[LLVMdev] Address space extension

[Tom Stellard]

On Thu, Aug 08, 2013 at 08:05:33AM -0400, Justin Holewinski wrote:
> On Wed, Aug 7, 2013 at 9:52 PM, Pete Cooper <peter_cooper at apple.com> wrote:
> 
> >
> > On Aug 7, 2013, at 6:38 PM, Michele Scandale <michele.scandale at gmail.com>
> > wrote:
> >
> > On 08/08/2013 03:16 AM, Pete Cooper wrote:
> >
> >
> > On Aug 7, 2013, at 5:12 PM, Michele Scandale <michele.scandale at gmail.com>
> > wrote:
> >
> > On 08/08/2013 02:02 AM, Justin Holewinski wrote:
> >
> > This worries me a bit.  This would introduce language-specific
> > processing into SelectionDAG.  OpenCL maps address spaces one way, other
> > languages map them in other ways.  Currently, it is the job of the
> > front-end to map pointers into the correct address space for the target
> > (hence the address space map in clang).  With (my understanding of) this
> > proposal, there would be a pre-defined set of language-specific address
> > spaces that the target would need to know about. IMO it should be the
> > job of the front-end to do this mapping.
> >
> >
> > The begin of the discussion was about possible way to represent high level
> > address space information in the IR different from target address spaces
> > (to have the information orthogonally respect the mapping so to handle also
> > those targets that have the trivial mapping).
> >
> > My interpretation of the solution proposed by Pete is that the frontend
> > emits metadata that describe address spaces (overlapping information and
> > mapping target specific). The instruction selection simply applis the
> > mapping encoded in the metadata. So there is no pre-defined set, but there
> > is only a mapping algorithm implemented in the instruction selection phase
> > "table driven", the table is encoded as metadata.
> >
> > I think its fair to have this be dealt with by targets instead of the
> > front-end.  That way the optimizer can remain generic and use only the
> > metadata.  CPU targets will just map every address space to 0 as they have
> > only a single physical memory space.  GPU targets such as PTX and R600 can
> > map to the actual HW spaces they want.
> >
> >
> > Why a backend should be responsible (meaning have knowledge) for a mapping
> > between high level address spaces and low level address spaces?
> >
> > Thats true.  I’m thinking entirely from the persecutive of the backend
> > doing CL/CUDA.  But actually LLVM is language agnostic.  That is still
> > something the metadata could solve.  The front-end could generate the
> > metadata i suggested earlier which will tell the backend how to do the
> > mapping.  Then the backend only needs to read the metadata.
> >
> >
> > Why X86 backend should be aware of opencl address spaces or any other
> > address spaces?
> >
> > The only reason i can think of is that this allows the address space alias
> > analysis to occur, and all of the optimizations you might want to implement
> > on top of it.  Otherwise you’ll need the front-end to put everything in
> > address space 0 and you’ll have lost some opportunity to optimize in that
> > way for x86.
> >
> >
> > Like for other aspects I see more direct and intuitive to anticipate
> > target information in the frontend (this is already done and accepted) to
> > have a middle-end and back-end source language dependent (no specific
> > language knowledge is required, because different frontends could be built
> > on top of this).
> >
> > Maybe a way to decouple the frontend and the specific target is possible
> > in order to have in the target independent part of the code-generator a
> > support for a set of language with common concept (like opencl/cuda) but
> > it's still language dependent!
> >
> > Yes, that could work.  Actually the numbers are probably not the important
> > thing.  Its the names that really tell you what the address space is for.
> >  The backend needs to know what loading from a local means.  Its almost
> > unimportant what specific number a front-end chooses for that address
> > space.  We know the front-end is really going to choose 2 (from what you
> > said earlier), but the backend just needs to know how to load/store a local.
> >
> > So perhaps the front-end should really be generating metadata which tells
> > the target what address space it chose for a memory space.  That is
> >
> > !private_memory = metadata !{ i32 0 }
> > !global_memory = metadata !{ i32 1 }
> > !local_memory = metadata !{ i32 2 }
> > !constant_memory = metadata !{ i32 3 }
> >
> 
> This is specific to an OpenCL front-end.  How would this translate to a
> language with a different memory hierarchy?
> 
> I would also like to preserve the ability for front-ends to directly assign
> address spaces in a target-dependent manner.  Currently, I can write IR
> that explicitly assigns global variables to the PTX "shared" address space
> (for example).  Under this proposal, I would need to use address space 2
> (because that is what has been decreed as OpenCL "local"), and insert
> meta-data that tells the PTX back-end to map this to its "shared" address
> space.  Is that correct?
> 

I agree with Justin here.  I prefer having the address spaces be
consistent across all languages.  If we have to start using metadata to
describe the address spaces, there will be information loss (e.g. GLSL
private memory may not be the same as OpenCL private memory).

Also, I'm not sure I understand what the advantage would be of using
metadata, is it only to make alias analysis easier?

-Tom
> 
> >
> > Unfortunately you’d have to essentially reserve those metadata names for
> > your use (better names than i chose of course), but this might be
> > reasonable.  You could alternately use the example I first gave, but just
> > add a name field to it.
> >
> > I guess targets would have to either assert or default to address space 0
> > when they see an address space without associated metadata.
> >
> >
> > This way you have the target specific information in the backend where I
> > believe it should be, and the front-end can target agnostic (note, I know,
> > its not really agnostic and already contains target specific information,
> > but I just don’t want to add more unless its really needed)
> >
> > On the casting between address spaces topic "you can cast between the
> > generic address space and global/local/private, so there's also that to
> > consider.”.  This terrifies me.  I don’t know how to generate code for this
> > on a system which has disjoint physical memory without branching on every
> > memory access to that address space.
> >
> >
> > The OpenCL 2.0 specification says that a runtime resolution to a named
> > address spaced is required in order to use a pointer in the generic address
> > space.
> >
> > Ouch!  I can’t imagine thats good for performance on some architectures.
> >  But at least its been considered and defined.
> >
> > Pete
> >
> >
> >
> > -Michele
> >
> >
> >
> 
> 
> -- 
> 
> Thanks,
> 
> Justin Holewinski

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