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

[Adrian Prantl via llvm-commits]

> 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 <mailto: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.

-- 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 <http://reviews.llvm.org/D16209>
> 
> 
> 
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