diff --git a/docs/Range.md b/docs/Range.md
index 644711e..4ad61bc 100644
--- a/docs/Range.md
+++ b/docs/Range.md
@@ -63,6 +63,8 @@ In spite of this you can use ranges in for loops:
     >>> 3
     >>> void
 
+The loop variable is read-only inside the loop body (behaves like a `val`).
+
 but
 
     for( i in 1..<3 ) 
diff --git a/docs/advanced_topics.md b/docs/advanced_topics.md
index 90c8fd3..1bdfe9d 100644
--- a/docs/advanced_topics.md
+++ b/docs/advanced_topics.md
@@ -105,6 +105,7 @@ arguments list in almost arbitrary ways. For example:
         var result = ""
         for( a in args ) result += a
     }
+    // loop variables are read-only inside the loop body
 
     assertEquals( 
         "4231", 
diff --git a/docs/tutorial.md b/docs/tutorial.md
index aa6f787..5162813 100644
--- a/docs/tutorial.md
+++ b/docs/tutorial.md
@@ -739,6 +739,7 @@ See also: [Testing and Assertions](Testing.md)
         var result = []
         for( x in iterable ) result += transform(x)
     }
+    // loop variables are read-only inside the loop body
     assert( [11, 21, 31] == mapValues( [1,2,3], { it*10+1 }))
     >>> void
 
diff --git a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/BytecodeCompiler.kt b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/BytecodeCompiler.kt
index 03032f9..6a6526a 100644
--- a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/BytecodeCompiler.kt
+++ b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/BytecodeCompiler.kt
@@ -77,6 +77,8 @@ class BytecodeCompiler(
     private val slotInitClassByKey = mutableMapOf<ScopeSlotKey, ObjClass>()
     private val intLoopVarNames = LinkedHashSet<String>()
     private val valueFnRefs = LinkedHashSet<ValueFnRef>()
+    private val loopVarKeys = LinkedHashSet<ScopeSlotKey>()
+    private val loopVarSlots = HashSet<Int>()
     private val loopStack = ArrayDeque<LoopContext>()
     private var currentPos: Pos? = null
 
@@ -1950,6 +1952,10 @@ class BytecodeCompiler(
                 return value
             }
             val value = compileRef(assignValue(ref)) ?: return null
+            if (isLoopVarRef(localTarget)) {
+                emitLoopVarReassignError(localTarget.name, localTarget.pos())
+                return value
+            }
             if (!localTarget.isMutable || localTarget.isDelegated) {
                 val msgId = builder.addConst(BytecodeConst.StringVal("can't reassign val ${localTarget.name}"))
                 val msgSlot = allocSlot()
@@ -1990,6 +1996,11 @@ class BytecodeCompiler(
             val resolved = resolveAssignableSlotByName(nameTarget) ?: return null
             val slot = resolved.first
             val isMutable = resolved.second
+            if (isLoopVarSlot(slot)) {
+                val pos = (ref.target as? LocalVarRef)?.pos() ?: Pos.builtIn
+                emitLoopVarReassignError(nameTarget, pos)
+                return value
+            }
             if (!isMutable) {
                 val msgId = builder.addConst(BytecodeConst.StringVal("can't reassign val $nameTarget"))
                 val msgSlot = allocSlot()
@@ -2227,6 +2238,11 @@ class BytecodeCompiler(
         val localTarget = ref.target as? LocalSlotRef
         if (localTarget != null) {
             if (!allowLocalSlots) return compileEvalRef(ref)
+            if (isLoopVarRef(localTarget)) {
+                val rhs = compileRef(ref.value) ?: return compileEvalRef(ref)
+                emitLoopVarReassignError(localTarget.name, localTarget.pos())
+                return rhs
+            }
             if (localTarget.isDelegated) {
                 val slot = resolveSlot(localTarget) ?: return null
                 if (slot < scopeSlotCount) return null
@@ -2299,6 +2315,12 @@ class BytecodeCompiler(
         }
         val varTarget = ref.target as? LocalVarRef
         if (varTarget != null) {
+            val resolved = resolveAssignableSlotByName(varTarget.name)
+            if (resolved != null && isLoopVarSlot(resolved.first)) {
+                val rhs = compileRef(ref.value) ?: return compileEvalRef(ref)
+                emitLoopVarReassignError(varTarget.name, varTarget.pos())
+                return rhs
+            }
             return compileEvalRef(ref)
         }
         val objOp = when (ref.op) {
@@ -2591,6 +2613,10 @@ class BytecodeCompiler(
             }
             is LocalSlotRef -> {
                 if (!allowLocalSlots || !target.isMutable) return null
+                if (isLoopVarRef(target)) {
+                    emitLoopVarReassignError(target.name, target.pos())
+                    return CompiledValue(currentObj.slot, SlotType.OBJ)
+                }
                 if (target.isDelegated) {
                     val slot = resolveSlot(target) ?: return null
                     if (slot < scopeSlotCount) return null
@@ -3178,6 +3204,10 @@ class BytecodeCompiler(
         val target = ref.target as? LocalSlotRef
         if (target != null) {
             if (!allowLocalSlots) return null
+            if (isLoopVarRef(target)) {
+                val errorSlot = emitLoopVarReassignError(target.name, target.pos())
+                return CompiledValue(errorSlot, SlotType.OBJ)
+            }
             if (!target.isMutable) return null
             if (target.isDelegated) {
                 val slot = resolveSlot(target) ?: return null
@@ -3345,6 +3375,14 @@ class BytecodeCompiler(
                 else -> null
             }
         }
+        val varTarget = ref.target as? LocalVarRef
+        if (varTarget != null) {
+            val resolved = resolveAssignableSlotByName(varTarget.name)
+            if (resolved != null && isLoopVarSlot(resolved.first)) {
+                val errorSlot = emitLoopVarReassignError(varTarget.name, varTarget.pos())
+                return CompiledValue(errorSlot, SlotType.OBJ)
+            }
+        }
 
         val thisFieldTarget = ref.target as? ThisFieldSlotRef
         if (thisFieldTarget != null) {
@@ -4380,18 +4418,18 @@ class BytecodeCompiler(
         } else {
             stmt.condition
         }
-        val conditionStmt = conditionTarget as? ExpressionStatement ?: return null
-        val condValue = compileRefWithFallback(conditionStmt.ref, SlotType.BOOL, stmt.pos) ?: return null
-        if (condValue.type != SlotType.BOOL) return null
-
         val resultSlot = allocSlot()
         val elseLabel = builder.label()
         val endLabel = builder.label()
-
-        builder.emit(
-            Opcode.JMP_IF_FALSE,
-            listOf(CmdBuilder.Operand.IntVal(condValue.slot), CmdBuilder.Operand.LabelRef(elseLabel))
-        )
+        val conditionStmt = conditionTarget as? ExpressionStatement ?: return null
+        if (!emitIntCompareJump(conditionStmt.ref, jumpOnTrue = false, target = elseLabel)) {
+            val condValue = compileRefWithFallback(conditionStmt.ref, SlotType.BOOL, stmt.pos) ?: return null
+            if (condValue.type != SlotType.BOOL) return null
+            builder.emit(
+                Opcode.JMP_IF_FALSE,
+                listOf(CmdBuilder.Operand.IntVal(condValue.slot), CmdBuilder.Operand.LabelRef(elseLabel))
+            )
+        }
         val thenValue = compileStatementValueOrFallback(stmt.ifBody) ?: return null
         emitMove(thenValue, resultSlot)
         builder.emit(Opcode.JMP, listOf(CmdBuilder.Operand.LabelRef(endLabel)))
@@ -5403,6 +5441,8 @@ class BytecodeCompiler(
         }
 
