Emit better code for binary await expression with constants

mcs/mcs/async.cs

@@ -40,6 +40,12 @@ public Await (Expression expr, Location loc)
 			}
 		}
 
+		public AwaitStatement Statement {
+			get {
+				return stmt;
+			}
+		}
+
 		protected override void CloneTo (CloneContext clonectx, Expression target)
 		{
 			var t = (Await) target;
@@ -113,7 +119,7 @@ public override object Accept (StructuralVisitor visitor)
 		}
 	}
 
-	class AwaitStatement : YieldStatement<AsyncInitializer>
+	public class AwaitStatement : YieldStatement<AsyncInitializer>
 	{
 		sealed class AwaitableMemberAccess : MemberAccess
 		{

mcs/mcs/ecore.cs

@@ -1933,6 +1933,12 @@ private ReducedExpression (Expression expr, Expression orig_expr)
 
 		#region Properties
 
+		public override bool IsSideEffectFree {
+			get {
+				return expr.IsSideEffectFree;
+			}
+		}
+
 		public Expression OriginalExpression {
 			get {
 				return orig_expr;
@@ -2005,6 +2011,11 @@ public override void Emit (EmitContext ec)
 			expr.Emit (ec);
 		}
 
+		public override Expression EmitToField (EmitContext ec)
+		{
+ 			return expr.EmitToField(ec);
+		}
+
 		public override void EmitBranchable (EmitContext ec, Label target, bool on_true)
 		{
 			expr.EmitBranchable (ec, target, on_true);

mcs/mcs/expression.cs

@@ -3929,6 +3929,27 @@ public override void EmitSideEffect (EmitContext ec)
 			}
 		}
 
+		public override Expression EmitToField (EmitContext ec)
+		{
+			if ((oper & Operator.LogicalMask) == 0) {
+				var await_expr = left as Await;
+				if (await_expr != null && right.IsSideEffectFree) {
+					await_expr.Statement.EmitPrologue (ec);
+					left = await_expr.Statement.GetResultExpression (ec);
+					return this;
+				}
+
+				await_expr = right as Await;
+				if (await_expr != null && left.IsSideEffectFree) {
+					await_expr.Statement.EmitPrologue (ec);
+					right = await_expr.Statement.GetResultExpression (ec);
+					return this;
+				}
+			}
+
+			return base.EmitToField (ec);
+		}
+
 		protected override void CloneTo (CloneContext clonectx, Expression t)
 		{
 			Binary target = (Binary) t;

mcs/tests/test-async-13.cs

@@ -340,6 +340,19 @@ async Task<int> BinaryTest_4 ()
 		return 0;
 	}
 
+	async Task<int> BinaryTest_5 ()
+	{
+		var r1 = await Task.FromResult (1) == 9;
+		if (r1)
+			return 1;
+
+		var r2 = 1 == await Task.FromResult (1);
+		if (!r2)
+			return 2;
+
+		return 0;
+	}
+
 	async Task<int> CallTest_1 ()
 	{
 		return Call (