[PATCH] D59431: [DebugInfo] Terminate constant-value location-list ranges at the end of basic blocks

[Adrian Prantl via Phabricator via llvm-commits]

aprantl added a comment.

Thanks for the thoughtful writeup!

> I think LiveDebugValues/DbgEntityHistoryCalculator are inferring facts about variable locations to avoid having to record all variable locations in all basic blocks at the start of compilation.

"at the start of compilation" really means in mem2reg, at least for Clang. As you pointed out, there is no way to preserve the dbg.value in the exit block in mem2reg without introducing an phi node, but doing so would change codegen, which is not an option. So I guess the answer to my question whether this is indicative of a bug in an earlier pass is: not necessarily. If we want to preserve our ability to get a chance of having debug info for bar in the exit block, we have to accept that mem2reg doesn't insert undefs. IMO, that is not elegant, but a good trade-off.

It would be great to include a section on mem2reg and your examples to SourceLevelDebugging.rst since this will no doubt come up again in the future!

If we go forward with this patch — the reason why we don't have dbg.values in the exit block in your example is because we can't introduce a new PHI for bar. But if bar is a *constant*, mem2reg could do a miniature version of LiveDebugValues that only applies to constants. Conceptually, we could move the ability to propagate constants from LiveDebugValues into mem2reg. I'm not sure if it will be worth the extra effort, but we can think about it.


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