PATCH RFA: C++ frontend warning: comparing function pointer to NULL

[Ian Lance Taylor]

If you write code like this:
    extern int foo();
    if (foo == 0)
      ...
the C frontend will issue a warning:
  the address of 'foo' will never be NULL

This is useful for the case where somebody wrote "foo" instead of
"foo()".

The C++ frontend does not issue this warning.  This patch adds the
warning to the C++ frontend.

In the testsuite I added a couple of tests for
   if (foo)
which is a warning which the C++ frontend already generates.

Tested with bootstrap and testsuite run on i686-pc-linux-gnu.

OK for mainline?

Ian


cp/ChangeLog:
2007-01-04  Ian Lance Taylor  <iant@google.com>

	* typeck.c (build_binary_op): Warn about comparing a non-weak
	address to NULL.

testsuite/ChangeLog:
2007-01-04  Ian Lance Taylor  <iant@google.com>

	* g++.dg/warn/Walways-true-1.C: New test.
	* g++.dg/warn/Walways-true-2.C: New test.


Index: gcc/cp/typeck.c
===================================================================
--- gcc/cp/typeck.c	(revision 120376)
+++ gcc/cp/typeck.c	(working copy)
@@ -3334,10 +3334,24 @@ build_binary_op (enum tree_code code, tr
 					      "comparison");
       else if ((code0 == POINTER_TYPE || TYPE_PTRMEM_P (type0))
 	       && null_ptr_cst_p (op1))
-	result_type = type0;
+	{
+	  if (TREE_CODE (op0) == ADDR_EXPR
+	      && DECL_P (TREE_OPERAND (op0, 0)) 
+	      && !DECL_WEAK (TREE_OPERAND (op0, 0)))
+	    warning (OPT_Walways_true, "the address of %qD will never be NULL",
+		     TREE_OPERAND (op0, 0));
+	  result_type = type0;
+	}
       else if ((code1 == POINTER_TYPE || TYPE_PTRMEM_P (type1))
 	       && null_ptr_cst_p (op0))
-	result_type = type1;
+	{
+	  if (TREE_CODE (op1) == ADDR_EXPR 
+	      && DECL_P (TREE_OPERAND (op1, 0))
+	      && !DECL_WEAK (TREE_OPERAND (op1, 0)))
+	    warning (OPT_Walways_true, "the address of %qD will never be NULL",
+		     TREE_OPERAND (op1, 0));
+	  result_type = type1;
+	}
       else if (code0 == POINTER_TYPE && code1 == INTEGER_TYPE)
 	{
 	  result_type = type0;
Index: gcc/testsuite/g++.dg/warn/Walways-true-1.C
===================================================================
--- gcc/testsuite/g++.dg/warn/Walways-true-1.C	(revision 0)
+++ gcc/testsuite/g++.dg/warn/Walways-true-1.C	(revision 0)
@@ -0,0 +1,30 @@
+// Test -Walways-true for testing an address against NULL.
+// Origin: Ian Lance Taylor <iant@google.com>
+
+// { dg-do compile}
+// { dg-options "-Walways-true" }
+
+extern int foo (int);
+
+int i;
+
+void
+bar ()
+{
+  if (foo)	// { dg-warning "always evaluate as" "correct warning" }
+    foo (0);
+  if (foo (1))
+    foo (1);
+  if (&i)	// { dg-warning "always evaluate as" "correct warning" }
+    foo (2);
+  if (i)
+    foo (3);
+  if (foo == 0)	// { dg-warning "never be NULL" "correct warning" }
+    foo (4);
+  if (foo (1) == 0)
+    foo (5);
+  if (&i == 0)	// { dg-warning "never be NULL" "correct warning" }
+    foo (6);
+  if (i == 0)
+    foo (7);
+}
Index: gcc/testsuite/g++.dg/warn/Walways-true-2.C
===================================================================
--- gcc/testsuite/g++.dg/warn/Walways-true-2.C	(revision 0)
+++ gcc/testsuite/g++.dg/warn/Walways-true-2.C	(revision 0)
@@ -0,0 +1,33 @@
+// Make sure we don't assume that a weak symbol is always non-NULL.
+// This is just like Walways-true-1.C, except that it uses a weak
+// symbol.
+// Origin: Ian Lance Taylor <iant@google.com>
+
+// { dg-do compile}
+// { dg-options "-Walways-true" }
+// { dg-require-weak "" }
+
+extern int foo (int) __attribute__ ((weak));
+
+int i __attribute__ ((weak));
+
+void
+bar ()
+{
+  if (foo)
+    foo (0);
+  if (foo (1))
+    foo (1);
+  if (&i)
+    foo (2);
+  if (i)
+    foo (3);
+  if (foo == 0)
+    foo (4);
+  if (foo (1) == 0)
+    foo (5);
+  if (&i == 0)
+    foo (6);
+  if (i == 0)
+    foo (7);
+}