         try {
+        val needsBreakFlag = stmt.canBreak || stmt.elseStatement != null
+        val breakFlagSlot = allocSlot()
         if (range == null && rangeRef == null && typedRangeLocal == null) {
             val sourceValue = compileStatementValueOrFallback(stmt.source) ?: return null
             val sourceObj = ensureObjSlot(sourceValue)
@@ -5430,13 +5470,18 @@ class BytecodeCompiler(
             builder.emit(Opcode.CALL_MEMBER_SLOT, sourceObj.slot, iteratorMethodId, 0, 0, iterSlot)
             builder.emit(Opcode.ITER_PUSH, iterSlot)
 
-            val breakFlagSlot = allocSlot()
-            val falseId = builder.addConst(BytecodeConst.Bool(false))
-            builder.emit(Opcode.CONST_BOOL, falseId, breakFlagSlot)
-
-            val resultSlot = allocSlot()
-            val voidId = builder.addConst(BytecodeConst.ObjRef(ObjVoid))
-            builder.emit(Opcode.CONST_OBJ, voidId, resultSlot)
+            if (needsBreakFlag) {
+                val falseId = builder.addConst(BytecodeConst.Bool(false))
+                builder.emit(Opcode.CONST_BOOL, falseId, breakFlagSlot)
+            }
+            val resultSlot = if (wantResult) {
+                val slot = allocSlot()
+                val voidId = builder.addConst(BytecodeConst.ObjRef(ObjVoid))
+                builder.emit(Opcode.CONST_OBJ, voidId, slot)
+                slot
+            } else {
+                null
+            }
 
             val loopLabel = builder.label()
             val continueLabel = builder.label()
@@ -5465,7 +5510,7 @@ class BytecodeCompiler(
                     endLabel,
                     continueLabel,
                     breakFlagSlot,
-                    if (wantResult) resultSlot else null,
+                    resultSlot,
                     hasIterator = true
                 )
             )
@@ -5473,42 +5518,52 @@ class BytecodeCompiler(
             loopStack.removeLast()
             if (wantResult) {
                 val bodyObj = ensureObjSlot(bodyValue)
-                builder.emit(Opcode.MOVE_OBJ, bodyObj.slot, resultSlot)
+                builder.emit(Opcode.MOVE_OBJ, bodyObj.slot, resultSlot!!)
             }
             builder.mark(continueLabel)
             builder.emit(Opcode.JMP, listOf(CmdBuilder.Operand.LabelRef(loopLabel)))
 
             builder.mark(endLabel)
-            val afterPop = builder.label()
-            builder.emit(
-                Opcode.JMP_IF_TRUE,
-                listOf(CmdBuilder.Operand.IntVal(breakFlagSlot), CmdBuilder.Operand.LabelRef(afterPop))
-            )
-            builder.emit(Opcode.ITER_POP)
-            builder.mark(afterPop)
-            if (stmt.elseStatement != null) {
-                val afterElse = builder.label()
+            if (needsBreakFlag) {
+                val afterPop = builder.label()
                 builder.emit(
                     Opcode.JMP_IF_TRUE,
-                    listOf(CmdBuilder.Operand.IntVal(breakFlagSlot), CmdBuilder.Operand.LabelRef(afterElse))
+                    listOf(CmdBuilder.Operand.IntVal(breakFlagSlot), CmdBuilder.Operand.LabelRef(afterPop))
                 )
+                builder.emit(Opcode.ITER_POP)
+                builder.mark(afterPop)
+            } else {
+                builder.emit(Opcode.ITER_POP)
+            }
+            if (stmt.elseStatement != null) {
+                val afterElse = if (needsBreakFlag) builder.label() else null
+                if (needsBreakFlag) {
+                    builder.emit(
+                        Opcode.JMP_IF_TRUE,
+                        listOf(CmdBuilder.Operand.IntVal(breakFlagSlot), CmdBuilder.Operand.LabelRef(afterElse!!))
+                    )
+                }
                 val elseValue = compileStatementValueOrFallback(stmt.elseStatement, wantResult) ?: return null
                 if (wantResult) {
                     val elseObj = ensureObjSlot(elseValue)
-                    builder.emit(Opcode.MOVE_OBJ, elseObj.slot, resultSlot)
+                    builder.emit(Opcode.MOVE_OBJ, elseObj.slot, resultSlot!!)
+                }
+                if (needsBreakFlag) {
+                    builder.mark(afterElse!!)
                 }
-                builder.mark(afterElse)
             }
-            return resultSlot
+            return resultSlot ?: breakFlagSlot
         }
 
-        val iSlot = allocSlot()
+        val iSlot = loopSlotId
         val endSlot = allocSlot()
         if (range != null) {
             val startId = builder.addConst(BytecodeConst.IntVal(range.start))
             val endId = builder.addConst(BytecodeConst.IntVal(range.endExclusive))
             builder.emit(Opcode.CONST_INT, startId, iSlot)
             builder.emit(Opcode.CONST_INT, endId, endSlot)
+            updateSlotType(iSlot, SlotType.INT)
+            updateSlotTypeByName(stmt.loopVarName, SlotType.INT)
         } else {
             if (rangeRef != null) {
                 val left = rangeRef.left ?: return null
@@ -5521,6 +5576,8 @@ class BytecodeCompiler(
                 if (rangeRef.isEndInclusive) {
                     builder.emit(Opcode.INC_INT, endSlot)
                 }
+                updateSlotType(iSlot, SlotType.INT)
+                updateSlotTypeByName(stmt.loopVarName, SlotType.INT)
             } else {
                 val rangeLocal = typedRangeLocal ?: return null
                 val rangeValue = compileRef(rangeLocal) ?: return null
@@ -5532,27 +5589,33 @@ class BytecodeCompiler(
                     Opcode.JMP_IF_FALSE,
                     listOf(CmdBuilder.Operand.IntVal(okSlot), CmdBuilder.Operand.LabelRef(badRangeLabel))
                 )
-                val breakFlagSlot = allocSlot()
-                val falseId = builder.addConst(BytecodeConst.Bool(false))
-                builder.emit(Opcode.CONST_BOOL, falseId, breakFlagSlot)
-
-                val resultSlot = allocSlot()
-                val voidId = builder.addConst(BytecodeConst.ObjRef(ObjVoid))
-                builder.emit(Opcode.CONST_OBJ, voidId, resultSlot)
+                if (needsBreakFlag) {
+                    val falseId = builder.addConst(BytecodeConst.Bool(false))
+                    builder.emit(Opcode.CONST_BOOL, falseId, breakFlagSlot)
+                }
+                val resultSlot = if (wantResult) {
+                    val slot = allocSlot()
+                    val voidId = builder.addConst(BytecodeConst.ObjRef(ObjVoid))
+                    builder.emit(Opcode.CONST_OBJ, voidId, slot)
+                    slot
+                } else {
+                    null
+                }
 
