[cfe-dev] Parallelism TS implementation and feasibility of GPUexecution policies

[Eli Bendersky]

On Mon, Apr 6, 2015 at 3:22 PM, C Bergström <cbergstrom at pathscale.com>
wrote:

> On Tue, Apr 7, 2015 at 4:47 AM, Eli Bendersky <eliben at google.com> wrote:
> >
> >
> > On Mon, Apr 6, 2015 at 11:48 AM, Régis Portalez
> > <regis.portalez at altimesh.com> wrote:
> >>
> >> Hi.
> >>
> >> Sorry to interrupt, but I understood there is way to emit llvm ir from
> >> cuda code?
> >>
> >
> > In general, the Clang frontend (-cc1) can generate LLVM IR for the nvptx
> > triples/targets, when passed -fcuda-is-device. To use this in practice,
> > you'll need to supply a bunch of things in headers (definitions of
> builtins,
> > CUDA types and such), and no such headers exist in the open yet. Clang
> won't
> > be able to parse the NVIDIA headers as these collide with the standard
> C++
> > headers in some ways.
>
> The old cuda headers used to be permissively licensed. (below)
>
> Those headers are probably sufficient to get things "rolling", but I
> don't know if they are really a good start. (Not to mention you'd be
> missing a runtime)
>
> The problem is that you'd have to do 2 passes with different
> (conflicting) defines. Once for host and once for device. To get
> host+device to play nice together is a b*. We have this resolved in
> our clang, but it's really specific to our compilation flow. A general
> solution would most likely involve extensive changes to the headers or
> a rewrite. :/
>

Yep, this (2 passes with different defines) is the path taken in our
approach, and the one we're pushing in http://reviews.llvm.org/D8463

Note that, headers notwithstanding, the 2 pass compilation flow is enforced
by the definition of the CUDA language, because __CUDA_ARCH__ is defined
only for device code, and undefined for host code, even though these two
can live in the same TU. So you *have* to compile the code twice.

Back to the headers - these will remain a problem. We're now looking at
different options.


Eli







>
> I've fought with this for the past 5 years. I'll try to help in a
> general way if/where I can.
>
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