[LLVMdev] Vectorization metadata

[Renato Golin]

Hi Ether,

On 18 April 2012 19:11, Hongbin Zheng <etherzhhb at gmail.com> wrote:
> Instead of exporting the polyhedral model of the program with
> metadata, another possible solution is designing a generic "Loop
> Parallelism" analysis interface just like the AliasAnalysis group.
> For a particular loop, the interface simply answer how many loop
> iterations can run in parallel. With information provided by this
> interface we can unroll the loop to expose vectorizable iterations and
> apply vectorization to the unrolled loop with BBVectorizer.

In the long run, this kind of paralellism detector should be replaced
by Polly, but it could be a starting point. I only fear that the
burden might out-weight the benefits in the short term.


> To allow polly export its analysis result to FunctionPass/LoopPass, we
> need to make the polyhedral loop parallelism analysis became a
> FunctionPass, and schedule it before all polly passes but do nothing
> in its runOnFunction method, after that we can let another pass of
> polly to fill the actually analysis results into the polyhedral loop
> parallelism analysis pass. By doing this, other
> FunctionPasses/LoopPasses can query the parallelism information
> calculated by Polly.

That's the idea. Would be good if you could have both
analysis+transform AND analysis-only pre-passes, to allow a more fine
grained control over vectorization (and ease tests of other passes
that use Polly's info).


> If the parallelism information is available outside polly, we can also
> find some way to move code generation support for OpenMP, Vecorization
> and CUDA from Polly to LLVM transformation library, after that we can
> also generate such code base on the analysis result of the SCEV based
> parallelism analysis.

LLVM already has OpenMP support, maybe we should follow a similar
standard, or common them up.

CUDA would be closer to OpenCL than OpenMP or Polly, I'm not sure
there is a feasible way to make sure the semantics remains the same on
such drastic changes of paradigm.

-- 
cheers,
--renato

http://systemcall.org/