[LLVMdev] multi-threading in llvm

[Dmitry N. Mikushin]

Recently I met the following spec:
http://gcc.gnu.org/onlinedocs/libgomp/OMP_005fNUM_005fTHREADS.html#OMP_005fNUM_005fTHREADS
It was new to me OpenMP actually allows to specify "the number of
threads to use for the corresponding nested level" in the
multidimensional loops. Is that correct in Polly this feature is not
supported, and OpenMP clause is always applied only the the most outer
loop?

With it supported I think we can step much closer to natural
unification of OpenMP and multidimensional compute grids of GPU
programming models.

2011/9/12 Tobias Grosser <tobias at grosser.es>:
> On 09/12/2011 04:56 PM, Sebastian Pop wrote:
>>
>> On Mon, Sep 12, 2011 at 10:44, Tobias Grosser<tobias at grosser.es>  wrote:
>>>>
>>>> You can have the parallel code generation part of Polly working as
>>>> a LoopPass.
>>>
>>> Are you sure about this? In CodeGeneration we basically translate a CLooG
>>> AST into LLVM-IR. Without a CLooG AST this does not work.
>>
>> I mean you could rewrite that code to work on a per loop basis, like
>> graphite does: flag the parallel loops in the code generation of polly
>> and then independently of polly, as a LoopPass, iterate over all the
>> loops and code generate the parallel ones.
>
> Yes, that's true and probably a good idea. It is more work as generating it
> directly in Polly, but it facilitates reuse of code. We can probably reuse
> quite a bit of the code in the existing OpenMP code generation.
>
> We (or actually someone) may work on three things:
>
> 1) The high level OpenMP intrinsics that can be lowered to actual library
> calls
>
> This can be reused by an clang OpenMP extensions as well as by polly
> directly.
>
> 2) An OpenMP builder, that allows to build OpenMP loops from scratch
>
> 3) The actual parallelizer, that takes an LLVM-IR loop and parallelizes it
> with OpenMP intrinsics.
>
> Cheers
> Tobi
>
>
>