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

Tue Aug 8 17:00:13 PDT 2023

MaskRay added a comment.

In D157020#4570667 <https://reviews.llvm.org/D157020#4570667>, @aeubanks wrote:

> we'd like to have a single configuration across all builds so that people don't have to manually specify `-Wl,--[no-]relax`
> the problems with `--no-relax` are:
>
> - increases `.rela.dyn` and `.got` section sizes

The `.rela.dyn` and `.got` size increase is negligible, mostly in the scale of less than 0.1% VM size.

> - causes regressions on some microbenchmarks
>
> that's why I'd like to make this work in general without users having to specify flags

What microbenchmarks? Are they good indicators of real-world application performance?

Note, for relocation overflow mitigation, it's natural to have a separate configuration. You can utilize a linker script using `INSERT [AFTER|BEFORE]` or `--no-relax`.

> if we limit the extra scanning pass to cases where there may be a relocation overflow, so that it doesn't run for most normal-sized binaries, is that still a concern? if other arches already do this sort of thunk creation/relaxation I don't see why x86-64 can't do it

An extra relocation scanning pass adds conceivable overhead. I am not sure how the pass can be limited. In theory one can add relocation distance metric to somewhere in `TargetInfo::relocateAlloc`, but this would bring overhead as well.

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https://reviews.llvm.org/D157020