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

[Jeremy Morse via llvm-commits]

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+# Key Instructions in Clang
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+Key Instructions is an LLVM feature that reduces the jumpiness of optimized code debug stepping. This document explains how Clang applies the necessary metadata.
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jmorse wrote:

This provides a negative reason (reduces jumpiness), IMO we also want to provide a few words about the things positively added by KIs. "reduces jumpiness [...] by discriminating the 'significance' of the instructions making up a statement", perhaps?

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