Index: gcc/wide-int.h
===================================================================
--- gcc/wide-int.h	2018-02-02 14:03:53.964530009 +0000
+++ gcc/wide-int.h	2018-02-02 14:03:54.185521788 +0000
@@ -3308,6 +3308,8 @@ gt_pch_nx (trailing_wide_ints <N> *, voi
   wide_int set_bit_in_zero (unsigned int, unsigned int);
   wide_int insert (const wide_int &x, const wide_int &y, unsigned int,
 		   unsigned int);
+  wide_int round_down_for_mask (const wide_int &, const wide_int &);
+  wide_int round_up_for_mask (const wide_int &, const wide_int &);
 
   template <typename T>
   T mask (unsigned int, bool);
Index: gcc/wide-int.cc
===================================================================
--- gcc/wide-int.cc	2018-02-02 14:03:53.964530009 +0000
+++ gcc/wide-int.cc	2018-02-02 14:03:54.185521788 +0000
@@ -2132,6 +2132,70 @@ wi::only_sign_bit_p (const wide_int_ref
   return only_sign_bit_p (x, x.precision);
 }
 
+/* Return VAL if VAL has no bits set outside MASK.  Otherwise round VAL
+   down to the previous value that has no bits set outside MASK.
+   This rounding wraps for signed values if VAL is negative and
+   the top bit of MASK is clear.
+
+   For example, round_down_for_mask (6, 0xf1) would give 1 and
+   round_down_for_mask (24, 0xf1) would give 17.  */
+
+wide_int
+wi::round_down_for_mask (const wide_int &val, const wide_int &mask)
+{
+  /* Get the bits in VAL that are outside the mask.  */
+  wide_int extra_bits = wi::bit_and_not (val, mask);
+  if (extra_bits == 0)
+    return val;
+
+  /* Get a mask that includes the top bit in EXTRA_BITS and is all 1s
+     below that bit.  */
+  unsigned int precision = val.get_precision ();
+  wide_int lower_mask = wi::mask (precision - wi::clz (extra_bits),
+				  false, precision);
+
+  /* Clear the bits that aren't in MASK, but ensure that all bits
+     in MASK below the top cleared bit are set.  */
+  return (val & mask) | (mask & lower_mask);
+}
+
+/* Return VAL if VAL has no bits set outside MASK.  Otherwise round VAL
+   up to the next value that has no bits set outside MASK.  The rounding
+   wraps if there are no suitable values greater than VAL.
+
+   For example, round_up_for_mask (6, 0xf1) would give 16 and
+   round_up_for_mask (24, 0xf1) would give 32.  */
+
+wide_int
+wi::round_up_for_mask (const wide_int &val, const wide_int &mask)
+{
+  /* Get the bits in VAL that are outside the mask.  */
+  wide_int extra_bits = wi::bit_and_not (val, mask);
+  if (extra_bits == 0)
+    return val;
+
+  /* Get a mask that is all 1s above the top bit in EXTRA_BITS.  */
+  unsigned int precision = val.get_precision ();
+  wide_int upper_mask = wi::mask (precision - wi::clz (extra_bits),
+				  true, precision);
+
+  /* Get the bits of the mask that are above the top bit in EXTRA_BITS.  */
+  upper_mask &= mask;
+
+  /* Conceptually we need to:
+
+     - clear bits of VAL outside UPPER_MASK
+     - add the lowest bit in UPPER_MASK to VAL (or add 0 if UPPER_MASK is 0)
+     - propagate the carry through the bits of VAL in UPPER_MASK
+
+     If (~VAL & UPPER_MASK) is nonzero, the carry eventually
+     reaches that bit and the process leaves all lower bits clear.
+     If (~VAL & UPPER_MASK) is zero then the result is also zero.  */
+  wide_int tmp = wi::bit_and_not (upper_mask, val);
+
+  return (val | tmp) & -tmp;
+}
+
 /*
  * Private utilities.
  */
@@ -2384,6 +2448,53 @@ test_overflow ()
     }
 }
 