                 val loopLabel = builder.label()
                 val continueLabel = builder.label()
                 val endLabel = builder.label()
                 val doneLabel = builder.label()
                 builder.mark(loopLabel)
-                val cmpSlot = allocSlot()
-                builder.emit(Opcode.CMP_GTE_INT, iSlot, endSlot, cmpSlot)
                 builder.emit(
-                    Opcode.JMP_IF_TRUE,
-                    listOf(CmdBuilder.Operand.IntVal(cmpSlot), CmdBuilder.Operand.LabelRef(endLabel))
+                    Opcode.JMP_IF_GTE_INT,
+                    listOf(
+                        CmdBuilder.Operand.IntVal(iSlot),
+                        CmdBuilder.Operand.IntVal(endSlot),
+                        CmdBuilder.Operand.LabelRef(endLabel)
+                    )
                 )
-                emitMove(CompiledValue(iSlot, SlotType.INT), loopSlotId)
-                updateSlotType(loopSlotId, SlotType.INT)
+                updateSlotType(iSlot, SlotType.INT)
                 updateSlotTypeByName(stmt.loopVarName, SlotType.INT)
                 loopStack.addLast(
                     LoopContext(
@@ -5560,7 +5623,7 @@ class BytecodeCompiler(
                         endLabel,
                         continueLabel,
                         breakFlagSlot,
-                        if (wantResult) resultSlot else null,
+                        resultSlot,
                         hasIterator = false
                     )
                 )
@@ -5568,7 +5631,7 @@ class BytecodeCompiler(
                 loopStack.removeLast()
                 if (wantResult) {
                     val bodyObj = ensureObjSlot(bodyValue)
-                    builder.emit(Opcode.MOVE_OBJ, bodyObj.slot, resultSlot)
+                    builder.emit(Opcode.MOVE_OBJ, bodyObj.slot, resultSlot!!)
                 }
                 builder.mark(continueLabel)
                 builder.emit(Opcode.INC_INT, iSlot)
@@ -5576,49 +5639,60 @@ class BytecodeCompiler(
 
                 builder.mark(endLabel)
                 if (stmt.elseStatement != null) {
-                    val afterElse = builder.label()
-                    builder.emit(
-                        Opcode.JMP_IF_TRUE,
-                        listOf(CmdBuilder.Operand.IntVal(breakFlagSlot), CmdBuilder.Operand.LabelRef(afterElse))
-                    )
+                    val afterElse = if (needsBreakFlag) builder.label() else null
+                    if (needsBreakFlag) {
+                        builder.emit(
+                            Opcode.JMP_IF_TRUE,
+                            listOf(CmdBuilder.Operand.IntVal(breakFlagSlot), CmdBuilder.Operand.LabelRef(afterElse!!))
+                        )
+                    }
                     val elseValue = compileStatementValueOrFallback(stmt.elseStatement, wantResult) ?: return null
                     if (wantResult) {
                         val elseObj = ensureObjSlot(elseValue)
-                        builder.emit(Opcode.MOVE_OBJ, elseObj.slot, resultSlot)
+                        builder.emit(Opcode.MOVE_OBJ, elseObj.slot, resultSlot!!)
+                    }
+                    if (needsBreakFlag) {
+                        builder.mark(afterElse!!)
                     }
-                    builder.mark(afterElse)
                 }
                 builder.emit(Opcode.JMP, listOf(CmdBuilder.Operand.LabelRef(doneLabel)))
                 builder.mark(badRangeLabel)
                 val msgId = builder.addConst(BytecodeConst.StringVal("expected Int range"))
-                builder.emit(Opcode.CONST_OBJ, msgId, resultSlot)
+                val errorSlot = resultSlot ?: allocSlot()
+                builder.emit(Opcode.CONST_OBJ, msgId, errorSlot)
                 val posId = builder.addConst(BytecodeConst.PosVal(stmt.pos))
-                builder.emit(Opcode.THROW, posId, resultSlot)
+                builder.emit(Opcode.THROW, posId, errorSlot)
                 builder.mark(doneLabel)
-                return resultSlot
+                return resultSlot ?: breakFlagSlot
             }
         }
 
-        val breakFlagSlot = allocSlot()
-        val falseId = builder.addConst(BytecodeConst.Bool(false))
-        builder.emit(Opcode.CONST_BOOL, falseId, breakFlagSlot)
-
-        val resultSlot = allocSlot()
-        val voidId = builder.addConst(BytecodeConst.ObjRef(ObjVoid))
-        builder.emit(Opcode.CONST_OBJ, voidId, resultSlot)
+        if (needsBreakFlag) {
+            val falseId = builder.addConst(BytecodeConst.Bool(false))
+            builder.emit(Opcode.CONST_BOOL, falseId, breakFlagSlot)
+        }
+        val resultSlot = if (wantResult) {
+            val slot = allocSlot()
+            val voidId = builder.addConst(BytecodeConst.ObjRef(ObjVoid))
+            builder.emit(Opcode.CONST_OBJ, voidId, slot)
+            slot
+        } else {
+            null
+        }
 
         val loopLabel = builder.label()
         val continueLabel = builder.label()
         val endLabel = builder.label()
         builder.mark(loopLabel)
-        val cmpSlot = allocSlot()
-        builder.emit(Opcode.CMP_GTE_INT, iSlot, endSlot, cmpSlot)
         builder.emit(
-            Opcode.JMP_IF_TRUE,
-            listOf(CmdBuilder.Operand.IntVal(cmpSlot), CmdBuilder.Operand.LabelRef(endLabel))
+            Opcode.JMP_IF_GTE_INT,
+            listOf(
+                CmdBuilder.Operand.IntVal(iSlot),
+                CmdBuilder.Operand.IntVal(endSlot),
+                CmdBuilder.Operand.LabelRef(endLabel)
+            )
         )
-        builder.emit(Opcode.MOVE_INT, iSlot, loopSlotId)
-        updateSlotType(loopSlotId, SlotType.INT)
+        updateSlotType(iSlot, SlotType.INT)
         updateSlotTypeByName(stmt.loopVarName, SlotType.INT)
         loopStack.addLast(
             LoopContext(
@@ -5626,7 +5700,7 @@ class BytecodeCompiler(
                 endLabel,
                 continueLabel,
                 breakFlagSlot,
-                if (wantResult) resultSlot else null,
+                resultSlot,
                 hasIterator = false
             )
         )
@@ -5634,7 +5708,7 @@ class BytecodeCompiler(
         loopStack.removeLast()
         if (wantResult) {
             val bodyObj = ensureObjSlot(bodyValue)
-            builder.emit(Opcode.MOVE_OBJ, bodyObj.slot, resultSlot)
+            builder.emit(Opcode.MOVE_OBJ, bodyObj.slot, resultSlot!!)
         }
         builder.mark(continueLabel)
         builder.emit(Opcode.INC_INT, iSlot)
@@ -5642,19 +5716,23 @@ class BytecodeCompiler(
 
