[llvm-commits] [PATCH] SIV tests in LoopDependenceAnalysis

[Sanjoy Das]

Hi Hal!

Thanks for the comments.  I have removed the #include <iostream> and
the const change (I had overlooked it in the earlier patches).

> What happens for nested loops?

For nested loops we check each subscript for dependence.  If we have
independence in any one subscript, the entire access is independent.
For instance:

for (i = 0 to n) {
  a [5][i + 1] = a [4][i];
}

There can never be a dependency since for no `i' is the tuple (5, i +
1) equal to the tuple (4, i).  This index by index comparison happens
in LoopDependenceAnalysis::analysePair.

In an individual SIV test we check for only one pair of subscripts.
So, analyzing the above example would involve analyzing the dependency
between [5 and 4] and the dependency between [(i + 1) and i].

SIV analyzes pairs with a total of one induction variable between
them, so it can analyze pairs like [5 and (i + 3)] and [(i + 2) and (2
* i - 1)].  The snippet

const Loop *theLoop; // There is only one loop we're concerned about.
{
  DenseSet<const Loop*> loops;
  getLoops(A, &loops);
  getLoops(B, &loops);
  assert(loops.size() == 1 && "Expected single loop in SIV test!");
  theLoop = *loops.begin();
}

finds this single loop (and hence single induction variable) in the
two subscript expressions.

A more general case is an MIV subscript, where a subscript pair has a
total of more than one loop induction variable.  This isn't
implemented yet.

> Also, it might be a good idea to break up analyseSIV in to multiple
> functions, one for each case being tested.

I refrained from doing this because it would involve passing around
too much information and the resultant functions won't have any use
anywhere else.

> Do you have any ideas about how to test this? Maybe that can wait until
> something actually uses the analysis.

I think the current approach (the second patch) is okayish -- have opt
dump information into stdout which FileCheck then compares.  I agree
that once something actually uses LDA, we'll have better testing
opportunities.

More importantly, I think a next good step would be calculate the
direction of the loop-carried dependencies so that things like loop
interchange can be implemented.

Thanks!
--
Sanjoy Das.
http://playingwithpointers.com
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