Improved flow analysis in general and especially for "for" loops. Added more

tests.

src/cc65/stmt.c

@@ -229,8 +229,10 @@ static int IfStatement (void)
         g_defcodelabel (Label2);
     }
 
-    /* Done */
-    return StmtFlags;
+    /* Done. We will never return "empty" for an if statement because of side
+    ** effects when evaluating the condition.
+    */
+    return StmtFlags & ~SF_EMPTY;
 }
 
 
@@ -299,8 +301,10 @@ static int DoStatement (void)
         }
     }
 
-    /* "break" and "continue" are not relevant for the following code */
-    StmtFlags &= ~(SF_ANY_BREAK | SF_ANY_CONTINUE);
+    /* "break" and "continue" are not relevant for the following code. "empty"
+    ** is removed because of side effects when evaluating the condition.
+    */
+    StmtFlags &= ~(SF_ANY_BREAK | SF_ANY_CONTINUE | SF_EMPTY);
 
     /* Done */
     return StmtFlags;
@@ -393,8 +397,10 @@ static int WhileStatement (void)
         */
         StmtFlags &= ~SF_MASK_UNREACH;
     }
-    /* "break" and "continue" are not relevant for the following code */
-    StmtFlags &= ~(SF_ANY_BREAK | SF_ANY_CONTINUE);
+    /* "break" and "continue" are not relevant for the following code. "empty"
+    ** is removed because of side effects when evaluating the condition.
+    */
+    StmtFlags &= ~(SF_ANY_BREAK | SF_ANY_CONTINUE | SF_EMPTY);
 
     /* Done */
     return StmtFlags;
@@ -602,6 +608,11 @@ static int ForStatement (void)
     /* Skip the closing paren */
     ConsumeRParen ();
 
+    /* Output a warning if the loop body is never executed */
+    if (TestResult == TESTEXPR_FALSE) {
+        UnreachableCodeWarning ();
+    }
+
     /* Loop body */
     g_defcodelabel (BodyLabel);
     StmtFlags = AnyStatement (&PendingToken, 0);
@@ -628,10 +639,30 @@ static int ForStatement (void)
     /* Remove the loop from the loop stack */
     DelLoop ();
 
-    /* If the condition is always true, any special statements are always
-    ** executed. Otherwise we don't know.
+    /* Fix the flags for the loop. */
+    if (TestResult == TESTEXPR_TRUE) {
+        /* If the loop condition is always true, and we do not have a
+        ** "break" statement, the loop won't terminate. So the only valid
+        ** "unreach" flag is that for the endless loop. Otherwise - if there
+        ** is a "break" statement, the code after the loop is reachable.
+        */
+        StmtFlags &= ~SF_MASK_UNREACH;
+        if (!SF_Any_Break (StmtFlags)) {
+            StmtFlags |= (SF_OTHER | SF_ANY_OTHER);
+        }
+    } else {
+        /* If the loop condition is not always true, the code after the loop
+        ** is always reachable.
+        */
+        StmtFlags &= ~SF_MASK_UNREACH;
+    }
+    /* "break" and "continue" are not relevant for the following code. "empty"
+    ** is removed because of side effects when evaluating the condition.
     */
-    return (TestResult == TESTEXPR_TRUE)? StmtFlags : SF_NONE;
+    StmtFlags &= ~(SF_ANY_BREAK | SF_ANY_CONTINUE | SF_EMPTY);
+
+    /* Done */
+    return StmtFlags;
 }
 
 
@@ -641,15 +672,23 @@ static int CompoundStatement (int* PendingToken, struct SwitchCtrl* Switch)
 ** function returns true if the last statement was a break or return.
 */
 {
+    int OldStack;
+    unsigned OldBlockStackSize;
     int StmtFlags;
 
-    /* Remember the stack at block entry */
-    int OldStack = StackPtr;
-    unsigned OldBlockStackSize = CollCount (&CurrentFunc->LocalsBlockStack);
-
     /* Skip '{' */
     NextToken ();
 
+    /* If the closing curly bracket follows we have an empty statement */
+    if (CurTok.Tok == TOK_RCURLY) {
+        NextToken ();
+        return SF_EMPTY;
+    }
+
+    /* Remember the stack at block entry */
+    OldStack = StackPtr;
+    OldBlockStackSize = CollCount (&CurrentFunc->LocalsBlockStack);
+
     /* Enter a new lexical level */
     EnterBlockLevel ();
 
@@ -742,36 +781,41 @@ int StatementBlock (struct SwitchCtrl* Switch)
         LineInfo* LI1 = UseLineInfo (GetDiagnosticLI ());
         int StmtFlags1 = AnyStatement (0, Switch);
         int Unreachable = 0;            /* True if code is unreachable */
-        int UnreachableWarning = 0;     /* True if warning was output */
+        int Warning = 0;                /* True if warning was output */
         while (CurTok.Tok != TOK_RCURLY && CurTok.Tok != TOK_CEOF) {
             LineInfo* LI2 = UseLineInfo (GetDiagnosticLI ());
             int StmtFlags2 = AnyStatement (0, Switch);
-            if (!UnreachableWarning) {
-                /* If this statement is not already unreachable, check if the
-                ** previous statement made it unreachable.
-                */
-                if (!Unreachable) {
-                    Unreachable = SF_Unreach (StmtFlags1);
-                }
-                /* If the previous statement made this one unreachable, but
-                ** this one has a label, it is not unreachable.
-                */
-                if (Unreachable && SF_Label (StmtFlags2)) {
-                    Unreachable = 0;
-                }
-                /* If this statement is unreachable but not the empty
-                ** statement, output a warning.
-                */
-                if (Unreachable && !SF_Empty (StmtFlags2)) {
-                    LIUnreachableCodeWarning (LI2);
-                    UnreachableWarning = 1;
-                }
+
+            /* If this statement is not already unreachable, check if the
+            ** previous statement made it unreachable.
+            */
+            if (!Unreachable) {
+                Unreachable = SF_Unreach (StmtFlags1);
             }
+            /* If the previous statement made this one unreachable, but this
+            ** one has a label, it is not unreachable.
+            */
+            if (Unreachable && SF_Label (StmtFlags2)) {
+                Unreachable = 0;
+            }
+            /* If this statement is unreachable but not the empty statement,
+            ** and we didn't give a warning before, to that now
+            */
+            if (Unreachable && !SF_Empty (StmtFlags2) && !Warning) {
+                LIUnreachableCodeWarning (LI2);
+                Warning = 1;
+            }
+
+            /* If the current statement wasn't unreachable update the flags */
+            if (!Unreachable) {
+                StmtFlags1 = SF_Any (StmtFlags1) | StmtFlags2;
+            }
+
+            /* Prepare for the next round */
             if (LI1) {
                 ReleaseLineInfo (LI1);
             }
             LI1 = LI2;
-            StmtFlags1 = (StmtFlags1 & SF_MASK_ANY) | StmtFlags2;
         }
         if (LI1) {
             ReleaseLineInfo (LI1);

