Check whether any statements need masking (PR 83922)

Check whether any statements need masking (PR 83922)

This PR is an odd case in which, due to the low optimisation level,
we enter vectorisation with:

  outer1:
    x_1 = PHI <x_3(outer2), ...>;
    ...

  inner:
    x_2 = 0;
    ...

  outer2:
    x_3 = PHI <x_2(inner)>;

These statements are tentatively treated as a double reduction by
vect_force_simple_reduction, but in the end only x_3 and x_2 are marked
as relevant.  vect_analyze_loop_operations skips over x_3, leaving the
vectorizable_reduction check to a presumed future test of x_1, which
in this case never happens.  We therefore end up vectorising x_2 only
(complete with peeling for niters!) and leave the scalar x_3 in place.

This caused a segfault in the support for fully-masked loops,
since there were no statements that needed masking.  Fixed by
checking for that.

But I think this is also a flaw in vect_analyze_loop_operations.
Outer loop vectorisation reduces the number of times that the
inner loop is executed, so it wouldn't necessarily be valid
to leave the scalar x_3 in place for all vectorisable x_2.
There's already code to forbid that when x_1 isn't present:

              /* FORNOW: we currently don't support the case that these phis
                 are not used in the outerloop (unless it is double reduction,
                 i.e., this phi is vect_reduction_def), cause this case
                 requires to actually do something here.  */

I think we need to do the same if x_1 is present but not relevant.

2018-01-19  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR tree-optimization/83922
	* tree-vect-loop.c (vect_verify_full_masking): Return false if
	there are no statements that need masking.
	(vect_active_double_reduction_p): New function.
	(vect_analyze_loop_operations): Use it when handling phis that
	are not in the loop header.

gcc/testsuite/
	PR tree-optimization/83922
	* gcc.dg/pr83922.c: New test.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@256885 138bc75d-0d04-0410-961f-82ee72b054a4