[LLVMdev] Bitcasts between pointers with different address spaces

[Villmow, Micah]

So an instruction like addrspacecast would be the proper way to do this? I know that Eli had pointed to this being preferred.

Another quest would be, could the pointer size changes and the addrspace case be submitted separately or would they need to be part of the same patchset?

Micah

> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
> On Behalf Of Mon Ping Wang
> Sent: Saturday, September 08, 2012 1:46 AM
> To: Nick Lewycky
> Cc: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] Bitcasts between pointers with different address
> spaces
> 
> 
> On Sep 8, 2012, at 12:21 AM, Nick Lewycky <nicholas at mxc.ca> wrote:
> 
> > Mon Ping Wang wrote:
> >>
> >> On Sep 7, 2012, at 11:10 PM, Nick Lewycky<nicholas at mxc.ca>  wrote:
> >>
> >>> Mon Ping Wang wrote:
> >>>> Hi,
> >>>>
> >>>> I don't think we should make bit casts between pointers with
> different
> >>>> address spaces illegal. Address spaces are not required to be
> disjoint.
> >>>> In the N1169 spec, it says
> >>>> A non-null pointer into an address space A can be cast to a
> pointer into
> >>>> another address space B, but such a cast is undefined if the
> source
> >>>> pointer does not point to a location in B.
> >>>
> >>> Use a ptrtoint + inttoptr to implement the cast.
> >>>
> >>
> >> I find that strange.  One has to pick an intermediate integer type
> that is safe to convert to and from.  It would seem more natural then
> to define a ptrtoptr operation that will truncate/extended as
> appropriate.  It would make auto upgraded of old LLVM IR files easier
> to change bitcast of pointer to pointer.
> >
> > Yes, that makes sense to me. It's more cumbersome to implement, what
> with being a new instruction, but I guess that's the price for doing it
> right.
> >
> > The name also needs a bit of work; bitcast works for pointer-to-
> pointer conversions, this is specifically changing the address space.
> Ideas? '%bar = ptraddrspace i32 addrspace(5)* %foo to i32
> addrspace(6)*' and the types must match except for the addrspace?
> >
> 
> I think you have the semantics right here that the types must match
> expect for the addrspace. How about addrspacecast since it is only
> changing the address space?
> 
>   -- Mon Ping
> 
> > Nick
> >
> >>>> If the address spaces overlap, one should be able to bticast
> between them.
> >>>
> >>> I am not okay with having the verifier accept or reject a bitcast
> based on the contents of the TargetData. Given that we intend to permit
> different address spaces to have different sized pointers, we'll need
> to reject all cross address-space bitcasts and casts will need to go
> through ptrtoint+inttoptr which makes the potential for truncation and
> extension of the value explicit.
> >>>
> >>
> >> I agree that the verifier shouldn't depend on TargetData for this
> and bitcast is no longer appropriate to use since pointer sizes can't
> be used.
> >>
> >> -- Mon Ping
> >>
> >>> Is that acceptable? In particular, it still permits you to have
> overlapping address spaces, right?
> >>>
> >>
> >>
> >>> Nick
> >>>
> >>>> On Sep 7, 2012, at 10:47 AM, Villmow, Micah wrote:
> >>>>
> >>>>> Should LLVM make bitcasts between pointers with different address
> >>>>> spaces illegal?
> >>>>> This will require a small clarification in the documentation and
> an
> >>>>> assertion check added to the verifier, but I think this would be
> a
> >>>>> good approach.
> >>>>> The reason being is that in different address spaces, pointers
> are not
> >>>>> always the same size.
> >>>>> This could be limited to make it legal only if the size of the
> pointer
> >>>>> in source and destination address spaces are equivalent, but that
> >>>>> seems like more of a work-around than a proper solution.
> >>>>> Ideas?
> >>>>> Micah
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