Benedikt Meurer 00342452e2 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.

2017-10-30 09:16:44 -04:00

14 KiB

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