RFR: 8299962: Speed up compiler/intrinsics/unsafe/DirectByteBufferTest.java and HeapByteBufferTest.java

[Vladimir Kozlov]

On Wed, 11 Jan 2023 13:51:16 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

> The test used a blanket `100_000` iterations to ensure C2 triggers for the relevant intrinsics of `jdk.internal.misc.Unsafe`.
> 
> I experimented lowering the number of iterations, and got the following results:
> `100_000` (overkill, 8.5 sec) `5000`(very safe, 3 sec), `2000`(decent, 2.6 sec) and even `200` (ok, 2.5 sec).
> I measured the time per `@run` statement (there are 6 over the two test files).
> 
> For `5000` I got the same count of intrinsifications per intrinsic.
> For `2000` it dropped slightly, rarely an intrinsic was not intrinsified.
> For `200` it dropped a bit more, and sometimes multiple intrinsics are not intrinsified.
> 
> If one uses `-Xbatch -X:-TieredCompilation`, then the cound does not change at all, even for `200`.
> 
> Since the marginal speedup from `5000` to `200` is very small, I decided to be on the **safe side**.
> This is still a **speedup of 2.7x**.

Default C2 compilation threshold is around 10000. So 5000 is not safe.
Instead I suggest to use `-XX:CompileThresholdScaling=` together with small `iterations` numbers which guarantees C2 compilation.

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PR: https://git.openjdk.org/jdk/pull/11944