4 ms·

I think the most common operations are comparisons for equality and copying anyways. UTF-8 is faster for those. I tried out how fast I could make UTF-8 strlen,

I think the most common operations are comparisons for equality and copying anyways. UTF-8 is faster for those. I tried out how fast I could make UTF-8 strlen, with an assumption of a valid UTF-8 string. The routine ran at 18 GB/s on a single core using SSE. > With UTF8, CPU mispredicts branches all the time because spaces, punctuations and newlines are single bytes I don't understand this sentence. Why would there be any more mispredictions because of those being single bytes? These days code is so often bandwidth limited if anything, so smaller data helps.

HelloNurse 9y ago
In non-Latin text, if most characters are 2 bytes but a large minority are 1 byte, the branch prediction in charge of guessing between the different codepoint representation lengths expects 2 bytes and fails very often. Speculative execution (counting in two or three ways simultaneously) might mitigate the performance hit.
- vardump 9y ago
  > In non-Latin text, if most characters are 2 bytes but a large minority are 1 byte, the branch prediction in charge of guessing between the different codepoint representation lengths expects 2 bytes and fails very often You wouldn't want to process a single code point (or unit) at a time anyways, but 16, 32 or 64 code units (or bytes) at once. That UTF-8 strlen I wrote had no mispredicts, because it was vectored. Indexing is slow, but the difference to UTF-16 is not significant. I guess locale based comparisons or case insensitive operations could be slow, but then again, they'll need a slow array lookup anyways. Which string operation(s) are you talking about?
- legulere 9y ago
  You don't need to check the representation doing anything specifically with spaces or newlines. All 0x0A bytes are newline characters in UTF8 and all 0x20 bytes are spaces. The only place you really need to decode UTF8 characters is when you convert it to another format (which you hopefully won't need to do anymore in the far future) or display it (where the decoding is a minuscule factor in performance)
Const-me 9y ago
> most common operations are comparisons for equality and copying anyways Indexing & substrings are common, too. > These days code is so often bandwidth limited if anything Right, and for 1 billion Chinese speaking people UTF16 is 2 bytes/character, UTF8 is 3 bytes/character.
- jcranmer 9y ago
  Represent indexes not as number-of-code-points from start but as byte offsets, and index/substring doesn't need to decode code points. You lose the ability to easily say "get me the 100th code point in this string", but I'm hard-pressed to actually think of any case where that is actually valuable. > Right, and for 1 billion Chinese speaking people UTF16 is 2 bytes/character, UTF8 is 3 bytes/character. The information density of a single hanzi character is roughly equivalent to 5 letters in English. A Chinese plaintext document in UTF-8 is still smaller in memory footprint than an equivalent English document in ASCII. Of course, most documents aren't plaintext, and where people use characters for metadata (e.g., email, HTML), there is a substantial corpus of ASCII metadata in those documents that UTF-8 is still smaller than UTF-16 even for East Asian languages. Of course, it's moot since the people who don't like UTF-8 in China and Japan aren't using UTF-16 either. They're using GB18030 or ISO-2022-JP for their documents.
  - Const-me 9y ago
    > A Chinese plaintext document in UTF-8 is still smaller in memory footprint than an equivalent English document in ASCII. When you need to process Chinese text you don’t care how much an equivalent English document would take. You only care about the difference between different encodings of Chinese language. And UTF16 is more compact for East Asian languages. > most documents aren't plaintext That’s true for the web, and that’s why UTF8 is the clear winner there. In a desktop software, in a videogame, in a database — not so much.
- vardump 9y ago
  > Indexing & substrings are common, too. Indexing code points in both UTF-8 and UTF-16 requires reading the whole string up to index location. Substrings are the same as well. > Right, and for 1 billion Chinese speaking people UTF16 is 2 bytes/character, UTF8 is 3 bytes/character. That's true for a text file without markup. But most text is not like that in 2017. HTML is probably the most common text format nowadays. So let's see how a popular Chinese language website does. curl http://language.chinadaily.com.cn/ --silent | wc -c 52678 curl http://language.chinadaily.com.cn/ --silent | iconv -f utf8 -t utf-16le | wc -c 93368 So UTF-8 seems to be quite a bit more efficient in this case, 52678 bytes. When converted to UTF-16, same page was 93368 bytes.

