llvm dev - Jul 2015 - [LLVMdev] SPMD Autovectorizer

On 07/07/2015 01:32 PM, Renato Golin wrote:
> Wouldn't OpenMP account for some of that? At least on a single
> machine, could you have both parallel and simd optimisations done on
> the same loop?
The point in SPMD program description (e.g. CUDA or OpenCL C)
autovectorization is to produce something like OpenMP parallel
loops or SIMD pragmas automatically from the single thread/WI
description, adhering to its barrier synchronization semantics
etc.

That is, the output of this pass could be also converted to
OpenMP SIMD constructs, if wanted. In pocl's case the output
is simply a new kernel function (we call "work group function")
that executes all WIs using parallel loops (which can be
autovectorized more easily, or even multithreaded if seeing fit,
or both).

-- 
--Pekka

On Wed, Jul 8, 2015 at 1:19 AM, Pekka Jääskeläinen
<pekka.jaaskelainen at tut.fi> wrote:
> On 07/07/2015 01:32 PM, Renato Golin wrote:
>>
>> Wouldn't OpenMP account for some of that? At least on a single
>> machine, could you have both parallel and simd optimisations done on
>> the same loop?
>
>
> The point in SPMD program description (e.g. CUDA or OpenCL C)
> autovectorization is to produce something like OpenMP parallel
> loops or SIMD pragmas automatically from the single thread/WI
> description, adhering to its barrier synchronization semantics
> etc.
>
> That is, the output of this pass could be also converted to
> OpenMP SIMD constructs, if wanted. In pocl's case the output
> is simply a new kernel function (we call "work group function")
> that executes all WIs using parallel loops (which can be
> autovectorized more easily, or even multithreaded if seeing fit,
> or both).
If you're going to "autopar" (turn a loop into a threads which run
on
many cores or something) then please don't add a dependency on OMP.
While it may seem enticing that will just add a layer of overhead that
in-the-end you won't need (and probably won't want). Just lower to
pthreads on linux and whatever on Windows.

On 07/07/2015 09:30 PM, C Bergström wrote:
> If you're going to "autopar" (turn a loop into a threads
which run on
> many cores or something) then please don't add a dependency on OMP.
I wouldn't, but simply utilize the parallel loop metadata that
was originally designed for this purpose. What is done with
that MD is up to other passes.

-- 
--Pekka