Summary

I'd like to deprecate the following product (x86 and ppc) flags for removal: UseRTMLocking UseRTMDeopt UseRTMForStackLocks UseRTMXendForLockBusy

Problem

HotSpot supports RTM (restricted transactional memory) to be used for locking and deoptimization. RTM has since been disabled in Intel processors due to security vulnerabilities [0] and IBM removed support for it since its Power 10 line. RTM adds unnecessarily to complexity and maintenance burden.

[0] https://en.wikipedia.org/wiki/Transactional_Synchronization_Extensions#History_and_bugs

Solution

I am proposing to deprecate the relevant flags for removal, and actually remove the flags and all related code in a later release, unless somebody comes up with a good reason and performance comparison to show that it's worth keeping.

Specification

diff --git a/src/hotspot/share/runtime/arguments.cpp b/src/hotspot/share/runtime/arguments.cpp
index 019c8cab92ef..200eac857771 100644
--- a/src/hotspot/share/runtime/arguments.cpp
+++ b/src/hotspot/share/runtime/arguments.cpp
@@ -536,6 +536,10 @@ 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() },
+  { "UseRTMLocking",                JDK_Version::jdk(20), JDK_Version::jdk(21), JDK_Version::jdk(22) },
+  { "UseRTMDeopt",                  JDK_Version::jdk(20), JDK_Version::jdk(21), JDK_Version::jdk(22) },
+  { "UseRTMForStackLocks",          JDK_Version::jdk(20), JDK_Version::jdk(21), JDK_Version::undefined() },
+  { "UseRTMXendForLockBusy",        JDK_Version::jdk(20), JDK_Version::jdk(21), JDK_Version::undefined() },

   // --- 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() },