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

Peter Collingbourne via llvm-dev
Mon, 09 May 2016 20:53:45 -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.
>

It looks like this is how debug info for static data members used to look
until r172591 landed back in 2013. There are a couple of tests that
exercise this path (DebugInfo/X86/multiple-at-const-val.ll
and DebugInfo/X86/pr12831.ll) but it seems likely that they were just
upgraded manually.

Do we care about backwards compatibility for debug info from that far back?
I assume we don't, since this predates the last debug info version bump in
2015 (r231082 which introduced the debug info hierarchy). If that's the
case, I intend to "upgrade" this metadata by dropping it, and we can
probably think of some other representation later for a constant folded
global.

Thanks,
--
--
Peter
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