[LLVMdev] Pointer Context Metadata (was: Parallel Loop Metadata)

[Hal Finkel]

----- Original Message -----
> From: "Pekka Jääskeläinen" <pekka.jaaskelainen at tut.fi>
> To: "Nadav Rotem" <nrotem at apple.com>
> Cc: "Hal Finkel" <hfinkel at anl.gov>, "Tobias Grosser" <tobias at grosser.es>, "llvmdev at cs.uiuc.edu Dev"
> <llvmdev at cs.uiuc.edu>
> Sent: Tuesday, February 19, 2013 2:27:09 AM
> Subject: Re: [LLVMdev] Pointer Context Metadata (was: Parallel Loop Metadata)
> 
> Hi,
> 
> On 02/19/2013 02:31 AM, Nadav Rotem wrote:
> > vectorization.  However, I don't completely understand something.
> >  If we
> > already have the information that consecutive iterations of the
> > loops are
> > independent, then the loop vectorizer should already vectorize the
> > loop.
> 
> Yes, the loop vectorizer should be a better match for the parallel
> (inner)loops.
> 
> That's why I've been pushing the parallel loop metadata: to go
> towards
> using the generic loop vectorizer instead of the hacked bbvectorizer
> for
> work-group autovectorization in pocl. Unfortunately, it needs some
> more work
> still to be efficient for this purpose (like discussed), but a step
> towards it
> has been now made and it can vectorize some work-group functions with
> pocl.
> That's good.
> 
> BTW, there's at least one thing the bbvectorizer handles better now:
> intra-kernel/function vector datatypes. IIRC, it doesn't choke when
> it sees
> vectors already present in the input, but calmly tries to combine
> multiple
> vector instructions to a larger one. Thus, it might be useful in the
> case
> where the loop at hand is not nicely and cleanly vectorizable (e.g.,
> nasty
> memory access patterns) to still provide some level of vector ISA
> utilization.

Indeed, that's the idea.

> 
> Hal, this OpenCL WG autovectorization work, unfortunately, is not my
> first
> priority task at work currently (more like a pet project), so I
> cannot make any
> promises on when I might find time to work on it again. So, if you
> want to
> see the parallel loop iteration MD happen sooner, I'd recommend you
> dig into
> it. I think we'd like to start from the scratch for the bbvectorizer
> utilization
> in pocl anyways, that is, would add the metadata support first and
> then use it
> in a fresh bbvectorizer version. The current hacked version in pocl
> seems not
> to be upstreamable easily as it has lagged behind some LLVM versions
> and is
> rather dirty.

Understood. If you have some time, it seems that there are several sub-tasks:

 - Update the language reference
 - Update the loop vectorizer (to update the metadata when it unrolls)
 - Update the regular unroller
 - Update the alias analysis (maybe this is sufficient for basic support in BBVectorize) - is your current code close enough for this?
 - Update the BB vectorizer to prefer pairings from different iterations

Thanks again,
Hal

> 
> BR,
> --
> --Pekka
> 
>