Re: RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v57]
On Thu, 7 Nov 2024 17:25:40 GMT, Roman Kennke wrote: >> This is the main body of the JEP 450: Compact Object Headers (Experimental). >> >> It is also a follow-up to #20640, which now also includes (and supersedes) >> #20603 and #20605, plus the Tiny Class-Pointers parts that have been >> previously missing. >> >> Main changes: >> - Introduction of the (experimental) flag UseCompactObjectHeaders. All >> changes in this PR are protected by this flag. The purpose of the flag is to >> provide a fallback, in case that users unexpectedly observe problems with >> the new implementation. The intention is that this flag will remain >> experimental and opt-in for at least one release, then make it on-by-default >> and diagnostic (?), and eventually deprecate and obsolete it. However, there >> are a few unknowns in that plan, specifically, we may want to further >> improve compact headers to 4 bytes, we are planning to enhance the Klass* >> encoding to support virtually unlimited number of Klasses, at which point we >> could also obsolete UseCompressedClassPointers. >> - The compressed Klass* can now be stored in the mark-word of objects. In >> order to be able to do this, we are add some changes to GC forwarding (see >> below) to protect the relevant (upper 22) bits of the mark-word. Significant >> parts of this PR deal with loading the compressed Klass* from the mark-word. >> This PR also changes some code paths (mostly in GCs) to be more careful when >> accessing Klass* (or mark-word or size) to be able to fetch it from the >> forwardee in case the object is forwarded. >> - Self-forwarding in GCs (which is used to deal with promotion failure) now >> uses a bit to indicate 'self-forwarding'. This is needed to preserve the >> crucial Klass* bits in the header. This also allows to get rid of >> preserved-header machinery in SerialGC and G1 (Parallel GC abuses >> preserved-marks to also find all other relevant oops). >> - Full GC forwarding now uses an encoding similar to compressed-oops. We >> have 40 bits for that, and can encode up to 8TB of heap. When exceeding 8TB, >> we turn off UseCompressedClassPointers (except in ZGC, which doesn't use the >> GC forwarding at all). >> - Instances can now have their base-offset (the offset where the field >> layouter starts to place fields) at offset 8 (instead of 12 or 16). >> - Arrays will now store their length at offset 8. >> - CDS can now write and read archives with the compressed header. However, >> it is not possible to read an archive that has been written with an opposite >> setting of UseCompactObjectHeaders. Some build machinery is added so that >> _co... > > Roman Kennke has updated the pull request with a new target base due to a > merge or a rebase. The pull request now contains 107 commits: > > - Merge branch 'master' into JDK-8305895-v4 > - Merge tag 'jdk-25+23' into JDK-8305895-v4 > >Added tag jdk-24+23 for changeset c0e6c3b9 > - Fix gen-ZGC removal > - Merge tag 'jdk-24+22' into JDK-8305895-v4 > >Added tag jdk-24+22 for changeset 388d44fb > - Enable riscv in CompressedClassPointersEncodingScheme test > - s390 port > - Conditionalize platform specific parts of > CompressedClassPointersEncodingScheme test > - Update copyright > - Avoid assert/endless-loop in JFR code > - Update copyright headers > - ... and 97 more: https://git.openjdk.org/jdk/compare/d3c042f9...c1a6323b > So yes, some platforms [have alignment requirements for vectors]. But they > seem to be the exception. All AArch64 implementations work with unaligned vectors – that's in the architecture spec – but some designs thaht were made years ago performed badly. It's not a problem with new designs. - PR Comment: https://git.openjdk.org/jdk/pull/20677#issuecomment-2485185002
Re: RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v57]
On Mon, 18 Nov 2024 15:30:14 GMT, Magnus Ihse Bursie wrote: >> test/hotspot/jtreg/gtest/MetaspaceUtilsGtests.java line 1: >> >> >> This file was reduced to empty but not actually deleted. Can you fix it >> please. > > @rkennke Just making sure this is not being missed. Can you please open a JBS > issue to correct this and the file below? I filed: https://bugs.openjdk.org/browse/JDK-8344425 @tstuefe is working on it (mostly checking that nothing important has been removed) - PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1846945329
Re: RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v57]
On Thu, 7 Nov 2024 17:25:40 GMT, Roman Kennke wrote:
>> This is the main body of the JEP 450: Compact Object Headers (Experimental).
>>
>> It is also a follow-up to #20640, which now also includes (and supersedes)
>> #20603 and #20605, plus the Tiny Class-Pointers parts that have been
>> previously missing.
>>
>> Main changes:
>> - Introduction of the (experimental) flag UseCompactObjectHeaders. All
>> changes in this PR are protected by this flag. The purpose of the flag is to
>> provide a fallback, in case that users unexpectedly observe problems with
>> the new implementation. The intention is that this flag will remain
>> experimental and opt-in for at least one release, then make it on-by-default
>> and diagnostic (?), and eventually deprecate and obsolete it. However, there
>> are a few unknowns in that plan, specifically, we may want to further
>> improve compact headers to 4 bytes, we are planning to enhance the Klass*
>> encoding to support virtually unlimited number of Klasses, at which point we
>> could also obsolete UseCompressedClassPointers.
>> - The compressed Klass* can now be stored in the mark-word of objects. In
>> order to be able to do this, we are add some changes to GC forwarding (see
>> below) to protect the relevant (upper 22) bits of the mark-word. Significant
>> parts of this PR deal with loading the compressed Klass* from the mark-word.
>> This PR also changes some code paths (mostly in GCs) to be more careful when
>> accessing Klass* (or mark-word or size) to be able to fetch it from the
>> forwardee in case the object is forwarded.
>> - Self-forwarding in GCs (which is used to deal with promotion failure) now
>> uses a bit to indicate 'self-forwarding'. This is needed to preserve the
>> crucial Klass* bits in the header. This also allows to get rid of
>> preserved-header machinery in SerialGC and G1 (Parallel GC abuses
>> preserved-marks to also find all other relevant oops).
>> - Full GC forwarding now uses an encoding similar to compressed-oops. We
>> have 40 bits for that, and can encode up to 8TB of heap. When exceeding 8TB,
>> we turn off UseCompressedClassPointers (except in ZGC, which doesn't use the
>> GC forwarding at all).
>> - Instances can now have their base-offset (the offset where the field
>> layouter starts to place fields) at offset 8 (instead of 12 or 16).
>> - Arrays will now store their length at offset 8.
>> - CDS can now write and read archives with the compressed header. However,
>> it is not possible to read an archive that has been written with an opposite
>> setting of UseCompactObjectHeaders. Some build machinery is added so that
>> _co...
>
> Roman Kennke has updated the pull request with a new target base due to a
> merge or a rebase. The pull request now contains 107 commits:
>
> - Merge branch 'master' into JDK-8305895-v4
> - Merge tag 'jdk-25+23' into JDK-8305895-v4
>
>Added tag jdk-24+23 for changeset c0e6c3b9
> - Fix gen-ZGC removal
> - Merge tag 'jdk-24+22' into JDK-8305895-v4
>
>Added tag jdk-24+22 for changeset 388d44fb
> - Enable riscv in CompressedClassPointersEncodingScheme test
> - s390 port
> - Conditionalize platform specific parts of
> CompressedClassPointersEncodingScheme test
> - Update copyright
> - Avoid assert/endless-loop in JFR code
> - Update copyright headers
> - ... and 97 more: https://git.openjdk.org/jdk/compare/d3c042f9...c1a6323b
Ah there are some exceptions:
x86:
`src/hotspot/cpu/x86/vm_version_x86.cpp:AlignVector =
!UseUnalignedLoadStores;`
if (supports_sse4_2()) { // new ZX cpus
if (FLAG_IS_DEFAULT(UseUnalignedLoadStores)) {
UseUnalignedLoadStores = true; // use movdqu on newest ZX cpus
}
}
So I suppose some older platforms may be affected, though I have not seen one
yet. They would have to be missing the unaligned `movdqu` instructions.
