[llvm-dev] RFC: PointerType should derive from Type rather than SequentialType

[Peter Collingbourne via llvm-dev]

Thanks David. I ended up trying this over the weekend (see
https://github.com/llvm-project/llvm-project/compare/
master...pcc:pointertype). It ended up being a net reduction in code size
so seems like a useful cleanup even independent of the typeless pointer
work, I'll see if I can send a patch to the list.

FWIW I think this directly helps towards the migration because we would
have a clearer idea of where we are legitimately calling getElementType()
on arrays or vectors as opposed to the pointer uses which we want to remove.

Peter

On Mon, Oct 31, 2016 at 11:24 AM, David Blaikie <dblaikie at gmail.com> wrote:

> Seems pretty plausible to me - my only question would be whether it's
> worth the churn to do this intermediate step before finishing off typeless
> pointers (granted, I've stalled out on that for nearly a year, admittedly -
> but good to have extra hands/incentives - there's still a fair bit of work
> to do though, so understandable if it might not be useful to block progress
> on other things to finish it off) - not sure how much work this would
> produce as an intermediate step compared to progress towards the end goal
> directly. Perhaps not much.
>
> - Dave
>
> On Sat, Oct 29, 2016 at 8:41 PM Peter Collingbourne via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi all,
>>
>> An oddity that has existed for a long time in the IR is that PointerType
>> derives from SequentialType. Per subject I propose to make PointerType
>> derive from Type for a couple of reasons:
>>
>> - Values of type PointerType are unlike the other SequentialTypes (arrays
>> and vectors) in that they do not hold values of the element type. By moving
>> PointerType we can unify certain aspects of how the other SequentialTypes
>> are handled.
>> - PointerType will have no place in the SequentialType hierarchy once
>> pointee types are removed, so this is a necessary step towards removing
>> pointee types.
>>
>> Although this would slightly complicate GEP enumeration, this can be
>> handled at the API level and is also inevitable given the move to remove
>> pointee types.
>>
>> Specifically I want to change the API exposed by the GEP type iterator so
>> that each iterator can be in one of three states: unbounded array (i.e. the
>> existing PointerType state, which would represent the first "element" of
>> the GEP), bounded array (the existing ArrayType state) or struct (the
>> existing StructType state). In the former two cases clients would not have
>> access to the "underlying" type and would only be able to access the
>> "boundedness", the upper bound if any and the element type.
>>
>> Thanks,
>> --
>> --
>> Peter
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>


-- 
-- 
Peter
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