[PATCH] D29110: LangRef: Document the allowed metadata dropping transforms.

[Mehdi AMINI via Phabricator via llvm-commits]

mehdi_amini added a comment.

In https://reviews.llvm.org/D29110#656337, @mehdi_amini wrote:

> That still does not address that the metadata is opaque and carry any semantic that you don't about and may invalidate. Which is fine as long as metadata don't carry any correctness related semantic.


I need to clarify, because that was poorly phrased (or even wrong): metadata can already carry correctness related semantic: for example TBAA. 
It is not just that the metadata can be ignored, but that they *have to* be dropped if not understood by a transformation. So a metadata-agnostic transformation is always valid as long as it drop the MD. 
The issue with these global MDs is not that they affect the correctness per se, but that they can't be dropped by a transformation operating on globals that would invalidate them.


https://reviews.llvm.org/D29110