PR tree-optimization/71831 - __builtin_object_size poor results with no

PR tree-optimization/71831 - __builtin_object_size poor results with no
	optimization

gcc/testsuite/ChangeLog:

	PR tree-optimization/71831
	* gcc.dg/builtin-object-size-16.c: New test.
	* gcc.dg/builtin-object-size-17.c: New test.

gcc/ChangeLog:

	PR tree-optimization/71831
	* tree-object-size.h: Return bool instead of the size and add
	argument for the size.
	* tree-object-size.c (compute_object_offset): Update signature.
	(addr_object_size): Same.
	(compute_builtin_object_size): Return bool instead of the size
	and add argument for the size.  Handle POINTER_PLUS_EXPR when
	optimization is disabled.
	(expr_object_size): Adjust.
	(plus_stmt_object_size): Adjust.
	(pass_object_sizes::execute): Adjust.
	* builtins.c (fold_builtin_object_size): Adjust.
	* doc/extend.texi (Object Size Checking): Update.
	* ubsan.c (instrument_object_size): Adjust.

From-SVN: r239953

gcc/ChangeLog

 2016-09-01 Martin Sebor  <msebor@redhat.com>
 
+	PR tree-optimization/71831
+	* tree-object-size.h: Return bool instead of the size and add
+	argument for the size.
+	* tree-object-size.c (compute_object_offset): Update signature.
+	(addr_object_size): Same.
+	(compute_builtin_object_size): Return bool instead of the size
+	and add argument for the size.  Handle POINTER_PLUS_EXPR when
+	optimization is disabled.
+	(expr_object_size): Adjust.
+	(plus_stmt_object_size): Adjust.
+	(pass_object_sizes::execute): Adjust.
+	* builtins.c (fold_builtin_object_size): Adjust.
+	* doc/extend.texi (Object Size Checking): Update.
+	* ubsan.c (instrument_object_size): Adjust.
+
+2016-09-01 Martin Sebor  <msebor@redhat.com>
+
 	* genmatch.c (parser::parse_expr): Increase buffer size to guarantee
 	it fits the output of the formatted function regardless of its
 	arguments.

gcc/builtins.c

@@ -9615,7 +9615,7 @@ fold_builtin_object_size (tree ptr, tree ost)
 
   if (TREE_CODE (ptr) == ADDR_EXPR)
     {
-      bytes = compute_builtin_object_size (ptr, object_size_type);
+      compute_builtin_object_size (ptr, object_size_type, &bytes);
       if (wi::fits_to_tree_p (bytes, size_type_node))
 	return build_int_cstu (size_type_node, bytes);
     }
@@ -9624,8 +9624,7 @@ fold_builtin_object_size (tree ptr, tree ost)
       /* If object size is not known yet, delay folding until
        later.  Maybe subsequent passes will help determining
        it.  */
-      bytes = compute_builtin_object_size (ptr, object_size_type);
-      if (bytes != (unsigned HOST_WIDE_INT) (object_size_type < 2 ? -1 : 0)
+      if (compute_builtin_object_size (ptr, object_size_type, &bytes)
 	  && wi::fits_to_tree_p (bytes, size_type_node))
 	return build_int_cstu (size_type_node, bytes);
     }

gcc/doc/extend.texi

@@ -10030,8 +10030,15 @@ __atomic_store_n(&lockvar, 0, __ATOMIC_RELEASE|__ATOMIC_HLE_RELEASE);
 @findex __builtin___fprintf_chk
 @findex __builtin___vfprintf_chk
 
-GCC implements a limited buffer overflow protection mechanism
-that can prevent some buffer overflow attacks.
+GCC implements a limited buffer overflow protection mechanism that can
+prevent some buffer overflow attacks by determining the sizes of objects
+into which data is about to be written and preventing the writes when
+the size isn't sufficient.  The built-in functions described below yield
+the best results when used together and when optimization is enabled.
+For example, to detect object sizes across function boundaries or to
+follow pointer assignments through non-trivial control flow they rely
+on various optimization passes enabled with @option{-O2}.  However, to
+a limited extent, they can be used without optimization as well.
 
