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

[Jeremy Morse via llvm-commits]

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+# Key Instructions debug info in LLVM and Clang
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+Key Instructions is an LLVM feature that reduces the jumpiness of optimized code debug stepping by discriminating the significance of instrucions that make up source language statements. This document explains the feature and how it is implemented in [LLVM](#llvm) and [Clang](#clang-and-other-front-ends).
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+## Status
+
+Feature complete except for coroutines, which fall back to not-key-instructions handling for now but will get support soon (there is no fundemental reason why they cannot be supported, we've just not got to it at time of writing).
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jmorse wrote:

fundamental

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