HelloNurse 9y ago

In non-Latin text, if most characters are 2 bytes but a large minority are 1 byte, the branch prediction in charge of guessing between the different codepoint representation lengths expects 2 bytes and fails very often. Speculative execution (counting in two or three ways simultaneously) might mitigate the performance hit.

vardump 9y ago
> In non-Latin text, if most characters are 2 bytes but a large minority are 1 byte, the branch prediction in charge of guessing between the different codepoint representation lengths expects 2 bytes and fails very often You wouldn't want to process a single code point (or unit) at a time anyways, but 16, 32 or 64 code units (or bytes) at once. That UTF-8 strlen I wrote had no mispredicts, because it was vectored. Indexing is slow, but the difference to UTF-16 is not significant. I guess locale based comparisons or case insensitive operations could be slow, but then again, they'll need a slow array lookup anyways. Which string operation(s) are you talking about?
legulere 9y ago
You don't need to check the representation doing anything specifically with spaces or newlines. All 0x0A bytes are newline characters in UTF8 and all 0x20 bytes are spaces. The only place you really need to decode UTF8 characters is when you convert it to another format (which you hopefully won't need to do anymore in the far future) or display it (where the decoding is a minuscule factor in performance)

Const-me 9y ago

> most common operations are comparisons for equality and copying anyways Indexing & substrings are common, too. > These days code is so often bandwidth limited if anything Right, and for 1 billion Chinese speaking people UTF16 is 2 bytes/character, UTF8 is 3 bytes/character.

jcranmer 9y ago
Represent indexes not as number-of-code-points from start but as byte offsets, and index/substring doesn't need to decode code points. You lose the ability to easily say "get me the 100th code point in this string", but I'm hard-pressed to actually think of any case where that is actually valuable. > Right, and for 1 billion Chinese speaking people UTF16 is 2 bytes/character, UTF8 is 3 bytes/character. The information density of a single hanzi character is roughly equivalent to 5 letters in English. A Chinese plaintext document in UTF-8 is still smaller in memory footprint than an equivalent English document in ASCII. Of course, most documents aren't plaintext, and where people use characters for metadata (e.g., email, HTML), there is a substantial corpus of ASCII metadata in those documents that UTF-8 is still smaller than UTF-16 even for East Asian languages. Of course, it's moot since the people who don't like UTF-8 in China and Japan aren't using UTF-16 either. They're using GB18030 or ISO-2022-JP for their documents.
- Const-me 9y ago
  > A Chinese plaintext document in UTF-8 is still smaller in memory footprint than an equivalent English document in ASCII. When you need to process Chinese text you don’t care how much an equivalent English document would take. You only care about the difference between different encodings of Chinese language. And UTF16 is more compact for East Asian languages. > most documents aren't plaintext That’s true for the web, and that’s why UTF8 is the clear winner there. In a desktop software, in a videogame, in a database — not so much.
vardump 9y ago
> Indexing & substrings are common, too. Indexing code points in both UTF-8 and UTF-16 requires reading the whole string up to index location. Substrings are the same as well. > Right, and for 1 billion Chinese speaking people UTF16 is 2 bytes/character, UTF8 is 3 bytes/character. That's true for a text file without markup. But most text is not like that in 2017. HTML is probably the most common text format nowadays. So let's see how a popular Chinese language website does. curl http://language.chinadaily.com.cn/ --silent | wc -c 52678 curl http://language.chinadaily.com.cn/ --silent | iconv -f utf8 -t utf-16le | wc -c 93368 So UTF-8 seems to be quite a bit more efficient in this case, 52678 bytes. When converted to UTF-16, same page was 93368 bytes.