From a4ad212316bce9ae9a2d54fa35999a2b5c15ba96 Mon Sep 17 00:00:00 2001
From: acqn <acqn163@outlook.com>
Date: Mon, 22 Feb 2021 13:47:32 +0800
Subject: [PATCH] Now the compiler removes code that has no observable effect.

---
 src/cc65/expr.c | 22 ++++++++++++++--------
 src/cc65/stmt.c | 23 ++++++++++++++---------
 2 files changed, 28 insertions(+), 17 deletions(-)

diff --git a/src/cc65/expr.c b/src/cc65/expr.c
index ef3c4ec38..eb0afb38b 100644
--- a/src/cc65/expr.c
+++ b/src/cc65/expr.c
@@ -4188,19 +4188,25 @@ void hie0 (ExprDesc *Expr)
         /* Append deferred inc/dec at sequence point */
         DoDeferred (SQP_KEEP_NONE, Expr);
 
-        /* If the expression didn't generate code or isn't cast to type void,
-        ** emit a warning.
+        /* If the expression has no observable effect and isn't cast to type
+        ** void, emit a warning and remove useless code if any.
         */
         GetCodePos (&End);
-        if (!ED_MayHaveNoEffect (Expr)              &&
-            (CodeRangeIsEmpty (&Start, &End) ||
-             (Expr->Flags & E_SIDE_EFFECTS) == 0)   &&
-            IS_Get (&WarnNoEffect)                  &&
-            PrevErrorCount == ErrorCount) {
-            Warning ("Left-hand operand of comma expression has no effect");
+        if (CodeRangeIsEmpty (&Start, &End) ||
+            (Expr->Flags & E_SIDE_EFFECTS) == 0) {
+
+            if (!ED_MayHaveNoEffect (Expr)  &&
+                IS_Get (&WarnNoEffect)      &&
+                PrevErrorCount == ErrorCount) {
+                Warning ("Left-hand operand of comma expression has no effect");
+            }
+
+            /* Remove code with no effect */
+            RemoveCode (&Start);
         }
 
         PrevErrorCount = ErrorCount;
+
         /* Remember the current code position */
         GetCodePos (&Start);
 
diff --git a/src/cc65/stmt.c b/src/cc65/stmt.c
index a08197c09..7355e88a8 100644
--- a/src/cc65/stmt.c
+++ b/src/cc65/stmt.c
@@ -609,18 +609,23 @@ static void Statement (int* PendingToken)
     Expr.Flags |= E_NEED_NONE;
     Expression0 (&Expr);
 
-    /* If the statement didn't generate code, and is not of type
-    ** void, emit a warning.
+    /* If the statement has no observable effect and isn't cast to type
+    ** void, emit a warning and remove useless code if any.
     */
     GetCodePos (&End);
-    if (!ED_YetToLoad (&Expr)                   &&
-        !ED_MayHaveNoEffect (&Expr)             &&
-        (CodeRangeIsEmpty (&Start, &End) ||
-         (Expr.Flags & E_SIDE_EFFECTS) == 0)    &&
-        IS_Get (&WarnNoEffect)                  &&
-        PrevErrorCount == ErrorCount) {
-        Warning ("Statement has no effect");
+    if (CodeRangeIsEmpty (&Start, &End) ||
+        (Expr.Flags & E_SIDE_EFFECTS) == 0) {
+
+        if (!ED_MayHaveNoEffect (&Expr) &&
+            IS_Get (&WarnNoEffect)      &&
+            PrevErrorCount == ErrorCount) {
+            Warning ("Statement has no effect");
+        }
+
+        /* Remove code with no effect */
+        RemoveCode (&Start);
     }
+
     CheckSemi (PendingToken);
 }