diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index bbcff7463e6..76944d5ea01 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,11 @@
+2017-07-02  Richard Sandiford  <richard.sandiford@linaro.org>
+
+	PR tree-optimization/81136
+	* tree-vect-data-refs.c (vect_update_misalignment_for_peel): Only
+	assert that two references with the same misalignment have the same
+	compile-time misalignment if those compile-time misalignments
+	are known.
+
 2017-07-01  Andi Kleen  <ak@linux.intel.com>
 
         * print-tree.c (print_node): Print all attributes.
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index c71540aa811..46259217cc0 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2017-07-02  Richard Sandiford  <richard.sandiford@linaro.org>
+
+	PR tree-optimization/81136
+	* gcc.dg/vect/pr81136.c: New test.
+
 2017-07-01  Jakub Jelinek  <jakub@redhat.com>
 
 	PR sanitizer/81262
diff --git a/gcc/testsuite/gcc.dg/vect/pr81136.c b/gcc/testsuite/gcc.dg/vect/pr81136.c
new file mode 100644
index 00000000000..24bd8fa3b19
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/pr81136.c
@@ -0,0 +1,16 @@
+/* { dg-do compile } */
+
+struct __attribute__((aligned (32)))
+{
+  char misaligner;
+  int foo[100];
+  int bar[100];
+} *a;
+
+void
+fn1 (int n)
+{
+  int *b = a->foo;
+  for (int i = 0; i < n; i++)
+    a->bar[i] = b[i];
+}
diff --git a/gcc/tree-vect-data-refs.c b/gcc/tree-vect-data-refs.c
index 623acf695ed..1595bb4247c 100644
--- a/gcc/tree-vect-data-refs.c
+++ b/gcc/tree-vect-data-refs.c
@@ -906,8 +906,10 @@ vect_update_misalignment_for_peel (struct data_reference *dr,
     {
       if (current_dr != dr)
         continue;
-      gcc_assert (DR_MISALIGNMENT (dr) / dr_size ==
-                  DR_MISALIGNMENT (dr_peel) / dr_peel_size);
+      gcc_assert (!known_alignment_for_access_p (dr)
+		  || !known_alignment_for_access_p (dr_peel)
+		  || (DR_MISALIGNMENT (dr) / dr_size
+		      == DR_MISALIGNMENT (dr_peel) / dr_peel_size));
       SET_DR_MISALIGNMENT (dr, 0);
       return;
     }