Preliminary patch for PR23820 and PR24309

[Sebastian Pop]

On 10/12/07, Sebastian Pop <sebpop@gmail.com> wrote:
> True, if I'm dumping the value ranges that VRP ends with, we have
>
> N.0_28: [1, +INF]  EQUIVALENCES: { N.0_8 } (1 elements)
>
> that is enough to deduce that "if (N != 0)" is true.  I'm looking into
> why VRP doesn't trigger the elimination of the condition.
>

That's really strange, as after first VRP we don't have any N!=0
condition.  I suppose that DCE is in fault there: in passes.c we have
the following sequence around the first VRP:

      NEXT_PASS (pass_vrp);
      NEXT_PASS (pass_dce);
      NEXT_PASS (pass_cselim);

Now if I'm duplicating VRP just after DCE,

      NEXT_PASS (pass_vrp);
      NEXT_PASS (pass_dce);
      NEXT_PASS (pass_vrp);
      NEXT_PASS (pass_cselim);

the N!=0 in the outer loop disappears, and the ltrans-3.c testcase
passes with the stricter definition of the perfect nested loop.

Sebastian