Use nonzero bits to refine range in split_constant_offset (PR 81635)

This patch is part 2 of the fix for PR 81635.  It means that
split_constant_offset can handle loops like:

  for (unsigned int i = 0; i < n; i += 4)
    {
      a[i] = ...;
      a[i + 1] = ...;
    }

CCP records that "i" must have its low 2 bits clear, but we don't
include this information in the range of "i", which remains [0, +INF].
I tried making set_nonzero_bits update the range info in the same
way that set_range_info updates the nonzero bits, but it regressed
cases like vrp117.c and made some other tests worse.

vrp117.c has a multiplication by 10, so CCP can infer that the low bit
of the result is clear.  If we included that in the range, the range
would go from [-INF, +INF] to [-INF, not-quite-+INF].  However,
the multiplication is also known to overflow in all cases, so VRP
saturates the result to [INT_MAX, INT_MAX].  This obviously creates a
contradiction with the nonzero bits, and intersecting the new saturated
range with an existing not-quite-+INF range would make us drop to
VR_UNDEFINED.  We're prepared to fold a comparison with an [INT_MAX,
INT_MAX] value but not with a VR_UNDEFINED value.

The other problems were created when intersecting [-INF, not-quite-+INF]
with a useful VR_ANTI_RANGE like ~[-1, 1].  The intersection would
keep the former range rather than the latter.

The patch therefore keeps the adjustment local to split_constant_offset
for now, but adds a helper routine so that it's easy to move this later.

2018-02-08  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR tree-optimization/81635
	* wide-int.h (wi::round_down_for_mask, wi::round_up_for_mask): Declare.
	* wide-int.cc (wi::round_down_for_mask, wi::round_up_for_mask)
	(test_round_for_mask): New functions.
	(wide_int_cc_tests): Call test_round_for_mask.
	* tree-vrp.h (intersect_range_with_nonzero_bits): Declare.
	* tree-vrp.c (intersect_range_with_nonzero_bits): New function.
	* tree-data-ref.c (split_constant_offset_1): Use it to refine the
	range returned by get_range_info.

gcc/testsuite/
	PR tree-optimization/81635
	* gcc.dg/vect/bb-slp-pr81635-3.c: New test.
	* gcc.dg/vect/bb-slp-pr81635-4.c: Likewise.

From-SVN: r257491

1 files changed, 7 insertions, 2 deletions

diff --git a/gcc/tree-data-ref.c b/gcc/tree-data-ref.c
index f3070d3..fdb2ac1 100644
--- a/gcc/tree-data-ref.c
+++ b/gcc/tree-data-ref.c
@@ -721,7 +721,13 @@ split_constant_offset_1 (tree type, tree op0, enum tree_code code, tree op1,
 		if (TREE_CODE (tmp_var) != SSA_NAME)
 		  return false;
 		wide_int var_min, var_max;
-		if (get_range_info (tmp_var, &var_min, &var_max) != VR_RANGE)
+		value_range_type vr_type = get_range_info (tmp_var, &var_min,
+							   &var_max);
+		wide_int var_nonzero = get_nonzero_bits (tmp_var);
+		signop sgn = TYPE_SIGN (itype);
+		if (intersect_range_with_nonzero_bits (vr_type, &var_min,
+						       &var_max, var_nonzero,
+						       sgn) != VR_RANGE)
 		  return false;
 
 		/* See whether the range of OP0 (i.e. TMP_VAR + TMP_OFF)
@@ -729,7 +735,6 @@ split_constant_offset_1 (tree type, tree op0, enum tree_code code, tree op1,
 		   operations done in ITYPE.  The addition must overflow
 		   at both ends of the range or at neither.  */
 		bool overflow[2];
-		signop sgn = TYPE_SIGN (itype);
 		unsigned int prec = TYPE_PRECISION (itype);
 		wide_int woff = wi::to_wide (tmp_off, prec);
 		wide_int op0_min = wi::add (var_min, woff, sgn, &overflow[0]);