 @deftypefn {Built-in Function} {size_t} __builtin_object_size (void * @var{ptr}, int @var{type})
 is a built-in construct that returns a constant number of bytes from

gcc/testsuite/ChangeLog

+2016-09-01 Martin Sebor  <msebor@redhat.com>
+
+	PR tree-optimization/71831
+	* gcc.dg/builtin-object-size-16.c: New test.
+	* gcc.dg/builtin-object-size-17.c: New test.
+
 2016-09-01  Jerry DeLisle  <jvdelisle@gcc.gnu.org>
 
 	PR libgfortran/77393

gcc/testsuite/gcc.dg/builtin-object-size-16.c

+/* PR 71831 - __builtin_object_size poor results with no optimization
+   Verify that even without optimization __builtin_object_size returns
+   a meaningful result for a subset of simple expressins.  In cases
+   where the result could not easily be made to match the one obtained
+   with optimization the built-in was made to fail instead.  */
+/* { dg-do run } */
+/* { dg-options "-O0" } */
+
+static int nfails;
+
+#define TEST_FAILURE(line, obj, type, expect, result)		\
+  __builtin_printf ("FAIL: line %i: __builtin_object_size("	\
+		    #obj ", %i) == %zu, got %zu\n",		\
+		    line, type, expect, result), ++nfails
+
+#define bos(obj, type) __builtin_object_size (obj, type)
+#define size(obj, n) ((size_t)n == X ? sizeof *obj : (size_t)n)
+
+#define test(expect, type, obj)						\
+  do {									\
+    if (bos (obj, type)	!= size (obj, expect))				\
+      TEST_FAILURE (__LINE__, obj, type, size (obj, expect), bos (obj, type)); \
+  } while (0)
+
+#define T(r0, r1, r2, r3, obj)			\
+  do {						\
+    test (r0, 0, obj);				\
+    test (r1, 1, obj);				\
+    test (r2, 2, obj);				\
+    test (r3, 3, obj);				\
+  } while (0)
+
+/* For convenience.  Substitute for 'sizeof object' in test cases where
+   the size can vary from target to target.  */
+#define X  (size_t)0xdeadbeef
+
+/* __builtin_object_size checking results are inconsistent for equivalent
+   expressions (see bug 71831).  To avoid having duplicate the inconsistency
+   at -O0 the built-in simply fails.  The results hardcoded in this test
+   are those obtained with optimization (for easy comparison) but without
+   optimization the macros below turn them into expected failures .  */
+#if __OPTIMIZE__
+#  define F0(n)    n
+#  define F1(n)    n
+#  define F2(n)    n
+#  define F3(n)    n
+#else
+#  define F0(n)   -1
+#  define F1(n)   -1
+#  define F2(n)    0
+#  define F3(n)    0
+#endif
+
+typedef __SIZE_TYPE__ size_t;
+
+extern char ax[];
+char ax2[];               /* { dg-warning "assumed to have one element" } */
+
+extern char a0[0];
+static char a1[1];
+static char a2[2];
+static char a9[9];
+
+#if __SIZEOF_SHORT__ == 4
+extern short ia0[0];
+static short ia1[1];
+static short ia9[9];
+#elif __SIZEOF_INT__ == 4
+extern int ia0[0];
+static int ia1[1];
+static int ia9[9];
+#endif
+
+static char a2x2[2][2];
+static char a3x5[3][5];
+
+struct Sx { char n, a[]; } sx;
+struct S0 { char n, a[0]; } s0;
+struct S1 { char n, a[1]; } s1;
+struct S2 { char n, a[2]; } s2;
+struct S9 { char n, a[9]; } s9;
+
+struct S2x2 { char n, a[2][2]; } s2x2;
+struct S3x5 { char n, a[3][5]; } s3x5;
+
+static __attribute__ ((noclone, noinline)) void
+test_arrays ()
+{
+  T (    -1,      -1,       0,       0,   ax);
+
+  T (     0,       0,       0,       0,   a0);
+  T (     1,       1,       1,       1,   ax2);
+
+  T (     1,       1,       1,       1,   a1);
+  T (     2,       2,       2,       2,   a2);
+  T (     9,       9,       9,       9,   a9);
+
+  T (     0,       0,       0,       0,   a0);
+  T (     1,       1,       1,       1,   ax2);
+
+  T (     0,       0,       0,       0,   ia0);
+  T (     4,       4,       4,       4,   ia1);
+  T (    36,      36,      36,      36,   ia9);
+
+  /* Not all results for multidimensional arrays make sense (see
