llvm dev - Dec 2008 - [LLVMdev] Questions on Parallelism and Data Dependence Analysis

Hi, I have two questions about llvm, and expect your reply very much.

1. Is there any plan of llvm to support Thread-Level Parallelism by using
OpenMP, MPI, pthread or llvm-defined directives?
    If automatic parallelism exploring is very hard, what is the key problem? We
can't get the precise data dependence
    information at compile-time?

2. Can I use the functions provided by llvm to get the real data dependence(Read
After Write) info on basic block level directly?
   In other words, can I get the task graph(node: task viewed as basic block or
more coarse grain, edge: real data dependence)
   from sequential program using llvm to explore task-level parallelism by some
proposed task scheduling algorithm?

thanks.

2008-12-25 



Wenzhi Tao 
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On Wed, Dec 24, 2008 at 6:11 PM, Wenzhi Tao <linus_wind at zju.edu.cn>
wrote:
> Hi, I have two questions about llvm, and expect your reply very much.
>
> 1. Is there any plan of llvm to support Thread-Level Parallelism by using
> OpenMP, MPI, pthread or llvm-defined directives?
>     If automatic parallelism exploring is very hard, what is the key
> problem? We can't get the precise data dependence
>     information at compile-time?
AFAIK, OpenMP, MPI, and pthreads should all work just fine with LLVM.
Automatically parallelizing code is a fundamentally difficult problem
for many reasons which I won't go into as it's only tangentially
relevant to LLVM.
> 2. Can I use the functions provided by llvm to get the real data
> dependence(Read After Write) info on basic block level directly?
>    In other words, can I get the task graph(node: task viewed as basic
block
> or more coarse grain, edge: real data dependence)
>    from sequential program using llvm to explore task-level parallelism by
> some proposed task scheduling algorithm?
I don't think there's anything quite like that.  There is
MemoryDependenceAnalysis; that returns the dependencies for an
individual instruction.

-Eli

On Dec 24, 2008, at 6:11 PM, Wenzhi Tao wrote:
> 2. Can I use the functions provided by llvm to get the real data  
> dependence(Read After Write) info on basic block level directly?
>    In other words, can I get the task graph(node: task viewed as  
> basic block or more coarse grain, edge: real data dependence)
>    from sequential program using llvm to explore task-level  
> parallelism by some proposed task scheduling algorithm?
>

It is possible. Wojciech Matyjewicz has done this.  See relevant  
thread (with attached patch) 
http://lists.cs.uiuc.edu/pipermail/llvmdev/2008-August/016616.html

LLVM will benefit if data dependence analysis support is integrated in  
llvm mainline sources.

-
Devang