         builder.mark(endLabel)
         if (stmt.elseStatement != null) {
-            val afterElse = builder.label()
-            builder.emit(
-                Opcode.JMP_IF_TRUE,
-                listOf(CmdBuilder.Operand.IntVal(breakFlagSlot), CmdBuilder.Operand.LabelRef(afterElse))
-            )
+            val afterElse = if (needsBreakFlag) builder.label() else null
+            if (needsBreakFlag) {
+                builder.emit(
+                    Opcode.JMP_IF_TRUE,
+                    listOf(CmdBuilder.Operand.IntVal(breakFlagSlot), CmdBuilder.Operand.LabelRef(afterElse!!))
+                )
+            }
             val elseValue = compileStatementValueOrFallback(stmt.elseStatement, wantResult) ?: return null
             if (wantResult) {
                 val elseObj = ensureObjSlot(elseValue)
-                builder.emit(Opcode.MOVE_OBJ, elseObj.slot, resultSlot)
+                builder.emit(Opcode.MOVE_OBJ, elseObj.slot, resultSlot!!)
+            }
+            if (needsBreakFlag) {
+                builder.mark(afterElse!!)
             }
-            builder.mark(afterElse)
         }
-        return resultSlot
+        return resultSlot ?: breakFlagSlot
         } finally {
             if (usedOverride) {
                 loopSlotOverrides.remove(stmt.loopVarName)
@@ -5693,12 +5771,16 @@ class BytecodeCompiler(
             builder.emit(Opcode.MOVE_OBJ, bodyObj.slot, resultSlot)
         }
         builder.mark(continueLabel)
-        val condition = compileCondition(stmt.condition, stmt.pos) ?: return null
-        if (condition.type != SlotType.BOOL) return null
-        builder.emit(
-            Opcode.JMP_IF_TRUE,
-            listOf(CmdBuilder.Operand.IntVal(condition.slot), CmdBuilder.Operand.LabelRef(loopLabel))
-        )
+        val conditionTarget = if (stmt.condition is BytecodeStatement) stmt.condition.original else stmt.condition
+        val conditionStmt = conditionTarget as? ExpressionStatement ?: return null
+        if (!emitIntCompareJump(conditionStmt.ref, jumpOnTrue = true, target = loopLabel)) {
+            val condition = compileRefWithFallback(conditionStmt.ref, SlotType.BOOL, stmt.pos) ?: return null
+            if (condition.type != SlotType.BOOL) return null
+            builder.emit(
+                Opcode.JMP_IF_TRUE,
+                listOf(CmdBuilder.Operand.IntVal(condition.slot), CmdBuilder.Operand.LabelRef(loopLabel))
+            )
+        }
 
         builder.mark(endLabel)
         if (stmt.elseStatement != null) {
@@ -5748,12 +5830,16 @@ class BytecodeCompiler(
             builder.emit(Opcode.MOVE_OBJ, bodyObj.slot, resultSlot)
         }
         builder.mark(continueLabel)
-        val condition = compileCondition(stmt.condition, stmt.pos) ?: return null
-        if (condition.type != SlotType.BOOL) return null
-        builder.emit(
-            Opcode.JMP_IF_TRUE,
-            listOf(CmdBuilder.Operand.IntVal(condition.slot), CmdBuilder.Operand.LabelRef(loopLabel))
-        )
+        val conditionTarget = if (stmt.condition is BytecodeStatement) stmt.condition.original else stmt.condition
+        val conditionStmt = conditionTarget as? ExpressionStatement ?: return null
+        if (!emitIntCompareJump(conditionStmt.ref, jumpOnTrue = true, target = loopLabel)) {
+            val condition = compileRefWithFallback(conditionStmt.ref, SlotType.BOOL, stmt.pos) ?: return null
+            if (condition.type != SlotType.BOOL) return null
+            builder.emit(
+                Opcode.JMP_IF_TRUE,
+                listOf(CmdBuilder.Operand.IntVal(condition.slot), CmdBuilder.Operand.LabelRef(loopLabel))
+            )
+        }
 
         builder.mark(endLabel)
         if (stmt.elseStatement != null) {
@@ -5773,14 +5859,18 @@ class BytecodeCompiler(
     }
 
     private fun compileIfStatement(stmt: IfStatement): CompiledValue? {
-        val condition = compileCondition(stmt.condition, stmt.pos) ?: return null
-        if (condition.type != SlotType.BOOL) return null
         val elseLabel = builder.label()
         val endLabel = builder.label()
-        builder.emit(
-            Opcode.JMP_IF_FALSE,
-            listOf(CmdBuilder.Operand.IntVal(condition.slot), CmdBuilder.Operand.LabelRef(elseLabel))
-        )
+        val conditionTarget = if (stmt.condition is BytecodeStatement) stmt.condition.original else stmt.condition
+        val conditionStmt = conditionTarget as? ExpressionStatement ?: return null
+        if (!emitIntCompareJump(conditionStmt.ref, jumpOnTrue = false, target = elseLabel)) {
+            val condition = compileRefWithFallback(conditionStmt.ref, SlotType.BOOL, stmt.pos) ?: return null
+            if (condition.type != SlotType.BOOL) return null
+            builder.emit(
+                Opcode.JMP_IF_FALSE,
+                listOf(CmdBuilder.Operand.IntVal(condition.slot), CmdBuilder.Operand.LabelRef(elseLabel))
+            )
+        }
         val thenRestore = applyFlowTypeOverride(flowTypeOverrideForIf(stmt.condition, applyForThen = true))
         compileStatementValueOrFallback(stmt.ifBody, false) ?: return null
         restoreFlowTypeOverride(thenRestore)
@@ -5792,7 +5882,10 @@ class BytecodeCompiler(
             restoreFlowTypeOverride(elseRestore)
         }
         builder.mark(endLabel)
-        return condition
+        val slot = allocSlot()
+        val voidId = builder.addConst(BytecodeConst.ObjRef(ObjVoid))
+        builder.emit(Opcode.CONST_OBJ, voidId, slot)
+        return CompiledValue(slot, SlotType.OBJ)
     }
 
     private fun updateSlotTypeByName(name: String, type: SlotType) {
@@ -5809,15 +5902,19 @@ class BytecodeCompiler(
     }
 
     private fun compileIfExpression(stmt: IfStatement): CompiledValue? {
-        val condition = compileCondition(stmt.condition, stmt.pos) ?: return null
-        if (condition.type != SlotType.BOOL) return null
         val resultSlot = allocSlot()
         val elseLabel = builder.label()
         val endLabel = builder.label()
-        builder.emit(
-            Opcode.JMP_IF_FALSE,
-            listOf(CmdBuilder.Operand.IntVal(condition.slot), CmdBuilder.Operand.LabelRef(elseLabel))
-        )
+        val conditionTarget = if (stmt.condition is BytecodeStatement) stmt.condition.original else stmt.condition
+        val conditionStmt = conditionTarget as? ExpressionStatement ?: return null
+        if (!emitIntCompareJump(conditionStmt.ref, jumpOnTrue = false, target = elseLabel)) {
+            val condition = compileRefWithFallback(conditionStmt.ref, SlotType.BOOL, stmt.pos) ?: return null
+            if (condition.type != SlotType.BOOL) return null
+            builder.emit(
+                Opcode.JMP_IF_FALSE,
+                listOf(CmdBuilder.Operand.IntVal(condition.slot), CmdBuilder.Operand.LabelRef(elseLabel))
+            )
+        }
         val thenRestore = applyFlowTypeOverride(flowTypeOverrideForIf(stmt.condition, applyForThen = true))
         val thenValue = compileStatementValueOrFallback(stmt.ifBody) ?: return null
         restoreFlowTypeOverride(thenRestore)
@@ -5840,6 +5937,70 @@ class BytecodeCompiler(
         return CompiledValue(resultSlot, SlotType.OBJ)
     }
 
