Improve support for post-increment/decrement on pointers

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -1,4 +1,4 @@
-// Copyright (c) 2014-2020 Daniel Grunwald
+// Copyright (c) 2014-2020 Daniel Grunwald
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -1901,7 +1901,16 @@ namespace ICSharpCode.Decompiler.CSharp
 			if (inst.EvalMode == CompoundEvalMode.EvaluatesToOldValue)
 			{
 				Debug.Assert(op == AssignmentOperatorType.Add || op == AssignmentOperatorType.Subtract);
-				Debug.Assert(inst.Value.MatchLdcI(1) || inst.Value.MatchLdcF4(1) || inst.Value.MatchLdcF8(1));
+#if DEBUG
+				if (inst.Type is PointerType ptrType)
+				{
+					ILInstruction instValue = PointerArithmeticOffset.Detect(inst.Value, ptrType.ElementType, inst.CheckForOverflow);
+					Debug.Assert(instValue is not null);
+					Debug.Assert(instValue.MatchLdcI(1));
+				}
+				else
+					Debug.Assert(inst.Value.MatchLdcI(1) || inst.Value.MatchLdcF4(1) || inst.Value.MatchLdcF8(1));
+#endif
 				UnaryOperatorType unary;
 				ExpressionType exprType;
 				if (op == AssignmentOperatorType.Add)

ICSharpCode.Decompiler/IL/Transforms/TransformAssignment.cs

@@ -1,4 +1,4 @@
-// Copyright (c) 2015 Siegfried Pammer
+// Copyright (c) 2015 Siegfried Pammer
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -871,10 +871,18 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			}
 			if (UnwrapSmallIntegerConv(value, out var conv) is BinaryNumericInstruction binary)
 			{
-				if (!binary.Left.MatchLdLoc(tmpVar) || !(binary.Right.MatchLdcI(1) || binary.Right.MatchLdcF4(1) || binary.Right.MatchLdcF8(1)))
-					return false;
 				if (!(binary.Operator == BinaryNumericOperator.Add || binary.Operator == BinaryNumericOperator.Sub))
 					return false;
+				if (!binary.Left.MatchLdLoc(tmpVar))
+					return false;
+				if (targetType is PointerType ptrType)
+				{
+					var right = PointerArithmeticOffset.Detect(binary.Right, ptrType.ElementType, binary.CheckForOverflow);
+					if (right is null || !right.MatchLdcI(1))
+						return false;
+				}
+				else if (!(binary.Right.MatchLdcI(1) || binary.Right.MatchLdcF4(1) || binary.Right.MatchLdcF8(1)))
+					return false;
 				if (!ValidateCompoundAssign(binary, conv, targetType, context.Settings))
 					return false;
 				context.Step("TransformPostIncDecOperator (builtin)", inst);