+     bug 77293).  The expected results below simply reflect those
+     obtained at -O2 (modulo the known limitations at -O1).  */
+  T (     4,       4,       4,       4,   a2x2);
+  T (     4,       4,       4,       4,   &a2x2[0]);
+  T (     4,       2,       4,       2,   &a2x2[0][0]);
+  T (     0,  F1  (0),      0,       0,   &a2x2 + 1);
+  T (     2,  F1 ( 2),      2,  F3 ( 2),  &a2x2[0] + 1);
+  T (     3,  F1 ( 1),      3,  F3 ( 3),  &a2x2[0][0] + 1);
+
+  T (    15,      15,      15,      15,   a3x5);
+  T (    15,       5,      15,       5,   &a3x5[0][0] + 0);
+  T (    14,  F1 ( 4),     14,  F3 (14),  &a3x5[0][0] + 1);
+
+  T (     1,       1,       1,       1,   a1 + 0);
+  T (     0,  F1  (0),      0,       0,   a1 + 1);
+  T (     0,  F1 ( 0),      0,       0,   &a1 + 1);
+  /* In the following the offset is out of bounds which makes
+     the expression undefined.  Still, verify that the returned
+     size is zero (and not some large number).  */
+  T (     0,  F1  (0),      0,       0,   a1 + 2);
+
+  T (     2,       2,       2,       2,   a2 + 0);
+  T (     1,  F1 ( 1),      1, F3 ( 1),   a2 + 1);
+  T (     0,  F1 ( 0),      0,       0,   a2 + 2);
+}
+
+static __attribute__ ((noclone, noinline)) void
+test_structs (struct Sx *psx, struct S0 *ps0, struct S1 *ps1, struct S9 *ps9)
+{
+  /* The expected size of a declared object with a flexible array member
+     is sizeof sx in all __builtin_object_size types.  */
+  T (     X,       X,       X,       X,   &sx);
+
+  /* The expected size of an unknown object with a flexible array member
+     is unknown in all __builtin_object_size types.  */
+  T (    -1,      -1,       0,       0,   psx);
+
+  /* The expected size of a flexible array member of a declared object
+     is zero.  */
+  T (     0,       0,       0,       0,   sx.a);
+
+  /* The expected size of a flexible array member of an unknown object
+     is unknown.  */
+  T (    -1,      -1,       0,       0,   psx->a);
+
+  /* The expected size of a declared object with a zero-length array member
+     is sizeof sx in all __builtin_object_size types.  */
+  T (     X,       X,       X,       X,   &s0);
+
+  /* The expected size of an unknown object with a zero-length array member
+     is unknown in all __builtin_object_size types.  */
+  T (    -1,      -1,       0,       0,   ps0);
+
+  /* The expected size of a zero-length array member of a declared object
+     is zero.  */
+  T (     0,       0,       0,       0,   s0.a);
+
+  /* The expected size of a zero-length array member of an unknown object
+     is unknown.  */
+  T (    -1,      -1,       0,       0,   ps0->a);
+
+  T (     X,       X,       X,       X,   &s1);
+  T (     1,       1,       1,       1,   s1.a);
+  T (     0,  F1 (0),       0,       0,   s1.a + 1);
+
+  /* GCC treats arrays of all sizes that are the last member of a struct
+     as flexible array members.  */
+  T (    -1,      -1,       0,       0,   ps1);
+  T (    -1,      -1,       0,       0,   ps1->a);
+  T (    -1,      -1,       0,       0,   ps1->a + 1);
+
+  T (     X,       X,       X,       X,   &s9);
+  T (     9,       9,       9,       9,   s9.a);
+  T (     9,       9,       9,       9,   s9.a + 0);
+  T (     8,  F1 ( 8),      8, F3 (  8),  s9.a + 1);
+  T (     7,  F1 ( 7),      7, F3 (  7),  s9.a + 2);
+  T (     0,  F1 ( 0),      0, F3 (  0),  s9.a + 9);
+
+  /* The following make little sense but see bug 77301.  */
+  T (    -1,      -1,       0,       0,   ps9);
+  T (    -1,      -1,       0,       0,   ps9->a);
+  T (    -1,      -1,       0,       0,   ps9->a + 1);
+}
+
+int
+main()
+{
+  test_arrays ();
+
+  test_structs (&sx, &s0, &s1, &s9);
+
+  if (nfails)
+    __builtin_abort ();
+
+  return 0;
+}

