Re: [WIP, OpenMP] OpenMP metadirectives support

[Kwok Cheung Yeung <kcy@codesourcery.com>]

Hello

Thanks for your reply.

On 26/07/2021 3:29 pm, Jakub Jelinek wrote:
> On Fri, Jul 09, 2021 at 12:16:15PM +0100, Kwok Cheung Yeung wrote:
>> 3) In the OpenMP examples (version 5.0.1), section 9.7, the example
>> metadirective.3.c does not work as expected.
>>
>> #pragma omp declare target
>> void exp_pi_diff(double *d, double my_pi){
>>     #pragma omp metadirective \
>>                 when( construct={target}: distribute parallel for ) \
>>                 default( parallel for simd)
>> ...
>> int main()
>> {
>>     ...
>>     #pragma omp target teams map(tofrom: d[0:N])
>>     exp_pi_diff(d,my_pi);
>>     ...
>>     exp_pi_diff(d,my_pi);
> 
> The spec says in this case that the target construct is added to the
> construct set because of the function appearing in between omp declare target
> and omp end declare target, so the above is something that resolves
> statically to distribute parallel for.
> It is true that in OpenMP 5.1 the earlier
> For functions within a declare target block, the target trait is added to the beginning of the
> set as c 1 for any versions of the function that are generated for target regions so the total size
> of the set is increased by 1.
> has been mistakenly replaced with:
> For device routines, the target trait is added to the beginning of the set as c 1 for any versions of
> the procedure that are generated for target regions so the total size of the set is increased by 1.
> by that has been corrected in 5.2:
> C/C++:
> For functions that are declared in a code region that is delimited by a declare target directive and
> its paired end directive, the target trait is added to the beginning of the set as c 1 for any target
> variants that result from the directive so the total size of the set is increased by one.
> Fortran:
> If a declare target directive appears in the specification part of a procedure or in the
> specification part of a procedure interface body, the target trait is added to the beginning of the
> set as c 1 for any target variants that result from the directive so the total size of the set is
> increased by one.
> 
> So, it is really a static decision that can be decided already during
> parsing.

In Section 1.2.2 of the OpenMP TR10 spec, 'target variant' is defined as:

A version of a device routine that can only be executed as part of a target region.

So isn't this really saying the same thing as the previous versions of the spec? 
The target trait is added to the beginning of the construct set _for any target 
variants_ that result from the directive (implying that it shouldn't be added 
for non-target variants). In this example, the same function exp_pi_diff is 
being used in both a target and non-target context, so shouldn't the 
metadirective resolve differently in the two contexts, independently of the 
function being declared in a 'declare target' block? If not, there does not seem 
to be much point in that example (in section 9.7 of the OpenMP Examples v5.0.1).

 From reading the spec, I infer that they expect the device and non-device 
versions of a function with 'declare target' to be separate, but that is not 
currently the case for GCC - on the host compiler, the same version of the 
function gets called in both target and non-target regions (though in the target 
region case, it gets called indirectly via a compiler-generated function with a 
name like main._omp_fn.0). The offload compiler gets its own streamed version, 
so there is no conflict there - by definition, its version must be in a target 
context.

Thanks,

Kwok