Summary

Deprecate the flag -XX:+/-UseHeavyMonitors in JDK 18, obsolete it in 19 and convert it to a develop flag in 19.

Problem

UseHeavyMonitors is supposed to be used to disable stack-locking (thin locks) and let the synchronization subsystem use only 'heavy' monitors. However, the current Hotspot only implements this in the interpreter. Even worse, even then it calls into the runtime when running with +UseHeavyMonitors, which would still use stack-locking. C1 has its own flag UseFastLocking to achieve the same, and C2 doesn't have a comparable mechanism at all, and would always emit code that attempts stack-locking.

As such, the flag is pretty much useless in its current form. Users who use this in production would not see any noticeable effect. If it were implemented correctly, it would perhaps expose somewhat surprising behaviours, like potential failure to allocate or deflate monitors under much increased monitor pressure.

Solution

This change implements UseHeavyMonitors consistently (on x86_64), and deprecates the flag UseHeavyMonitors. The intention is to demote the flag to develop in JDK 19, at which time it becomes "obsolete" in a product build. It should really not be used in production, and is only really useful for experimentation by the Hotspot team. I also intend to remove the (develop) UseFastLocking flag of C1, and use UseHeavyMonitors there instead.

Alternatively, we could get rid of UseHeavyMonitors altogether. However, I currently want to conduct some experiments with disabling stack-locking, so I think it's still useful. I am not alone with this. For example, the implementors of JEP 143 also used it for experiments (probably misguided, because the flag did not really work).

See: https://openjdk.java.net/jeps/143

As an aside, I intend to introduce a new develop flag VerifyHeavyMonitors to enable verification that the UseHeavyMonitors flag works correctly. I would prefer to unconditionally verify this using asserts, but this is currently not possible because the functionality is only implemented on x86_64.

Specification

diff --git a/src/hotspot/share/c1/c1_globals.hpp b/src/hotspot/share/c1/c1_globals.hpp
index 2b5de079a0c..e90aaf6536d 100644
--- a/src/hotspot/share/c1/c1_globals.hpp
+++ b/src/hotspot/share/c1/c1_globals.hpp
@@ -242,9 +242,6 @@
   develop(bool, UseFastNewObjectArray, true,                                \
           "Use fast inlined object array allocation")                       \
                                                                             \
-  develop(bool, UseFastLocking, true,                                       \
-          "Use fast inlined locking code")                                  \
-                                                                            \
   develop(bool, UseSlowPath, false,                                         \
           "For debugging: test slow cases by always using them")            \
                                                                             \
    diff --git a/src/hotspot/share/runtime/globals.hpp b/src/hotspot/share/runtime/globals.hpp
index 81bfbaaf033..d547c4c6376 100644
--- a/src/hotspot/share/runtime/globals.hpp
+++ b/src/hotspot/share/runtime/globals.hpp
@@ -1071,6 +1071,10 @@ const intx ObjectAlignmentInBytes = 8;
   product(bool, UseHeavyMonitors, false,                                    \
           "use heavyweight instead of lightweight Java monitors")           \
                                                                             \
+  develop(bool, VerifyHeavyMonitors, false,                                 \
+          "Checks that no stack locking happens when using "                \
+          "+UseHeavyMonitors")                                              \
+                                                                            \
   product(bool, PrintStringTableStatistics, false,                          \
           "print statistics about the StringTable and SymbolTable")         \

diff --git a/src/hotspot/share/runtime/arguments.cpp b/src/hotspot/share/runtime/arguments.cpp
index 89352c1f95e..79d938939a4 100644
--- a/src/hotspot/share/runtime/arguments.cpp
+++ b/src/hotspot/share/runtime/arguments.cpp
@@ -532,6 +532,9 @@ static SpecialFlag const special_jvm_flags[] = {
   { "DynamicDumpSharedSpaces",      JDK_Version::jdk(18), JDK_Version::jdk(19), JDK_Version::undefined() },
   { "RequireSharedSpaces",          JDK_Version::jdk(18), JDK_Version::jdk(19), JDK_Version::undefined() },
   { "UseSharedSpaces",              JDK_Version::jdk(18), JDK_Version::jdk(19), JDK_Version::undefined() },
+#ifdef PRODUCT
+  { "UseHeavyMonitors",             JDK_Version::jdk(18), JDK_Version::jdk(19), JDK_Version::jdk(20) },
+#endif

   // --- Deprecated alias flags (see also aliased_jvm_flags) - sorted by obsolete_in then expired_in:
   { "DefaultMaxRAMFraction",        JDK_Version::jdk(8),  JDK_Version::undefined(), JDK_Version::undefined() },

The complete change is: https://github.com/openjdk/jdk/pull/6320/files