[PATCH] D26238: [LLD][ARM] Correct ARM TLS GOT entries without dynamic relocations.

[Rafael Espíndola via llvm-commits]

looking, sorry for taking this long.

On 2 November 2016 at 07:24, Peter Smith <peter.smith at linaro.org> wrote:
> peter.smith created this revision.
> peter.smith added reviewers: ruiu, rafael.
> peter.smith added a subscriber: llvm-commits.
> Herald added subscribers: rengolin, aemerson.
>
> ARM has three dynamic relocations for TLS:
>
> - R_ARM_TPOFF32 (Offset of S from Thread Pointer)
> - R_ARM_DTPMOD32 (Module-Id, always 1 for executables)
> - R_ARM_DTPOFF32 (Offset of S from TLS block)
>
> When static linking we must resolve these relocations statically by writing the values directly into the .got. In many ways ARM is similar to Mips:
>
> - TLS is not relaxable so the linker can't rely on relaxing global and local dynamic away.
> - ARM is type 1 TLS (TLS block is immediately after Thread Pointer and precedes TLS data) so the TCB size has to be accounted for when resolving R_ARM_TPOFF32.
> - The R_ARM_DTPMOD32 needs to be hard coded to 1 in applications.
>
> To implement this I've:
>
> - Removed the fixme in handleNoRelaxTlsRelocation() which always added dynamic relocations for ARM TLS
> - Recorded where symbols have been relocated by a Target->TlsGotRel, R_ARM_TLS_TPOFF32 for ARM
> - Introduce a writeARMGot(), similar to writeMipsGot() to isolate the special cases from the other Targets.
>   - Write the module index to be 1 (from Global Dynamic or Local Dynamic)
>   - Account for the Target->TcbSize when writing the value of a symbol that has been recorded as being referenced by R_ARM_TPOFF32.
>
> The changes show up in the existing tests. As we no longer need the dynamic relocation for the module index and the GOT entries that  are statically resolved R_ARM_TLS_TPOFF32 relocations include the ARM TcbSize of 8.
>
> The implementation of writeARMGot() is similar to writeMipsGot() in the way it handles TLS.
>
> The Mips implementation uses Config->Pic consistently to use dynamic relocations. For ARM I've used Config->Shared as an executable can still use Pic, and can still be statically linked with Pic.
>
>
> https://reviews.llvm.org/D26238
>
> Files:
>   ELF/OutputSections.cpp
>   ELF/OutputSections.h
>   ELF/Relocations.cpp
>   ELF/Symbols.h
>   test/ELF/arm-tls-norelax-gd-le.s
>   test/ELF/arm-tls-norelax-ie-le.s
>   test/ELF/arm-tls-norelax-ld-le.s
>