[LLVMdev] multi-threading in llvm

[Vlad Krylov]

Hi,

It was proposed to implement OpenMP framework.

Is there any progress in this area? Was the question raised at the
Euro-LLVM? Does anybody work on implementation?

2011/9/12 Tobias Grosser <tobias at grosser.es>:
> On 09/12/2011 05:28 PM, Jimborean Alexandra wrote:
>> Thanks for your ideas.
>>
>> Polly looked interesting as it seems it has already introduced the basic
>> support for OpenMP that I need. But indeed, it seems difficult to apply
>> the same code on a regular LLVM loop, instead of a SCoP.
>>
>> What I am working on is speculative parallelism, so I cannot provide the
>> SCoPs required by Polly. I analyze the loops and try to parallelize them
>> speculatively at the LLVM IR level. So I am searching for the easiest
>> (and possibly fastest) way to create parallel code. I was not planning
>> to reimplement the OpenMP support you provide, as I was hoping to reuse
>> it. Maybe we can further discuss which solution is better and how
>> difficult it is to adapt the parallel code generation from Polly for
>> regular loops.
> Sure, let's do this. We can also have a chat this Friday on Euro-LLVM.
>
>> What I am looking for is a pass that can automatically build the
>> structure of parameters required for the thread creation and execution,
>> and modify the loop code accordingly. Namely, to load the parameters
>> necessary for the thread into a structure and replace the use of these
>> values inside the loop with the corresponding field in the structure.
>> Similarly, once the threads finish the execution to update the values of
>> the variables with the correct values from the structure. Also, the loop
>> code has to be modified to execute only the slice allocated to the
>> thread. I did not go yet into further details on the private and shared
>> variables between the threads and other OpenMP features.
> I would also like to have such a pass. It is more complicated as the
> code in Polly right now, but most probably worth the effort.
>
>> As far as I know this code is not yet available in LLVM. Do you think
>> the parallel code generation of Polly can be adapted to perform this?
> Yes and No. I think we should be able to extract the generic functions
> to build the new OpenMP loop structure, the parameter structures and the
> function calls. With those we could create an OpenMPBuilder as we today
> have an LLVM-IR builder.
>
> Based on this, building a pass that translates a regular LLVM-IR loop
> into a OpenMP parallelized one should not be too difficult. (Especially,
> as you perform speculative parallelization and you do not need any
> dependency analysis)
>
>  > My
>> work is not focused on adding OpenMP support in LLVM, but I was looking
>> for codes that already enable parallelism in the LLVM IR. Surely, if
>> necessary I will work on building the code and inserting calls to the
>> gomp library, to parallelize regular loops. In that case, we can discuss
>> the best approach to stay general and to meet all our requirements.
> Yes, I assume there is no finished solution available. Though, I am
> interested in work to get one.
>
> Cheers
> Tobi
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