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Fix OOB string character access in Printer#_maybeAddParen. (#6589)

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* 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.
This commit is contained in:
Benedikt Meurer 2017-10-30 14:16:44 +01:00 committed by Henry Zhu
parent 04d2c030be
commit 00342452e2
1 changed files with 9 additions and 7 deletions
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16
packages/babel-generator/src/printer.js
View File

@ -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) {