OpenACC data clause syntax question/patch

[Jakub Jelinek]

On Fri, Sep 23, 2016 at 06:02:32PM +0200, Bernd Schmidt wrote:
> While trying to get the following benchmarks to run:
>   https://codesign.llnl.gov/lulesh.php
> 
> I came across some cases where the code and the compiler differed in its
> interpretation of the OpenACC spec. Specifically, this occurs for acc data
> clauses with arrays, like:
> 
> #pragma acc data copyin(a[N][M]) copyout(b[N][M])
> 
> Here, gcc expects an entire array to be specified as a[0:N][0:M], while the
> benchmark uses a[N][M]. The latter is interpreted by gcc as a single array
> element (low bound N, length 1), and produces an error.
> 
> As usual, the "spec" isn't super clear but my reading of it tends to agree
> with gcc's. However, I suspect compatibility with other compilers is more
> important than what's in that document. I don't have any to test against
> unfortunately.
> 
> In any case, the following patch changes gcc's behaviour, if we think it
> should be changed. Not fully tested yet, in particular I have some issues
> with timeouts while running OpenACC offload tests.

I haven't studied OpenACC in this case, but a[N][M] in OpenMP array section
syntax is certainly the same thing as in C/C++ without any extensions, i.e.
an array element, to cover the whole array one needs to use e.g. a[:N][:M]
or a[0:N][0:M].  So your patch would certainly break OpenMP.

E.g. in
https://github.com/EPCCed/epcc-openacc-benchmarks/blob/master/level0.c
I see even in OpenACC the [0:len1][0:len2] syntax being used when they mean
the whole array.

	Jakub