Support for the null coalescing operator

ICSharpCode.Decompiler/Ast/AstMethodBodyBuilder.cs

@@ -276,6 +276,7 @@ namespace ICSharpCode.Decompiler.Ast
 				case ILCode.LogicAnd:   return new Ast.BinaryOperatorExpression(arg1, BinaryOperatorType.ConditionalAnd, arg2);
 				case ILCode.LogicOr:    return new Ast.BinaryOperatorExpression(arg1, BinaryOperatorType.ConditionalOr, arg2);
 				case ILCode.TernaryOp:  return new Ast.ConditionalExpression() { Condition = arg1, TrueExpression = arg2, FalseExpression = arg3 };
+				case ILCode.NullCoalescing: 	return new Ast.BinaryOperatorExpression(arg1, BinaryOperatorType.NullCoalescing, arg2);
 				#endregion
 				#region Branch
 				case ILCode.Br:         return new Ast.GotoStatement(((ILLabel)byteCode.Operand).Name);

ICSharpCode.Decompiler/ILAst/ILAstOptimizer.cs

@@ -19,6 +19,10 @@ namespace ICSharpCode.Decompiler.ILAst
 		SplitToMovableBlocks,
 		TypeInference,
 		PeepholeOptimizations,
+		SimplifyShortCircuit,
+		SimplifyTernaryOperator,
+		SimplifyNullCoalescing,
+		MoreSimplifyPasses,
 		FindLoops,
 		FindConditions,
 		FlattenNestedMovableBlocks,
@@ -79,8 +83,17 @@ namespace ICSharpCode.Decompiler.ILAst
 				bool modified;
 				do {
 					modified = false;
+					
+					if (abortBeforeStep == ILAstOptimizationStep.SimplifyShortCircuit) return;
 					modified |= block.RunPeepholeOptimization(TrySimplifyShortCircuit);
+					
+					if (abortBeforeStep == ILAstOptimizationStep.SimplifyTernaryOperator) return;
 					modified |= block.RunPeepholeOptimization(TrySimplifyTernaryOperator);
+					
+					if (abortBeforeStep == ILAstOptimizationStep.SimplifyNullCoalescing) return;
+					modified |= block.RunPeepholeOptimization(TrySimplifyNullCoalescing);
+					
+					if (abortBeforeStep == ILAstOptimizationStep.MoreSimplifyPasses) return;
 				} while(modified);
 			}
 			
@@ -160,7 +173,9 @@ namespace ICSharpCode.Decompiler.ILAst
 							i++;  // Ignore the label as well
 					} else if (body[i].Match(ILCode.Nop)){
 						// Ignore nop
-					} else if (body[i].Match(ILCode.Pop, out popExpr) && popExpr.HasNoSideEffects()) {
+					} else if (body[i].Match(ILCode.Pop, out popExpr)) {
+						if (!popExpr.HasNoSideEffects())
+							throw new Exception("Pop should have just ldloc at this stage");
 						// Ignore pop
 					} else {
 						newBody.Add(body[i]);
@@ -323,7 +338,10 @@ namespace ICSharpCode.Decompiler.ILAst
 			   labelToBasicBlock[trueLabel].Match(ILCode.Stloc, out trueLocVar, out trueExpr, out trueFall) &&
 			   labelToBasicBlock[falseLabel].Match(ILCode.Stloc, out falseLocVar, out falseExpr, out falseFall) &&
 			   trueLocVar == falseLocVar &&
-			   trueFall == falseFall)
+			   trueFall == falseFall &&
+			   scope.Contains(labelToBasicBlock[trueLabel]) &&
+			   scope.Contains(labelToBasicBlock[falseLabel])
+			  )
 			{
 				int boolVal;
 				ILExpression newExpr;
@@ -353,18 +371,75 @@ namespace ICSharpCode.Decompiler.ILAst
 				head.FallthoughGoto = new ILExpression(ILCode.Br, trueFall);
 				
 				// Remove the old basic blocks
-				scope.RemoveOrThrow(labelToBasicBlock[trueLabel]);
-				scope.RemoveOrThrow(labelToBasicBlock[falseLabel]);
-				labelToBasicBlock.RemoveOrThrow(trueLabel);
-				labelToBasicBlock.RemoveOrThrow(falseLabel);
-				labelGlobalRefCount.RemoveOrThrow(trueLabel);
-				labelGlobalRefCount.RemoveOrThrow(falseLabel);
+				foreach(ILLabel deleteLabel in new [] { trueLabel, falseLabel }) {
+					scope.RemoveOrThrow(labelToBasicBlock[deleteLabel]);
+					labelGlobalRefCount.RemoveOrThrow(deleteLabel);
+					labelToBasicBlock.RemoveOrThrow(deleteLabel);
+				}
 				
