From 00342452e2e543452f91fcd7bd2b75e5d7c9ecd4 Mon Sep 17 00:00:00 2001
From: Benedikt Meurer <benedikt.meurer@googlemail.com>
Date: Mon, 30 Oct 2017 14:16:44 +0100
Subject: [PATCH] Fix OOB string character access in Printer#_maybeAddParen.
 (#6589)

* Fix OOB string character access in Printer#_maybeAddParen.

The `_maybeAddParen` method of the `Printer` class does

```js
const chaPost = str[i + 1]
```

without checking that `i + 1` is still within the bounds of `str`. It
seems like this triggers fairly often that the `str[i + 1]` access is
out of bounds. The first out of bounds access will turn the KeyedLoadIC
(in case of V8) into *MEGAMORPHIC* state, which is significantly slower
for strings (there's a fix in flight for V8 to mitigate the cost a bit
in that case). Even worse than that, the out of bounds access also
pollutes the later comparisons, namely

```js
chaPost === "/"
```

and

```js
chaPost === "*"
```

which are now no longer monomorphic on strings, since `chaPost` was
sometimes `undefined`.

This is a non-breaking performance fix, which improves babel execution
on the [web-tooling-benchmark](github.com/v8/web-tooling-benchmark)
workload by around 6-9%.

* Restructure and optimize the code a bit.
---
 packages/babel-generator/src/printer.js | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/packages/babel-generator/src/printer.js b/packages/babel-generator/src/printer.js
index d550e6635c..a1b7f82eca 100644
--- a/packages/babel-generator/src/printer.js
+++ b/packages/babel-generator/src/printer.js
@@ -245,15 +245,17 @@ export default class Printer {
     for (i = 0; i < str.length && str[i] === " "; i++) continue;
     if (i === str.length) return;
 
+    // Check for newline or comment.
     const cha = str[i];
-    const chaPost = str[i + 1];
-
-    // Check for newline or comment
-    if (cha === "\n" || (cha === "/" && (chaPost === "/" || chaPost === "*"))) {
-      this.token("(");
-      this.indent();
-      parenPushNewlineState.printed = true;
+    if (cha !== "\n") {
+      if (cha !== "/") return;
+      if (i + 1 === str.length) return;
+      const chaPost = str[i + 1];
+      if (chaPost !== "/" && chaPost !== "*") return;
     }
+    this.token("(");
+    this.indent();
+    parenPushNewlineState.printed = true;
   }
 
   _catchUp(prop: string, loc: Object) {