[PATCH] D72197: [MC][ELF] Emit a relocation if target is defined in the same section and is non-local

[Fangrui Song via Phabricator via llvm-commits]

MaskRay added a comment.

In D72197#1825012 <https://reviews.llvm.org/D72197#1825012>, @pcc wrote:

> It looks like this change caused us to start rejecting:
>
>   .thumb
>   .thumb_func
>   .globl foo
>   .hidden foo
>   foo:
>   adr lr, foo + 1
>
>
> with:
>
>   test.s:6:1: error: unsupported relocation on symbol
>   adr lr, foo + 1
>   ^
>  
>
>
> This is exposed by this Android ART code: https://cs.android.com/android/platform/superproject/+/master:art/runtime/arch/arm/quick_entrypoints_arm.S;l=1826
>
> I see a couple of possible fixes for this:
>
> 1. We could go back to the previous behaviour for global hidden symbols.
> 2. We could teach MC and LLD about the `R_ARM_THM_ALU_PREL_11_0` relocation required to relocate this instruction. Arguably the ART code shouldn't be adding the 1 for Thumb here (because `R_ARM_THM_ALU_PREL_11_0` adds the 1 itself), so ART would then need to be fixed.


`foo` is a hidden definition in the current object file. Can the ART code be changed to use a local symbol (`adr lr, .Lfoo`) instead?

GNU as resolves `R_ARM_THM_ALU_PREL_11_0` at assembly time, regardless of the visibility. It also rejects an undefined symbol. This looks likes a relocation-specific behavior.
I think a local symbol can make the intention more explicit, just don't know whether this is a broader issue.

(`ELFObjectWriter::isSymbolRefDifferenceFullyResolvedImpl` is in an anonymous namespace, so is not convenient to be overridden in an ARM specific file.)


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D72197/new/

https://reviews.llvm.org/D72197