AssemblyScript & Interface Types - UTF-16 String Support / Consider UTF-8 Strings

[torch2424]

Hello! So this is a follow up from: WebAssembly/interface-types#13

This is a long thread, exploring string encoding types in the upcoming interface types proposal. Where currently, the only supported string encoding (in the MVP), is UTF-8.

However, AssemblyScript uses UTF-16, to stay parallel with the Web APIs, and interface types could require some double encoding for UTF-16 languages (if I understand correctly.)

It was suggested, but there are a few issues, not the full list, we see on the AS side (I'll let @dcodeIO give implementation details where necessary):

• AssemblyScript tries to stay as close as possible as to WebAPIs, and mimicks the JS String API. Functions like .substring and charCodeAt are implemented in a way that would be difficult to re-implement in UTF-8, but also could break libraries that depend on specific JS behavior (if they were to be ported to AS).
• If AssemblyScript were to support two string representations, having a "UTF8String" class would be unintuitive and cause lots of headaches. Or, trying to support both UTF-16 and UTF-8 could greatly increase module size (which would be a huge downside for the browser case).
• The most important to me (personally), is that another big / notable use case for wasm is C#/.NET in the Blazor project, which could greatly benefit from interface types, but uses UTF-16 strings. As well as other popular lanugages that could output Wasm that use UTF-16 as their string representation, such as Java, and Kotlin.
• This list is incomplete, @dcodeIO would know more implementation details that'd make this a bit difficult.

Would be interested to hear everyone's thoughts. Looking forward to a respectful, thoughtful discussion here, and finding a good solution 😄 Thanks everyone! 👍

cc @lukewagner @dcodeIO @MaxGraey

[MaxGraey]

such as Java, and Kotlin

Also Dart lang which also inherit many aspects from JavaScript and also use UTF-16. But for current not going to WebAssembly direction (however has AOT compilation) but it could changed in future.

[lukewagner]

Oh wow, this is probably silly of me, but I didn't realize that Java and C# also use UTF-16. Yes, with Blazor and Kotlin targeting wasm, I can definitely see this being a common need :) I'll go comment about this back in WebAssembly/interface-types#13.

I did just want to clarify one thing: I was only suggesting that AS change the representation of strings in linear memory, not the programmer-visible semantics. I think this is still a good idea, even if Interface Types has UTF-16 lifting/lowering.

More specifically, even assuming interface types has both UTF-8 and UTF-16 lifting/lowering, I think a nice impl for AS strings could be:

• when a JS string flows into wasm linear memory, lower into UTF-8.
• when a JS string is random-accessed (which is not most strings):
  • assume the JS string header has three flag bits: scanned, ascii, utf-16, all initially false
  • if the JS string has not been scanned, scan it to determine if the string contains any non-ascii code points, setting scanned and, possibly, ascii
  • if the JS string has ascii set, then perform a random-access on the UTF-8 byte array, knowing that this will produce the semantically-correct integer
  • if the JS string is scanned without ascii or utf-16 set, inflate the UTF-8 chars in-place to UTF-16, setting the utf-16 bit
  • at this point, utf-16 must be set, so perform a UTF-16 random-access
• when a JS string flows out of wasm linear memory to JS lift using UTF-8 or UTF-16 based on whether the utf-16 flag is set

With such a scheme, I think only a small % of strings will get inflated, and, when that happens, the one-time inflation cost will be amortized. I think code size impact should from the extra code to handle the two cases should be small since it'll be factored out into runtime functions. And, in exchange, string memory use is ~50%. WDYT?

[MaxGraey]

Java has similar approach since 9th version (btw its second attempt). But only switching between ASCII/Latin1 and UTF16. It's turned off by default and called Compact String. And even with JIT runtime like in Java turning on Compact String may slowdown string manipulations sometimes. For us it means also significantly increasing code size of our runtime which we try keep as small as possible. I know JavaScript engines internally represent strings in different formats and even used rope structures when need concatenate strings frequently but for us it means significant bloating of runtime unfortunately

[aminya]

I am not involved with the discussion, and maybe my question is offtopic. Does this solve the performance issue of string conversion between WASM and JavaScript?

[lukewagner]

It's turned off by default and called Compact String.

If I read correctly, Compact Strings are enabled by default (with a flag to disable) which demonstrates a widespread win in practice. Perhaps you're thinking of the previous Compressed Strings feature which it sounds like had more of a perf impact and was never enabled by default.

For us it means also significantly increasing code size of our runtime

Do you have any data to show the code-size increase is significant? I'm assuming that, in general, the AS compiler would want to balance code-size and runtime-perf and thus the concrete magnitudes matter.

[MaxGraey]

If I read correctly, Compact Strings are enabled by default (with a flag to disable) which demonstrates a widespread win in practice. Perhaps you're thinking of the previous Compressed Strings feature which it sounds like had more of a perf impact and was never enabled by default.

Yes, it seems it turned by default now. But -XX:-CompactStrings has a description why you might want to disable it:

"In the unexpected event where a performance regression is observed in migrating from Java SE 8 to Java SE 9 and an analysis shows that Compact Strings introduces the regression."

Do you have any data to show the code-size increase is significant? I'm assuming that, in general, the AS compiler would want to balance code-size and runtime-perf and thus the concrete magnitudes matter.

I expect this changes introduce more cache misses due to split most of string methods to 2 or 4 branches depends on method's arity:

for (String#concat, String#replace, comparision and etc)

1. Latin1 <-> Latin1
2. Latin1 <-> UTF16
3. UTF16 <-> Latin1 (but usually same as 2 if commutative)
4. UTF16 <-> UTF16

for indexfOf, search, repeat and etc:

1. Latin1
2. UTF16

It also increase code size for Strings and related to it functions to approx 2-3 times.

Maybe I'm missing something, though?

[MaxGraey]

We can definitely get the advantage (twice faster and twice less memory footprint) if most of string which has only Latin1 encodings. But how often we use only English (without emojis, ligatures and special symbols like á / Á or ©) nowadays? For example, the first and second languages ​​in prevalence in the world - Chinese and Spanish already require UTF8/UTF16. English is only in third place by number of native speakers however first in the web

[jtenner]

Some of us care about compile times too. People testing their software locally will care about untouched builds, and anyone using strings will see a very large increase in testing module size just by upgrading to a new compiler.

Even if this adds only a single second to compile time (per run,) this adds up over long periods of time and gets very frustrating.

[lukewagner]

I'm just a casual commentator here, so I won't press the matter any further, but I'd encourage measurement before ruling the option out. Cheers!

[MaxGraey]

@lukewagner Luke thank you so much for the reasonable suggestion about "compacting strings". I'm really appreciate for this and it totally makes sense for runtime which not distribute via web like JVM or DartVM. By the way, I already thought about it here.