[LLVMdev] parallel loop metadata simplification

Redmond, Paul paul.redmond at intel.com
Fri Mar 1 08:49:24 PST 2013

On 2013-03-01, at 11:35 AM, Hal Finkel wrote:

> ----- Original Message -----
>> From: "Paul Redmond" <paul.redmond at intel.com>
>> To: "Hal Finkel" <hfinkel at anl.gov>
>> Cc: "llvmdev at cs.uiuc.edu Dev" <llvmdev at cs.uiuc.edu>
>> Sent: Friday, March 1, 2013 10:06:51 AM
>> Subject: Re: [LLVMdev] parallel loop metadata simplification
>> Hi Hal,
>> On 2013-02-28, at 9:33 PM, Hal Finkel wrote:
>>> ----- Original Message -----
>>>> From: "Paul Redmond" <paul.redmond at intel.com>
>>>> To: "llvmdev at cs.uiuc.edu Dev" <llvmdev at cs.uiuc.edu>
>>>> Sent: Thursday, February 28, 2013 1:30:57 PM
>>>> Subject: [LLVMdev] parallel loop metadata simplification
>>>> Hi,
>>>> I've been working on clang codegen for #pragma ivdep and creating
>>>> the
>>>> llvm.mem.parallel_loop_access metadata seems quite difficult. The
>>>> main problem is that there are so many places where loads and
>>>> stores
>>>> are created and all of them need to be changed when emitting a
>>>> parallel loop. Note that creating llvm.loop.parallel is not a
>>>> problem.
>>>> One option is to modify IRBuilder to enable attaching the metadata
>>>> for loads and stores but that seems like a huge hack.
>>> Can you please explain why this is a bad option? To be honest, this
>>> is what I expected you to do. The IRBuilder sits on top of all of
>>> the loads and stores, and seems like the perfect place to keep
>>> state for something that affects "all" loads and stores in some
>>> code region. In addition, putting this in IRBuilder should also
>>> make it easier to use this feature from other frontends.
>> One concern I have is that the llvm.mem.parallel_loop_access can
>> refer to nested loops. Is it possible that instructions in the same
>> loop require different metadatas (one is parallel in both inner and
>> outer loop and another is only parallel in the inner loop?) This is
>> a more general problem I think but the IRBuilder would only be
>> useful for brute force adding the same metadata to all instructions
>> with mayReadOrWriteMemory().
> Well, it would be more complicated than that anyway because you specifically don't want to tag the loads and stores to the local alloca'd variable storage locations, only the "explicit" memory references. I think an extra boolean parameter to CreateLoad, etc. could take care of those? Nevertheless, IRBuilder could keep a stack of parallel loops to handle nested cases, right?

Hmm, I guess I'm missing something. It is my understanding that all loads and stores require the metadata (and based on the implementation of isAnnotatedParallel). What you describe suggests that a loop goes from non-parallel to parallel after SROA?


> -Hal
>> Thoughts?
>> paul
>>> -Hal
>>>> I'd like to reopen the discussion on requiring the
>>>> llvm.mem.parallel_loop_access metadata. I understand the reason
>>>> for
>>>> the metadata is to protect against transformations that may
>>>> introduce unsafe parallel memory accesses (the reg2mem example.)
>>>> I'm
>>>> wondering if perhaps we can make the metadata more user-friendly
>>>> by
>>>> providing a single loop metadata which can be expanded into
>>>> "safer"
>>>> metadata as required. Specifically a loop pass could be added that
>>>> expands llvm.loop.parallel into llvm.loop.parallel_protected +
>>>> llvm.mem.parallel_loop_access.
>>>> Perhaps there are simpler options I've overlooked?
>>>> paul
>>>> _______________________________________________
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>>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
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