[cfe-dev] [LLVMdev] RFC: Representation of OpenCL Memory Spaces

Justin Holewinski justin.holewinski at gmail.com
Thu Oct 13 13:16:51 PDT 2011 

On Thu, Oct 13, 2011 at 2:59 PM, Villmow, Micah <Micah.Villmow at amd.com>wrote:

> Justin,
>  Out of these options, I would take the metadata approach for AA support.
>
> This doesn't solve the problem of different frontend/backends choosing
> different
> address space representations for the same language, but is the correct
> approach for providing extra information to the optimizations.
>
> The issue about memory spaces in general is a little different. For
> example, based on
> the code you posted below, address space 0(default) is global in CUDA, but
> in OpenCL, the default address space is private. So, how does the ptx
> backend
> handle the differences? I think this is problematic as address spaces
> are language constructs and hardcoded at the frontend, but the backend
> needs to be
> able to interpret them differently based on the source language.
>
> One way this could be done is to have the backends have options, but then
> each backend would need to implement this. I think a better approach is
> to have some way to represent address spaces generically in the module.
>

That's sort of where I was trying to go with this.  I'm thinking of some
sort of annotation like address spaces, but with semantic properties
associated with them instead of leaving the definitions solely up to the
target.  Then again, this may be too high-level for LLVM IR, which is target
dependent to begin with.


