* fold-const.c (pointer_may_wrap_p): New static function.

	(fold_comparison): Add another test for pointer overflow.  Use
	pointer_may_wrap_p to disable some false positives.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@134440 138bc75d-0d04-0410-961f-82ee72b054a4

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index a053bf4..270b12e 100644 (file)
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,9 @@
+2008-04-18  Ian Lance Taylor  <iant@google.com>
+
+       * fold-const.c (pointer_may_wrap_p): New static function.
+       (fold_comparison): Add another test for pointer overflow.  Use
+       pointer_may_wrap_p to disable some false positives.
+
 2008-04-18  Kris Van Hees <kris.van.hees@oracle.com>
          
        * c-common.c (CHAR16_TYPE, CHAR32_TYPE): New macros.

diff --git a/gcc/fold-const.c b/gcc/fold-const.c
index 4e46761..f5ec30c 100644 (file)
--- a/gcc/fold-const.c
+++ b/gcc/fold-const.c
@@ -8394,6 +8394,66 @@ maybe_canonicalize_comparison (enum tree_code code, tree type,
   return t;
 }
 
+/* Return whether BASE + OFFSET + BITPOS may wrap around the address
+   space.  This is used to avoid issuing overflow warnings for
+   expressions like &p->x which can not wrap.  */
+
+static bool
+pointer_may_wrap_p (tree base, tree offset, HOST_WIDE_INT bitpos)
+{
+  tree size;
+  unsigned HOST_WIDE_INT offset_low, total_low;
+  HOST_WIDE_INT offset_high, total_high;
+
+  if (!POINTER_TYPE_P (TREE_TYPE (base)))
+    return true;
+
+  if (bitpos < 0)
+    return true;
+
+  size = size_in_bytes (TREE_TYPE (TREE_TYPE (base)));
+  if (size == NULL_TREE || TREE_CODE (size) != INTEGER_CST)
+    return true;
+
+  /* We can do slightly better for SIZE if we have an ADDR_EXPR of an
+     array.  */
+  if (TREE_CODE (base) == ADDR_EXPR)
+    {
+      tree base_size = size_in_bytes (TREE_TYPE (TREE_OPERAND (base, 0)));
+      if (base_size != NULL_TREE
+         && TREE_CODE (base_size) == INTEGER_CST
+         && INT_CST_LT_UNSIGNED (size, base_size))
+       size = base_size;
+    }
+
+  if (offset == NULL_TREE)
+    {
+      offset_low = 0;
+      offset_high = 0;
+    }
+  else if (TREE_CODE (offset) != INTEGER_CST || TREE_OVERFLOW (offset))
+    return true;
+  else
+    {
+      offset_low = TREE_INT_CST_LOW (offset);
+      offset_high = TREE_INT_CST_HIGH (offset);
+    }
+
+  if (add_double_with_sign (offset_low, offset_high,
+                           bitpos / BITS_PER_UNIT, 0,
+                           &total_low, &total_high,
+                           true))
+    return true;
+
+  if ((unsigned HOST_WIDE_INT) total_high
+      < (unsigned HOST_WIDE_INT) TREE_INT_CST_HIGH (size))
+    return false;
+  if ((unsigned HOST_WIDE_INT) total_high
+      > (unsigned HOST_WIDE_INT) TREE_INT_CST_HIGH (size))
+    return true;
+  return total_low > TREE_INT_CST_LOW (size);
+}
+
 /* Subroutine of fold_binary.  This routine performs all of the
    transformations that are common to the equality/inequality
    operators (EQ_EXPR and NE_EXPR) and the ordering operators
@@ -8544,10 +8604,24 @@ fold_comparison (enum tree_code code, tree type, tree op0, tree op1)
        {
          /* We can fold this expression to a constant if the non-constant
             offset parts are equal.  */
-         if (offset0 == offset1
-             || (offset0 && offset1
-                 && operand_equal_p (offset0, offset1, 0)))
+         if ((offset0 == offset1
+              || (offset0 && offset1
+                  && operand_equal_p (offset0, offset1, 0)))
+             && (code == EQ_EXPR
+                 || code == NE_EXPR
+                 || POINTER_TYPE_OVERFLOW_UNDEFINED))
+               
            {
+             if (code != EQ_EXPR
+                 && code != NE_EXPR
+                 && bitpos0 != bitpos1
+                 && (pointer_may_wrap_p (base0, offset0, bitpos0)
+                     || pointer_may_wrap_p (base1, offset1, bitpos1)))
+               fold_overflow_warning (("assuming pointer wraparound does not "
+                                       "occur when comparing P +- C1 with "
+                                       "P +- C2"),
+                                      WARN_STRICT_OVERFLOW_CONDITIONAL);
+
              switch (code)
                {
                case EQ_EXPR:
@@ -8593,7 +8667,10 @@ fold_comparison (enum tree_code code, tree type, tree op0, tree op1)
              else
                offset1 = fold_convert (signed_size_type_node, offset1);
 
-             if (code != EQ_EXPR && code != NE_EXPR)
+             if (code != EQ_EXPR
+                 && code != NE_EXPR
+                 && (pointer_may_wrap_p (base0, offset0, bitpos0)
+                     || pointer_may_wrap_p (base1, offset1, bitpos1)))
                fold_overflow_warning (("assuming pointer wraparound does not "
                                        "occur when comparing P +- C1 with "
                                        "P +- C2"),