re PR c++/19199 (Wrong warning about returning a reference to a temporary)

2005-04-03  Roger Sayle  <roger@eyesopen.com>
	    Alexandre Oliva  <aoliva@redhat.com>

	PR c++/19199
	* fold-const.c (non_lvalue): Split tests into...
	(maybe_lvalue_p): New function.
	(fold_cond_expr_with_comparison): Preserve lvalue-ness for the
	C++ front-end prior to lowering into gimple form.

	* g++.dg/expr/lval2.C: New.

	* expr2.C: Fixed.

From-SVN: r97522

gcc/ChangeLog

@@ -1,3 +1,12 @@
+2005-04-03  Roger Sayle  <roger@eyesopen.com>
+	    Alexandre Oliva  <aoliva@redhat.com>
+
+	PR c++/19199
+	* fold-const.c (non_lvalue): Split tests into...
+	(maybe_lvalue_p): New function.
+	(fold_cond_expr_with_comparison): Preserve lvalue-ness for the
+	C++ front-end prior to lowering into gimple form.
+
 2005-04-03  Kaveh R. Ghazi  <ghazi@caip.rutgers.edu>
 
 	* builtins.def (BUILT_IN_STPNCPY, BUILT_IN_STRCASECMP,

gcc/fold-const.c

@@ -2003,16 +2003,13 @@ fold_convert (tree type, tree arg)
     }
 }
 
+/* Return false if expr can be assumed not to be an value, true
+   otherwise.  */
 /* Return an expr equal to X but certainly not valid as an lvalue.  */
 
-tree
-non_lvalue (tree x)
+static bool
+maybe_lvalue_p (tree x)
 {
-  /* While we are in GIMPLE, NON_LVALUE_EXPR doesn't mean anything to
-     us.  */
-  if (in_gimple_form)
-    return x;
-
   /* We only need to wrap lvalue tree codes.  */
   switch (TREE_CODE (x))
   {
@@ -2052,8 +2049,24 @@ non_lvalue (tree x)
     /* Assume the worst for front-end tree codes.  */
     if ((int)TREE_CODE (x) >= NUM_TREE_CODES)
       break;
-    return x;
+    return false;
   }
+
+  return true;
+}
+
+/* Return an expr equal to X but certainly not valid as an lvalue.  */
+
+tree
+non_lvalue (tree x)
+{
+  /* While we are in GIMPLE, NON_LVALUE_EXPR doesn't mean anything to
+     us.  */
+  if (in_gimple_form)
+    return x;
+
+  if (! maybe_lvalue_p (x))
+    return x;
   return build1 (NON_LVALUE_EXPR, TREE_TYPE (x), x);
 }
 
@@ -4273,7 +4286,13 @@ fold_cond_expr_with_comparison (tree type, tree arg0, tree arg1, tree arg2)
      a number and A is not.  The conditions in the original
      expressions will be false, so all four give B.  The min()
      and max() versions would give a NaN instead.  */
-  if (operand_equal_for_comparison_p (arg01, arg2, arg00))
+  if (operand_equal_for_comparison_p (arg01, arg2, arg00)
+      /* Avoid these transformations if the COND_EXPR may be used
+	 as an lvalue in the C++ front-end.  PR c++/19199.  */
+      && (in_gimple_form
+	  || strcmp (lang_hooks.name, "GNU C++") != 0
+	  || ! maybe_lvalue_p (arg1)
+	  || ! maybe_lvalue_p (arg2)))
     {
       tree comp_op0 = arg00;
       tree comp_op1 = arg01;

gcc/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2005-04-03  Alexandre Oliva  <aoliva@redhat.com>
+
+	PR c++/19199
+	* g++.dg/expr/lval2.C: New.
+
 2005-04-03  Kaveh R. Ghazi  <ghazi@caip.rutgers.edu>
 
 	* gcc.dg/nonnull-3.c: Also test __builtin_stpncpy,

gcc/testsuite/g++.dg/expr/lval2.C

@@ -0,0 +1,27 @@
+// PR c++/19199
+
+// { dg-do run }
+
+// We used to turn the COND_EXPR lvalue into a MIN_EXPR rvalue, and
+// then return a reference to a temporary in qMin.
+
+#include <assert.h>
+
+enum Foo { A, B };
+
+template<typename T> T &qMin(T &a, T &b) 
+{
+  return a < b ? a : b;
+}
+
+int main (int,  char **)
+{
+  Foo f = A;
+  Foo g = B;
+  Foo &h = qMin(f, g);
+  assert (&h == &f || &h == &g);
+  const Foo &i = qMin((const Foo&)f, (const Foo&)g);
+  assert (&i == &f || &i == &g);
+  return 0;
+}
+

gcc/testsuite/g++.old-deja/g++.oliva/ChangeLog

@@ -1,3 +1,8 @@
+2005-04-03  Alexandre Oliva  <aoliva@redhat.com>
+
+	PR c++/19199
+	* expr2.C: Fixed.
+
 2003-06-04  J"orn Rennecke <joern.rennecke@superh.com>
 
 	* template1.C (bar): Remove xfail marker.

gcc/testsuite/g++.old-deja/g++.oliva/expr2.C

@@ -1,4 +1,4 @@
-// { dg-do run { xfail *-*-* } }
+// { dg-do run }
 
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