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

[Jonathan Roelofs via llvm-commits]

On 1/15/16 2:42 PM, Adrian Prantl wrote:
>
>> On Jan 15, 2016, at 1:29 PM, David Blaikie <dblaikie at gmail.com
>> <mailto: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?

Ahh, I see. Thanks for the explanation.

>>
>> 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.

Am I understanding correctly that this is a limitation of the IR's 
representation of it, and not of DWARF?


Jon

>
> -- 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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>>
>>
>

-- 
Jon Roelofs
jonathan at codesourcery.com
CodeSourcery / Mentor Embedded