Enhancement 8386858: [lworld] Omit load_prototype_header When UseObjectMonitorTable Is Used

Overview#

In the lworld branch of Valhalla, when an object appears to be locked, the prototype header is read from klass, and the Valhalla-specific markWord flags are checked.

However, if UseObjectMonitorTable is enabled, the object monitor is not represented as a markWord on the object header. In this case, the necessary mark word bits remain in the object header, so there's no need to proceed with load_prototype_header; the read-in header can be inspected directly.

This fix modifies the code such that test_oop_prototype_bit reads the prototype header only when !UseObjectMonitorTable. The affected architectures are aarch64, ppc, riscv, and x86. This change makes the process more lightweight.

ldr(temp_reg, Address(oop, oopDesc::mark_offset_in_bytes()));
-tst(temp_reg, markWord::unlocked_value);
-br(Assembler::NE, test_mark_word);
-load_prototype_header(temp_reg, oop);
+if (!UseObjectMonitorTable) {
+  tst(temp_reg, markWord::unlocked_value);
+  br(Assembler::NE, test_mark_word);
+  load_prototype_header(temp_reg, oop);
+}

As a result, when UseObjectMonitorTable is enabled, unnecessary prototype header reads are avoided, and flags can be directly inspected from the object header.

PR Created on June 29, 2026#

In the PR, I explained that with UseObjectMonitorTable enabled, the object monitor does not get placed as a displaced mark word in the object header. Therefore, bits can be obtained directly from the read-in header¹.

The fix applies the same logic to each CPU's MacroAssembler::test_oop_prototype_bit. First, it loads the mark word and executes the previous behavior when UseObjectMonitorTable is disabled.

Review on June 29-30, 2026#

Coleen Phillimore² approved the PR. She noted that ObjectMonitorTable will be removed in the future, and this change results in faster code.

Afterward, /integrate was performed, putting it into a sponsor-waiting state.

Integration on June 30, 2026#

David Holmes³ also approved the PR and proceeded with /sponsor. The final commit 28b617e⁴ integrated this change into the Valhalla lworld branch.

Overview#

PR Created on June 29, 2026#

Review on June 29-30, 2026#

Integration on June 30, 2026#

Footnotes#