Fix #3465: Translate comp.o(a op b) if op not in (==, !=) into Unsafe.As<object, UIntPtr>(ref a) op Unsafe.As<object, UIntPtr>(ref b)

Author: siegfriedpammer

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -1108,6 +1108,21 @@ TranslatedExpression TranslateComp(Comp inst)
 				left = left.ConvertTo(inputType, this);
 				right = right.ConvertTo(inputType, this);
 			}
+			else if (inst.InputType == StackType.O)
+			{
+				// Unsafe.As<object, UIntPtr>(ref left) op Unsafe.As<object, UIntPtr>(ref right)
+				// TTo Unsafe.As<TFrom, TTo>(ref TFrom source)
+				var uintptr = compilation.FindType(KnownTypeCode.UIntPtr);
+				left = WrapInUnsafeAs(left, inst.Left);
+				right = WrapInUnsafeAs(right, inst.Right);
+
+				TranslatedExpression WrapInUnsafeAs(TranslatedExpression expr, ILInstruction inst)
+				{
+					var type = expr.Type;
+					expr = WrapInRef(expr, new ByReferenceType(type));
+					return CallUnsafeIntrinsic("As", [expr], uintptr, typeArguments: [type, uintptr]);
+				}
+			}
 			return new BinaryOperatorExpression(left.Expression, op, right.Expression)
 				.WithILInstruction(inst)
 				.WithRR(new OperatorResolveResult(compilation.FindType(TypeCode.Boolean),

ICSharpCode.Decompiler/IL/ILReader.cs

@@ -1248,14 +1248,11 @@ DecodedInstruction DecodeInstruction()
 			}
 		}
 
-		StackType PeekStackType()
+		StackType PeekStackType(int i = 0)
 		{
-			if (expressionStack.Count > 0)
-				return expressionStack.Last().ResultType;
-			if (currentStack.IsEmpty)
-				return StackType.Unknown;
-			else
-				return currentStack.Peek().StackType;
+			if (expressionStack.Count > i)
+				return expressionStack[^(i + 1)].ResultType;
+			return currentStack.Skip(i).FirstOrDefault()?.StackType ?? StackType.Unknown;
 		}
 
 		sealed class CollectStackVariablesVisitor : ILVisitor<ILInstruction>
@@ -1845,6 +1842,12 @@ DecodedInstruction DecodeCallIndirect()
 
 		ILInstruction Comparison(ComparisonKind kind, bool un = false)
 		{
+			var stackType = PeekStackType();
+			if (!kind.IsEqualityOrInequality() && stackType == StackType.O && stackType == PeekStackType(1))
+			{
+				FlushExpressionStack();
+			}
+
 			var right = Pop();
 			var left = Pop();
 			// left will run before right, thus preserving the evaluation order

ICSharpCode.Decompiler/IL/Instructions/Comp.cs

@@ -229,5 +229,13 @@ public static Comp LogicNot(ILInstruction arg, bool isLifted)
 			var liftingKind = isLifted ? ComparisonLiftingKind.ThreeValuedLogic : ComparisonLiftingKind.None;
 			return new Comp(ComparisonKind.Equality, liftingKind, StackType.I4, Sign.None, arg, new LdcI4(0));
 		}
+
+		internal override bool CanInlineIntoSlot(int childIndex, ILInstruction expressionBeingMoved)
+		{
+			// ExpressionBuilder translates comp.o(a op b) for op not in (==, !=) into
+			// Unsafe.As(ref a) op Unsafe.As(ref b), which requires that a and b are variables
+			// and not expressions. Returning false in those cases prevents inlining.
+			return kind.IsEqualityOrInequality() || this.InputType != StackType.O;
+		}
 	}
 }