[PATCH] D61584: [DebugInfo] Some fields do not need relocations even relax is enabled.

Wed Jul 10 21:31:44 PDT 2019

dblaikie added a comment.

In D61584#1579860 <https://reviews.llvm.org/D61584#1579860>, @HsiangKai wrote:

> In D61584#1574203 <https://reviews.llvm.org/D61584#1574203>, @dblaikie wrote:
>
> > Does this solution extend/address other label deltas, like those that would appear in the other DWARF/non-frame sections? (eg: currently we use a length in the DWARF high_pc because it means not having to use a relocation for high_pc, which reduces object size - so, does this fix cause relocations to be used for that length expression to ensure they're correct if the linker changes the instructions in that range?)
>
>
> After D45181 <https://reviews.llvm.org/D45181> committed, LLVM will generate relocation pairs for symbol difference expressions if target enables relaxation. So, it will generate relocations for high_pc due to it is a symbol difference expression of end symbol and start symbol of the function range. Such as,
>
>   .word   .Lfunc_begin0           # DW_AT_low_pc
>   .word   .Lfunc_end4-.Lfunc_begin0 # DW_AT_high_pc
>
>
> This commit modified the condition to generate relocations for symbol difference expressions. Not all symbol difference expressions are related to executable code. However, only executable code will be modified by linker for relaxation. So, we do not need to generate relocations for symbol differences not located in executable code sections.

Good to know - so has LLVM been creating incorrect DWARF in these cases so far - or has no target enabled linker relaxation yet?

Is there any way to make this more targeted - does linker relaxation allow arbitrary machine code changes? Or is there something more fine-grained - so we could compute compile-time constants for some label differences, but only use linker relocations for cases where we know the instruction sequence includes something relaxable?

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