>
> Micah
> > -----Original Message-----
> > From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
> > On Behalf Of Peter Collingbourne
> > Sent: Thursday, October 13, 2011 8:58 AM
> > To: Justin Holewinski
> > Cc: clang-dev Developers; LLVM Developers Mailing List
> > Subject: Re: [LLVMdev] [cfe-dev] RFC: Representation of OpenCL Memory
> > Spaces
> >
> > Hi Justin,
> >
> > Thanks for bringing this up, I think it's important to discuss
> > these issues here.
> >
> > On Thu, Oct 13, 2011 at 09:46:28AM -0400, Justin Holewinski wrote:
> > > It is becoming increasingly clear to me that LLVM address spaces are
> > not the
> > > general solution to OpenCL/CUDA memory spaces. They are a convenient
> > hack to
> > > get things working in the short term, but I think a more long-term
> > approach
> > > should be discussed and decided upon now before the OpenCL and CUDA
> > > implementations in Clang/LLVM get too mature. To be clear, I am not
> > > advocating that *targets* change to a different method for
> > representing
> > > device memory spaces. The current use of address spaces to represent
> > > different types of device memory is perfectly valid, IMHO. However,
> > this
> > > knowledge should not be encoded in front-ends and pre-SelectionDAG
> > > optimization passes.
> >
> > I disagree.  The targets should expose all the address spaces they
> > provide, and the frontend should know about the various address spaces
> > it needs to know about.  It is incumbent on the frontend to deliver
> > a valid IR for a particular language implementation, and part of
> > that involves knowing about the ABI requirements for the language
> > implementation (which may involve using specific address spaces)
> > and the capabilities of each target (including the capabilities of
> > the target's address spaces), together with the language semantics.
> > It is not the job of the optimisers or backend to know the semantics
> > for a specific language, a specific implementation of that language
> > or a specific ABI.
> >
> > >
> > >
> > > *2. Solutions*
> > >
> > > A couple of solutions to this problem are presented here, with the
> > hope that
> > > the Clang/LLVM community will offer a constructive discussion on how
> > best to
> > > proceed with OpenCL/CUDA support in Clang/LLVM. The following list is
> > in no
> > > way meant to be exhaustive; it merely serves as a starting basis for
> > > discussion.
> > >
> > >
> > > *2A. Extend TBAA*
> > >
> > > In theory, the type-based alias analysis pass could be extended to
> > > (properly) support aliasing queries for pointers in OpenCL kernels.
> > >  Currently, it has no way of knowing if two pointers in different
> > address
> > > spaces can alias, and in fact cannot know if this is the case given
> > the
> > > definition of LLVM address spaces.  Instead of programming it with
> > > target-specific knowledge, it can be extended with language-specific
> > > knowledge.  Instead of considering address spaces, the Clang portion
> > of TBAA
> > > can be programmed to use OpenCL attributes to extend its pointer
> > metadata.
> > >  Specifically, pointers to different memory spaces are in essence
> > different
> > > types and cannot alias.  For the kernel shown above, the resulting
> > LLVM IR
> > > could be:
> > >
> > > ; ModuleID = 'test1.cl'
> > > target datalayout = "e-p:32:32-i64:64:64-f64:64:64-n1:8:16:32:64"
> > > target triple = "ptx32--"
> > >
> > > define ptx_kernel void @foo(float* nocapture %a, float addrspace(4)*
> > > nocapture %b) nounwind noinline {
> > > entry:
> > >   %0 = load float* %a, align 4, !tbaa !1
> > >   store float %0, float addrspace(4)* %b, align 4, !tbaa *!2*
> > >   ret void
> > > }
> > >
> > > !opencl.kernels = !{!0}
> > >
> > > !0 = metadata !{void (float*, float addrspace(4)*)* @foo}
> > > *!1 = metadata !{metadata !"float$__global", metadata !3}*
> > > *!2 = metadata !{metadata !"float$__local", metadata !3}*
> > > !3 = metadata !{metadata !"omnipotent char", metadata !4}
> > > !4 = metadata !{metadata !"Simple C/C++ TBAA", null}
> > >
> > > Differences are bolded.  Here, the TBAA pass would be able to
> > identify that
> > > the loads and stores do not alias.  Of course, when compiling in
> > > non-OpenCL/CUDA mode, TBAA would work just as before.
> >
> > I have to say that I much prefer the TBAA solution, as it encodes the
> > language semantics using the existing metadata for language semantics.
> >
> > > *Pros:*
> > >
> > > Relatively easy to implement
> > >
> > > *Cons:*
> > >
> > > Does not solve the full problem, such as how to represent OpenCL
> > memory
> > > spaces in other backends, such as X86 which uses LLVM address spaces
> > for
> > > different purposes.
> >
> > This presupposes that we need a way of representing OpenCL address
> > spaces in IR targeting X86 (and targets which lack GPU-like address
> > spaces).  As far as I can tell, the only real representations of
> > OpenCL address spaces on such targets that we need are a way of
> > distinguishing the different address spaces for alias analysis
> > and a representation for __local variables allocated on the stack.
> > TBAA metadata would solve the first problem, and we already have
> > mechanisms in the frontend that could be used to solve the second.
> >
> > > I see this solution as more of a short-term hack to solve the pointer
> > > aliasing issue without actually addressing the larger issues.
> >
> > I remain to be persuaded that there are any "larger issues" to solve.
> >
> > > *2B. Emit OpenCL/CUDA-specific Metadata or Attributes*
> > >
> > > Instead of using LLVM address spaces to represent OpenCL/CUDA memory
> > spaces,
> > > language-specific annotations can be provided on types.  This can
> > take the
> > > form of metadata, or additional LLVM IR attributes on types and
> > parameters,
> > > such as:
> > >
> > > ; ModuleID = 'test1.cl'
> > > target datalayout = "e-p:32:32-i64:64:64-f64:64:64-n1:8:16:32:64"
> > > target triple = "ptx32--"
> > >
> > > define *ocl_kernel* void @foo(float* nocapture *ocl_global* %a,
> > float*
> > > nocapture *ocl_local* %b) nounwind noinline {
> > > entry:
> > >   %0 = load float* %a, align 4
> > >   store float %0, float* %b, align 4
> > >   ret void
> > > }
> > >
> > > Instead of extending the LLVM IR language, this information could
> > also be
> > > encoded as metadata by either (1) emitting some global metadata that
> > binds
> > > useful properties to globals and parameters, or (2) extending LLVM IR
> > to
> > > allow attributes on parameters and globals.
> > >
> > > Optimization passes can make use of these additional attributes to
> > derive
> > > useful properties, such as %a cannot alias %b. Then, back-ends can
> > use these
> > > attributes to emit proper code sequences based on the pointer
> > attributes.
> > >
> > > *Pros:*
> > > *
> > > *
> > > If done right, would solve the general problem
> > >
> > > *Cons:*
> > > *
> > > *
> > > Large implementation commitment; could potentially touch many parts
> > of LLVM.
> >
> > You are being vague about what is required here.  A complete solution
> > following 2B would involve allowing these attributes on all pointer
> > types.  It would be very expensive to allow custom attributes or
> > metadata on pointer types, since they are used frequently in the IR,
> > and the common case is not to have attributes or metadata.  Also,
> > depending on how this is implemented, this would encode far too much
> > language specific information in the IR.
> >
> > Thanks,
> > --
> > Peter
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
>


-- 

Thanks,

Justin Holewinski
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