[PATCH] D16209: Teach GlobalOpt not to drop DebugInfo on the floor when it promotes globals to allocas

[David Blaikie via llvm-commits]

On Fri, Jan 15, 2016 at 1:42 PM, Adrian Prantl <aprantl at apple.com> wrote:

>
> On Jan 15, 2016, at 1:29 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>
>
> On Fri, Jan 15, 2016 at 6:54 AM, Jonathan Roelofs via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
>
>> jroelofs added a comment.
>>
>> @Dblaikie Not sure if my explanation was clear: I'm trying to convert the
>> DI metadata from the kind that references a global, to the kind that
>> references a local.
>
>
> Right
>
>
>> To do that, I /think/ I need to add the dbg.declare/value to the alloca
>> that gets's created, and delete the one that references the soon-to-be-dead
>> global. Does /that/ seem to you to be something out of the range of things
>> that the DI metadata scheme can cope with?
>>
>
> The problem I'm getting at is that I think you'll end up with DWARF that
> doesn't make sense. You'll end up with a local variable instead of a
> global. Not just a global described by a local location.
>
> Does that make sense?
>
> DWARF:
>
>   DW_TAG_compile_unit
>     DW_TAG_global_variable "foo"
>     DW_TAG_subprogram
>       DW_TAG_variable "bar"
>       ...
>
> ->
>
>   DW_TAG_compile_unit
>     DW_TAG_subprogram
>       DW_TAG_variable "foo"
>       DW_TAG_variable "bar"
>
> Which isn't correct. "foo" should still be a DW_TAG_global_variable in the
> CU scope, its location should be a local location.
>
> I don't think the metadata can describe this currently... - but I could be
> wrong.
>
>
> I don’t think it can, which is why I suggested this hack with the local
> variable on IRC yesterday. But, of course, it would be better to fix LLVM
> to support the latter.
>

Ah, yeah - if we're going to implement this, I'm not sure turning a global
into a local is a sufficiently good user experience to make it an
intermediate step towards the end goal, rather than doing the work up
front. I'm open to discussing it, though - I can see how it'd be an option,
but just seems like a bad user experience..

- Dave


>
> -- adrian
>
>
>
>>
>> If not, do you know how to delete the DIGlobalVariable here? I started
>> down this rabbit hole because I've got a bug where the debug info emitted
>> into the asm references the global... which no longer exists, causing the
>> assembler to barf on it.
>>
>>
>> http://reviews.llvm.org/D16209
>>
>>
>>
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>
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