test/misc/Makefile

@@ -157,6 +157,11 @@ $(WORKDIR)/flow-do-01.$1.$2.prg: flow-do-01.c $(ISEQUAL) | $(WORKDIR)
 	$(CC65) -t sim$2 -$1 -o $$@ $$< $(NULLOUT) 2>$(WORKDIR)/flow-do-01.$1.$2.out
 	$(ISEQUAL) $(WORKDIR)/flow-do-01.$1.$2.out flow-do-01.ref
 
+$(WORKDIR)/flow-for-01.$1.$2.prg: flow-for-01.c $(ISEQUAL) | $(WORKDIR)
+	$(if $(QUIET),echo misc/flow-for-01.$1.$2.prg)
+	$(CC65) -t sim$2 -$1 -o $$@ $$< $(NULLOUT) 2>$(WORKDIR)/flow-for-01.$1.$2.out
+	$(ISEQUAL) $(WORKDIR)/flow-for-01.$1.$2.out flow-for-01.ref
+
 $(WORKDIR)/flow-if-01.$1.$2.prg: flow-if-01.c $(ISEQUAL) | $(WORKDIR)
 	$(if $(QUIET),echo misc/flow-if-01.$1.$2.prg)
 	$(CC65) -t sim$2 -$1 -o $$@ $$< $(NULLOUT) 2>$(WORKDIR)/flow-if-01.$1.$2.out

test/misc/flow-for-01.c

@@ -0,0 +1,90 @@
+int a;
+
+static int f1(void)
+{
+    for (;;) {
+        ++a;
+    }
+    /* Unreachable */
+    a = 2;
+    return a;
+}
+
+static int f2(void)
+{
+    for (;1;) {
+        ++a;
+    }
+    /* Unreachable */
+    a = 2;
+    return a;							 
+}
+
+static int f3(void)
+{
+    for (;;) {
+        ++a;
+        if (a == 5) break;
+    }
+    /* Reachable */
+    a = 2;
+    return a;
+}
+
+static int f4(void)
+{
+    for (;;) {
+        ++a;
+        return a;
+    }
+    /* Unreachable */
+    a = 2;
+}
+
+static int f5(void)
+{
+    for (;0;) {
+        /* Unreachable */
+        ++a;
+        return a;
+    }
+    /* Reachable */
+    a = 2;
+    return 0;
+}
+
+static int f6(void)
+{
+    for (;;) {
+        ++a;
+        if (a == 4) goto L;
+        return a;
+    }
+    /* Reachable via L */
+L:  a = 2;
+}
+
+static int f7(void)
+{
+    for (;0;) {
+        /* Unreachable but no warning */
+    }
+    a = 2;
+    return a;
+}
+
+static int f8(void)
+{
+    for (;a;) {
+        return a;
+    }
+    /* Reachable */
+    a = 2;
+    return a;
+}
+
+int main(void)
+{
+    return f1() + f2() + f3() + f4() + f5() + f6() + f7() + f8();
+}
+

test/misc/flow-for-01.ref

@@ -0,0 +1,4 @@
+flow-for-01.c:9: Warning: Unreachable code
+flow-for-01.c:19: Warning: Unreachable code
+flow-for-01.c:41: Warning: Unreachable code
+flow-for-01.c:48: Warning: Unreachable code

test/misc/flow-while-01.c

@@ -63,8 +63,18 @@ static int f6(void)
     return a;
 }
 
-int main(void)
+static int f7(void)
 {
-    return f1() + f2() + f3() + f4() + f5() + f6();
+    while (a) {
+        return a;
+    }
+    /* Reachable */
+    a = 2;
+    return a;
+}
+
+int main(void)
+{
+    return f1() + f2() + f3() + f4() + f5() + f6() + f7();
 }
 

test/misc/flow-while-02.c

@@ -22,7 +22,7 @@ static int f2(void)
         /* Unreachable */
         break;
     }
-    /* Unreachable but no warning */
+    /* Unreachable */
     a = 2;
     return a;
 }

test/misc/flow-while-02.ref

@@ -1,6 +1,7 @@
 flow-while-02.c:8: Warning: Unreachable code
 flow-while-02.c:12: Warning: Unreachable code
 flow-while-02.c:23: Warning: Unreachable code
+flow-while-02.c:26: Warning: Unreachable code
 flow-while-02.c:38: Warning: Unreachable code
 flow-while-02.c:50: Warning: Unreachable code
 flow-while-02.c:66: Warning: Unreachable code