[PATCH] D151165: [ThinLTO] Make the cache key independent of the module identifier paths

[Nikita Popov via Phabricator via llvm-commits]

nikic added subscribers: tejohnson, nikic.
nikic added a comment.

@tejohnson Can you please confirm the correctness of this change?

This change has caused major compile-time regressions in Rust (compiled modules no longer get reused in incremental builds), because modules get added to the index in different orders across compilations. On the surface, it seems like we could easily work around this by adding a sort over the module key. However, the modules involved in the compilation can actually change, e.g. if references to symbols in a module are added/removed so that the module does not need to be linked at all (or starts needing to be linked). Even with a sort, this is going to shift the module indices, and result in at least unnecessary cache invalidation (I'm not sure it can result in result in incorrect cache reuse).

The basic premise of this patch (that we can use the module order instead of the module key) seems to be premised on a specific compilation model that is not valid for all thin lto consumers.


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  https://reviews.llvm.org/D151165/new/

https://reviews.llvm.org/D151165