On Tue, 1 Oct 2024 10:08:54 GMT, Maurizio Cimadamore <mcimadam...@openjdk.org>
wrote:
> The fix for JDK-8331865 introduced an accidental performance regression.
> The main issue is that now *all* memory segment var handles go through some
> round of adaptation.
> Adapting a var handle results in a so called *indirect* var handle.
> When an indirect var handle is called through a *var handle guard*, an extra
> `MethodHandle::asType` call is triggered.
> In some cases, if `asType` has already been compiled into a big method, it
> cannot be inlined into the caller, which then results in a failure to inline
> through the var handle call, resulting in a big performance regression.
>
> The solution is to make sure that the compiled size of `MethodHandle::asType`
> stays small: this is done by making sure that the slowpath (the one which
> populates the cache used by `asType`) is not inlined by the JVM. This is done
> by consolidating the slow path into a separate method, which is annotated
> with the internal `@DontInline` annotation.
test/micro/org/openjdk/bench/java/lang/foreign/LoopOverNonConstantAsType.java
line 57:
> 55: @State(org.openjdk.jmh.annotations.Scope.Thread)
> 56: @OutputTimeUnit(TimeUnit.MILLISECONDS)
> 57: @Fork(value = 3, jvmArgsAppend = { "-XX:-TieredCompilation" })
The benchmark only reproduces if tiered compilation is disabled. This is due to
the fact that some threshold are updated accordingly, and set to a smaller
value, which is enough to exhibit the issue.
test/micro/org/openjdk/bench/java/lang/foreign/LoopOverNonConstantAsType.java
line 96:
> 94: static {
> 95: types = new Class<?>[TYPE_SIZE];
> 96: ClassLoader customLoader = new URLClassLoader(new URL[0],
> LoopOverNonConstantAsType.class.getClassLoader());
Technically, using a different class loadrer is not required - but using
classes with different class loaders can end up taking even longer code paths
when calling `MethodHandle::asType`, so I've added that here.
-------------
PR Review Comment: https://git.openjdk.org/jdk/pull/21283#discussion_r1782505769
PR Review Comment: https://git.openjdk.org/jdk/pull/21283#discussion_r1782504664
