perf(Subscription): use instanceof to avoid megamorphic LoadIC
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In `Subscription#add()` use `instanceof` to test whether `subscription` must be wrapped in a `Subscription` (aka for presence of `_addParent` method), instead of `typeof subscription._addParent === 'function'`, as the latter is going to turn into a *megamorphic property access* in any realistic application and cause quite a bit of contention on the megamorphic stub cache in Node (aka V8). The `instanceof` is definitely faster, even if the property access would hit the megamorphic stub cache. For example in the case of a simple [Angular Universal prerender][1] test, just changing this can reduce the number of megamorphic stub cache misses by around **2%**. Even in the case where the megamorphic stub cache hits all the time, using `instanceof` is still faster. I put together a little micro- benchmark [here][2]. Running this with the latest V8 we get: ``` $ out/Release/d8 bench-instanceof-versus-typeof-megamorphic.js instanceOf: 91 ms. typeOf: 138 ms. ``` [1]: https://gist.githubusercontent.com/bmeurer/5b9480ef1a74c5187180193abc73dcd4/raw/fc5d9a01a6b7fe841efa1c7773ae55e70435d16a/prerender.js [2]: https://gist.github.com/bmeurer/d6d8d4e5d3a9937499ed738c93b3b7cf
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benlesh
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Thanks, @bmeurer!!! |
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In
Subscription#add()useinstanceofto test whethersubscriptionmust be wrapped in a
Subscription(aka for presence of_addParentmethod), instead of
typeof subscription._addParent === 'function',as the latter is going to turn into a megamorphic property access
in any realistic application and cause quite a bit of contention on
the megamorphic stub cache in Node (aka V8).
The
instanceofis definitely faster, even if the property accesswould hit the megamorphic stub cache. For example in the case of a
simple Angular Universal prerender test, just changing this can
reduce the number of megamorphic stub cache misses by around 2%.
Even in the case where the megamorphic stub cache hits all the time,
using
instanceofis still faster. I put together a little micro-benchmark here. Running this with the latest V8 we get: