diff --git a/gcc/testsuite/gcc.dg/builtin-dynamic-object-size-0.c b/gcc/testsuite/gcc.dg/builtin-dynamic-object-size-0.c
index 2fca0a9c5b4..e5dc23a908d 100644
--- a/gcc/testsuite/gcc.dg/builtin-dynamic-object-size-0.c
+++ b/gcc/testsuite/gcc.dg/builtin-dynamic-object-size-0.c
@@ -323,6 +323,34 @@ test_substring (size_t sz, size_t off)
   return __builtin_dynamic_object_size (&str[off], 0);
 }
 
+struct S2
+{
+  char arr[7];
+};
+
+struct S1
+{
+  int pad;
+  struct S2 s2;
+};
+
+static long
+g (struct S1 *s1)
+{
+  struct S2 *s2 = &s1->s2;
+  return __builtin_dynamic_object_size (s2->arr, 0);
+}
+
+long
+__attribute__ ((noinline))
+test_alloc_nested_structs (int x)
+{
+  struct S1 *s1 = __builtin_malloc (x);
+  return g (s1);
+}
+
+/* POINTER_PLUS expressions.  */
+
 size_t
 __attribute__ ((noinline))
 test_substring_ptrplus (size_t sz, size_t off)
@@ -342,6 +370,8 @@ test_substring_ptrplus2 (size_t sz, size_t off, size_t off2)
   return __builtin_dynamic_object_size (ptr + off2, 0);
 }
 
+/* Function parameters.  */
+
 size_t
 __attribute__ ((access (__read_write__, 1, 2)))
 __attribute__ ((noinline))
@@ -382,6 +412,8 @@ test_parmsz_unknown (void *obj, void *unknown, size_t sz, int cond)
   return __builtin_dynamic_object_size (cond ? obj : unknown, 0);
 }
 
+/* Loops.  */
+
 size_t
 __attribute__ ((noinline))
 __attribute__ ((access (__read_write__, 1, 2)))
@@ -491,6 +523,8 @@ main (int argc, char **argv)
     FAIL ();
   if (test_dynarray_cond (1) != 8)
     FAIL ();
+  if (test_alloc_nested_structs (42) != 42 - sizeof (int))
+    FAIL ();
   if (test_deploop (128, 4) != 128)
     FAIL ();
   if (test_deploop (128, 129) != 32)
diff --git a/gcc/tree-object-size.cc b/gcc/tree-object-size.cc
index 9728f79da75..b0b50774936 100644
--- a/gcc/tree-object-size.cc
+++ b/gcc/tree-object-size.cc
@@ -371,8 +371,8 @@ size_for_offset (tree sz, tree offset, tree wholesize = NULL_TREE)
     }
 
   /* Safe to convert now, since a valid net offset should be non-negative.  */
-  if (!types_compatible_p (TREE_TYPE (offset), sizetype))
-    fold_convert (sizetype, offset);
+  if (!useless_type_conversion_p (sizetype, TREE_TYPE (offset)))
+    offset = fold_convert (sizetype, offset);
 
   if (TREE_CODE (offset) == INTEGER_CST)
     {