llvm dev - Oct 2016 - RFC: Absolute or "fixed address" symbols as immediate operands

On 24 October 2016 at 16:54, Peter Collingbourne via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
>
> On Mon, Oct 24, 2016 at 1:36 PM, Friedman, Eli <efriedma at
codeaurora.org>
> wrote:
>>
>> On 10/24/2016 1:07 PM, Peter Collingbourne via llvm-dev 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.)
>>
>> A GlobalConstant can either be a definition or a declaration. A
definition
>> would look like this:
>>
>> @foo = globalconst i32 42
>>
>>
>> This is equivalent to writing "foo = 42" in assembly?
>
>
> Yes.

Back in the day the idea was to use an alias whose ConstantExpr was
just 42. Would that work?

Cheers,
Rafael

>>> @foo = globalconst i32 42
>>>
>>>
>>> This is equivalent to writing "foo = 42" in assembly?
>>
>>
>> Yes.
>
>
> Back in the day the idea was to use an alias whose ConstantExpr was
> just 42. Would that work?
In fact, it already works:

@foo = alias i64, inttoptr (i64 42 to i64*)

produces

foo = 42

Cheers,
Rafael

On Tue, Oct 25, 2016 at 5:49 AM, Rafael Espíndola <
rafael.espindola at gmail.com> wrote:
> >>> @foo = globalconst i32 42
> >>>
> >>>
> >>> This is equivalent to writing "foo = 42" in
assembly?
> >>
> >>
> >> Yes.
> >
> >
> > Back in the day the idea was to use an alias whose ConstantExpr was
> > just 42. Would that work?
>
> In fact, it already works:
>
> @foo = alias i64, inttoptr (i64 42 to i64*)
>
> produces
>
> foo = 42
>
> Cheers,
> Rafael
>
Right, I should have brought up that we can already use aliases for
definitions -- that's what I've been using in my prototype. What I think
we
need to decide on here is a representation for declarations. From that, I
think our representation for definitions should be clear. For example, I
think that if we decide to require GlobalConstant to have a pointer type,
it may be reasonable to use it only for declarations and to use GlobalAlias
for definitions. This I think would be analogous to the current state where
a GlobalAlias can be referenced by either a Function or a GlobalVariable.

Thanks,
-- 
-- 
Peter
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