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

[Peter Collingbourne via llvm-dev]

On Mon, May 9, 2016 at 3:17 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:

>
>
> On Mon, May 9, 2016 at 2:33 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>>
>>
>> On Mon, May 9, 2016 at 1:53 PM, Peter Collingbourne <peter at pcc.me.uk>
>> wrote:
>>
>>>
>>>
>>> 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.
>>>>
>>>
>>> It looks like this is how debug info for static data members used to
>>> look until r172591 landed back in 2013.
>>>
>>
>> Could you explain more what you mean by "<this> is how it worked until" -
>> 'this' being how it used to work and how you're proposing to make it work?
>> I'm not quite following.
>>
>
> Sorry, I meant that prior to r172591 we would represent a static data
> member with something that looked like what a DIGlobalVariable looks like
> today, but with a ConstantInt representing the initializer in the variable
> field (which is where the GlobalVariable is normally stored). The change
> moved the initializer to an associated "static member type" (yes, it's not
> really a type), which as a side effect caused us to (as far as I can tell)
> no longer store ConstantInts in the variable field.
>

Actually this isn't accurate, we still produce a ConstantInt in the
variable field of DIGlobalVariable for constants in the global scope (e.g.
test/CodeGen/2010-08-10-DbgConstant.c in clang). I'll have to think more
about what the right representation is for that then.

Thanks,
-- 
-- 
Peter
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