RFR: 8313224: Avoid calling JavaThread::current() in MemAllocator::Allocation constructor

[Thomas Schatzl]

On Thu, 27 Jul 2023 18:34:16 GMT, Ioi Lam <iklam at openjdk.org> wrote:

> Summary: Avoid calling `JavaThread::current()` since we already have a `Thread*` in `MemAllocator::_thread`.
> 
> Although the latter is declared to be a `Thead*` and not a `JavaThread*`, the calling context requires the current thread to be a `JavaThread`, and  `MemAllocator::_thread` was initialized to be  `Thread::current`, so I changed the code from 
> 
> 
> _thread(JavaThread::current()),
> 
> 
> to
> 
> 
> assert(Thread::current()->is_Java_thread(), "must be used by JavaThreads only");
> assert(Thread::current() == allocator._thread, "do not pass MemAllocator across threads");
> _thread = JavaThread::cast(allocator._thread);
> 
> 
> I also clean up a few other lines to limit the explicit use of `JavaThread*`.
> 
> This fix improved `java --version` by about 0.6% for my prototype of [JDK-8310823](https://bugs.openjdk.org/browse/JDK-8310823), which allocates ~24000 heap objects at VM start-up.

lgtm, with some minor comments about superfluous asserts.

src/hotspot/share/gc/shared/memAllocator.cpp line 79:

> 77:       _tlab_end_reset_for_sample(false)
> 78:   {
> 79:     assert(Thread::current()->is_Java_thread(), "must be used by JavaThreads only");

This first assert duplicates the one done by `JavaThread::cast()` given the other assert just below.

src/hotspot/share/gc/shared/memAllocator.cpp line 81:

> 79:     assert(Thread::current()->is_Java_thread(), "must be used by JavaThreads only");
> 80:     assert(Thread::current() == allocator._thread, "do not pass MemAllocator across threads");
> 81:     _thread = JavaThread::cast(allocator._thread);

The assignment could also be done in the initializer list as it seems simple enough.

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Marked as reviewed by tschatzl (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/15058#pullrequestreview-1557033074
PR Review Comment: https://git.openjdk.org/jdk/pull/15058#discussion_r1280662683
PR Review Comment: https://git.openjdk.org/jdk/pull/15058#discussion_r1280663418