[llvm-dev] RFC: metadata attachments for global variables

[Adrian Prantl via llvm-dev]

> On May 6, 2016, at 2:29 PM, Pete Cooper via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> +1 or attaching metadata to globals.  I’d be curious if we can attach !range to a global representing an enum for example.  But…
> 
> Should we care about GlobalMerge for !dbg here?  Currently if we merge globals ‘a’ and ‘b’ in to __merged_global, i guess the DIGlobalVariable’s for ‘a’ and ‘b’ would now both point to the new __merged_global.
> 
> If thats the case, then by having the !dbg on the global, we wouldn’t be able to handle this case, unless you allow the !dbg to (optionally) point to a list of DIGlobalVariable’s.

Users seem to care. I did receive several independent bug reports about LLVM not handling debug info for global merge even before this change, so I definitely want to at least have a story for supporting it before we proceed.

> BTW, this isn’t something I think should block this bug, but just wanted to point out that it could be an issue.  MergeFunction’s likely has a similar issue with !dbg on functions and I guess we don’t handle it there either.
> 
> Cheers
> Pete
>> On May 6, 2016, at 2:23 PM, Philip Reames via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> 
>> +1.  I've seen a couple of cases where metadata on a global would be useful as an optimization hint as well.  
>> 
>> Philip
>> 
>> On 05/06/2016 01:17 PM, Peter Collingbourne via llvm-dev wrote:
>>> Hi all,
>>> 
>>> I'd like to add support for metadata attachments for global variables in the same way as we did for functions.
>>> 
>>> Syntax would be pretty simple:
>>> @foo = global i32 0, !foo !0, !bar !1
>>> (the extra commas are required to disambiguate from a named metadata on the next line)
>>> 
>>> Benefits:
>>> 1) Lets us reverse the DIGlobalVariable -> GlobalVariable edge, which should hopefully clear the way for removing the llvm.dbg.cu <http://llvm.dbg.cu/> named metadata node.
>>> 
>>> 2) Allows for bitset metadata to be attached to globals rather than being represented as a named metadata node. This means that bitset metadata is naturally dropped when globals are dropped, and makes it easier to manipulate bitset metadata and build summaries of bitset definitions.
>>> 
>>> Comments appreciated.
>>> 
>>> Thanks,
>>> -- 
>>> -- 
>>> Peter
>>> 
>>> 
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