Improve RemoveDeadVariableInit and CopyPropagation

ICSharpCode.Decompiler/IL/Transforms/CopyPropagation.cs

@@ -49,7 +49,7 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 				if (block.Instructions[i].MatchStLoc(out v, out copiedExpr)) {
 					if (v.IsSingleDefinition && v.LoadCount == 0 && v.Kind == VariableKind.StackSlot) {
 						// dead store to stack
-						if (copiedExpr.Flags == InstructionFlags.None) {
+						if (SemanticHelper.IsPure(copiedExpr.Flags)) {
 							// no-op -> delete
 							context.Step("remove dead store to stack: no-op -> delete", block.Instructions[i]);
 							block.Instructions.RemoveAt(i--);

ICSharpCode.Decompiler/IL/Transforms/RemoveDeadVariableInit.cs

@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+using System.Collections.Generic;
+using System.Linq;
 using ICSharpCode.Decompiler.FlowAnalysis;
 
 namespace ICSharpCode.Decompiler.IL.Transforms
@@ -37,13 +39,27 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 					v.HasInitialValue = false;
 				}
 			}
-			if (function.IsIterator || function.IsAsync) {
-				// In yield return + async, the C# compiler tends to store null/default(T) to variables
-				// when the variable goes out of scope. Remove such useless stores.
-				foreach (var v in function.Variables) {
-					if (v.Kind == VariableKind.Local && v.StoreCount == 1 && v.LoadCount == 0 && v.AddressCount == 0) {
-						if (v.StoreInstructions[0] is StLoc stloc && (stloc.Value.MatchLdNull() || stloc.Value is DefaultValue) && stloc.Parent is Block block) {
+			// Remove dead stores to variables that are never read from.
+			// If the stored value has some side-effect, the value is unwrapped.
+			// This is necessary to remove useless stores generated by some compilers, e.g., the F# compiler.
+			// In yield return + async, the C# compiler tends to store null/default(T) to variables
+			// when the variable goes out of scope.
+			var variableQueue = new Queue<ILVariable>(function.Variables.Where(v => v.Kind == VariableKind.Local || v.Kind == VariableKind.StackSlot));
+			while (variableQueue.Count > 0) {
+				var v = variableQueue.Dequeue();
+				if (v.Kind != VariableKind.Local && v.Kind != VariableKind.StackSlot)
+					continue;
+				if (v.LoadCount != 0 || v.AddressCount != 0)
+					continue;
+				foreach (var stloc in v.StoreInstructions.OfType<StLoc>().ToArray()) {
+					if (stloc.Parent is Block block) {
+						if (SemanticHelper.IsPure(stloc.Value.Flags)) {
 							block.Instructions.Remove(stloc);
+						} else {
+							stloc.ReplaceWith(stloc.Value);
+						}
+						if (stloc.Value is LdLoc ldloc && (ldloc.Variable.Kind == VariableKind.Local || ldloc.Variable.Kind == VariableKind.StackSlot)) {
+							variableQueue.Enqueue(ldloc.Variable);
 						}
 					}
 				}