re PR c/23113 (The -Wunused (value computed is not used) option missed an important case)

PR c/23113
	* stmt.c (warn_if_unused_value): Check TREE_NO_WARNING at start.
	Don't handle NOP_EXPR, CONVERT_EXPR and NON_LVALUE_EXPR
	specially.  Check for side effects only for COND_EXPR.
	* c-typeck.c (c_finish_stmt_expr): Mark statement expression
	return with TREE_NO_WARNING.

testsuite:
	* gcc.dg/Wunused-value-1.c: New test.

From-SVN: r102805

gcc/ChangeLog

@@ -1,3 +1,12 @@
+2005-08-06  Joseph S. Myers  <joseph@codesourcery.com>
+
+	PR c/23113
+	* stmt.c (warn_if_unused_value): Check TREE_NO_WARNING at start.
+	Don't handle NOP_EXPR, CONVERT_EXPR and NON_LVALUE_EXPR
+	specially.  Check for side effects only for COND_EXPR.
+	* c-typeck.c (c_finish_stmt_expr): Mark statement expression
+	return with TREE_NO_WARNING.
+
 2005-08-06  Richard Sandiford  <richard@codesourcery.com>
 
 	PR rtl-optimization/23233

gcc/c-typeck.c

@@ -7315,7 +7315,13 @@ c_finish_stmt_expr (tree body)
   if (last == error_mark_node
       || (last == BIND_EXPR_BODY (body)
 	  && BIND_EXPR_VARS (body) == NULL))
-    return last;
+    {
+      /* Do not warn if the return value of a statement expression is
+	 unused.  */
+      if (EXPR_P (last))
+	TREE_NO_WARNING (last) = 1;
+      return last;
+    }
 
   /* Extract the type of said expression.  */
   type = TREE_TYPE (last);

gcc/stmt.c

@@ -1373,7 +1373,7 @@ int
 warn_if_unused_value (tree exp, location_t locus)
 {
  restart:
-  if (TREE_USED (exp))
+  if (TREE_USED (exp) || TREE_NO_WARNING (exp))
     return 0;
 
   /* Don't warn about void constructs.  This includes casting to void,
@@ -1416,8 +1416,6 @@ warn_if_unused_value (tree exp, location_t locus)
       goto restart;
 
     case COMPOUND_EXPR:
-      if (TREE_NO_WARNING (exp))
-	return 0;
       if (warn_if_unused_value (TREE_OPERAND (exp, 0), locus))
 	return 1;
       /* Let people do `(foo (), 0)' without a warning.  */
@@ -1426,27 +1424,12 @@ warn_if_unused_value (tree exp, location_t locus)
       exp = TREE_OPERAND (exp, 1);
       goto restart;
 
-    case NOP_EXPR:
-    case CONVERT_EXPR:
-    case NON_LVALUE_EXPR:
-      /* Don't warn about conversions not explicit in the user's program.  */
-      if (TREE_NO_WARNING (exp))
+    case COND_EXPR:
+      /* If this is an expression with side effects, don't warn; this
+	 case commonly appears in macro expansions.  */
+      if (TREE_SIDE_EFFECTS (exp))
 	return 0;
-      /* Assignment to a cast usually results in a cast of a modify.
-	 Don't complain about that.  There can be an arbitrary number of
-	 casts before the modify, so we must loop until we find the first
-	 non-cast expression and then test to see if that is a modify.  */
-      {
-	tree tem = TREE_OPERAND (exp, 0);
-
-	while (TREE_CODE (tem) == CONVERT_EXPR || TREE_CODE (tem) == NOP_EXPR)
-	  tem = TREE_OPERAND (tem, 0);
-
-	if (TREE_CODE (tem) == MODIFY_EXPR || TREE_CODE (tem) == INIT_EXPR
-	    || TREE_CODE (tem) == CALL_EXPR)
-	  return 0;
-      }
-      goto maybe_warn;
+      goto warn;
 
     case INDIRECT_REF:
       /* Don't warn about automatic dereferencing of references, since
@@ -1470,11 +1453,7 @@ warn_if_unused_value (tree exp, location_t locus)
       if (EXPRESSION_CLASS_P (exp) && TREE_CODE_LENGTH (TREE_CODE (exp)) == 0)
 	return 0;
 
-    maybe_warn:
-      /* If this is an expression with side effects, don't warn.  */
-      if (TREE_SIDE_EFFECTS (exp))
-	return 0;
-
+    warn:
       warning (0, "%Hvalue computed is not used", &locus);
       return 1;
     }

gcc/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2005-08-06  Joseph S. Myers  <joseph@codesourcery.com>
+
+	PR c/23113
+	* gcc.dg/Wunused-value-1.c: New test.
+
 2005-08-06  Richard Sandiford  <richard@codesourcery.com>
 
 	PR rtl-optimization/23233

gcc/testsuite/gcc.dg/Wunused-value-1.c

@@ -0,0 +1,31 @@
+/* Test -Wunused-value.  Bug 23113.  */
+/* Origin: Joseph Myers <joseph@codesourcery.com> */
+/* { dg-do compile } */
+/* { dg-options "-Wunused-value" } */
+
+int f (void);
+void g (void);
+int *p;
+
+void
+h (void)
+{
+  1 + f (); /* { dg-warning "value computed is not used" } */
+  f () + f (); /* { dg-warning "value computed is not used" } */
+  f () + f (), f (); /* { dg-warning "value computed is not used" } */
+  (char) f (); /* { dg-warning "value computed is not used" } */
+  g ();
+  f ();
+  (void) f ();
+  *p++; /* { dg-warning "value computed is not used" } */
+  ++*p;
+  (*p ? f() : 0);
+  ({ f(); });
+  /* Statement expressions may be used in macro expansions which like
+     functions return values which may or may not be of use, so don't
+     warn for them but do warn inside them.  */
+  ({ f() + 1; });
+  ({ f(); 0; });
+  ({ f() + 1; 0; }); /* { dg-warning "value computed is not used" } */
+  1 + ({ f(); }); /* { dg-warning "value computed is not used" } */
+}