gcc/testsuite/gcc.dg/builtin-object-size-17.c

+/* PR 71831 - __builtin_object_size poor results with no optimization
+   Verify that even without optimization __builtin_object_size result
+   is folded into a constant and dead code that depends on it is
+   eliminated.  */
+/* { dg-do compile } */
+/* { dg-options "-O0 -fdump-tree-ssa" } */
+
+#define concat(a, b)   a ## b
+#define CAT(a, b)      concat (a, b)
+
+/* Create a symbol name unique to each tes and object size type.  */
+#define SYM(type)      CAT (CAT (CAT (failure_on_line_, __LINE__), _type_), type)
+
+/* References to the following undefined symbol which is unique for each
+   test case are expected to be eliminated.  */
+#define TEST_FAILURE(type)			\
+  do {						\
+    extern void SYM (type)(void);		\
+    SYM (type)();				\
+  } while (0)
+
+#define bos(obj, type) __builtin_object_size (obj, type)
+#define size(obj, n) ((size_t)n == X ? sizeof *obj : (size_t)n)
+
+#define test(expect, type, obj)			\
+  do {						\
+    if (bos (obj, type)	!= size (obj, expect))	\
+      TEST_FAILURE (type);			\
+  } while (0)
+
+#define FOLD_ALL(r0, r1, r2, r3, obj)		\
+  do {						\
+    test (r0, 0, obj);				\
+    test (r1, 1, obj);				\
+    test (r2, 2, obj);				\
+    test (r3, 3, obj);				\
+  } while (0)
+
+#define FOLD_0_2(r0, r1, r2, r3, obj)		\
+  do {						\
+    test (r0, 0, obj);				\
+    test (r2, 2, obj);				\
+  } while (0)
+
+/* For convenience.  Substitute for 'sizeof object' in test cases where
+   the size can vary from target to target.  */
+#define X  (size_t)0xdeadbeef
+
+typedef __SIZE_TYPE__ size_t;
+
+extern char ax[];
+char ax2[];               /* { dg-warning "assumed to have one element" } */
+
+extern char a0[0];
+static char a1[1];
+static char a2[2];
+static char a9[9];
+
+#if __SIZEOF_SHORT__ == 4
+extern short ia0[0];
+static short ia1[1];
+static short ia9[9];
+#elif __SIZEOF_INT__ == 4
+extern int ia0[0];
+static int ia1[1];
+static int ia9[9];
+#endif
+
+static char a2x2[2][2];
+static char a3x5[3][5];
+
+struct Sx { char n, a[]; } sx;
+struct S0 { char n, a[0]; } s0;
+struct S1 { char n, a[1]; } s1;
+struct S2 { char n, a[2]; } s2;
+struct S9 { char n, a[9]; } s9;
+
+struct S2x2 { char n, a[2][2]; } s2x2;
+struct S3x5 { char n, a[3][5]; } s3x5;
+
+static __attribute__ ((noclone, noinline)) void
+test_arrays ()
+{
+  FOLD_ALL (     1,       1,       1,       1,   ax2);
+
+  FOLD_ALL (     1,       1,       1,       1,   a1);
+  FOLD_ALL (     2,       2,       2,       2,   a2);
+  FOLD_ALL (     9,       9,       9,       9,   a9);
+
+  FOLD_ALL (     0,       0,       0,       0,   a0);
+  FOLD_ALL (     1,       1,       1,       1,   ax2);
+
+  FOLD_ALL (     0,       0,       0,       0,   ia0);
+  FOLD_ALL (     4,       4,       4,       4,   ia1);
+  FOLD_ALL (    36,      36,      36,      36,   ia9);
+
+  /* Not all results for multidimensional arrays make sense (see
+     bug 77293).  The expected results below simply reflect those
+     obtained at -O2 (modulo the known limitations at -O1).  */
+  FOLD_ALL (     4,       4,       4,       4,   a2x2);
+  FOLD_ALL (     4,       4,       4,       4,   &a2x2[0]);
+  FOLD_ALL (     4,       2,       4,       2,   &a2x2[0][0]);
+  FOLD_0_2 (     0,  F1  (0),      0,       0,   &a2x2 + 1);
+  FOLD_0_2 (     2,  F1 ( 2),      2,  F3 ( 2),  &a2x2[0] + 1);
+  FOLD_0_2 (     3,  F1 ( 1),      3,  F3 ( 3),  &a2x2[0][0] + 1);
+
+  FOLD_ALL (    15,      15,      15,      15,   a3x5);
+  FOLD_ALL (    15,       5,      15,       5,   &a3x5[0][0] + 0);
+  FOLD_0_2 (    14,  F1 ( 4),     14,  F3 (14),  &a3x5[0][0] + 1);
+
+  FOLD_ALL (     1,       1,       1,       1,   a1 + 0);
+  FOLD_0_2 (     0,  F1 ( 0),      0,       0,   &a1 + 1);
+  FOLD_ALL (     2,       2,       2,       2,   a2 + 0);
+  FOLD_0_2 (     1,  F1 ( 1),      1, F3 ( 1),   a2 + 1);
+  FOLD_0_2 (     0,  F1 ( 0),      0,       0,   a2 + 2);
+}
+
+static __attribute__ ((noclone, noinline)) void
+test_structs (void)
+{
+  /* The expected size of a declared object with a flexible array member
+     is sizeof sx in all __builtin_object_size types.  */
+  FOLD_ALL (     X,       X,       X,       X,   &sx);
+
+  /* The expected size of a flexible array member of a declared object
+     is zero.  */
+  FOLD_ALL (     0,       0,       0,       0,   sx.a);
+
+  /* The expected size of a declared object with a zero-length array member
+     is sizeof sx in all __builtin_object_size types.  */
+  FOLD_ALL (     X,       X,       X,       X,   &s0);
+
+  /* The expected size of a zero-length array member of a declared object
+     is zero.  */
+  FOLD_ALL (     0,       0,       0,       0,   s0.a);
+
+  FOLD_ALL (     X,       X,       X,       X,   &s1);
+  FOLD_ALL (     1,       1,       1,       1,   s1.a);
+  FOLD_0_2 (     0,  F1 (0),       0,       0,   s1.a + 1);
+
+  FOLD_ALL (     X,       X,       X,       X,   &s9);
+  FOLD_ALL (     9,       9,       9,       9,   s9.a);
+  FOLD_ALL (     9,       9,       9,       9,   s9.a + 0);
+  FOLD_0_2 (     8,  F1 ( 8),      8, F3 (  8),  s9.a + 1);
+  FOLD_0_2 (     7,  F1 ( 7),      7, F3 (  7),  s9.a + 2);
+  FOLD_0_2 (     0,  F1 ( 0),      0, F3 (  0),  s9.a + 9);
+}
+
+int
+main()
+{
+  test_arrays ();
+  test_structs ();
+
+  return 0;
+}
+
+/* { dg-final { scan-tree-dump-not "failure_on_line" "ssa" } } */

