diff -uprN a/gcc/testsuite/gcc.dg/pr94574.c b/gcc/testsuite/gcc.dg/pr94574.c
--- a/gcc/testsuite/gcc.dg/pr94574.c	1970-01-01 00:00:00.000000000 +0000
+++ b/gcc/testsuite/gcc.dg/pr94574.c	2020-04-15 21:08:48.972000000 +0000
@@ -0,0 +1,15 @@
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+typedef unsigned int v4si __attribute__((vector_size(16)));
+typedef unsigned int v2si __attribute__((vector_size(8)));
+
+/* The aliasing is somewhat dubious here, but it must compile.  */
+
+v2si
+foo (v4si v)
+{
+  v2si res;
+  *(v4si *) &res = v;
+  return res;
+}
diff -uprN a/gcc/tree-ssa.c b/gcc/tree-ssa.c
--- a/gcc/tree-ssa.c	2020-03-31 01:51:30.000000000 +0000
+++ b/gcc/tree-ssa.c	2020-04-15 21:26:09.828000000 +0000
@@ -1528,7 +1528,9 @@ non_rewritable_lvalue_p (tree lhs)
 	  && known_gt (wi::to_poly_offset (TYPE_SIZE_UNIT (TREE_TYPE (decl))),
 		       mem_ref_offset (lhs))
 	  && multiple_of_p (sizetype, TREE_OPERAND (lhs, 1),
-			    TYPE_SIZE_UNIT (TREE_TYPE (lhs))))
+			    TYPE_SIZE_UNIT (TREE_TYPE (lhs)))
+	  && known_ge (wi::to_poly_offset (TYPE_SIZE (TREE_TYPE (decl))),
+		       wi::to_poly_offset (TYPE_SIZE (TREE_TYPE (lhs)))))
 	return false;
     }