fix a bug in tail-call optimization code

runtime/vm.go

@@ -894,6 +894,8 @@ func (v *VM) callFunction(fn *objects.CompiledFunction, freeVars []*objects.Obje
 			//  |--------|
 			//  | *ARG1  |        for next function (tail-call)
 			//  |--------|
+			//  |  FUNC  |        function itself
+			//  |--------|
 			//  | LOCAL3 |        for current function
 			//  |--------|
 			//  | LOCAL2 |        for current function
@@ -904,7 +906,7 @@ func (v *VM) callFunction(fn *objects.CompiledFunction, freeVars []*objects.Obje
 			//  |--------|
 
 			copy(v.stack[curFrame.basePointer:], v.stack[v.sp-numArgs:v.sp])
-			v.sp -= numArgs
+			v.sp -= numArgs + 1
 			curFrame.ip = -1
 
 			//  stack after tail-call
@@ -914,7 +916,9 @@ func (v *VM) callFunction(fn *objects.CompiledFunction, freeVars []*objects.Obje
 			//  |--------|
 			//  | *ARG2  |
 			//  |--------|
-			//  | *ARG1  | <- SP  current
+			//  | *ARG1  |
+			//  |--------|
+			//  |  FUNC  | <- SP  current
 			//  |--------|
 			//  | LOCAL3 |        for current function
 			//  |--------|

runtime/vm_tail_call_test.go

@@ -66,6 +66,19 @@ func TestTailCall(t *testing.T) {
 		return f2(5, 0)
 	}
 	out = f1()`, 15)
+
+	// tail-call replacing loop
+	// without tail-call optimization, this code will cause stack overflow
+	expect(t, `
+iter := func(n, max) {
+	if n == max {
+		return n
+	}
+
+	return iter(n+1, max)
+}
+out = iter(0, 9999)
+`, 9999)
 }
 
 // tail call with free vars

runtime/vm_test.go

@@ -148,7 +148,7 @@ func traceCompileRun(file *ast.File, symbols map[string]objects.Object) (res map
 			var ipstr string
 			if v != nil {
 				frameIdx, ip := v.FrameInfo()
-				ipstr = fmt.Sprintf("\n  (Frame=%d, IP=%d)", frameIdx, ip)
+				ipstr = fmt.Sprintf("\n  (Frame=%d, IP=%d)", frameIdx, ip+1)
 			}
 			trace = append(trace, fmt.Sprintf("[Panic]\n\n  %v%s\n", e, ipstr))
 			trace = append(trace, fmt.Sprintf("[Error Trace]\n\n  %s\n", strings.Join(stackTrace, "\n  ")))
@@ -216,6 +216,9 @@ func traceCompileRun(file *ast.File, symbols map[string]objects.Object) (res map
 			}
 		}
 		trace = append(trace, fmt.Sprintf("\n[Globals]\n\n%s", strings.Join(globalsStr, "\n")))
+
+		frameIdx, ip := v.FrameInfo()
+		trace = append(trace, fmt.Sprintf("\n[IP]\n\nFrame=%d, IP=%d", frameIdx, ip+1))
 	}
 	if err != nil {
 		return