[LLVMdev] Address space extension

[Michele Scandale]

On 08/08/2013 02:16 PM, Justin Holewinski wrote:
> Why should SelectionDAGBuilder generate an explicit bitcast for a no-op
> bitcast?

The bitcast operation isn't just the reinterpretation (change in the 
semantic) of the bits without any change in the value of the bits 
itself? If the size and value do not change should be fine.

> By definition, no bits are changed; so if the EVTs match,
> there is nothing to do.  The fundamental problem is how address spaces
> are handled, and specifically how they are converted, in LLVM IR.
>   Address space casts are currently implemented with bitcasts (in
> general).  While this works out for the LLVM IR type system, it does not
> match the semantics of an address space cast for some targets.  For PTX,
> and address space cast may involve changing bits in the address.
>   Therefore, a bitcast is not a valid way to perform an address space
> cast.  We introduced target intrinsics to perform address space casts
> for this purpose.  But I feel that this problem is likely not specific
> to PTX.  A target that uses different pointer sizes for different
> address spaces would be hit by this issue even more, since a bitcast
> would not even be valid IR.

I agree with this, probably a 'ptrcast' instruction would better fits 
the various cases allowing the targets to handle correctly all the cases 
where the size/value change.

It seems that there are various 'little' problems in the way address 
spaces are currently handled. I think it may be useful to list the 
various issues in order to plan a common solution.

Thanks.

-Michele