[PATCH] D157020: [WIP][lld/ELF] Don't relax R_X86_64_(REX_)GOTPCRELX when offset is too far

Thu Aug 3 11:30:59 PDT 2023

MaskRay added a comment.

This should state the motication, which is likely to alleviate relocation overflow pressure.

> On x86-64, linkers optimize some GOT-indirect instructions (R_X86_64_REX_GOTPCRELX; e.g. movq var at GOTPCREL(%rip), %rax) to PC-relative instructions. The distance between a code section and .got is usually smaller than The distance between a code section and .data/.bss. ld.lld's one-pass relocation scanning scheme has a limitation: if it decides to suppress a GOT entry and it turns out that optimizing the instruction will lead to relocation overflow, the decision cannot be reverted. It should be easy to work around the issue with -Wl,--no-relax.

However, I'm not sure adding another relocation scanning pass for this purpose is a good idea. It's quite a bit of code targeted with very specific workloads where `-Wl,--no-relax` can be used instead. Our relocation scanning is more complex than mold and makes us slower. I'm fairly concerned of more relocation passes.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D157020/new/

https://reviews.llvm.org/D157020