RFR: 8367982: Unify ObjectSynchronizer and LightweightSynchronizer

[Patricio Chilano Mateo]

On Tue, 21 Oct 2025 13:11:45 GMT, Fredrik Bredberg <fbredberg at openjdk.org> wrote:

> This is the last PR in a series of PRs (see: [JDK-8344261](https://bugs.openjdk.org/browse/JDK-8344261)) to obsolete the LockingMode flag and related code.
> 
> The main focus is to to unify `ObjectSynchronizer` and `LightweightSynchronizer`.
> There used to be a number of "dispatch functions" to redirect calls depending on the setting of the `LockingMode` flag.
> Since we now only have lightweight locking, there is no longer any need for those dispatch functions, so I removed them.
> To remove the dispatch functions I renamed the corresponding lightweight functions and call them directly.
> This ultimately led me to remove "lightweight" from the function names and go back to "fast" instead, just to avoid having some with, and some without the "lightweight" part of the name.
> 
> This PR also include a small simplification of `ObjectSynchronizer::FastHashCode`.
> 
> Tested tier1-7 (on supported platforms) without seeing any problems that can be traced to this code change.
> All other platforms (`arm`, `ppc`, `riscv`, `s390`) has been sanity checked using QEMU.

Thanks for doing this cleanup. I only found some instances in comments where `lightweight` referred to the locking mode and not just the fast path, so the word should be removed rather than replaced with `fast`.

src/hotspot/share/oops/markWord.hpp line 215:

> 213:   ObjectMonitor* monitor() const {
> 214:     assert(has_monitor(), "check");
> 215:     assert(!UseObjectMonitorTable, "Fast locking with OM table does not use markWord for monitors");

Suggestion:

    assert(!UseObjectMonitorTable, "Locking with OM table does not use markWord for monitors");

src/hotspot/share/oops/markWord.hpp line 241:

> 239:   }
> 240:   static markWord encode(ObjectMonitor* monitor) {
> 241:     assert(!UseObjectMonitorTable, "Fast locking with OM table does not use markWord for monitors");

Suggestion:

    assert(!UseObjectMonitorTable, "Locking with OM table does not use markWord for monitors");

src/hotspot/share/runtime/objectMonitor.hpp line 164:

> 162:   // Because of frequent access, the metadata field is at offset zero (0).
> 163:   // Enforced by the assert() in metadata_addr().
> 164:   // * Fast locking with UseObjectMonitorTable:

Suggestion:

  // * Locking with UseObjectMonitorTable:

src/hotspot/share/runtime/objectMonitor.hpp line 166:

> 164:   // * Fast locking with UseObjectMonitorTable:
> 165:   //   Contains the _object's hashCode.
> 166:   // * * Fast locking without UseObjectMonitorTable:

Suggestion:

  // * Locking without UseObjectMonitorTable:

src/hotspot/share/runtime/objectMonitor.inline.hpp line 77:

> 75: 
> 76: inline markWord ObjectMonitor::header() const {
> 77:   assert(!UseObjectMonitorTable, "Fast locking with OM table does not use header");

Suggestion:

  assert(!UseObjectMonitorTable, "Locking with OM table does not use header");

src/hotspot/share/runtime/objectMonitor.inline.hpp line 82:

> 80: 
> 81: inline void ObjectMonitor::set_header(markWord hdr) {
> 82:   assert(!UseObjectMonitorTable, "Fast locking with OM table does not use header");

Suggestion:

  assert(!UseObjectMonitorTable, "Locking with OM table does not use header");

src/hotspot/share/runtime/objectMonitor.inline.hpp line 87:

> 85: 
> 86: inline intptr_t ObjectMonitor::hash() const {
> 87:   assert(UseObjectMonitorTable, "Only used by fast locking with OM table");

Suggestion:

  assert(UseObjectMonitorTable, "Only used by locking with OM table");

src/hotspot/share/runtime/objectMonitor.inline.hpp line 92:

> 90: 
> 91: inline void ObjectMonitor::set_hash(intptr_t hash) {
> 92:   assert(UseObjectMonitorTable, "Only used by fast locking with OM table");

Suggestion:

  assert(UseObjectMonitorTable, "Only used by locking with OM table");

src/hotspot/share/runtime/synchronizer.cpp line 648:

> 646:       // stability and then install the hash.
> 647:     } else {
> 648:       assert(!mark.is_unlocked() && !mark.is_fast_locked(), "invariant");

Note that `mark.monitor()` below already asserts `mark.has_monitor()` which is stronger.

-------------

PR Review: https://git.openjdk.org/jdk/pull/27915#pullrequestreview-3371295012
PR Review Comment: https://git.openjdk.org/jdk/pull/27915#discussion_r2456141768
PR Review Comment: https://git.openjdk.org/jdk/pull/27915#discussion_r2456143321
PR Review Comment: https://git.openjdk.org/jdk/pull/27915#discussion_r2456146275
PR Review Comment: https://git.openjdk.org/jdk/pull/27915#discussion_r2456190353
PR Review Comment: https://git.openjdk.org/jdk/pull/27915#discussion_r2456194705
PR Review Comment: https://git.openjdk.org/jdk/pull/27915#discussion_r2456195722
PR Review Comment: https://git.openjdk.org/jdk/pull/27915#discussion_r2456201765
PR Review Comment: https://git.openjdk.org/jdk/pull/27915#discussion_r2456204642
PR Review Comment: https://git.openjdk.org/jdk/pull/27915#discussion_r2456288423