RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v50]

[Amit Kumar]

On Tue, 22 Oct 2024 16:22:20 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with two additional commits since the last revision:
>> 
>>  - Update copyright
>>  - Avoid assert/endless-loop in JFR code
>
> @egahlin / @mgronlun could you please review the JFR parts of this PR? One change is for getting the right prototype header, the other is for avoiding an endless loop/assert in a corner case.

@rkennke can you include this small update for s390x as well: 

```diff 
diff --git a/src/hotspot/cpu/s390/c1_MacroAssembler_s390.cpp b/src/hotspot/cpu/s390/c1_MacroAssembler_s390.cpp
index 0f7e5c9f457..476e3d5daa4 100644
--- a/src/hotspot/cpu/s390/c1_MacroAssembler_s390.cpp
+++ b/src/hotspot/cpu/s390/c1_MacroAssembler_s390.cpp
@@ -174,8 +174,11 @@ void C1_MacroAssembler::try_allocate(
 void C1_MacroAssembler::initialize_header(Register obj, Register klass, Register len, Register Rzero, Register t1) {
   assert_different_registers(obj, klass, len, t1, Rzero);
   if (UseCompactObjectHeaders) {
-    z_lg(t1, Address(klass, in_bytes(Klass::prototype_header_offset())));
-    z_stg(t1, Address(obj, oopDesc::mark_offset_in_bytes()));
+    z_mvc(
+          Address(obj, oopDesc::mark_offset_in_bytes()),  /* move to */
+          Address(klass, in_bytes(Klass::prototype_header_offset())), /* move from */
+          sizeof(markWord) /* how much to move */
+        );
   } else {
     load_const_optimized(t1, (intx)markWord::prototype().value());
     z_stg(t1, Address(obj, oopDesc::mark_offset_in_bytes()));
diff --git a/src/hotspot/cpu/s390/c2_MacroAssembler_s390.cpp b/src/hotspot/cpu/s390/c2_MacroAssembler_s390.cpp
index 378d5e4cfe1..c5713161bf9 100644
--- a/src/hotspot/cpu/s390/c2_MacroAssembler_s390.cpp
+++ b/src/hotspot/cpu/s390/c2_MacroAssembler_s390.cpp
@@ -46,7 +46,7 @@ void C2_MacroAssembler::load_narrow_klass_compact_c2(Register dst, Address src)
   // The incoming address is pointing into obj-start + klass_offset_in_bytes. We need to extract
   // obj-start, so that we can load from the object's mark-word instead.
   z_lg(dst, src.plus_disp(-oopDesc::klass_offset_in_bytes()));
-  z_srlg(dst, dst, markWord::klass_shift); // TODO: could be z_sra
+  z_srlg(dst, dst, markWord::klass_shift);
 }
 
 //------------------------------------------------------
diff --git a/src/hotspot/cpu/s390/templateTable_s390.cpp b/src/hotspot/cpu/s390/templateTable_s390.cpp
index 3cb1aba810d..5b8f7a20478 100644
--- a/src/hotspot/cpu/s390/templateTable_s390.cpp
+++ b/src/hotspot/cpu/s390/templateTable_s390.cpp
@@ -3980,8 +3980,11 @@ void TemplateTable::_new() {
     // Initialize object header only.
     __ bind(initialize_header);
     if (UseCompactObjectHeaders) {
-      __ z_lg(tmp, Address(iklass, in_bytes(Klass::prototype_header_offset())));
-      __ z_stg(tmp, Address(RallocatedObject, oopDesc::mark_offset_in_bytes()));
+      __ z_mvc(
+            Address(RallocatedObject, oopDesc::mark_offset_in_bytes()), // move to
+            Address(iklass, in_bytes(Klass::prototype_header_offset())), // move from
+            sizeof(markWord) // how much to move
+          );
     } else {
       __ store_const(Address(RallocatedObject, oopDesc::mark_offset_in_bytes()),
                      (long) markWord::prototype().value());

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

PR Comment: https://git.openjdk.org/jdk/pull/20677#issuecomment-2444180131