Performance degradation for indexed string property access

[skinny85]

First of all, thank you for all the work you do on Graal JS and Graal itself, it's an awesome project 🙂.

I authored of a series of blog posts that aim to be an introduction to working with Truffle and GraalVM. While working on the latest chapter on strings, I've ran into some interesting numbers for a simple benchmark.

The idea is to count to n in a loop, while using some simple string operations. The benchmark has two variants, one with direct property access:

function countWhileCharAtDirectProp(n) {
    var ret = 0;
    while (n > 0) {
        n = n - ('a'.charAt(0) + ''.charAt()).length;
        ret = ret + 1;
    }
    return ret;
}

And the other uses indexed property access:

function countWhileCharAtIndexProp(n) {
    var ret = 0;
    while (n > 0) {
        n = n - ('a'['charAt'](0) + ''['charAt']())['length'];
        ret = ret + 1;
    }
    return ret;
}

The benchmark invokes the function with n equal to 1_000_000.

Here are the results on my laptop (ezs is the toy implementation developed in the article series, called EasyScript) - the units are microseconds per iteration, so lower is better:

Benchmark                                                  Mode  Cnt       Score      Error  Units
StringLengthBenchmark.count_while_char_at_direct_prop_ezs  avgt    5     576.093 ±    5.992  us/op
StringLengthBenchmark.count_while_char_at_direct_prop_js   avgt    5     576.772 ±    3.865  us/op
StringLengthBenchmark.count_while_char_at_index_prop_ezs   avgt    5     576.813 ±    7.087  us/op
StringLengthBenchmark.count_while_char_at_index_prop_js    avgt    5  112404.250 ± 1012.309  us/op

So, the "index access" variant is 200 times slower than the "direct access" variant.

I thought you'd be interested in seeing this - I assume this is a simple bug in the implementation 🙂.

The version of GraalVM used:

$ java -version

openjdk version "17.0.5" 2022-10-18
OpenJDK Runtime Environment GraalVM CE 22.3.0 (build 17.0.5+8-jvmci-22.3-b08)
OpenJDK 64-Bit Server VM GraalVM CE 22.3.0 (build 17.0.5+8-jvmci-22.3-b08, mixed mode, sharing)

And org.graalvm.js:js is in the same version (22.3.0).

---

Let me know if you have any questions!

Thanks,
Adam

[iamstolis]

Thank you for the report. There was really a simple optimization opportunity in our implementation of reads of the form primitiveValue[key]. I can see a comparable performance of countWhileCharAtDirectProp and countWhileCharAtIndexProp with d4442d6.

[skinny85]

Thanks for the quick fix @iamstolis! Any idea when, and in what version, it will be released?

[iamstolis]

It hasn't made it into GraalVM 23.0.0 that we are about to release. So, it will be in GraalVM 23.1.0. You can use/test a development build in the meantime. The fix should appear there in a week or so.

[skinny85]

Thank you so much @iamstolis!