[clang] [lld] [llvm] [ThinLTO] Reduce the number of renaming due to promotions (PR #178587)

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yonghong-song wrote:

> Ah, this raises an issue I hadn't thought about for distributed ThinLTO. The legacy behavior for distributed ThinLTO is that we import definitions for any and all values with summaries in the sharded index files. So for any values that don't have a corresponding summary, we just import the declaration. Meaning if we decide not to import a local, but import something that references it, we won't get the summary and won't know whether to rename it or not (the default will be to rename). Your force-import-all works around this but is not going to fix the general case. This is going to be an issue for distributed ThinLTO.
> 
> However, there was not long ago support added for encoding in the summary whether it should be imported as a definition or declaration. This allows us to read the summary for purposes like attribute propagation, and would work well for this case too. But this is currently off by default. I see roughly what we would want to do here to enable importing as a declaration when renaming is off, but I am thinking this is best left as a follow on change. We can disable this now for distributed ThinLTO with a FIXME. But I'm thinking we might want to disable this late, so at least we are testing the path that creates this information. Let me do some testing for a large target to see how much overhead the new analysis adds and get back to you, hopefully in the next ~day. Then we can decide where to disable it.

Thanks for detailed explanation. I agree that we can delay to support distributed mode for now. Once all related pieces are in place, we can easily support distributed mode.

https://github.com/llvm/llvm-project/pull/178587