Re: OpenACC

[Richard Biener <richard dot guenther at gmail dot com>]

On Wed, Nov 20, 2013 at 11:41 PM, Nathan Sidwell
<nathan@codesourcery.com> wrote:
> Hi,
> there seems to have been some confusion about the OpenACC development that
> we're currently engaged in.  I thought I'd write here to clarify some
> things.
>
> As Thomas previously announced, we're working on an implementation of
> OpenACC 2.0 for x86-64/Linux host systems and PTX accelerator devices.
> OpenACC is specified at http://www.openacc-standard.org/.  There are several
> proprietary implementations of OpenACC targeting a variety of accelerator
> devices.  This is an opportunity to make OpenACC available in a free
> software compiler.
>
> For this development the accelerated code will be PTX -- one has to start
> somewhere.  PTX is an ISA for a virtual machine.  Its specification is
> public and available at
> http://docs.nvidia.com/cuda/parallel-thread-execution/index.html.  To get
> PTX code executed on a compute device (currently) requires use of Nvidia's
> driver library.  That library is available for zero cost and its API is
> documented at http://docs.nvidia.com/cuda/cuda-driver-api/index.html.
> Although all those links contain the name 'cuda', don't be misled by that --
> it's an accident of history.
>
> Nothing in this project is preventing others from working on OpenACC support
> for different accelerator devices.  Neither will anyone be forced to build
> OpenACC support -- as with OpenMP, there will be a configuration option
> allowing one to configure a compiler without it.
>
> Targeting PTX, an ISA for use with a single manufacturer's devices, is not
> different from targeting the other single-manufacturer ISAs that GCC already
> supports.  It is, of course, a steering committee decision as to whether a
> new backend is acceptable once it meets technical review.

Btw, we (SUSE and AMD) are working on targeting HSAIL/BRIG from GCC,
I don't think targeting PTX has any issue.

Richard.

> GCC supports systems with proprietary runtimes.  Historically GCC had to
> work with proprietary C libraries -- for instance, I started in GCC
> development using a sparc-solaris system.  Now that Linux has become so
> prevalent, and its C library is glibc, there's the opportunity to build GCC
> with and for free software libraries.  However, that in itself, hasn't
> caused any of the non-free host or target OS support to be removed. Nor
> should it, IMHO, prevent GCC from adding support for systems that have
> proprietary components (IIUC some CPUs rely on an opaque blob of microcode
> in order to function).
>
> As many of you will know, CodeSourcery, which Mentor purchased a few years
> ago, has been contributing to GCC and other GNU projects for over 15 years.
> Several Sourcerers are maintainers of particular pieces of the GNU project
> (mainly toolchain components).
>
> nathan
>
> --
> Nathan Sidwell