[LLVMdev] FW: RFC: Supporting different sized address space arithmetic

[Eli Friedman]

On Thu, Aug 30, 2012 at 3:27 PM, Villmow, Micah <Micah.Villmow at amd.com> wrote:
>
>
>> -----Original Message-----
>> From: Eli Friedman [mailto:eli.friedman at gmail.com]
>> Sent: Thursday, August 30, 2012 3:03 PM
>> To: Villmow, Micah
>> Cc: LLVM Developers Mail
>> Subject: Re: [LLVMdev] FW: RFC: Supporting different sized address space
>> arithmetic
>>
>> On Thu, Aug 30, 2012 at 2:38 PM, Villmow, Micah <Micah.Villmow at amd.com>
>> wrote:
>> > Eli,
>> >  Here is an updated patch. This is a lot smaller based on your
>> feedback and still solves the same problem.
>>
>> The patch appears to be corrupt; can you regenerate it?
> [Villmow, Micah] Attached.
>>
>> > For your comment on the IR changes, I'm reluctant to introduce changes
>> there because really the backend is overriding the default behavior at a
>> device specific level. The optimizations themselves can be dangerous,
>> but still should produce correct results, this only allows the backend
>> to optimize certain cases and is opt-in.
>>
>> Suppose I have an array of ten pointers into some address-space which
>> uses 16-bit pointers, and the default pointer size is 64 bits.  How many
>> bytes in memory does that take?  To me, it seems like the obvious answer
>> is 20 bytes, but if you compute it using our current TargetData, you'll
>> come up with an answer of 80.  That can't work.
>>
>> If your answer is that it should be 80, and the size of a pointer isn't
>> something the frontend/IR optimizers should be aware of, I'm not sure
>> your approach makes sense; you could just introduce custom load/store
>> nodes in your target which truncate the pointer, and you wouldn't need
>> to mess with the size of a pointer at all.
> [Villmow, Micah] Yeah I see your point here. I don't deal with array of pointers in OpenCL, so didn't think of this case.
>
> So, what about extending data layout to support something like:
> p#:size:abi:pref <- specify the size, abi and preference for the pointer of address space '#'. Default behavior is p:size:abi:pref.

That's fine.

(You'll also need to deal with the fact that LLVM assumes bit casts
across address-spaces are lossless.)

-Eli