RFR: 8264288: Performance issue with MethodHandle.asCollector

[Paul Sandoz]

On Thu, 1 Apr 2021 13:25:05 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

> This patch speeds up MethodHandle.asCollector handles where the array type is not Object[], as well as speeding up all collectors where the arity is greater than 10.
> 
> The old code is creating a collector handle by combining a set of hard coded methods for collecting arguments into an Object[], up to a maximum of ten elements, and then copying this intermediate array into a final array.
> 
> In principle, it shouldn't matter how slow (or fast) this handle is, because it should be replaced by existing bytecode intrinsification which does the right thing. But, through investigation it turns out that the intrinsification is only being applied in a very limited amount of cases: Object[] with max 10 elements only, only for the intermediate collector handles. Every other collector shape incurs overhead because it essentially ends up calling the ineffecient fallback handle.
> 
> Rather than sticking on a band aid (I tried this, but it turned out to be very tricky to untangle the existing code), the new code replaces the existing implementation with a collector handle implemented using a LambdaForm, which removes the need for intrinsification, and also greatly reduces code-complexity of the implementation. (I plan to expose this collector using a public API in the future as well, so users don't have to go through MHs::identity to make a collector).
> 
> The old code was also using a special lambda form transform for collecting arguments into an array. I believe this was done to take advantage of the existing-but-limited bytecode intrinsification, at least for Object[] with less than 10 elements.
> 
> The new code just uses the existing collect arguments transform with the newly added collector handle as filter, and this works just as well for the existing case, but as a bonus is also much simpler, since no separate transform is needed. Using the collect arguments transform should also improve sharing.
> 
> As a result of these changes a lot of code was unused and has been removed in this patch.
> 
> Testing: tier 1-3, benchmarking using TypedAsCollector (part of the patch), as well as another variant of the benchmark that used a declared static identity method instead of MHs::identity (not included). Before/after comparison of MethodHandleAs* benchmarks (no differences there).
> 
> Here are some numbers from the added benchmark:
> 
> Before:
> Benchmark                                    Mode  Cnt    Score    Error  Units
> TypedAsCollector.testIntCollect              avgt   30  189.156 �  1.796  ns/op
> TypedAsCollector.testIntCollectHighArity     avgt   30  660.549 � 10.159  ns/op
> TypedAsCollector.testObjectCollect           avgt   30    7.092 �  0.042  ns/op
> TypedAsCollector.testObjectCollectHighArity  avgt   30   65.225 �  0.546  ns/op
> TypedAsCollector.testStringCollect           avgt   30   28.511 �  0.243  ns/op
> TypedAsCollector.testStringCollectHighArity  avgt   30   57.054 �  0.635  ns/op
> (as you can see, just the Object[] with arity less than 10 case is fast here)
> After:
> Benchmark                                    Mode  Cnt  Score   Error  Units
> TypedAsCollector.testIntCollect              avgt   30  6.569 � 0.131  ns/op
> TypedAsCollector.testIntCollectHighArity     avgt   30  8.923 � 0.066  ns/op
> TypedAsCollector.testObjectCollect           avgt   30  6.813 � 0.035  ns/op
> TypedAsCollector.testObjectCollectHighArity  avgt   30  9.718 � 0.066  ns/op
> TypedAsCollector.testStringCollect           avgt   30  6.737 � 0.016  ns/op
> TypedAsCollector.testStringCollectHighArity  avgt   30  9.618 � 0.052  ns/op
> 
> Thanks,
> Jorn

That's an elegant solution.

At first i thought it might unduly perturb lambda form generation and caching. but you slotted a different lambda form implementation underneath the varargs implementation.

src/java.base/share/classes/java/lang/invoke/MethodHandleImpl.java line 1277:

> 1275:             if (intrinsicName == Intrinsic.IDENTITY) {
> 1276:                 MethodType resultType = type().asCollectorType(arrayType, type().parameterCount() - 1, arrayLength);
> 1277:                 MethodHandle collector = MethodHandleImpl.makeCollector(arrayType, arrayLength);

Should `MethodHandleImpl.varargsArray` still be used here since it performs arity checking and caching? 

Maybe the arity checks are performed by `asCollectorType`, but that would still leave the caching.

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PR: https://git.openjdk.java.net/jdk/pull/3306