+    private fun emitIntCompareJump(ref: ObjRef, jumpOnTrue: Boolean, target: CmdBuilder.Label): Boolean {
+        setPos(refPosOrCurrent(ref))
+        val binary = ref as? BinaryOpRef ?: return false
+        val op = binaryOp(binary)
+        if (op != BinOp.EQ && op != BinOp.NEQ && op != BinOp.LT && op != BinOp.LTE && op != BinOp.GT && op != BinOp.GTE) {
+            return false
+        }
+        val leftRef = binaryLeft(binary)
+        val rightRef = binaryRight(binary)
+        if (!isSimpleIntCompareRef(leftRef) || !isSimpleIntCompareRef(rightRef)) return false
+        val left = compileRef(leftRef) ?: return false
+        val right = compileRef(rightRef) ?: return false
+        if (left.type != SlotType.INT || right.type != SlotType.INT) return false
+        val opcode = if (jumpOnTrue) {
+            intCompareJumpOpcode(op)
+        } else {
+            intCompareJumpOpcode(invertIntCompareOp(op))
+        }
+        builder.emit(
+            opcode,
+            listOf(
+                CmdBuilder.Operand.IntVal(left.slot),
+                CmdBuilder.Operand.IntVal(right.slot),
+                CmdBuilder.Operand.LabelRef(target)
+            )
+        )
+        return true
+    }
+
+    private fun isSimpleIntCompareRef(ref: ObjRef): Boolean {
+        return when (ref) {
+            is ConstRef -> ref.constValue is ObjInt
+            is LocalVarRef -> ref.name != "this"
+            is FastLocalVarRef -> ref.name != "this"
+            is BoundLocalVarRef -> true
+            is LocalSlotRef -> !ref.isDelegated && ref.name != "this"
+            else -> false
+        }
+    }
+
+    private fun invertIntCompareOp(op: BinOp): BinOp {
+        return when (op) {
+            BinOp.EQ -> BinOp.NEQ
+            BinOp.NEQ -> BinOp.EQ
+            BinOp.LT -> BinOp.GTE
+            BinOp.LTE -> BinOp.GT
+            BinOp.GT -> BinOp.LTE
+            BinOp.GTE -> BinOp.LT
+            else -> op
+        }
+    }
+
+    private fun intCompareJumpOpcode(op: BinOp): Opcode {
+        return when (op) {
+            BinOp.EQ -> Opcode.JMP_IF_EQ_INT
+            BinOp.NEQ -> Opcode.JMP_IF_NEQ_INT
+            BinOp.LT -> Opcode.JMP_IF_LT_INT
+            BinOp.LTE -> Opcode.JMP_IF_LTE_INT
+            BinOp.GT -> Opcode.JMP_IF_GT_INT
+            BinOp.GTE -> Opcode.JMP_IF_GTE_INT
+            else -> Opcode.JMP_IF_NEQ_INT
+        }
+    }
+
     private fun compileCondition(stmt: Statement, pos: Pos): CompiledValue? {
         val target = if (stmt is BytecodeStatement) stmt.original else stmt
         return when (target) {
@@ -6325,6 +6486,21 @@ class BytecodeCompiler(
 
     private fun refSlot(ref: LocalSlotRef): Int = ref.slot
     private fun refScopeId(ref: LocalSlotRef): Int = ref.scopeId
+
+    private fun isLoopVarRef(ref: LocalSlotRef): Boolean {
+        return loopVarKeys.contains(ScopeSlotKey(refScopeId(ref), refSlot(ref)))
+    }
+
+    private fun isLoopVarSlot(slot: Int): Boolean = loopVarSlots.contains(slot)
+
+    private fun emitLoopVarReassignError(name: String, pos: Pos): Int {
+        val msgId = builder.addConst(BytecodeConst.StringVal("can't reassign loop variable $name"))
+        val msgSlot = allocSlot()
+        builder.emit(Opcode.CONST_OBJ, msgId, msgSlot)
+        val posId = builder.addConst(BytecodeConst.PosVal(pos))
+        builder.emit(Opcode.THROW, posId, msgSlot)
+        return msgSlot
+    }
     private fun binaryLeft(ref: BinaryOpRef): ObjRef = ref.left
     private fun binaryRight(ref: BinaryOpRef): ObjRef = ref.right
     private fun binaryOp(ref: BinaryOpRef): BinOp = ref.op
@@ -6682,6 +6858,8 @@ class BytecodeCompiler(
         declaredLocalKeys.clear()
         localRangeRefs.clear()
         intLoopVarNames.clear()
+        loopVarKeys.clear()
+        loopVarSlots.clear()
         valueFnRefs.clear()
         addrSlotByScopeSlot.clear()
         loopStack.clear()
@@ -6936,6 +7114,16 @@ class BytecodeCompiler(
                 }
             }
         }
+        if (loopVarKeys.isNotEmpty()) {
+            for (key in loopVarKeys) {
+                val localIndex = localSlotIndexByKey[key]
+                if (localIndex != null) {
+                    loopVarSlots.add(scopeSlotCount + localIndex)
+                    continue
+                }
+                scopeSlotMap[key]?.let { loopVarSlots.add(it) }
+            }
+        }
         nextSlot = scopeSlotCount + localSlotNames.size
     }
 
