[RFC] #pragma ivdep

[Hal Finkel]

Tobi, Paul,

Did anything ever happen with this?

 -Hal

----- Original Message -----
> On 03/12/2013 09:42 PM, Redmond, Paul wrote:
> >
> > On 2013-03-07, at 4:57 AM, Tobias Grosser wrote:
> >
> >> On 03/07/2013 12:35 AM, Redmond, Paul wrote:
> >>>
> >>> On 2013-03-06, at 3:42 PM, Pekka Jääskeläinen wrote:
> >>>
> >>>> Hi Paul,
> >>>>
> >>>> On 03/06/2013 10:11 PM, Redmond, Paul wrote:
> >>>>> I have updated the patch to not add metadata on loads and
> >>>>> stores where the
> >>>>> pointer comes from an alloca. I wonder if the check should be
> >>>>> more
> >>>>> conservative and only include pointers coming from Arguments
> >>>>> and GlobalValues
> >>>>> (perhaps Constants too?)
> >>>>
> >>>> I think it's safer that way around for now.
> >>>
> >>> Hmm.. I'm not sure that this approach (or my original) is general
> >>> enough. Here's a simple loop and the corresponding IR as
> >>> generated by clang:
> >>
> >> Before taking decisions on what exactly we need to do during code
> >> generation, I would appreciate if we could write down the exact
> >> semantics of #pragma ivdep on the C level. We should get a list
> >> of interesting test cases and investigate what other compilers
> >> gcc and icc do in such cases. Pekka and me gave a couple of test
> >> cases, that may be a good start.
> >>
> >
> > I haven't been able to find a detailed description of #pragma ivdep
> > semantics other than "ignore assumed vector dependencies" which is
> > pretty vague.
> 
> OK. That's the same I found.
> 
> In the absence of good documentation, it seems we need to generate
> this
> documentation ourselves. What about starting from very conservative
> examples, investigate for those examples what gcc and icc do and
> follow
> their behavior.
> 
> > I think the most basic semantic requirement at the C level is that
> > the loop:
> >
> > for (i...) {
> >    A;
> >    B;
> >    C;
> >    ...
> > }
> >
> > may be distributed as (without reordering statements):
> >
> > for (i...) A;
> > for (i...) B;
> > for (i...) C;
> > for (i...) ...
> >
> > to produce the same result.
> 
> I do not get this example. Can A, B, ... be arbitrary statements?
> 
> > Regarding the case:
> >
> > #pragma ivdep
> > for (i = 0; i < 100; i++) {
> >    long t[100];
> >    t[0] = i + 2;
> >    A[i] = A[i+b] + t[0];
> > }
> >
> > ICC is supposed to privatize t. I'm currently investigating how to
> > handle this during codegen--how to decide when to add the metadata
> > and when not to.
> 
> What about not adding meta-data, except if the loads/stores are
> explicit
> array references in the C code for which it is trivial to prove these
> references where allocated outside of the loop (globals, function
> parameters, stack allocated arrays).
> 
> This is obviously very conservative, but it seems to be correct. We
> could commit this support and start to gradually extend this feature.
> 
> Cheers,
> Tobias
> 
> 
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory