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

[Roman Kennke]

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());
> ```

Hi Amit,
sorry I only now get to reply to this, I have been traveling. What does the change do? Is it critical? Would it be possible to fix it after I intergrated the JEP? Because any change that I do now invalidates existing reviews, and might delay integration, and we're already running pretty close to RDP1. If at all possible, I would prefer to take it after I intergrated the JEP - we can have fixes well after RDP1, but not new features. If you agree, then please file a follow-up issue.

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PR Comment: https://git.openjdk.org/jdk/pull/20677#issuecomment-2457674486