Make the JS version faster

[nex3]

The JS-compiled version of Dart Sass hovers around 2.5x and 3x slower than when running on the Dart VM. Some slowdown is inevitable, but this seems extreme. We should put dedicated effort into profiling the JS output and optimizing hot spots.

[ranquild]

Common thing with dart2js performance issues is dynamic dispatch. For example, in dynamic s = func(); s.method();. call of method will be compiled to something like J.getInterceptor(s).method$0();, where getInterceptor - is sort-of runtime prototype wrapper that allows to call non-existing methods on JS primitives. Types like List or String are not wrapped in compiled dart code, they are Array and String in JS. So, when you have dynamic variable and call method/property like first, interceptor returns correct prototype for object by doing types checks for String, Array, int and so on.

What that does mean is that usage of dynamic ruins compiled-to-js performance and should be avoided at all costs. Easy workaround is to use Object for variables that could contain objects of different types and doing is checks and calling method on variable of specific types. Using dynamic in VM does not have those performance issues and that's why there is so huge performance gap.

While using Object with type checks may sound like a quick win, better solution would be to refactor code to extract interfaces or something like that so all code will be statically typed without runtime checks.

[nex3]

See this test file as a test case where JS is vastly slower than the Dart VM.

[ogonkov]

For me, full compilation time went to 2 minutes from 20 seconds

[nex3]

@ogonkov Can you share the stylesheets that are running so much slower?

[ogonkov]

I'm afraid no, i will try to reproduce the issue, by excluding sass files, but don't know when

[nex3]

This comment also includes a zip file that's reported to have poor JS performance.