[llvm-dev] DebugInfo: Global variable expression management

[David Blaikie via llvm-dev]

Thanks for explaining the context - sounds plausible, but has some sharp
edges/tradeoffs if anyone does decide to try their hand at it - good to
know!

On Wed, Dec 5, 2018 at 4:25 PM Adrian Prantl <aprantl at apple.com> wrote:

>
> On Dec 5, 2018, at 4:20 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>
> Fair, I suppose today if an optimization pass deletes a GlobalVariable,
> the result is that the debug info still describes the global variable - but
> without any value. (this is inconsistent, though technically better, than
> we do with functions - which, if totally deleted, aren't emitted at all -
> but they track through inlining more than a global variable does, so it's
> not a perfect comparison) - whereas if we avoided having them in the
> globals() list, we'd /have/ to update that list, even in cases where the
> optimization didn't have anything useful to say about the value of the
> global (eg: "hey, this global has lots of different values throughout its
> lifetime, but none of them are used, I'm going to delete it" - today, it
> just deletes it and the debug info is correct. But with this change, it'd
> have to go wire in a null description to ensure the variable wasn't lost
> entirely)
>
>
>
>
>> That is messy and may subtly break some assumptions that I haven't
>> thought about yet. Outside of DIBuilder we get through the entire compiler
>> without modifying any MDNodes.
>
>
> I guess we only create new ones when it comes to creating complicated
> dwarf location descriptions/expressions as optimizations change local
> variables around? So not modification in that case?
>
>
> Correct. The most common thing we do in the optimizer is to create a new,
> uniqued, more complex DIExpression and emit a new call @llvm.dbg.value to
> tie that DIExpression to a DILocalVariable in the instruction stream.
>
>
>
>> That said, DICompileUnit is distinct so calling replaceGlobals() on it is
>> probably safe.
>>
>> What would the primary benefit of making this change be?
>>
>
> Oh, just noticed as an aside while I was filing the aforementioned bugs -
> struck me as weird that they were connected in two places is all. No major
> thing I can think of.
>
>
> That's fair.
>
> -- adrian
>
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