This way, we still perform UTF-8 validation, but don't go through the
slow generic code path that rebuilds the decoded string one code point
at a time.
This was a bottleneck when loading a canned copy of reddit.com, which
ended up being ~120 MiB large.
- Time spent decoding UTF-8 before this change: 1192 ms
- Time spent decoding UTF-8 after this change: 154 ms
That's still a long time, but 7.7x faster is nothing to sneeze at! :^)
Note that if the input fails UTF-8 validation, we still fall back to
the slow path and insert replacement characters per the WHATWG Encoding
spec: https://encoding.spec.whatwg.org/#utf-8-decode
The Encoding specification maps ISO-8859-1 to windows-1252 and expects
the windows-1252 translation table to be used, which differs from
ISO-8859-1 for 0x80-0x9F.
Other contexts expect to get the actual ISO-8859-1 encoding, with 1-to-1
mapping to U+0000-U+00FF, when requesting it.
`decoder_for_exact_name` is introduced, which skips the mapping from
aliases to the encoding name done by `get_standardized_encoding`.
UTF8Decoder was already converting invalid data into replacement
characters while converting, so we know for sure we have valid UTF-8
by the time conversion is finished.
This patch adds a new StringBuilder::to_string_without_validation()
and uses it to make UTF8Decoder avoid half the work it was doing.
The UTF-8 decoder will currently crash if it is provided invalid UTF-8
input. Instead, change its behavior to match that of all other decoders
to replace invalid code points with U+FFFD. This is required by the web.
Using char causes bytes equal to or over 0x80 to be treated as a
negative value and produce incorrect results when implicitly casting to
u32.
For example, `atob` in LibWeb uses this decoder to convert non-ASCII
values to UTF-8, but non-ASCII values are >= 0x80 and thus produces
incorrect results in such cases:
```js
Uint8Array.from(atob("u660"), c => c.charCodeAt(0));
```
This used to produce [253, 253, 253] instead of [187, 174, 180].
Required by Cloudflare's IUAM challenges.
