[PATCH] D67393: [DebugInfo] LiveDebugValues: Defer all DBG_VALUE creation during analysis

[Adrian Prantl via Phabricator via llvm-commits]

aprantl added a comment.

Okay, we should make sure that we do get the design right. I don't quite get the argument made in SourceLevelDebugInfo.rst for why the semantics are different:

> After this pass the DBG_VALUE instruction changes meaning: rather than corresponding to a source-level assignment where the variable may change value, it asserts the location of a variable in a block, and loses effect outside the block.

First, DBG_VALUEs aren't necessarily source-level assignments *before* LiveDebugValues either, they update the SSA value that a (fragment) of a source-level variable can be found in, but that SSA value could have been created by the compiler and has not necessarily any relation to a source-level assignment (think about salvageDebugInfo, for example).
The fact that the DBG_VALUE has no effect outside of the current basic block just falls out of DbgEntityHistoryCalculator not doing a LiveDebugVariable-style data flow analysis, but IMO that isn't a change in semantics, it would be *legal* for it to perform one, it just would be pointless after LiveDebugValues has propagated DBG_VALUEs across basic blocks and reached a fixed point.

So I'm suspecting that I'm missing something and it isn't mentioned in the text. Can you perhaps fill me in?


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