 				return true;
 			}
 			return false;
 		}
 		
+		bool TrySimplifyNullCoalescing(List<ILNode> scope, ILBasicBlock head, int index)
+		{
+			// ...
+			// v = ldloc(leftVar)
+			// br(condBBLabel)
+			//
+			// condBBLabel:
+			// brtrue(endBBLabel, ldloc(leftVar))
+			// br(rightBBLabel)
+			//
+			// rightBBLabel:
+			// v = rightExpr
+			// br(endBBLabel)
+			//
+			// endBBLabel:
+			// ...
+				
+			ILVariable v, v2;
+			ILExpression leftExpr, leftExpr2;
+			ILVariable leftVar, leftVar2;
+			ILLabel condBBLabel;
+			ILBasicBlock condBB;
+			ILLabel endBBLabel, endBBLabel2;
+			ILLabel rightBBLabel;
+			ILBasicBlock rightBB;
+			ILExpression rightExpr;
+			ILBasicBlock endBB;
+			if (head.Body.LastOrDefault().Match(ILCode.Stloc, out v, out leftExpr) &&
+			    leftExpr.Match(ILCode.Ldloc, out leftVar) &&
+			    head.FallthoughGoto.Match(ILCode.Br, out condBBLabel) &&
+			    labelToBasicBlock.TryGetValue(condBBLabel, out condBB) &&
+			    condBB.Match(ILCode.Brtrue, out endBBLabel, out leftExpr2, out rightBBLabel) &&
+			    leftExpr2.Match(ILCode.Ldloc, out leftVar2) &&
+			    leftVar == leftVar2 &&
+			    labelToBasicBlock.TryGetValue(rightBBLabel, out rightBB) &&
+			    rightBB.Match(ILCode.Stloc, out v2, out rightExpr, out endBBLabel2) &&
+			    v == v2 &&
+			    endBBLabel == endBBLabel2 &&
+			    labelToBasicBlock.TryGetValue(endBBLabel, out endBB) &&
+			    labelGlobalRefCount.GetOrDefault(condBBLabel) == 1 &&
+			    labelGlobalRefCount.GetOrDefault(rightBBLabel) == 1 &&
+			    labelGlobalRefCount.GetOrDefault(endBBLabel) == 2 &&
+			    scope.ContainsAll(condBB, rightBB, endBB)
+			   )
+			{
+				head.Body[head.Body.Count - 1] = new ILExpression(ILCode.Stloc, v, new ILExpression(ILCode.NullCoalescing, null, leftExpr, rightExpr));
+				head.FallthoughGoto = new ILExpression(ILCode.Br, endBBLabel);
+				
+				foreach(ILLabel deleteLabel in new [] { condBBLabel, rightBBLabel }) {
+					scope.RemoveOrThrow(labelToBasicBlock[deleteLabel]);
+					labelGlobalRefCount.RemoveOrThrow(deleteLabel);
+					labelToBasicBlock.RemoveOrThrow(deleteLabel);
+				}
+				return true;
+			}
+			return false;
+		}
+		
 		// scope is modified if successful
 		bool TrySimplifyShortCircuit(List<ILNode> scope, ILBasicBlock head, int index)
 		{
@@ -1093,6 +1168,15 @@ namespace ICSharpCode.Decompiler.ILAst
 			return ret;
 		}
 		
+		public static bool ContainsAll<T>(this List<T> list, params T[] items)
+		{
+			foreach (T item in items) {
+				if (!list.Contains(item))
+					return false;
+			}
+			return true;
+		}
+		
 		public static void RemoveOrThrow<T>(this ICollection<T> collection, T item)
 		{
 			if (!collection.Remove(item))

ICSharpCode.Decompiler/ILAst/ILCodes.cs

@@ -258,6 +258,7 @@ namespace ICSharpCode.Decompiler.ILAst
 		LogicNot,
 		LogicAnd,
 		LogicOr,
+		NullCoalescing,
 		InitArray, // Array Initializer
 		InitCollection, // Collection Initializer: first arg is newobj, remaining args are InitCollectionAddMethod method calls
 		InitCollectionAddMethod,

ICSharpCode.Decompiler/ILAst/TypeAnalysis.cs

@@ -138,6 +138,11 @@ namespace ICSharpCode.Decompiler.ILAst
 						InferTypeForExpression(expr.Arguments[1], expectedType, forceInferChildren),
 						InferTypeForExpression(expr.Arguments[2], expectedType, forceInferChildren)
 					);
+				case ILCode.NullCoalescing:
+					return TypeWithMoreInformation(
+						InferTypeForExpression(expr.Arguments[0], expectedType, forceInferChildren),
+						InferTypeForExpression(expr.Arguments[1], expectedType, forceInferChildren)
+					);
 					#endregion
 					#region Variable load/store
 				case ILCode.Stloc: