[LLVMdev] Parallelization

[Tobias Grosser]

On 03/22/2011 01:56 PM, Reid Kleckner wrote:
> On Tue, Mar 22, 2011 at 1:36 PM, Gokul Ramaswamy
> <gokulhcramaswamy at gmail.com <mailto:gokulhcramaswamy at gmail.com>> wrote:
>
>     Hi Duncan Sands,
>
>          As I have understood, GOMP and OpenMP provides support for
>     parallelizing program at source program level. But I am at the IR
>     level. That is I am trying to parallelize the IR code. This is the
>     case of automatic parallelization. The programmer writing the code
>     does not have any idea of parallelization going behind the hood.
>
>     So my question is instead of support at the source program level, is
>     the an support at the LLVM IR level to parallelize things ??
>
>
>
> No, you have to insert calls to things like pthreads or GOMP or OpenMP
> or whatever threading runtime you choose.

Which is what we also do in Polly.

In case you just have the simple case of two statements you want to 
execute in parallel, I propose to write this as OpenMP annotated C code, 
compile the code with dragonegg to LLVM-IR and have a look what code is 
generated. You will need to create similar code and similar function 
calls if you want to do it at the LLVM-IR level.

One thing that might simplify the code is to specify in OpenMP that you 
want to be able to select choices at runtime. A common construct is:

SCHEDULE(runtime)

This will stop dragonegg from inlining some OpenMP runtime calls, which 
could complicate the code unnecessarily.

Cheers
Tobi

P.S.: In case of directly inserting OpenMP function callsn it would be 
nice to have support for a set of LLVM intrinsics that will 
automatically be lowered to the relevant OpenMP/mpc.sf.net function 
calls. Let me know when you think about working on such a thing.