[PATCH] D75203: [X86] Relax existing instructions to reduce the number of nops needed for alignment purposes

[James Y Knight via Phabricator via llvm-commits]

jyknight added a comment.

Right, there's no fundamental reason why moving a label has to be forbidden -- but it'd be extremely complex if we allowed moving a label which could cause the re-layout of a fragment we thought we'd already finalized the offsets for.  This would happen if the label offset required relaxation of some instruction/data referencing it. That, then, might require //undoing// the padding from an instruction we've already padded out, due to less alignment-padding being required overall.

So, maybe we can allow changing the address of labels that CAN'T cause such an issue, then? Can we identify them straightforwardly? We would need to avoid changing the offset of any label which might require further relaxation of instructions in a text segment (because that's all this pass can modify).

But that doesn't mean we can modify all labels in text which are referred to only by non-text fixups. Consider relaxable uleb128 data section containing offsets of labels in text sections -- naively it seems fine to allow further relaxation of the uleb128 data after running the padding...but unfortunately that's wrong, because if relaxable instructions in text point _back_ to labels in the uleb128 section, they may need to be relaxed when/if the address offsets in the uleb128 data grow. I think the criteria we'd need to use is something like: find all sections referenced by a relaxable fixup in the text section. Then find all sections referenced by a relaxable fixup in one of those sections, recursively. This is the set of sections that cannot point TO a label in text, if we want to be able to move it.  That seems...complicated, itself.

Simpler, is it be valid to simply assume that text sections cannot ever refer, even transitively, to debug sections?


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