BuckleScript List Benchmark

This benchmark demonstrates how to optimize list iteration speed in almost all major JS engines by tweaking the internal runtime representation.

As for right now, the expression let myList = ["a", "b", "c"] is compiled to following JavaScript:

var myList = /* :: */[
  "a",
  /* :: */[
    "b",
    /* :: */[
      "c",
      /* [] */0
    ]
  ]
];

Note how the last element in the array is terminated with a 0 value. This will cause V8 to do a kind transition to a polymorphic kind which ideally should be avoided.

To prevent this, the internal list representation must be terminated with a non-SMI value, such as false, null, etc:

var myList = /* :: */[
  "a",
  /* :: */[
    "b",
    /* :: */[
      "c",
      /* [] */null
    ]
  ]
];

Setup

npm install jsvu -g

# Make sure to tick chakra, javascriptcore, spidermonkey, v8 and v8-debug
jsvu

Run full Benchmark

# Will run the performance test for each JS engine
./benchmark.sh

Test Results:

Macbook Pro 13" 2016, 3,3 GHz Intel Core i7, 16GB ram

The benchmark does 3 runs per original / optimized group to make sure that there are no warm-up side-effects. Each function is called 1e7 times to ensure an obversable time diff.

The optimized function uses null as list terminator, the original function uses a 0 (current behavior in BuckleScript). The test source code can be found in ./perf.js.

./benchmark.sh

Run V8:
optimized: 1242 ms.
original: 1722 ms.
optimized: 1263 ms.
original: 1734 ms.
optimized: 1233 ms.
original: 1760 ms.

Run JavaSciptCore:
optimized: 430 ms.
original: 835 ms.
optimized: 881 ms.
original: 936 ms.
optimized: 859 ms.
original: 938 ms.

Run SpiderMonkey:
optimized: 542 ms.
original: 492 ms.
optimized: 567 ms.
original: 482 ms.
optimized: 546 ms.
original: 518 ms.

Run Chakra:
optimized: 1112 ms.
original: 3575 ms.
optimized: 908 ms.
original: 3479 ms.
optimized: 894 ms.
original: 3513 ms.

There are some interesting observations:

• ~30% better V8 / Chakra performance
• -9% worse performance in SpiderMonkey, but the performance there is already 3 times as fast as V8
• Not sure why, but in JSC, the first call has a huge gap with +51% performance boost, while every other call is only around +6%

Credits

Thanks to @bmeurer for explaining element kinds mechanic in V8, how to interpret the bytecode debugging output and writing the performance tests.

Links:

• V8 Element Kinds | V8 Dev Blog