llvm dev - Jan 2012 - [LLVMdev] multi-threading in llvm

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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I am interested. I would be grateful for your hints.

So OpenMP has various constructs such as parallel, barrier, single,
for, etc. And there is at least two libraries to generate OpenMP code:
libgomp and mpc. We want to be independent of specific library.

We should create an interface with methods which present manual
inserting of OpenMP pragmas in C or Fortran code. These sounds like
"this code block should be run with #parallel" or "this for loop
should be run with #parallel for loop". Constructs like barrier,
atomic can be simply presented by methods 'barrier' and
'atomic'. But
parallel or for constructs allows many modifiers (schedule, private,
shared, etc). Сan not devise a good solution for these cases.

Guess this interface is close to IRBuilder. You suggested it will be
OpenMPBuilder. This mean IR will be created from scratch. Other way is
to provide methods to modify existing IR, like user says "I want this
loop to be parallel, do it for me". What you think about this?

Implementation is calls of OpenMP library functions in a way provided
by the library abi. For example, in mpc:

   pragma omp for schedule(runtime)
   for ( i=lb ; i COND b ; i+=incr ) { A ; }
   ->
   if ( __mpcomp_runtime_loop_begin( ... ) ) {
     do {
       for ( ... ) {
         A ;
       }
     } while ( __mpcomp_runtime_loop_next( ... ) ) ;
   }
   __mpcomp_runtime_loop_end() ;


I think this work is not simple for me, so any comments are welcomed.


2012/1/7 Tobias Grosser <tobias at grosser.es>:
> On 01/07/2012 12:38 AM, Vlad Krylov wrote:
>>
>> 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?
>
>
> I am not aware of any work that was done or is planned to be done in
public.
>
> At Euro-LLVM I had a short discussion with Alexandra about targeting the
> OpenMP run time. We would both be interested in code that can translate
> normal LLVM-IR loops in OpenMP parallel loops. I am not sure,
> if Alexandra has some code that she can share. I for myself looked into
> the problem. I believe the loop extractor is a good start to implement such
> functionality. Yet, I do not have any written code ready.
>
> Do you happen to be interested to work on this? ;-) I can definitely give
> you some hints.
>
> Cheers
> Tobi