+/* Test the round_{down,up}_for_mask functions.  */
+
+static void
+test_round_for_mask ()
+{
+  unsigned int prec = 18;
+  ASSERT_EQ (17, wi::round_down_for_mask (wi::shwi (17, prec),
+					  wi::shwi (0xf1, prec)));
+  ASSERT_EQ (17, wi::round_up_for_mask (wi::shwi (17, prec),
+					wi::shwi (0xf1, prec)));
+
+  ASSERT_EQ (1, wi::round_down_for_mask (wi::shwi (6, prec),
+					 wi::shwi (0xf1, prec)));
+  ASSERT_EQ (16, wi::round_up_for_mask (wi::shwi (6, prec),
+					wi::shwi (0xf1, prec)));
+
+  ASSERT_EQ (17, wi::round_down_for_mask (wi::shwi (24, prec),
+					  wi::shwi (0xf1, prec)));
+  ASSERT_EQ (32, wi::round_up_for_mask (wi::shwi (24, prec),
+					wi::shwi (0xf1, prec)));
+
+  ASSERT_EQ (0x011, wi::round_down_for_mask (wi::shwi (0x22, prec),
+					     wi::shwi (0x111, prec)));
+  ASSERT_EQ (0x100, wi::round_up_for_mask (wi::shwi (0x22, prec),
+					   wi::shwi (0x111, prec)));
+
+  ASSERT_EQ (100, wi::round_down_for_mask (wi::shwi (101, prec),
+					   wi::shwi (0xfc, prec)));
+  ASSERT_EQ (104, wi::round_up_for_mask (wi::shwi (101, prec),
+					 wi::shwi (0xfc, prec)));
+
+  ASSERT_EQ (0x2bc, wi::round_down_for_mask (wi::shwi (0x2c2, prec),
+					     wi::shwi (0xabc, prec)));
+  ASSERT_EQ (0x800, wi::round_up_for_mask (wi::shwi (0x2c2, prec),
+					   wi::shwi (0xabc, prec)));
+
+  ASSERT_EQ (0xabc, wi::round_down_for_mask (wi::shwi (0xabd, prec),
+					     wi::shwi (0xabc, prec)));
+  ASSERT_EQ (0, wi::round_up_for_mask (wi::shwi (0xabd, prec),
+				       wi::shwi (0xabc, prec)));
+
+  ASSERT_EQ (0xabc, wi::round_down_for_mask (wi::shwi (0x1000, prec),
+					     wi::shwi (0xabc, prec)));
+  ASSERT_EQ (0, wi::round_up_for_mask (wi::shwi (0x1000, prec),
+				       wi::shwi (0xabc, prec)));
+}
+
 /* Run all of the selftests within this file, for all value types.  */
 
 void
@@ -2393,6 +2504,7 @@ wide_int_cc_tests ()
   run_all_wide_int_tests <offset_int> ();
   run_all_wide_int_tests <widest_int> ();
   test_overflow ();
+  test_round_for_mask ();
 }
 
 } // namespace selftest
Index: gcc/tree-vrp.h
===================================================================
--- gcc/tree-vrp.h	2018-02-02 14:03:53.964530009 +0000
+++ gcc/tree-vrp.h	2018-02-02 14:03:54.184521826 +0000
@@ -61,6 +61,8 @@ extern void extract_range_from_unary_exp
 					   tree op0_type);
 
 extern bool vrp_operand_equal_p (const_tree, const_tree);
+extern enum value_range_type intersect_range_with_nonzero_bits
+  (enum value_range_type, wide_int *, wide_int *, const wide_int &, signop);
 
 struct assert_info
 {
Index: gcc/tree-vrp.c
===================================================================
--- gcc/tree-vrp.c	2018-02-02 14:03:53.964530009 +0000
+++ gcc/tree-vrp.c	2018-02-02 14:03:54.184521826 +0000
@@ -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.  */
 
