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, 47 insertions, 0 deletions

diff --git a/gcc/tree-vrp.c b/gcc/tree-vrp.c
index 3af81f7..cba58e0 100644
--- a/gcc/tree-vrp.c
+++ b/gcc/tree-vrp.c
@@ -171,6 +171,53 @@ vrp_val_is_min (const_tree val)
 	      && operand_equal_p (val, type_min, 0)));
 }
 
+/* VR_TYPE describes a range with mininum value *MIN and maximum
+   value *MAX.  Restrict the range to the set of values that have
+   no bits set outside NONZERO_BITS.  Update *MIN and *MAX and
+   return the new range type.
+
+   SGN gives the sign of the values described by the range.  */
+
+enum value_range_type
+intersect_range_with_nonzero_bits (enum value_range_type vr_type,
+				   wide_int *min, wide_int *max,
+				   const wide_int &nonzero_bits,
+				   signop sgn)
+{
+  if (vr_type == VR_RANGE)
+    {
+      *max = wi::round_down_for_mask (*max, nonzero_bits);
+
+      /* Check that the range contains at least one valid value.  */
+      if (wi::gt_p (*min, *max, sgn))
+	return VR_UNDEFINED;
+
+      *min = wi::round_up_for_mask (*min, nonzero_bits);
+      gcc_checking_assert (wi::le_p (*min, *max, sgn));
+    }
+  if (vr_type == VR_ANTI_RANGE)
+    {
+      *max = wi::round_up_for_mask (*max, nonzero_bits);
+
+      /* If the calculation wrapped, we now have a VR_RANGE whose
+	 lower bound is *MAX and whose upper bound is *MIN.  */
+      if (wi::gt_p (*min, *max, sgn))
+	{
+	  std::swap (*min, *max);
+	  *max = wi::round_down_for_mask (*max, nonzero_bits);
+	  gcc_checking_assert (wi::le_p (*min, *max, sgn));
+	  return VR_RANGE;
+	}
+
+      *min = wi::round_down_for_mask (*min, nonzero_bits);
+      gcc_checking_assert (wi::le_p (*min, *max, sgn));
+
+      /* Check whether we now have an empty set of values.  */
+      if (*min - 1 == *max)
+	return VR_UNDEFINED;
+    }
+  return vr_type;
+}
 
 /* Set value range VR to VR_UNDEFINED.  */