[Openmp-commits] [PATCH] D110193: [RFC] Initial documentation for declare target indirect support.

[Vyacheslav Zakharin via Phabricator via Openmp-commits]

vzakhari added inline comments.


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Comment at: openmp/libomptarget/docs/declare_target_indirect.md:39
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+Since all pointers are supposed to be translated/mapped, when program does not use **required unified_shared_memory**, it is possible to avoid generating the runtime dispatch code for indirect function calls. The mapping between host and device address of an indirect function will be established by `libomptarget` during processing of the offload entries table.
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dreachem wrote:
> vzakhari wrote:
> > RaviNarayanaswamy wrote:
> > > vzakhari wrote:
> > > > jdoerfert wrote:
> > > > > How would we translate/map all pointers? I fail to see how we could even identify all (function) pointers (see below). Or did we add wording to the standard to forbid this?
> > > > > 
> > > > > ```
> > > > > char data[sizeof(fp)];
> > > > > memcpy(data, &fp, sizeof(fp));
> > > > > 
> > > > > #pragma omp target map(to:data[:sizeof(fp)])
> > > > > {
> > > > >   void (*fpc)();
> > > > >   memcpy(&fpc, data, sizeof(fp));
> > > > >   fpc(); 
> > > > > }
> > > > > ```
> > > > > 
> > > > > Modulo my syntax errors ;)
> > > > I agree that in this case there will be no translation.  I will remove the words about non-unified_shared_memory "optimization".  Does it sound right?
> > > > I agree that in this case there will be no translation.  I will remove the words about non-unified_shared_memory "optimization".  Does it sound right?
> > > 
> > > We should get the spec to disallow copy/casting of host pointers and using it  to allow the optimization.
> > I can move this to a TODO section at the end of the doc.  Let's agree on the general case, first.
> > > I agree that in this case there will be no translation.  I will remove the words about non-unified_shared_memory "optimization".  Does it sound right?
> > 
> > We should get the spec to disallow copy/casting of host pointers and using it  to allow the optimization.
> 
> This restriction was added for 5.2:  "A program must not rely on the value of a function address in a **target** region except for assignments, comparisons to zero and indirect calls."
> 
> So, you can copy pointers around, as in the example posted. You want to forbid this as well?
I think Ravi only talked about disallowing verbatim copies of function addresses at the boundary of target regions, which will allow the //optimization//.  @RaviNarayanaswamy, can you please reply to the comment?


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  https://reviews.llvm.org/D110193/new/

https://reviews.llvm.org/D110193