@@ -7118,6 +7306,10 @@ class BytecodeCompiler(
         }
         when (stmt) {
             is net.sergeych.lyng.ForInStatement -> {
+                val loopSlotIndex = stmt.loopSlotPlan[stmt.loopVarName]
+                if (loopSlotIndex != null) {
+                    loopVarKeys.add(ScopeSlotKey(stmt.loopScopeId, loopSlotIndex))
+                }
                 collectLoopSlotPlans(stmt.source, scopeDepth)
                 val loopDepth = scopeDepth + 1
                 collectLoopSlotPlans(stmt.body, loopDepth)
diff --git a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdBuilder.kt b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdBuilder.kt
index b1e73ac..6bb2b5d 100644
--- a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdBuilder.kt
+++ b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdBuilder.kt
@@ -193,6 +193,10 @@ class CmdBuilder {
                 listOf(OperandKind.IP)
             Opcode.JMP_IF_TRUE, Opcode.JMP_IF_FALSE ->
                 listOf(OperandKind.SLOT, OperandKind.IP)
+            Opcode.JMP_IF_EQ_INT, Opcode.JMP_IF_NEQ_INT,
+            Opcode.JMP_IF_LT_INT, Opcode.JMP_IF_LTE_INT,
+            Opcode.JMP_IF_GT_INT, Opcode.JMP_IF_GTE_INT ->
+                listOf(OperandKind.SLOT, OperandKind.SLOT, OperandKind.IP)
             Opcode.CALL_DIRECT ->
                 listOf(OperandKind.ID, OperandKind.SLOT, OperandKind.COUNT, OperandKind.SLOT)
             Opcode.CALL_MEMBER_SLOT ->
@@ -406,6 +410,36 @@ class CmdBuilder {
             Opcode.JMP -> CmdJmp(operands[0])
             Opcode.JMP_IF_TRUE -> CmdJmpIfTrue(operands[0], operands[1])
             Opcode.JMP_IF_FALSE -> CmdJmpIfFalse(operands[0], operands[1])
+            Opcode.JMP_IF_EQ_INT -> if (operands[0] >= scopeSlotCount && operands[1] >= scopeSlotCount) {
+                CmdJmpIfEqIntLocal(operands[0] - scopeSlotCount, operands[1] - scopeSlotCount, operands[2])
+            } else {
+                CmdJmpIfEqInt(operands[0], operands[1], operands[2])
+            }
+            Opcode.JMP_IF_NEQ_INT -> if (operands[0] >= scopeSlotCount && operands[1] >= scopeSlotCount) {
+                CmdJmpIfNeqIntLocal(operands[0] - scopeSlotCount, operands[1] - scopeSlotCount, operands[2])
+            } else {
+                CmdJmpIfNeqInt(operands[0], operands[1], operands[2])
+            }
+            Opcode.JMP_IF_LT_INT -> if (operands[0] >= scopeSlotCount && operands[1] >= scopeSlotCount) {
+                CmdJmpIfLtIntLocal(operands[0] - scopeSlotCount, operands[1] - scopeSlotCount, operands[2])
+            } else {
+                CmdJmpIfLtInt(operands[0], operands[1], operands[2])
+            }
+            Opcode.JMP_IF_LTE_INT -> if (operands[0] >= scopeSlotCount && operands[1] >= scopeSlotCount) {
+                CmdJmpIfLteIntLocal(operands[0] - scopeSlotCount, operands[1] - scopeSlotCount, operands[2])
+            } else {
+                CmdJmpIfLteInt(operands[0], operands[1], operands[2])
+            }
+            Opcode.JMP_IF_GT_INT -> if (operands[0] >= scopeSlotCount && operands[1] >= scopeSlotCount) {
+                CmdJmpIfGtIntLocal(operands[0] - scopeSlotCount, operands[1] - scopeSlotCount, operands[2])
+            } else {
+                CmdJmpIfGtInt(operands[0], operands[1], operands[2])
+            }
+            Opcode.JMP_IF_GTE_INT -> if (operands[0] >= scopeSlotCount && operands[1] >= scopeSlotCount) {
+                CmdJmpIfGteIntLocal(operands[0] - scopeSlotCount, operands[1] - scopeSlotCount, operands[2])
+            } else {
+                CmdJmpIfGteInt(operands[0], operands[1], operands[2])
+            }
             Opcode.RET -> CmdRet(operands[0])
             Opcode.RET_VOID -> CmdRetVoid()
             Opcode.PUSH_SCOPE -> CmdPushScope(operands[0])
diff --git a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdDisassembler.kt b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdDisassembler.kt
index 072b532..ae36d01 100644
--- a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdDisassembler.kt
+++ b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdDisassembler.kt
@@ -176,6 +176,42 @@ object CmdDisassembler {
             is CmdCmpGteRealInt -> Opcode.CMP_GTE_REAL_INT to intArrayOf(cmd.a, cmd.b, cmd.dst)
             is CmdCmpNeqIntReal -> Opcode.CMP_NEQ_INT_REAL to intArrayOf(cmd.a, cmd.b, cmd.dst)
             is CmdCmpNeqRealInt -> Opcode.CMP_NEQ_REAL_INT to intArrayOf(cmd.a, cmd.b, cmd.dst)
+            is CmdJmpIfEqInt -> Opcode.JMP_IF_EQ_INT to intArrayOf(cmd.a, cmd.b, cmd.target)
+            is CmdJmpIfEqIntLocal -> Opcode.JMP_IF_EQ_INT to intArrayOf(
+                cmd.a + fn.scopeSlotCount,
+                cmd.b + fn.scopeSlotCount,
+                cmd.target
+            )
+            is CmdJmpIfNeqInt -> Opcode.JMP_IF_NEQ_INT to intArrayOf(cmd.a, cmd.b, cmd.target)
+            is CmdJmpIfNeqIntLocal -> Opcode.JMP_IF_NEQ_INT to intArrayOf(
+                cmd.a + fn.scopeSlotCount,
+                cmd.b + fn.scopeSlotCount,
+                cmd.target
+            )
+            is CmdJmpIfLtInt -> Opcode.JMP_IF_LT_INT to intArrayOf(cmd.a, cmd.b, cmd.target)
+            is CmdJmpIfLtIntLocal -> Opcode.JMP_IF_LT_INT to intArrayOf(
+                cmd.a + fn.scopeSlotCount,
+                cmd.b + fn.scopeSlotCount,
+                cmd.target
+            )
+            is CmdJmpIfLteInt -> Opcode.JMP_IF_LTE_INT to intArrayOf(cmd.a, cmd.b, cmd.target)
+            is CmdJmpIfLteIntLocal -> Opcode.JMP_IF_LTE_INT to intArrayOf(
+                cmd.a + fn.scopeSlotCount,
+                cmd.b + fn.scopeSlotCount,
+                cmd.target
+            )
+            is CmdJmpIfGtInt -> Opcode.JMP_IF_GT_INT to intArrayOf(cmd.a, cmd.b, cmd.target)
+            is CmdJmpIfGtIntLocal -> Opcode.JMP_IF_GT_INT to intArrayOf(
+                cmd.a + fn.scopeSlotCount,
+                cmd.b + fn.scopeSlotCount,
+                cmd.target
+            )
+            is CmdJmpIfGteInt -> Opcode.JMP_IF_GTE_INT to intArrayOf(cmd.a, cmd.b, cmd.target)
+            is CmdJmpIfGteIntLocal -> Opcode.JMP_IF_GTE_INT to intArrayOf(
+                cmd.a + fn.scopeSlotCount,
+                cmd.b + fn.scopeSlotCount,
+                cmd.target
+            )
             is CmdCmpEqObj -> Opcode.CMP_EQ_OBJ to intArrayOf(cmd.a, cmd.b, cmd.dst)
             is CmdCmpNeqObj -> Opcode.CMP_NEQ_OBJ to intArrayOf(cmd.a, cmd.b, cmd.dst)
             is CmdCmpRefEqObj -> Opcode.CMP_REF_EQ_OBJ to intArrayOf(cmd.a, cmd.b, cmd.dst)
@@ -329,6 +365,10 @@ object CmdDisassembler {
                 listOf(OperandKind.IP)
             Opcode.JMP_IF_TRUE, Opcode.JMP_IF_FALSE ->
                 listOf(OperandKind.SLOT, OperandKind.IP)
+            Opcode.JMP_IF_EQ_INT, Opcode.JMP_IF_NEQ_INT,
+            Opcode.JMP_IF_LT_INT, Opcode.JMP_IF_LTE_INT,
+            Opcode.JMP_IF_GT_INT, Opcode.JMP_IF_GTE_INT ->
+                listOf(OperandKind.SLOT, OperandKind.SLOT, OperandKind.IP)
             Opcode.CALL_DIRECT ->
                 listOf(OperandKind.ID, OperandKind.SLOT, OperandKind.COUNT, OperandKind.SLOT)
             Opcode.CALL_SLOT, Opcode.CALL_BRIDGE_SLOT ->
diff --git a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdRuntime.kt b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdRuntime.kt
index 28ae783..fb8fc90 100644
--- a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdRuntime.kt
+++ b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdRuntime.kt
@@ -36,10 +36,12 @@ class CmdVm {
         val cmds = fn.cmds
         try {
             while (result == null) {
-                val cmd = cmds[frame.ip]
-                frame.ip += 1
                 try {
-                    cmd.perform(frame)
+                    while (result == null) {
+                        val cmd = cmds[frame.ip]
+                        frame.ip += 1
+                        cmd.perform(frame)
+                    }
                 } catch (e: Throwable) {
                     if (!frame.handleException(e)) {
                         frame.cancelIterators()
@@ -1303,6 +1305,114 @@ class CmdJmpIfFalse(internal val cond: Int, internal val target: Int) : Cmd() {
     }
 }
 
