Re: "omp declare target" on DECL_EXTERNAL vars

[Alexander Monakov <amonakov at ispras dot ru>]

On Fri, 20 May 2016, Jakub Jelinek wrote:
> but that made me think on what handling do we want for the
> "omp declare target" DECL_EXTERNAL vars.
[snip]
> In the C/C++ FEs, we set not just node->offloadable, but also for
> ENABLE_OFFLOADING g->have_offload and offload_vars too.  Wonder if that
> means we register even non-local vars, that would be IMHO a bug.

(it's unclear to me what you mean by 'non-local vars' here, from the context
it looks like it's 'variables with an external declaration and no definition
in the current TU'; correct?)

Looking at the OpenMP 4.5 spec, there's a requirement that

    [2.10.6 declare target directive, Restrictions, C/C++]
    * All declarations and definitions for a function must have a declare
    target directive if one is specified for any of them. Otherwise, the
    result is unspecified.

(why are variables exempted?)

A natural way to conform to that requirement is to have a '#pragma omp declare
target' in the header file declaring the offloaded function. But that means
every TU that includes that header will have g->have_offload set, even if
otherwise it doesn't touch OpenMP at all.

So from that perspective it's undesirable to have 'omp declare target' on
declarations that don't define anything.

> On the other side, we need to watch for an extern declaration
> of a VAR_DECL marked for offloading and only later on locally defined,
> in that case if we haven't set g->have_offload and added entry to
> offload_vars, we'd need to do it when merging the extern decl with the
> definition.

Yes, but I wonder if setting g->have_offload etc. in the front-ends is the
right thing to do at all.  Shouldn't frontends simply set 'omp declare target'
and leave the rest to omp-low?

Hope that's constructive.
Alexander