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Lack of capability to represent arbitrary alias dependent information


HI,
GCC adds runtime alias checks for data references in passes like
vectorizer, it would be very useful to pass along the runtime alias
dependent information to later passes.  Given below expample:

int foo (int *a, int *b, int *c, int *d, int *e, int len, int v)
{
  int k, x;
  for (k = 0; k < len; k++)
    {
      c[k] = a[k-1] + b[k-1];
      if ((x = d[k-1] + e[k-1]) > c[k]) c[k] = x;
      if (c[k] < v) c[k] = v;
    }

  return 0;
}
After vectorizer's runtime alias check, we know that c doesn't alias
to a/b/d/e arrays.  This enables dead store elimination for c array.
The question is how to pass the information to dse (or predcom, etc.).
So far MR_DEPENDENCE_* is suggested, but I found it's not capable of
that.  The fundamental issue is MR_DEPENDENCE_* can only represent
alias relations between references in a strong connected component of
dependence graph, while in most cases (like this one) the dependence
graph is not SCC.  In general, if we have below dependence graph:
Dependence Graph: G<V, E>
V: {x(write), y(write), a(read), b(read)}
E: <x, a>
     <x, b>
     <y, a>
     <y, b>
Since a and b are reads, we don't need to know the relations between a
and b;  also it's possible to have any dependence relation between x
and y.  In this case, we can't assign x, a and b into one clique.  We
can't assign x and y into different clique either because they both
are not-aliasing to a/b.  As a matter of fact, we need a way to
represent arbitrary dependence graph, rather than SCC only.
So any suggestions?

Thanks,
bin


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