Optimizing select projectionsPart IV–Understand, don’t do
This post is what the entire series has been building building toward to. In the previous post we have refactored our code to make it easier to build additional behaviors. Here is one such behavior, which uses the Esprima project to parse the code and see if it uses a pattern that we can optimize for. Let us see the code, then discuss what it does.
The new stuff is mostly in the TryProjectOptimized method. This method is making heavy use of the new C# features to do easy parsing of ASTs, and it really shows that the C# team is using C# to build the compiler (that is one of the common things that compiler writers do, make their own lives easier). Take this code even a couple of language versions back, and it would be much hard to read and work with.
At any rate, what this code is doing is search for an object literal that does simple assignment of properties. If it detect that, instead of calling into the JS engine, it will produce a delegate that will do just that directly.
The result is pretty much the same, but we have to do so much less. If we can’t detect a pattern that we recognize, we just fall back to the JS engine again, and get the pervious behavior.
This is pretty awesome thing to have, because if we detect a particular common pattern, we can optimize that quite easily. The code really lend itself to that approach now.
What about the performance? Well…
And this runs in 0.31 seconds on the 10K, 5K, 1K run.
- 10K in 83 ms
- 5K in 200 ms
- 1K in 31 ms
So that is two orders of magnitude for the 10K run.
An an exercise, try taking this code and teaching it how to recognize simple expressions like + and –. The second projection example is meant to give you some idea about that.
Oh, and one last thought, we are actually biased against the optimization. The Jint code produce results in formats that we need to pull the data from, and we aren’t accounting for that here. The optimize code can generate the output to be already in the format we need it to be ( the difference between the Dictionary and the JsValue).