Improve IsCompatibleTypeForMemoryAccess

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -457,7 +457,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		protected internal override TranslatedExpression VisitLdObj(LdObj inst)
 		{
 			var target = Translate(inst.Target);
-			if (target.Expression is DirectionExpression && IsCompatibleTypeForMemoryAccess(target.Type, inst.Type)) {
+			if (target.Expression is DirectionExpression && IsCompatibleTypeForMemoryAccess(target.Type, inst.Type, isWrite: false)) {
 				// we can deference the managed reference by stripping away the 'ref'
 				var result = target.UnwrapChild(((DirectionExpression)target.Expression).Expression);
 				result = result.ConvertTo(inst.Type, this);
@@ -472,26 +472,12 @@ namespace ICSharpCode.Decompiler.CSharp
 			}
 		}
 
-		bool IsCompatibleTypeForMemoryAccess(IType pointerType, IType accessType)
-		{
-			IType memoryType;
-			if (pointerType is PointerType)
-				memoryType = ((PointerType)pointerType).ElementType;
-			else if (pointerType is ByReferenceType)
-				memoryType = ((ByReferenceType)pointerType).ElementType;
-			else
-				return false;
-			if (memoryType.IsReferenceType == true && accessType.IsReferenceType == true)
-				return true;
-			return memoryType.Equals(accessType);
-		}
-		
 		protected internal override TranslatedExpression VisitStObj(StObj inst)
 		{
 			var target = Translate(inst.Target);
 			var value = Translate(inst.Value);
 			TranslatedExpression result;
-			if (target.Expression is DirectionExpression && IsCompatibleTypeForMemoryAccess(target.Type, inst.Type)) {
+			if (target.Expression is DirectionExpression && IsCompatibleTypeForMemoryAccess(target.Type, inst.Type, isWrite: true)) {
 				// we can deference the managed reference by stripping away the 'ref'
 				result = target.UnwrapChild(((DirectionExpression)target.Expression).Expression);
 			} else {
@@ -504,6 +490,49 @@ namespace ICSharpCode.Decompiler.CSharp
 			return Assignment(result, value).WithILInstruction(inst);
 		}
 
+		/// <summary>
+		/// Gets whether reading/writing an element of accessType from the pointer
+		/// is equivalent to reading/writing an element of the pointer's element type.
+		/// </summary>
+		bool IsCompatibleTypeForMemoryAccess(IType pointerType, IType accessType, bool isWrite)
+		{
+			IType memoryType;
+			if (pointerType is PointerType)
+				memoryType = ((PointerType)pointerType).ElementType;
+			else if (pointerType is ByReferenceType)
+				memoryType = ((ByReferenceType)pointerType).ElementType;
+			else
+				return false;
+			if (memoryType.Equals(accessType))
+				return true;
+			// If the types are not equal, the access still might produce equal results:
+			if (memoryType.IsReferenceType == true && accessType.IsReferenceType == true)
+				return true;
+			ITypeDefinition memoryTypeDef = memoryType.GetDefinition();
+			if (memoryTypeDef != null) {
+				switch (memoryTypeDef.KnownTypeCode) {
+					case KnownTypeCode.Byte:
+					case KnownTypeCode.SByte:
+						// Reading small integers of different signs is not equivalent due to sign extension,
+						// but writes are equivalent (truncation works the same for signed and unsigned)
+						return isWrite && (accessType.IsKnownType(KnownTypeCode.Byte) || accessType.IsKnownType(KnownTypeCode.SByte));
+					case KnownTypeCode.Int16:
+					case KnownTypeCode.UInt16:
+						return isWrite && (accessType.IsKnownType(KnownTypeCode.Int16) || accessType.IsKnownType(KnownTypeCode.UInt16));
+					case KnownTypeCode.Int32:
+					case KnownTypeCode.UInt32:
+						return isWrite && (accessType.IsKnownType(KnownTypeCode.Int32) || accessType.IsKnownType(KnownTypeCode.UInt32));
+					case KnownTypeCode.IntPtr:
+					case KnownTypeCode.UIntPtr:
+						return accessType.IsKnownType(KnownTypeCode.IntPtr) || accessType.IsKnownType(KnownTypeCode.UIntPtr);
+					case KnownTypeCode.Int64:
+					case KnownTypeCode.UInt64:
+						return accessType.IsKnownType(KnownTypeCode.Int64) || accessType.IsKnownType(KnownTypeCode.UInt64);
+				}
+			}
+			return false;
+		}
+		
 		protected internal override TranslatedExpression VisitLdFld(LdFld inst)
 		{
 			return ConvertField(inst.Field).WithILInstruction(inst);