[LLVMdev] Bitcasts between pointers with different address spaces

[Mon Ping Wang]

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.


>> 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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>> 
>> 
>> 
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