[llvm-dev] [lld][ELF] Regarding removeUnusedSyntheticSections()

[Amilendra Kodithuwakku via llvm-dev]

Hi,

I am working on an LLD port for a research project based on the AARCH64 based architecture which requires adding a new SyntheticSection to InStruct, but doing so gives me 2 failed x86 tests:
  lld :: ELF/shuffle-sections-init-fini.s
  lld :: ELF/shuffle-sections.s
This new SyntheticSection is empty for X86, so in principle those tests should be unaffected.

The immediate reason is that the shuffle implementation, maybeShuffle(), is dependent on inputSections. I think the real cause is that removeUnusedSyntheticSections() does not remove empty sections from the inputSections vector (although there is a comment that says that it does that). AFAICT elements are removed only from isd->sections and script->orphanSections.

I could not find any uses of empty sections after this point, but you never know so thought of asking here. Is there a reason empty SyntheticSections in inputSections is preserved after removeUnusedSyntheticSections()? Or is there another approach that will not require maintaining the shuffle tests everytime a Synthetic section is added/removed?

If it is safe to remove empty SyntheticSections from inputSections I can send out a patch to do that. Locally I have a simple patch that confirms that the shuffle tests do not fail after adding a new SyntheticSection. I can't think of new regression tests to add because this behavior cannot be demonstrated without adding a new SyntheticSection to InStruct.

Thanks
Amilendra

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