PR c++/84151 - unnecessary volatile load with static member.

	* call.c (build_new_method_call_1): Avoid loading from a volatile
	lvalue used as the object argument for a static member function.

From-SVN: r257763

gcc/cp/ChangeLog

 2018-02-16  Jason Merrill  <jason@redhat.com>
 
+	PR c++/84151 - unnecessary volatile load with static member.
+	* call.c (build_new_method_call_1): Avoid loading from a volatile
+	lvalue used as the object argument for a static member function.
+
 	PR c++/81853 - using-directive and constexpr.
 	* constexpr.c (cxx_eval_constant_expression): Handle USING_STMT.
 

gcc/cp/call.c

@@ -9284,8 +9284,14 @@ build_new_method_call_1 (tree instance, tree fns, vec<tree, va_gc> **args,
 	      if (TREE_CODE (TREE_TYPE (fn)) != METHOD_TYPE
 		  && !is_dummy_object (instance)
 		  && TREE_SIDE_EFFECTS (instance))
-		call = build2 (COMPOUND_EXPR, TREE_TYPE (call),
-			       instance, call);
+		{
+		  /* But avoid the implicit lvalue-rvalue conversion when 'a'
+		     is volatile.  */
+		  tree a = instance;
+		  if (TREE_THIS_VOLATILE (a))
+		    a = build_this (a);
+		  call = build2 (COMPOUND_EXPR, TREE_TYPE (call), a, call);
+		}
 	      else if (call != error_mark_node
 		       && DECL_DESTRUCTOR_P (cand->fn)
 		       && !VOID_TYPE_P (TREE_TYPE (call)))

gcc/testsuite/g++.dg/tree-ssa/volatile1.C

+// PR c++/84151
+// { dg-additional-options "-fdump-tree-gimple" }
+// { dg-final { scan-tree-dump-not {\*this} "gimple" } }
+
+struct A {
+  static int& bar(int& a) {
+    return a;
+  }
+  static int i;
+
+  int foo() volatile {
+    int v = c;
+    return i + bar(v);
+  }
+
+  int c;
+};
+
+int A::i = 0;
+
+A a;
+
+int main() {
+  a.c = 2;
+  a.foo();
+
+  return 0;
+}