llvm dev - May 2016 - RFC: metadata attachments for global variables

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?

Thanks,
-- 
-- 
Peter
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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.

>
> Thanks,
> --
> --
> Peter
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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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.

- David

>
>
>>
>> Thanks,
>> --
>> --
>> Peter
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
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