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.

From-SVN: r256885

0 files changed, 0 insertions, 0 deletions