[LLVMdev] Address space extension

[James Courtier-Dutton]

On 8 August 2013 21:29, Eric Christopher <echristo at gmail.com> wrote:
> On Thu, Aug 8, 2013 at 1:26 PM, Michele Scandale
> <michele.scandale at gmail.com> wrote:
>> On 08/08/2013 09:59 PM, Eric Christopher wrote:
>>>
>>> I don't believe so, no. Metadata should not be required for correct
>>> program behavior. It can be necessary for fast program behavior, but
>>> not for correct. The IR level handles the proper semantics of the
>>> program, metadata can provide extra optimization information. As an
>>> example see the TBAA work that was done.
>>>
>>> -eric
>>
>>
>> Perfectly fine with this. My question was ignoring the implementation detail
>> (the use of metadata to represent mapping infos).
>>
>> I think that the mapping should be a requirement for the correctness, so I
>> think we need another way to represent this...
>>
>> What is your opinion on this?
>>
>
> Wish I had one here. I completely defer to Chris for any IR design
> issues here. I just wanted to make sure that you didn't waste time
> going down the metadata for correctness path. :)
>

A suggestion:
Define/describe each address space in the lower layer of LLVM. <- all
the target specific info.
E.g. Address space 0 is 64bits and needs specific instructions to access it.
The LLVM IR can query the lower layer when it needs to know a
description of address space 0 during an optimization step.
CLANG then queries LLVM for address space description when it needs it.
>From the description, LLVM IR or CLANG then make their own choices:
E.g. CLANG decides for itself if, based on the address space
description, address space 0 is local, private or global.
The description of each address space would need to cover all the
different types of address space there are in enough detail. I.e. Not
just the bitsize, but also whether pointers can be converted from one
address space to another or not, and if so, how.
One advantage of this approach is that the "CLANG deciding for itself
whether address space 0 is local, private or global" only needs to
happen once, instead of needing to process it every time the metadata
appears (if it was stored in metadata instead).
This also might help, by allowing target specific master description
to be located in just one place, and all components, CLANG and LLVM
use that information.

James