[PATCH 1/5] OpenMP, NVPTX: memcpy[23]D bias correction

[Julian Brown]

On Wed, 27 Sep 2023 00:57:58 +0200
Thomas Schwinge <thomas@codesourcery.com> wrote:

> On 2023-09-06T02:34:30-0700, Julian Brown <julian@codesourcery.com>
> wrote:
> > This patch works around behaviour of the 2D and 3D memcpy
> > operations in the CUDA driver runtime.  Particularly in Fortran,
> > the "base pointer" of an array (used for either source or
> > destination of a host/device copy) may lie outside of data that is
> > actually stored on the device.  The fix is to make sure that we use
> > the first element of data to be transferred instead, and adjust
> > parameters accordingly.  
> 
> Do you (a) have a stand-alone test case for this (that is, not
> depending on your other pending patches, so that this could go in
> directly -- together with the before-FAIL test case).

Thanks for the reply! Here's a version with a stand-alone test case.

> Do you (b)
> know if is this a bug in our use of the CUDA Driver API or rather in
> CUDA itself?  If the latter, have you reported this to Nvidia?

I don't think the CUDA behaviour is *wrong*, as such -- at least to the
C/C++ way of thinking (or indeed a graphics-oriented way of thinking),
one would normally think of an array as having a zero-based origin, and
these 2D/3D memory copies would be intended as a way of updating just a
part of an array (or texture) that has full duplicate copies on both
the host and device.  Our use-case just happens to be a bit different,
both because Fortran (internally) represents an array by a zero-based
origin but may use 1-based (or whatever-based) indices, and because we
support partial mappings of host arrays on the device in all three
supported languages -- which amounts to much the same thing, actually.

That said, it *could* be fixed in CUDA, though probably not in all the
versions currently deployed out there in the world.  So I guess we'd
still need a patch like this anyway.

Julian
-------------- next part --------------
A non-text attachment was scrubbed...
Name: cuda-memcpyxd-bias-2.diff
Type: text/x-patch
Size: 5759 bytes
Desc: not available
URL: <https://gcc.gnu.org/pipermail/fortran/attachments/20231002/3b71e13c/attachment.bin>