From c0a1b1b887af536612987847c9344a1226bf4397 Mon Sep 17 00:00:00 2001
From: Kugel Fuhr <98353208+kugelfuhr@users.noreply.github.com>
Date: Tue, 24 Jun 2025 18:02:24 +0200
Subject: [PATCH] Add an optimization step that removes compare instructions
 preceeding an RTS. Since nothing is passed back in the flags, these
 instructions have no effect. Fixes #2025.

---
 src/cc65/codeopt.c |  3 +++
 src/cc65/coptcmp.c | 37 +++++++++++++++++++++++++++++++++++++
 src/cc65/coptcmp.h |  5 +++++
 3 files changed, 45 insertions(+)

diff --git a/src/cc65/codeopt.c b/src/cc65/codeopt.c
index 19acfde74..f57417526 100644
--- a/src/cc65/codeopt.c
+++ b/src/cc65/codeopt.c
@@ -136,6 +136,7 @@ static OptFunc DOptCmp5         = { OptCmp5,         "OptCmp5",         100, 0,
 static OptFunc DOptCmp7         = { OptCmp7,         "OptCmp7",          85, 0, 0, 0, 0, 0 };
 static OptFunc DOptCmp8         = { OptCmp8,         "OptCmp8",          50, 0, 0, 0, 0, 0 };
 static OptFunc DOptCmp9         = { OptCmp9,         "OptCmp9",          85, 0, 0, 0, 0, 0 };
+static OptFunc DOptCmp10        = { OptCmp10,        "OptCmp10",         33, 0, 0, 0, 0, 0 };
 static OptFunc DOptComplAX1     = { OptComplAX1,     "OptComplAX1",      65, 0, 0, 0, 0, 0 };
 static OptFunc DOptCondBranch1  = { OptCondBranch1,  "OptCondBranch1",   80, 0, 0, 0, 0, 0 };
 static OptFunc DOptCondBranch2  = { OptCondBranch2,  "OptCondBranch2",   40, 0, 0, 0, 0, 0 };
@@ -251,6 +252,7 @@ static OptFunc* OptFuncs[] = {
     &DOptBranchDist,
     &DOptBranchDist2,
     &DOptCmp1,
+    &DOptCmp10,
     &DOptCmp2,
     &DOptCmp3,
     &DOptCmp4,
@@ -729,6 +731,7 @@ static unsigned RunOptGroup3 (CodeSeg* S)
         C += RunOptFunc (S, &DOptCondBranch3, 1);
         C += RunOptFunc (S, &DOptCondBranchC, 1);
         C += RunOptFunc (S, &DOptRTSJumps1, 1);
+        C += RunOptFunc (S, &DOptCmp10, 1);         /* After OptRTSJumps1 */
         C += RunOptFunc (S, &DOptBoolCmp, 1);
         C += RunOptFunc (S, &DOptBoolTrans, 1);
         C += RunOptFunc (S, &DOptBNegA2, 1);        /* After OptCondBranch's */
diff --git a/src/cc65/coptcmp.c b/src/cc65/coptcmp.c
index 2970b363b..25499dc3c 100644
--- a/src/cc65/coptcmp.c
+++ b/src/cc65/coptcmp.c
@@ -741,3 +741,40 @@ unsigned OptCmp9 (CodeSeg* S)
     /* Return the number of changes made */
     return Changes;
 }
+
+
+
+unsigned OptCmp10 (CodeSeg* S)
+/* Remove compare instructions before an RTS. This is safe since no C function
+** passes back something in the flags.
+*/
+{
+    unsigned Changes = 0;
+    unsigned I;
+
+    /* Walk over the entries */
+    I = 0;
+    while (I < CS_GetEntryCount (S)) {
+
+        CodeEntry* N;
+
+        /* Get next entry */
+        CodeEntry* E = CS_GetEntry (S, I);
+
+        /* Check for a compare followed by an RTS */
+        if ((E->Info & OF_CMP) != 0             &&      /* Compare insn */
+            (N = CS_GetNextEntry (S, I)) != 0   &&      /* Next entry ... */
+            N->OPC == OP65_RTS) {                       /* ... is RTS */
+
+            /* Found, remove the compare */
+            CS_DelEntry (S, I);
+            ++Changes;
+        }
+
+        /* Next entry */
+        ++I;
+    }
+
+    /* Return the number of changes made */
+    return Changes;
+}
diff --git a/src/cc65/coptcmp.h b/src/cc65/coptcmp.h
index dd188f7fc..ff58d37df 100644
--- a/src/cc65/coptcmp.h
+++ b/src/cc65/coptcmp.h
@@ -146,6 +146,11 @@ unsigned OptCmp9 (CodeSeg* S);
 ** flag instead of the carry flag and remove the asl.
 */
 
+unsigned OptCmp10 (CodeSeg* S);
+/* Remove compare instructions before an RTS. This is safe since no C function
+** passes back something in the flags.
+*/
+
 
 
 /* End of coptcmp.h */