[PATCH] D133740: [ELF][Distributed ThinLTO] Do not generate empty index when bitcode object is linked

Tue Sep 13 12:09:53 PDT 2022

MaskRay added a comment.

In D133740#3786811 <https://reviews.llvm.org/D133740#3786811>, @weiwang wrote:

> In D133740#3785549 <https://reviews.llvm.org/D133740#3785549>, @MaskRay wrote:
>
>> For Bazel like build systems, it seems weird to have one object file in two modes, lazy and non-lazy. Do you know why it happens for your build system?
>> I think the change will improve robustness of lld's distributed ThinLTO usage so we probably need to make the change.
>
> We've seen the case of 2 same lazy objects in buck, and the cause for that is still a mystery to us. @christylee may know more.

Such a build system usually deduplicates linker input. I have seen duplicate archives but they are -lc, -lc++, etc, not from `cc_library` like targets.
Technically with linker scripts `INPUT` and `GROUP` an archive can be referenced by two `cc_library`, but that's a very rare case usually implying a user mistake but I feel the linker can guard against it, which will also help traditional build systems which don't do a great job deduplicating input.

> I listed the case of lazy and non-lazy just for the sake of test coverage.

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https://reviews.llvm.org/D133740