[2/2] Fix bogus inner induction (PR 86725)

[2/2] Fix bogus inner induction (PR 86725)

This patch is the second part of the fix for PR 86725.  The problem
in the original test is that for:

  outer1:
    x_1 = PHI <x_4(outer2), ...>;
    ...

  inner:
    x_2 = PHI <x_1(outer1), x_3(...)>;
    ...
    x_3 = ...;
    ...

  outer2:
    x_4 = PHI <x_3(inner)>;
    ...

there are corner cases in which it is possible to classify the
inner phi as an induction but not the outer phi.  The -4.c test
is a more direct example.

After failing to classify x_1 as an induction, we go on to
classify it as a double reduction (which is basically true).
But we still classified the inner phi as an induction rather
than as part of a reduction, leading to an ICE when trying
to vectorise the outer phi.

We analyse the phis for outer loops first, so the simplest
fix is not to classify the phi as an induction if outer loop
analysis said that it should be a reduction.

The -2.c test is from the original PR.  The -3.c test is a
version in which "wo" really is used a reduction; this was
already correctly rejected, but for the wrong reason ("inner-loop
induction only used outside of the outer vectorized loop").
The -4.c test is another way of tickling the original problem
without relying on the undefinedness of signed overflow.
The -5.c test shows an (uninteresting) example in which the
patch prevents a spurious failure to vectorise the outer loop.

2018-08-22  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR tree-optimization/86725
	* tree-vect-loop.c (vect_inner_phi_in_double_reduction_p): New
	function.
	(vect_analyze_scalar_cycles_1): Check it.

gcc/testsuite/
	PR tree-optimization/86725
	* gcc.dg/vect/no-scevccp-pr86725-2.c: New test.
	* gcc.dg/vect/no-scevccp-pr86725-3.c: Likewise.
	* gcc.dg/vect/no-scevccp-pr86725-4.c: Likewise.
	* gcc.dg/vect/no-scevccp-pr86725-5.c: Likewise.

From-SVN: r263774