[llvm-dev] RFC: Absolute or "fixed address" symbols as immediate operands

[Peter Collingbourne via llvm-dev]

On Mon, Oct 24, 2016 at 10:46 PM, Chris Lattner <clattner at apple.com> wrote:

>
> On Oct 24, 2016, at 1:07 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:
>
> On Mon, Oct 10, 2016 at 8:12 PM, Peter Collingbourne <peter at pcc.me.uk>
> wrote:
>
>> The specific change I have in mind is to allow !range metadata on
>> GlobalObjects. This would
>> be similar to existing !range metadata, but it would apply to the
>> "address" of the attached GlobalObject, rather than any value loaded from
>> it. Its presence on a GlobalObject would also imply that the address of the
>> GlobalObject is "fixed" at link time.
>>
>
> Going back to IR-level representation: here is an alternative
> representation based on a suggestion from Eli.
>
> Introduce a new type of GlobalValue called GlobalConstant. GlobalConstant
> would fit into the GlobalValue hierarchy like this:
>
>    - GlobalValue
>    - GlobalConstant
>       - GlobalPointer
>          - GlobalIndirectSymbol
>             - GlobalAlias
>             - GlobalIFunc
>          - GlobalObject
>             - Function
>             - GlobalVariable
>
> GlobalValue would no longer be assumed to be of pointer type. The
> getType() overload that takes a PointerType, as well as getValueType()
> would be moved down to GlobalPointer. (A nice side benefit of this is that
> it would help flush out cases where we are unnecessarily depending on
> global pointee types.)
>
>
> Hi Peter,
>
> I agree that it makes sense to introduce a new GlobalConstant IR node for
> this sort of thing.  That said, have you considered a design where
> GlobalConstant is still required to be a pointer type?  If you did this,
> you would end up with a simpler and less invasive design of:
>
>    - GlobalValue
>    - GlobalConstant
>       - GlobalIndirectSymbol
>          - GlobalAlias
>          - GlobalIFunc
>       - GlobalObject
>          - Function
>          - GlobalVariable
>
> I think that this would be better for (e.g.) the X86 backend anyway, since
> global objects can be assigned to specific addresses with linker maps, and
> thus have small addresses (and this is expressible with the range
> metadata).  This means that GlobalConstant and other GlobalValues should
> all be usable in the same places in principle.
>
> -Chris
>

Hi Chris,

I think there are a couple of additional considerations we should make here:

   - What are we trying to model? To me it's clear that GlobalConstant is
   for modelling integers, not pointers. That alone may not necessarily be
   enough to motivate a representational change, but...
   - If we make our representation fit the model, does that improve the
   quality of the code? There's clearly been a long standing assumption that
   GlobalValues are of pointer type. Breaking that assumption has in fact
   found potential bugs (see e.g. my changes to ValueTracking.cpp in D25930,
   as well as D25917 which was a requirement for the representational change),
   and seems like it may help prevent bugs in the future, so I'd say that the
   answer is most likely yes.

On the other hand, using pointers and !range metadata for both
GlobalConstant and GlobalVariable seems attractive because this would allow
them to be treated "uniformly", but it isn't clear that that would provide
a huge benefit. From a correctness perspective we basically only need to
care about the range in the backend, and D25878 shows that at least for X86
we only need changes in a handful of places.

Thanks,
-- 
-- 
Peter
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