[clang] [llvm] [KeyInstr] Add docs (PR #137991)

[Orlando Cazalet-Hyams via llvm-commits]

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+# Key Instructions debug info in LLVM
+
+Key Instructions reduces the jumpiness of optimized code debug stepping. This document explains the feature and how it is implemented in LLVM. For Clang support please see the [Clang docs](../../clang/docs/KeyInstructionsClang.md)
+
+## Status
+
+In development, but mostly complete. The feature is currently disabled for coroutines.
+
+Tell Clang [not] to produce Key Instructions metadata with `-g[no-]key-instructions`. See the Clang docs for implementation info.
+
+The feature improves optimized code stepping; it's intended for the feature to be used with optimisations enabled. Although the feature works at O0 it is not recommended because in some cases the effect of editing variables may not always be immediately realised. (This is a quirk of the current implementation, rather than fundemental limitation, covered in more detail later).
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OCHyams wrote:

Done

https://github.com/llvm/llvm-project/pull/137991