+class CmdJmpIfEqInt(internal val a: Int, internal val b: Int, internal val target: Int) : Cmd() {
+    override suspend fun perform(frame: CmdFrame) {
+        if (frame.getInt(a) == frame.getInt(b)) {
+            frame.ip = target
+        }
+        return
+    }
+}
+
+class CmdJmpIfEqIntLocal(internal val a: Int, internal val b: Int, internal val target: Int) : Cmd() {
+    override suspend fun perform(frame: CmdFrame) {
+        if (frame.getLocalInt(a) == frame.getLocalInt(b)) {
+            frame.ip = target
+        }
+        return
+    }
+}
+
+class CmdJmpIfNeqInt(internal val a: Int, internal val b: Int, internal val target: Int) : Cmd() {
+    override suspend fun perform(frame: CmdFrame) {
+        if (frame.getInt(a) != frame.getInt(b)) {
+            frame.ip = target
+        }
+        return
+    }
+}
+
+class CmdJmpIfNeqIntLocal(internal val a: Int, internal val b: Int, internal val target: Int) : Cmd() {
+    override suspend fun perform(frame: CmdFrame) {
+        if (frame.getLocalInt(a) != frame.getLocalInt(b)) {
+            frame.ip = target
+        }
+        return
+    }
+}
+
+class CmdJmpIfLtInt(internal val a: Int, internal val b: Int, internal val target: Int) : Cmd() {
+    override suspend fun perform(frame: CmdFrame) {
+        if (frame.getInt(a) < frame.getInt(b)) {
+            frame.ip = target
+        }
+        return
+    }
+}
+
+class CmdJmpIfLtIntLocal(internal val a: Int, internal val b: Int, internal val target: Int) : Cmd() {
+    override suspend fun perform(frame: CmdFrame) {
+        if (frame.getLocalInt(a) < frame.getLocalInt(b)) {
+            frame.ip = target
+        }
+        return
+    }
+}
+
+class CmdJmpIfLteInt(internal val a: Int, internal val b: Int, internal val target: Int) : Cmd() {
+    override suspend fun perform(frame: CmdFrame) {
+        if (frame.getInt(a) <= frame.getInt(b)) {
+            frame.ip = target
+        }
+        return
+    }
+}
+
+class CmdJmpIfLteIntLocal(internal val a: Int, internal val b: Int, internal val target: Int) : Cmd() {
+    override suspend fun perform(frame: CmdFrame) {
+        if (frame.getLocalInt(a) <= frame.getLocalInt(b)) {
+            frame.ip = target
+        }
+        return
+    }
+}
+
+class CmdJmpIfGtInt(internal val a: Int, internal val b: Int, internal val target: Int) : Cmd() {
+    override suspend fun perform(frame: CmdFrame) {
+        if (frame.getInt(a) > frame.getInt(b)) {
+            frame.ip = target
+        }
+        return
+    }
+}
+
+class CmdJmpIfGtIntLocal(internal val a: Int, internal val b: Int, internal val target: Int) : Cmd() {
+    override suspend fun perform(frame: CmdFrame) {
+        if (frame.getLocalInt(a) > frame.getLocalInt(b)) {
+            frame.ip = target
+        }
+        return
+    }
+}
+
+class CmdJmpIfGteInt(internal val a: Int, internal val b: Int, internal val target: Int) : Cmd() {
+    override suspend fun perform(frame: CmdFrame) {
+        if (frame.getInt(a) >= frame.getInt(b)) {
+            frame.ip = target
+        }
+        return
+    }
+}
+
+class CmdJmpIfGteIntLocal(internal val a: Int, internal val b: Int, internal val target: Int) : Cmd() {
+    override suspend fun perform(frame: CmdFrame) {
+        if (frame.getLocalInt(a) >= frame.getLocalInt(b)) {
+            frame.ip = target
+        }
+        return
+    }
+}
+
 class CmdRet(internal val slot: Int) : Cmd() {
     override suspend fun perform(frame: CmdFrame) {
         frame.vm.result = frame.slotToObj(slot)
diff --git a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/Opcode.kt b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/Opcode.kt
index 3e76136..2445abd 100644
--- a/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/Opcode.kt
+++ b/lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/Opcode.kt
@@ -117,6 +117,12 @@ enum class Opcode(val code: Int) {
     JMP(0x80),
     JMP_IF_TRUE(0x81),
     JMP_IF_FALSE(0x82),
+    JMP_IF_EQ_INT(0xD0),
+    JMP_IF_NEQ_INT(0xD1),
+    JMP_IF_LT_INT(0xD2),
+    JMP_IF_LTE_INT(0xD3),
+    JMP_IF_GT_INT(0xD4),
+    JMP_IF_GTE_INT(0xD5),
     RET(0x83),
     RET_VOID(0x84),
     RET_LABEL(0xBA),
diff --git a/lynglib/src/commonTest/kotlin/NestedRangeBenchmarkTest.kt b/lynglib/src/commonTest/kotlin/NestedRangeBenchmarkTest.kt
index a3b6332..bafc8b5 100644
--- a/lynglib/src/commonTest/kotlin/NestedRangeBenchmarkTest.kt
+++ b/lynglib/src/commonTest/kotlin/NestedRangeBenchmarkTest.kt
@@ -17,11 +17,13 @@
 