gcc/tree-object-size.c

@@ -51,8 +51,8 @@ static const unsigned HOST_WIDE_INT unknown[4] = {
 };
 
 static tree compute_object_offset (const_tree, const_tree);
-static unsigned HOST_WIDE_INT addr_object_size (struct object_size_info *,
-						const_tree, int);
+static bool addr_object_size (struct object_size_info *,
+			      const_tree, int, unsigned HOST_WIDE_INT *);
 static unsigned HOST_WIDE_INT alloc_object_size (const gcall *, int);
 static tree pass_through_call (const gcall *);
 static void collect_object_sizes_for (struct object_size_info *, tree);
@@ -163,14 +163,18 @@ compute_object_offset (const_tree expr, const_tree var)
    OBJECT_SIZE_TYPE is the second argument from __builtin_object_size.
    If unknown, return unknown[object_size_type].  */
 
-static unsigned HOST_WIDE_INT
+static bool
 addr_object_size (struct object_size_info *osi, const_tree ptr,
-		  int object_size_type)
+		  int object_size_type, unsigned HOST_WIDE_INT *psize)
 {
   tree pt_var, pt_var_size = NULL_TREE, var_size, bytes;
 
   gcc_assert (TREE_CODE (ptr) == ADDR_EXPR);
 
+  /* Set to unknown and overwrite just before returning if the size
+     could be determined.  */
+  *psize = unknown[object_size_type];
+
   pt_var = TREE_OPERAND (ptr, 0);
   while (handled_component_p (pt_var))
     pt_var = TREE_OPERAND (pt_var, 0);
@@ -183,8 +187,8 @@ addr_object_size (struct object_size_info *osi, const_tree ptr,
       if (!osi || (object_size_type & 1) != 0
 	  || TREE_CODE (TREE_OPERAND (pt_var, 0)) != SSA_NAME)
 	{
-	  sz = compute_builtin_object_size (TREE_OPERAND (pt_var, 0),
-					    object_size_type & ~1);
+	  compute_builtin_object_size (TREE_OPERAND (pt_var, 0),
+				       object_size_type & ~1, &sz);
 	}
       else
 	{
@@ -224,7 +228,7 @@ addr_object_size (struct object_size_info *osi, const_tree ptr,
 	      < offset_limit)
     pt_var_size = TYPE_SIZE_UNIT (TREE_TYPE (pt_var));
   else
-    return unknown[object_size_type];
+    return false;
 
   if (pt_var != TREE_OPERAND (ptr, 0))
     {
@@ -339,7 +343,7 @@ addr_object_size (struct object_size_info *osi, const_tree ptr,
       if (var != pt_var)
 	var_size = TYPE_SIZE_UNIT (TREE_TYPE (var));
       else if (!pt_var_size)
-	return unknown[object_size_type];
+	return false;
       else
 	var_size = pt_var_size;
       bytes = compute_object_offset (TREE_OPERAND (ptr, 0), var);
@@ -369,14 +373,17 @@ addr_object_size (struct object_size_info *osi, const_tree ptr,
 	}
     }
   else if (!pt_var_size)
-    return unknown[object_size_type];
+    return false;
   else
     bytes = pt_var_size;
 
   if (tree_fits_uhwi_p (bytes))
-    return tree_to_uhwi (bytes);
+    {
+      *psize = tree_to_uhwi (bytes);
+      return true;
+    }
 
-  return unknown[object_size_type];
+  return false;
 }
 
 
@@ -484,24 +491,61 @@ pass_through_call (const gcall *call)
 }
 
 
-/* Compute __builtin_object_size value for PTR.  OBJECT_SIZE_TYPE is the
-   second argument from __builtin_object_size.  */
+/* Compute __builtin_object_size value for PTR and set *PSIZE to
+   the resulting value.  OBJECT_SIZE_TYPE is the second argument
+   to __builtin_object_size.  Return true on success and false
+   when the object size could not be determined.  */
 
-unsigned HOST_WIDE_INT
-compute_builtin_object_size (tree ptr, int object_size_type)
+bool
+compute_builtin_object_size (tree ptr, int object_size_type,
+			     unsigned HOST_WIDE_INT *psize)
 {
   gcc_assert (object_size_type >= 0 && object_size_type <= 3);
 
+  /* Set to unknown and overwrite just before returning if the size
+     could be determined.  */
+  *psize = unknown[object_size_type];
+
   if (! offset_limit)
     init_offset_limit ();
 
   if (TREE_CODE (ptr) == ADDR_EXPR)
-    return addr_object_size (NULL, ptr, object_size_type);
+    return addr_object_size (NULL, ptr, object_size_type, psize);
+
+  if (TREE_CODE (ptr) != SSA_NAME
+      || !POINTER_TYPE_P (TREE_TYPE (ptr)))
+      return false;
 