Index: gcc/tree-data-ref.c
===================================================================
--- gcc/tree-data-ref.c	2018-02-02 14:03:53.964530009 +0000
+++ gcc/tree-data-ref.c	2018-02-02 14:03:54.184521826 +0000
@@ -721,7 +721,13 @@ split_constant_offset_1 (tree type, tree
 		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
 		   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]);
Index: gcc/testsuite/gcc.dg/vect/bb-slp-pr81635-3.c
===================================================================
--- /dev/null	2018-02-02 09:03:36.168354735 +0000
+++ gcc/testsuite/gcc.dg/vect/bb-slp-pr81635-3.c	2018-02-02 14:03:54.183521863 +0000
@@ -0,0 +1,62 @@
+/* { dg-do compile } */
+/* { dg-additional-options "-fno-tree-loop-vectorize" } */
+/* { dg-require-effective-target vect_double } */
+/* { dg-require-effective-target lp64 } */
+
+void
+f1 (double *p, double *q, unsigned int n)
+{
+  p = (double *) __builtin_assume_aligned (p, sizeof (double) * 2);
+  q = (double *) __builtin_assume_aligned (q, sizeof (double) * 2);
+  for (unsigned int i = 0; i < n; i += 4)
+    {
+      double a = q[i] + p[i];
+      double b = q[i + 1] + p[i + 1];
+      q[i] = a;
+      q[i + 1] = b;
+    }
+}
+
+void
+f2 (double *p, double *q, unsigned int n)
+{
+  p = (double *) __builtin_assume_aligned (p, sizeof (double) * 2);
+  q = (double *) __builtin_assume_aligned (q, sizeof (double) * 2);
+  for (unsigned int i = 0; i < n; i += 2)
+    {
+      double a = q[i] + p[i];
+      double b = q[i + 1] + p[i + 1];
+      q[i] = a;
+      q[i + 1] = b;
+    }
+}
+
+void
+f3 (double *p, double *q, unsigned int n)
+{
+  p = (double *) __builtin_assume_aligned (p, sizeof (double) * 2);
+  q = (double *) __builtin_assume_aligned (q, sizeof (double) * 2);
+  for (unsigned int i = 0; i < n; i += 6)
+    {
+      double a = q[i] + p[i];
+      double b = q[i + 1] + p[i + 1];
+      q[i] = a;
+      q[i + 1] = b;
+    }
+}
+
+void
+f4 (double *p, double *q, unsigned int start, unsigned int n)
+{
+  p = (double *) __builtin_assume_aligned (p, sizeof (double) * 2);
+  q = (double *) __builtin_assume_aligned (q, sizeof (double) * 2);
+  for (unsigned int i = start & -2; i < n; i += 2)
+    {
+      double a = q[i] + p[i];
+      double b = q[i + 1] + p[i + 1];
+      q[i] = a;
+      q[i + 1] = b;
+    }
+}
+
+/* { dg-final { scan-tree-dump-times "basic block vectorized" 4 "slp1" } } */
Index: gcc/testsuite/gcc.dg/vect/bb-slp-pr81635-4.c
===================================================================
--- /dev/null	2018-02-02 09:03:36.168354735 +0000
+++ gcc/testsuite/gcc.dg/vect/bb-slp-pr81635-4.c	2018-02-02 14:03:54.183521863 +0000
@@ -0,0 +1,47 @@
+/* { dg-do compile } */
+/* { dg-additional-options "-fno-tree-loop-vectorize" } */
+/* { dg-require-effective-target lp64 } */
+
+void
+f1 (double *p, double *q, unsigned int n)
+{
+  p = (double *) __builtin_assume_aligned (p, sizeof (double) * 2);
+  q = (double *) __builtin_assume_aligned (q, sizeof (double) * 2);
+  for (unsigned int i = 0; i < n; i += 1)
+    {
+      double a = q[i] + p[i];
+      double b = q[i + 1] + p[i + 1];
+      q[i] = a;
+      q[i + 1] = b;
+    }
+}
+
+void
+f2 (double *p, double *q, unsigned int n)
+{
+  p = (double *) __builtin_assume_aligned (p, sizeof (double) * 2);
+  q = (double *) __builtin_assume_aligned (q, sizeof (double) * 2);
+  for (unsigned int i = 0; i < n; i += 3)
+    {
+      double a = q[i] + p[i];
+      double b = q[i + 1] + p[i + 1];
+      q[i] = a;
+      q[i + 1] = b;
+    }
+}
+
+void
+f3 (double *p, double *q, unsigned int start, unsigned int n)
+{
+  p = (double *) __builtin_assume_aligned (p, sizeof (double) * 2);
+  q = (double *) __builtin_assume_aligned (q, sizeof (double) * 2);
+  for (unsigned int i = start; i < n; i += 2)
+    {
+      double a = q[i] + p[i];
+      double b = q[i + 1] + p[i + 1];
+      q[i] = a;
+      q[i + 1] = b;
+    }
+}
+
+/* { dg-final { scan-tree-dump-not "basic block vectorized" "slp1" } } */
