[PATCH, 4/16] Implement -foffload-alias

Tom de Vries Tom_deVries@mentor.com
Thu Nov 12 16:04:00 GMT 2015


On 11/11/15 12:00, Jakub Jelinek wrote:
> On Wed, Nov 11, 2015 at 11:51:02AM +0100, Richard Biener wrote:
>>> The option -foffload-alias=pointer instructs the compiler to assume that
>>> objects references in an offload region do not alias.
>>>
>>> The option -foffload-alias=all instructs the compiler to make no
>>> assumptions about aliasing in offload regions.
>>>
>>> The default value is -foffload-alias=none.
>>
>> I think global options for this is nonsense.  Please follow what
>> we do for #pragma GCC ivdep for example, thus allow the alias
>> behavior to be specified per "region" (whatever makes sense here
>> in the context of offloading).

So, IIUC, instead of a global option foffload-alias, you're saying 
something like the following would be acceptable:
...
#pragma GCC offload-alias=<none|pointer|all>
#pragma acc kernels copyin (a[0:N], b[0:N]) copyout (c[0:N])
   {
     #pragma acc loop
     for (COUNTERTYPE ii = 0; ii < N; ii++)
       c[ii] = a[ii] + b[ii];
   }
...
?

I suppose that would work (though a global option would allow us to 
easily switch between none/pointer/all values in a large number of 
files, something that might be useful when f.i. running an openacc  test 
suite).

> Yeah, completely agreed.  I don't see why the offloaded region would be in
> any way special, they are C/C++/Fortran code as any other.
> What we can and should improve is teach IPA aliasing/points to analysis
> about the way we lower the host vs. offloading region boundary, so that
> if alias analysis on the caller of GOMP_target_ext/GOACC_parallel_keyed
> determines something it can be used on the offloaded function side and vice
> versa,

I agree this would be a nice way to solve the aliasing info problem, but 
considering the remark of Richard at 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=46032#c19 :
...
Not that I think IPA PTA is anywhere near production ready
...
I haven't considered proceeding in that direction.

Thanks,
- Tom

> but a switch like the above is just wrong.



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