[PATCH] D133925: [DebugInfo] Fix: Variables that have no non-empty values being emitted when they have a DBG_VALUE_LIST

[Stephen Tozer via Phabricator via llvm-commits]

StephenTozer added a comment.

In D133925#4037971 <https://reviews.llvm.org/D133925#4037971>, @dblaikie wrote:

> Hmm - shouldn't we still see `localh` emitted, but without a `DW_AT_location` - dropping the variable entirely seems incorrect? I guess just for this test case that happens because the variable isn't listed in the `DISubprogram`'s `variables` list?

I recall there was some discussion about this a while ago but don't recall the conclusions - regardless of what should ideally be done, this patch doesn't change the way we handle always-undef variables in itself, it only prevents variadic debug values from behaving differently to normal debug values.

> Though internally at Google we're seeing an increase in the `llvm-dwarfdump --statistics` result for `#params with binary locations/#params` and `#local vars with binary locations/#local vars` - which suggests maybe this change is having the effect of dropping some variables entirely, leading to an apparent increase in these statistics?

It's possible that this patch is causing that, but I think it's unlikely - the only change this patch makes is to make `DBG_VALUE_LIST`s behave the same way as normal `DBG_VALUE`s w.r.t. being dropped. DBG_VALUE_LIST instructions do not usually make up a high % of locations, so it seems unlikely you'd see a large change, and more importantly I think they would rarely be used for parameters since they are only produced by attempting to salvage debug info from dead instructions, which isn't as likely to happen to parameters - they could still be assigned values that get salvaged into DBG_VALUE_LISTs later, but at that point we'd be looking at a fraction of a fraction of parameters being affected.


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