From d711b6d6fa3526095cf6013c31fa4fba2c3141dc Mon Sep 17 00:00:00 2001
From: Alex Volkov <codepro@usa.net>
Date: Sun, 22 Mar 2026 19:42:57 -0400
Subject: [PATCH] Prevent Lhs load insns shared with the Rhs from being
 inadvertently removed by Rhs optimizers. Fixes issue #2947.

---
 src/cc65/codeoptutil.c | 62 ++++++++++++++++++++++++++++++++++++++++++
 src/cc65/codeoptutil.h |  5 ++++
 src/cc65/coptstop.c    |  6 ++++
 3 files changed, 73 insertions(+)

diff --git a/src/cc65/codeoptutil.c b/src/cc65/codeoptutil.c
index 44174c5fc..b098845b4 100644
--- a/src/cc65/codeoptutil.c
+++ b/src/cc65/codeoptutil.c
@@ -509,6 +509,68 @@ void SetIfOperandLoadUnremovable (LoadInfo* LI, unsigned Used)
 
 
 
+static int IsRegInsnSameAsOtherAX (int InsnIndex, const LoadInfo* OtherLI)
+/* Helper: Return true if the other side (Rhs or Lhs) refers to the same insn */
+{
+    if (InsnIndex < 0) {
+        /* No, do not have an insn */
+        return 0;
+    }
+
+    /* Check if this insn matches any load/change on the other side */
+    return InsnIndex == OtherLI->A.LoadIndex ||
+           InsnIndex == OtherLI->A.ChgIndex ||
+           InsnIndex == OtherLI->X.LoadIndex ||
+           InsnIndex == OtherLI->X.ChgIndex;
+}
+
+
+
+static int IsRegInsnSameAsOtherY (int InsnIndex, const LoadInfo* OtherLI)
+/* Helper: Return true if the other side (Rhs or Lhs) refers to the same insn */
+{
+    if (InsnIndex < 0) {
+        /* No, do not have an insn */
+        return 0;
+    }
+
+    /* Check if this insn matches any load/change on the other side */
+    return InsnIndex == OtherLI->Y.LoadIndex ||
+           InsnIndex == OtherLI->Y.ChgIndex;
+}
+
+
+
+void SetUnremovableIfUsedByOther (LoadInfo* LI, const LoadInfo* OtherLI)
+/* Check and flag operand loads unremovable if the other side (Rhs or Lhs)
+** uses the same load insn (checked by index).
+*/
+{
+    /* Disallow removing the loads if they are shared between Lhs and Rhs.
+    ** Note: For safety, we check the ChgIndex as well, which accounts for
+    ** conditions where a label is encountered in the block.
+    */
+    if (IsRegInsnSameAsOtherAX (LI->A.LoadIndex, OtherLI) ||
+        IsRegInsnSameAsOtherAX (LI->A.ChgIndex, OtherLI)) {
+        /* Parts of A load are used by other and cannot be removed */
+        LI->A.Flags |= LI_DONT_REMOVE;
+    }
+
+    if (IsRegInsnSameAsOtherAX (LI->X.LoadIndex, OtherLI) ||
+        IsRegInsnSameAsOtherAX (LI->X.ChgIndex, OtherLI)) {
+        /* Parts of X load are used by other and cannot be removed */
+        LI->X.Flags |= LI_DONT_REMOVE;
+    }
+
+    if (IsRegInsnSameAsOtherY (LI->Y.LoadIndex, OtherLI) ||
+        IsRegInsnSameAsOtherY (LI->Y.ChgIndex, OtherLI)) {
+        /* Parts of Y load are used by other and cannot be removed */
+        LI->Y.Flags |= LI_DONT_REMOVE;
+    }
+}
+
+
+
 unsigned int TrackLoads (LoadInfo* LI, CodeSeg* S, int I)
 /* Track loads for a code entry.
 ** Return used registers.
diff --git a/src/cc65/codeoptutil.h b/src/cc65/codeoptutil.h
index cf7743605..06a9d4ce3 100644
--- a/src/cc65/codeoptutil.h
+++ b/src/cc65/codeoptutil.h
@@ -180,6 +180,11 @@ void SetIfOperandSrcAffected (LoadInfo* LLI, CodeEntry* E);
 void SetIfOperandLoadUnremovable (LoadInfo* LI, unsigned Used);
 /* Check and flag operand load that may be unremovable */
 
+void SetUnremovableIfUsedByOther (LoadInfo* LI, const LoadInfo* OtherLI);
+/* Check and flag operand loads unremovable if the other side (Rhs or Lhs)
+** uses the same load insn (checked by index).
+*/
+
 unsigned int TrackLoads (LoadInfo* LI, CodeSeg* S, int I);
 /* Track loads for a code entry.
 ** Return used registers.
diff --git a/src/cc65/coptstop.c b/src/cc65/coptstop.c
index 945dc1c54..0bc533297 100644
--- a/src/cc65/coptstop.c
+++ b/src/cc65/coptstop.c
@@ -2090,6 +2090,12 @@ unsigned OptStackOps (CodeSeg* S)
                     }
                 }
 
+                /* Lhs and Rhs may shared some of the load insns; or due to how
+                ** the labels are processed, "change" insns may refer to the
+                ** others' loads. In either case, it is unsafe to remove them.
+                */
+                SetUnremovableIfUsedByOther (&Data.Lhs, &Data.Rhs);
+
                 /* Flag entries that can't be removed */
                 SetDontRemoveEntryFlags (&Data);