[llvm] [Docs] Document persistent metadata printing (PR #220967)

[Yaxun Liu via llvm-commits]

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@@ -52,6 +52,14 @@ Makes programs 10x faster by doing Special New Thing.
 
 ### Changes to the LLVM IR
 
+* LLVM now assigns persistent print IDs to metadata nodes. This keeps metadata
+  IDs stable across repeated debug and pass output, but can change their
+  numbering and ordering compared with earlier releases. Standalone metadata
+  printing now uses numbered definitions such as `!1 = !DIFile(...)` instead
+  of pointer-based forms such as `<0x...> = !DIFile(...)`. Tools and tests that
+  compare such output may need updating. Final assembly output is still
+  renumbered into canonical order.
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yxsamliu wrote:

Thanks, I meant complete-module LLVM IR, not assembly. Before a full module is printed, metadata IDs are renumbered into a contiguous canonical sequence, so its output remains unchanged from before this change. Only debug/pass snapshots and standalone metadata printing can differ. I updated the release note to clarify this.

https://github.com/llvm/llvm-project/pull/220967