> Is there a reason why R_ARM_THM_CALL would not work with range
>> extension thunks? The only reason llvm/clang/lld links successfully
>> with bfd/gold on ARM is that by putting the .plt early it ends up
>> being close to crtbegin.
>>
>> Should we do the same? I have attached the patch I used for an
>> experiment and with it we can link llvm/clang/lld on ARM with the
>> exception of clang, clang-check and libclang which hit cases that need
>> range extension chunks.
>>
>> Cheers,
>> Rafael