 import kotlinx.coroutines.test.runTest
 import net.sergeych.lyng.Benchmarks
+import net.sergeych.lyng.BytecodeBodyProvider
 import net.sergeych.lyng.Compiler
 import net.sergeych.lyng.ForInStatement
 import net.sergeych.lyng.Script
 import net.sergeych.lyng.Statement
 import net.sergeych.lyng.bytecode.CmdDisassembler
+import net.sergeych.lyng.bytecode.CmdFunction
 import net.sergeych.lyng.bytecode.BytecodeStatement
 import net.sergeych.lyng.obj.ObjInt
 import kotlin.time.TimeSource
@@ -57,6 +59,7 @@ class NestedRangeBenchmarkTest {
         scope.eval(script)
         val fnDisasm = scope.disassembleSymbol("naiveCountHappyNumbers")
         println("[DEBUG_LOG] [BENCH] nested-happy function naiveCountHappyNumbers cmd:\n$fnDisasm")
+        dumpFunctionSlots(scope, "naiveCountHappyNumbers")
         runMode(scope)
     }
 
@@ -96,4 +99,30 @@ class NestedRangeBenchmarkTest {
         }
     }
 
+    private fun dumpFunctionSlots(scope: net.sergeych.lyng.Scope, name: String) {
+        val record = scope[name]?.value as? Statement ?: return
+        val fn = bytecodeFromStatement(record) ?: return
+        val scopeNames = fn.scopeSlotNames.mapIndexedNotNull { idx, slotName ->
+            slotName?.let { "$it@${fn.scopeSlotIndices[idx]}" }
+        }
+        val localNames = fn.localSlotNames.mapIndexedNotNull { idx, slotName ->
+            slotName?.let { "$it@$idx" }
+        }
+        val captures = fn.localSlotNames.mapIndexedNotNull { idx, slotName ->
+            if (slotName != null && fn.localSlotCaptures.getOrNull(idx) == true) "$slotName@$idx" else null
+        }
+        println(
+            "[DEBUG_LOG] [BENCH] nested-happy function $name slots: " +
+                "scopeCount=${fn.scopeSlotCount} " +
+                "scope=[${scopeNames.joinToString(", ")}] " +
+                "locals=[${localNames.joinToString(", ")}] " +
+                "captures=[${captures.joinToString(", ")}]"
+        )
+    }
+
+    private fun bytecodeFromStatement(stmt: Statement): CmdFunction? {
+        return (stmt as? BytecodeStatement)?.bytecodeFunction()
+            ?: (stmt as? BytecodeBodyProvider)?.bytecodeBody()?.bytecodeFunction()
+    }
+
 }
diff --git a/lynglib/src/commonTest/kotlin/ScriptTest.kt b/lynglib/src/commonTest/kotlin/ScriptTest.kt
index 464e7e2..187c223 100644
--- a/lynglib/src/commonTest/kotlin/ScriptTest.kt
+++ b/lynglib/src/commonTest/kotlin/ScriptTest.kt
@@ -1937,7 +1937,6 @@ class ScriptTest {
                         println("limit reached after "+n+" rounds")
                         break sum
                     }
-                    n++
                 }
                 else {
                     println("limit not reached")
diff --git a/lynglib/src/commonTest/kotlin/TypesTest.kt b/lynglib/src/commonTest/kotlin/TypesTest.kt
index a7b3adb..c591896 100644
--- a/lynglib/src/commonTest/kotlin/TypesTest.kt
+++ b/lynglib/src/commonTest/kotlin/TypesTest.kt
@@ -129,7 +129,6 @@ class TypesTest {
                         println("limit reached after "+n+" rounds")
                         break sum
                     }
-                    n++
                 }
                 else {
                     println("limit not reached")
diff --git a/notes/bytecode_vm_notes.md b/notes/bytecode_vm_notes.md
new file mode 100644
index 0000000..fb28c61
--- /dev/null
+++ b/notes/bytecode_vm_notes.md
@@ -0,0 +1,3 @@
+# Bytecode VM notes
+
+- Opcode switch dispatch was measured slower than virtual dispatch; keep the per-op Cmd class path for now.
diff --git a/notes/nested_loop_vm_state.md b/notes/nested_loop_vm_state.md
new file mode 100644
index 0000000..13e9d4e
--- /dev/null
+++ b/notes/nested_loop_vm_state.md
@@ -0,0 +1,43 @@
+## Nested loop performance investigation state (2026-02-15)
+
+### Key findings
+- Bytecode for `NestedRangeBenchmarkTest` is fully int-local ops; no dynamic lookups or scopes in hot path.
+- Loop vars now live directly in local int slots (`n1..n6`), removing per-iteration `MOVE_INT`.
+- Per-instruction try/catch in VM was replaced with an outer try/catch loop; on JVM this improved the benchmark.
+- Native slowdown is likely dominated by suspend/virtual dispatch overhead in VM, not allocations in int ops.
+
+### Current bytecode shape (naiveCountHappyNumbers)
+- Ops: `CONST_INT`, `CMP_GTE_INT`, `INC_INT`, `ADD_INT`, `CMP_EQ_INT`, `JMP*`, `RET`.
+- All are `*Local` variants hitting `BytecodeFrame` primitive arrays.
+
+### Changes made
+- Loop vars are enforced read-only inside loop bodies at bytecode compile time (reassign, op=, ?=, ++/-- throw).
+- Range loops reuse the loop var slot as the counter; no per-iteration move.
+- VM loop now uses outer try/catch (no per-op try/catch).
+- VM stats instrumentation was added temporarily, then removed for MP safety.
+
+### Files changed
+- `lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/BytecodeCompiler.kt`
+  - loop var immutability checks
+  - loop var slot reuse for range loops
+  - skip break/result init when not needed
+- `lynglib/src/commonMain/kotlin/net/sergeych/lyng/bytecode/CmdRuntime.kt`
+  - outer try/catch VM loop (removed per-op try/catch)
+  - stats instrumentation removed
+- `lynglib/src/commonTest/kotlin/NestedRangeBenchmarkTest.kt`
+  - temporary VM stats debug removed
+  - slot dump remains for visibility
+- `notes/bytecode_vm_notes.md`
+  - note: opcode switch dispatch tested slower than virtual dispatch
+
+### Benchmark snapshots (JVM)
+- Before VM loop change: ~96–110 ms.
+- With VM stats enabled: ~234–240 ms (stats overhead).
+- After VM loop change, stats disabled: ~85 ms.
+- 2026-02-15 baseline (fused int-compare jumps): 74 ms.
+  - Command: `./gradlew :lynglib:jvmTest -Pbenchmarks=true --tests '*NestedRangeBenchmarkTest*'`
+  - Notes: loop range checks use `JMP_IF_GTE_INT` (no CMP+bool temp).
+
+### Hypothesis for Native slowdown
+- Suspend/virtual dispatch per opcode dominates on K/N, even with no allocations in int ops.
+- Next idea: a non-suspend fast path for hot opcodes, or a dual-path VM loop.