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

Peter Collingbourne via llvm-dev
Fri, 06 May 2016 21:14:20 -0000

On Fri, May 6, 2016 at 2:10 PM, David Blaikie <dblaikie at gmail.com>
wrote:

>
>
> On Fri, May 6, 2016 at 2:09 PM, David Blaikie <dblaikie at gmail.com>
wrote:
>
>>
>>
>> On Fri, May 6, 2016 at 1:55 PM, Peter Collingbourne via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>>
>>>
>>> On Fri, May 6, 2016 at 1:21 PM, Duncan P. N. Exon Smith <
>>> dexonsmith at apple.com> wrote:
>>>
>>>>
>>>> > On 2016-May-06, at 13:17, Peter Collingbourne <peter at
pcc.me.uk>
>>>> 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)
>>>>
>>>> SGTM.
>>>>
>>>> > Benefits:
>>>> > 1) Lets us reverse the DIGlobalVariable ->
GlobalVariable edge, which
>>>> should hopefully clear the way for removing the llvm.dbg.cu
named
>>>> metadata node.
>>>>
>>>> A little harder than it sounds (need to somehow support a
>>>> GlobalVariable that is RAUW'ed with a ConstantInt), but I
think this is
>>>> important to do.
>>>>
>>>
>>> How can a GlobalVariable be replaced with a ConstantInt?
Wouldn't that
>>> just be an absolute address?
>>>
>>
>> If the global variable has a known constant value & the address is
never
>> needed, then we might optimize away the storage and still want debug
info
>> describing the constant.
>>
>
> I'm not sure under what conditions, if any, that currently works - so
this
> may be more of a "nice to have" or "be good to think about
how any future
> design wouldn't preclude fixing this gap" sort of thing.
>

I see. This does seem a lot like something like macro expansions where the
entity doesn't have a direct correspondent in the IR. So perhaps any pass
that would want to preserve debug info in such a case could call some
utility function that would move the DIGlobalVariable to (I suppose it
would have to be) some named metadata node to which we'd attach things like
macro expansions.

Thanks,
--
--
Peter
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