[PATCH] D80052: [docs] Sketch outline for HowToUpdateDebugInfo.rst

[Tom Weaver via Phabricator via llvm-commits]

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Comment at: llvm/docs/HowToUpdateDebugInfo.rst:27
+
+When an ``Instruction`` is deleted, its debug uses change to ``undef``.  This
+is a loss of debug info: the value of a one or more source variables becomes
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Hi and thanks for this!

I'm not sure this line is actually correct.

It's my assumption that when a value pointed at by an intrinsic is deleted, the intrinsic is left with a valueless ValueAsMetaData() operand.

This empty ValueAsMetaData points at an empty metadata node, thus leaving us with an 'empty' intrinsic.

These empty intrinsics are usually removed from the program by later passes that clean up or remove 'junk' from the instruction stream.

My reading of this line gives me the impression that if I delete a value pointed to by an intrinsic, the intrinsic is auto-magically fixed up to be undef.

This may not have been your intent but it's how it reads to me.

I've fixed a bug in the Reassociate pass related to this issue in:

https://reviews.llvm.org/rG41c3f76dcd0daeec60eddfcb56008a31ad6e8738


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