[LLVMdev] Supporting heterogeneous computing in llvm.

[Eric Christopher]

On Sat, Jun 6, 2015 at 1:09 PM C Bergström <cbergstrom at pathscale.com> wrote:

> On Sun, Jun 7, 2015 at 2:52 AM, Eric Christopher <echristo at gmail.com>
> wrote:
> >
> >
> > On Sat, Jun 6, 2015 at 12:43 PM C Bergström <cbergstrom at pathscale.com>
> > wrote:
> >>
> >> On Sun, Jun 7, 2015 at 2:34 AM, Eric Christopher <echristo at gmail.com>
> >> wrote:
> >> >
> >> >
> >> > On Sat, Jun 6, 2015 at 12:31 PM C Bergström <cbergstrom at pathscale.com
> >
> >> > wrote:
> >> >>
> >> >> On Sun, Jun 7, 2015 at 2:22 AM, Eric Christopher <echristo at gmail.com
> >
> >> >> wrote:
> >> >> >
> >> >> >
> >> >> > On Sat, Jun 6, 2015 at 5:02 AM C Bergström <
> cbergstrom at pathscale.com>
> >> >> > wrote:
> >> >> >>
> >> >> >> On Sat, Jun 6, 2015 at 6:24 PM, Christos Margiolas
> >> >> >> <chrmargiolas at gmail.com> wrote:
> >> >> >> > Hello,
> >> >> >> >
> >> >> >> > Thank you a lot for the feedback. I believe that the
> heterogeneous
> >> >> >> > engine
> >> >> >> > should be strongly connected with parallelization and
> >> >> >> > vectorization
> >> >> >> > efforts.
> >> >> >> > Most of the accelerators are parallel architectures where having
> >> >> >> > efficient
> >> >> >> > parallelization and vectorization can be critical for
> performance.
> >> >> >> >
> >> >> >> > I am interested in these efforts and I hope that my code can
> help
> >> >> >> > you
> >> >> >> > managing the offloading operations. Your LLVM instruction set
> >> >> >> > extensions
> >> >> >> > may
> >> >> >> > require some changes in the analysis code but I think is going
> to
> >> >> >> > be
> >> >> >> > straightforward.
> >> >> >> >
> >> >> >> > I am planning to push my code on phabricator in the next days.
> >> >> >>
> >> >> >> If you're doing the extracting at the loop and llvm ir level - why
> >> >> >> would you need to modify the IR? Wouldn't the target level
> lowering
> >> >> >> happen later?
> >> >> >>
> >> >> >> How are you actually determining to offload? Is this tied to
> >> >> >> directives or using heuristics+some set of restrictions?
> >> >> >>
> >> >> >> Lastly, are you handling 2 targets in the same module or end up
> >> >> >> emitting 2 modules and dealing with recombining things later..
> >> >> >>
> >> >> >
> >> >> > It's not currently possible to do this using the current structure
> >> >> > without
> >> >> > some significant and, honestly, icky patches.
> >> >>
> >> >> What's not possible? I agree some of our local patches and design may
> >> >> not make it upstream as-is, but we are offloading to 2+ targets using
> >> >> llvm ir *today*.
> >> >>
> >> >
> >> > I'm not sure how much more clear I can be. It's not possible, in the
> >> > same
> >> > module, to handle multiple targets at the same time.
> >> >
> >> >>
> >> >> IMHO - you must (re)solve the problem about handling multiple targets
> >> >> concurrently. That means 2 targets in a single Module or 2 Modules
> >> >> basically glued one after the other.
> >> >
> >> >
> >> > Patches welcome.
> >>
> >> While I appreciate your taste in music - Canned (troll) replies are
> >> typically a waste of time..
> >
> >
> > This is uncalled for and unacceptable. I've done an immense amount of
> work
> > so that we can support different subtargets in the same module and get
> > better LTO and target features. If you have a feature above and beyond
> what
> > I've been able to do (and you say you do) then a request for patches is
> more
> > than acceptable as a response. I've yet to see any work from you and a
> lot
> > of talk about what other people should do.
>
> Umm.. don't get your feathers in a ruffle - you provided *zero*
> content and I was just saying it wasn't impossible. To pop back all
> huffy is just funny.
>
>
I can say the same and calling my post trolling was unacceptable.


> Anyway, to bring this conversation back to something technical instead
> of just stupid comments.. I'd agree that flipping targets back and
> forth (intermixed) in the same Module *is* probably a substantial
> amount of work. If the optimization passes worked at a PU (program
> unit) aka function level it wouldn't be.
>
>
It's just another level of indirection essentially - and a lot of work.
It's much easier to do what's being proposed and outline work into another
module. To do what you've said (and I've looked at) is basically turning
each function into it's own little module - ala what the ORC JIT does with
per-function compilation.


> Why can't you append 1 Module after another and switch?
>

This is, effectively, two modules and it'll behave the same. The reasons
are data transfer etc for module level attributes, data layout, etc. We've
still got some lingering issues at the function level let alone at the
module level with side data taking over. Akira and I are working on them as
we can.


>
> As you point out whole program analysis/optimization will face a
> similar problem - same question as above.
> ---------------------
> Currently - (I don't know about DSP - TI/Qualcomm), but most people in
> the industry are using custom runtimes to parse the GPU code and
> load/execute. It would be great if the linker/loader actually had
> better support for this built-in.
>
> I don't know the exact capabilities of gnu/sun linker/loader, but
> something along the lines of managling the function to also include
> target details
>
> so compiler would emit multiple mangled versions of foo() and
> linker/loader could pick the most optimized.
>
> Something like this
> nvc0_foo
> avx2_foo
> avx512_foo
> (Also I'd agree that the above would be quite hard)
>

There's quite a bit of work in this direction in a lot of different ways.
You can take a look at the gnu ifunc ELF extensions as a way of doing this
on a per-subtarget feature level. The obvious extension of this to
accelerators is something that we've had discussions about (GNU Tools
Cauldron a couple of years ago) and I believe it's been discussed as part
of a C++ working group.

At any rate, it's a much bigger discussion than a weekend on the mailing
list, but there's been some thought about how it'll need to happen on each
architecture/OS and, as you can tell, it's a matter of ongoing
experimentation and development. (References: CUDA work, Movidius work,
etc).

-eric
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