On Tue, 1 Oct 2024 10:18:15 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. >> >> This problem was originally reported here: >> https://mail.openjdk.org/pipermail/panama-dev/2024-September/020643.html >> >> While we did not test this fix directly, we have made sure that running the >> problematic benchmark with the flags: >> >> >> -XX:CompileCommand=dontinline,java/lang/invoke/MethodHandle.setAsTypeCache >> -XX:CompileCommand=dontinline,java/lang/invoke/MethodHandle.asTypeUncached >> >> >> Solves the performance regression. The fix in this PR is just a programmatic >> way to achieve the same results w/o the need of command line flags. > > Maurizio Cimadamore has updated the pull request incrementally with one > additional commit since the last revision: > > Update copyright LGTM In the micro perhaps the `unsafe_loop` variant is superfluous for this special case test? At least it's distinct from the test in `LoopOverNonConstant` by summing longs rather than ints. ------------- Marked as reviewed by redestad (Reviewer). PR Review: https://git.openjdk.org/jdk/pull/21283#pullrequestreview-2339816894
