[LLVMdev] RFC: Representation of OpenCL Memory Spaces

[Justin Holewinski]

The problem I want to address in this discussion is the representation of
OpenCL/CUDA memory spaces in LLVM IR. As support for OpenCL and CUDA mature
within Clang, it is important that we provide a way to represent memory
spaces in a way that is (1) sufficiently generic that other language
front-ends can easily emit the needed annotations, and (2) sufficiently
specific that LLVM optimization passes can perform aggressive optimizations.

*1. Introduction*

Support for OpenCL/CUDA, and potentially future language extensions,
requires the compiler to differentiate between different types of memory.
 For example, OpenCL has a "__global" memory space which corresponds to
globally-accessible data, and is usually off-chip memory in most
GPU configurations; and a "__local" memory space which corresponds to
work-group data (not accessible by work items outside of the current work
group), and is usually on-chip scratchpad memory in most GPU configurations.
 This information is currently represented in Clang/LLVM using the
addrspace() attribute on pointer types, where the OpenCL memory space to
target address space mapping is defined by the requested target (e.g. PTX,
X86, etc.).

This leads to a few issues.  First, some existing targets already use LLVM
address spaces for other purposes, so supporting OpenCL (as currently
supported in Clang) on these targets would require significant
re-structuring in the back-end.  Second, LLVM address spaces do not provide
enough semantic knowledge for optimization passes. For example, consider
pointer aliasing in the following kernel:

__kernel
void foo(__global float* a, __local float* b) {
  b[0] = a[0];
}

If we compile this with Clang targeting PTX, the resulting LLVM IR will be:

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 !1
  ret void
}

!opencl.kernels = !{!0}

!0 = metadata !{void (float*, float addrspace(4)*)* @foo}
!1 = metadata !{metadata !"float", metadata !2}
!2 = metadata !{metadata !"omnipotent char", metadata !3}
!3 = metadata !{metadata !"Simple C/C++ TBAA", null}

Does the load from %a alias the store to %b?  Using the semantics of OpenCL,
they cannot alias since they correspond to two different memory spaces.
 However, if we just look at the information in the LLVM IR, then basic
alias analysis cannot determine if aliasing occurs because disjoint memory
is not a property of LLVM address spaces. Therefore, we are not able to
optimize as much as we could.

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.


*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.

*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.

I see this solution as more of a short-term hack to solve the pointer
aliasing issue without actually addressing the larger issues.


*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.


Any comments?

-- 

Thanks,

Justin Holewinski
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