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

[David Blaikie via llvm-dev]

On Mon, May 9, 2016 at 3:38 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:

>
>
> 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.
>

Looks like this might be broken/have regressed at some point:

static const int x = 3;
void f1(int);
void f2(int*);
void f3() {
  f1(x);
  f2(&x);
}

Now we generate two global variables named 'x', one with a constant value,
the other with an address.

We probably only want the one with the address. (but, then again, we
probably want to turn the one with an address to having a constant value if
its storage gets optimized away)
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