-  if (TREE_CODE (ptr) == SSA_NAME
-      && POINTER_TYPE_P (TREE_TYPE (ptr))
-      && computed[object_size_type] != NULL)
+  if (computed[object_size_type] == NULL)
     {
+      if (optimize || object_size_type & 1)
+	return false;
+
+      /* When not optimizing, rather than failing, make a small effort
+	 to determine the object size without the full benefit of
+	 the (costly) computation below.  */
+      gimple *def = SSA_NAME_DEF_STMT (ptr);
+      if (gimple_code (def) == GIMPLE_ASSIGN)
+	{
+	  tree_code code = gimple_assign_rhs_code (def);
+	  if (code == POINTER_PLUS_EXPR)
+	    {
+	      tree offset = gimple_assign_rhs2 (def);
+	      ptr = gimple_assign_rhs1 (def);
+
+	      if (cst_and_fits_in_hwi (offset)
+		  && compute_builtin_object_size (ptr, object_size_type, psize))
+		{
+		  /* Return zero when the offset is out of bounds.  */
+		  unsigned HOST_WIDE_INT off = tree_to_shwi (offset);
+		  *psize = off < *psize ? *psize - off : 0;
+		  return true;
+		}
+	    }
+	}
+      return false;
+    }
+
   if (!bitmap_bit_p (computed[object_size_type], SSA_NAME_VERSION (ptr)))
     {
       struct object_size_info osi;
@@ -616,10 +660,8 @@ compute_builtin_object_size (tree ptr, int object_size_type)
       BITMAP_FREE (osi.visited);
     }
 
-      return object_sizes[object_size_type][SSA_NAME_VERSION (ptr)];
-    }
-
-  return unknown[object_size_type];
+  *psize = object_sizes[object_size_type][SSA_NAME_VERSION (ptr)];
+  return *psize != unknown[object_size_type];
 }
 
 /* Compute object_sizes for PTR, defined to VALUE, which is not an SSA_NAME.  */
@@ -643,7 +685,7 @@ expr_object_size (struct object_size_info *osi, tree ptr, tree value)
 	      || !POINTER_TYPE_P (TREE_TYPE (value)));
 
   if (TREE_CODE (value) == ADDR_EXPR)
-    bytes = addr_object_size (osi, value, object_size_type);
+    addr_object_size (osi, value, object_size_type, &bytes);
   else
     bytes = unknown[object_size_type];
 
@@ -809,7 +851,7 @@ plus_stmt_object_size (struct object_size_info *osi, tree var, gimple *stmt)
 	  unsigned HOST_WIDE_INT off = tree_to_uhwi (op1);
 
           /* op0 will be ADDR_EXPR here.  */
-	  bytes = addr_object_size (osi, op0, object_size_type);
+	  addr_object_size (osi, op0, object_size_type, &bytes);
 	  if (bytes == unknown[object_size_type])
 	    ;
 	  else if (off > offset_limit)
@@ -1282,10 +1324,9 @@ pass_object_sizes::execute (function *fun)
 		      && lhs)
 		    {
 		      tree type = TREE_TYPE (lhs);
-		      unsigned HOST_WIDE_INT bytes
-			= compute_builtin_object_size (ptr, object_size_type);
-		      if (bytes != (unsigned HOST_WIDE_INT) (object_size_type == 1
-							     ? -1 : 0)
+		      unsigned HOST_WIDE_INT bytes;
+		      if (compute_builtin_object_size (ptr, object_size_type,
+						       &bytes)
 			  && wi::fits_to_tree_p (bytes, type))
 			{
 			  tree tem = make_ssa_name (type);

gcc/tree-object-size.h

@@ -21,6 +21,6 @@ along with GCC; see the file COPYING3.  If not see
 #define GCC_TREE_OBJECT_SIZE_H
 
 extern void init_object_sizes (void);
-extern unsigned HOST_WIDE_INT compute_builtin_object_size (tree, int);
+extern bool compute_builtin_object_size (tree, int, unsigned HOST_WIDE_INT *);
 
 #endif  // GCC_TREE_OBJECT_SIZE_H

gcc/ubsan.c

@@ -1826,8 +1826,8 @@ instrument_object_size (gimple_stmt_iterator *gsi, bool is_lhs)
   if (decl_p)
     base_addr = build1 (ADDR_EXPR,
 			build_pointer_type (TREE_TYPE (base)), base);
-  unsigned HOST_WIDE_INT size = compute_builtin_object_size (base_addr, 0);
-  if (size != HOST_WIDE_INT_M1U)
+  unsigned HOST_WIDE_INT size;
+  if (compute_builtin_object_size (base_addr, 0, &size))
     sizet = build_int_cst (sizetype, size);
   else if (optimize)
     {