aarch64:
`src/hotspot/cpu/aarch64/vm_version_aarch64.cpp:AlignVector =
AvoidUnalignedAccesses;`
// Ampere eMAG
if (_cpu == CPU_AMCC && (_model == CPU_MODEL_EMAG) && (_variant == 0x3)) {
if (FLAG_IS_DEFAULT(AvoidUnalignedAccesses)) {
FLAG_SET_DEFAULT(AvoidUnalignedAccesses, true);
}
and
// ThunderX
if (_cpu == CPU_CAVIUM && (_model == 0xA1)) {
guarantee(_variant != 0, "Pre-release hardware no longer supported.");
if (FLAG_IS_DEFAULT(AvoidUnalignedAccesses)) {
FLAG_SET_DEFAULT(AvoidUnalignedAccesses, true);
}
and
// ThunderX2
if ((_cpu == CPU_CAVIUM && (_model == 0xAF)) ||
(_cpu == CPU_BROADCOM && (_model == 0x516))) {
if (FLAG_IS_DEFAULT(AvoidUnalignedAccesses)) {
FLAG_SET_DEFAULT(AvoidUnalignedAccesses, true);
}
and
// HiSilicon TSV110
if (_cpu == CPU_HISILICON && _model == 0xd01) {
if (FLAG_IS_DEFAULT(AvoidUnalignedAccesses)) {
FLAG_SET_DEFAULT(AvoidUnalignedAccesses, true);
}
So yes, some platforms are affected. But they seem to be the exception.
And again: we have on
Re: RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v57]
On Fri, 15 Nov 2024 04:49:51 GMT, David Holmes wrote: >> Roman Kennke has updated the pull request with a new target base due to a >> merge or a rebase. The pull request now contains 107 commits: >> >> - Merge branch 'master' into JDK-8305895-v4 >> - Merge tag 'jdk-25+23' into JDK-8305895-v4 >> >>Added tag jdk-24+23 for changeset c0e6c3b9 >> - Fix gen-ZGC removal >> - Merge tag 'jdk-24+22' into JDK-8305895-v4 >> >>Added tag jdk-24+22 for changeset 388d44fb >> - Enable riscv in CompressedClassPointersEncodingScheme test >> - s390 port >> - Conditionalize platform specific parts of >> CompressedClassPointersEncodingScheme test >> - Update copyright >> - Avoid assert/endless-loop in JFR code >> - Update copyright headers >> - ... and 97 more: https://git.openjdk.org/jdk/compare/d3c042f9...c1a6323b > > test/hotspot/jtreg/gtest/MetaspaceUtilsGtests.java line 1: > > > This file was reduced to empty but not actually deleted. Can you fix it > please. @rkennke Just making sure this is not being missed. Can you please open a JBS issue to correct this and the file below? - PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1846790097
Re: RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v57]
On Thu, 7 Nov 2024 17:25:40 GMT, Roman Kennke wrote: >> This is the main body of the JEP 450: Compact Object Headers (Experimental). >> >> It is also a follow-up to #20640, which now also includes (and supersedes) >> #20603 and #20605, plus the Tiny Class-Pointers parts that have been >> previously missing. >> >> Main changes: >> - Introduction of the (experimental) flag UseCompactObjectHeaders. All >> changes in this PR are protected by this flag. The purpose of the flag is to >> provide a fallback, in case that users unexpectedly observe problems with >> the new implementation. The intention is that this flag will remain >> experimental and opt-in for at least one release, then make it on-by-default >> and diagnostic (?), and eventually deprecate and obsolete it. However, there >> are a few unknowns in that plan, specifically, we may want to further >> improve compact headers to 4 bytes, we are planning to enhance the Klass* >> encoding to support virtually unlimited number of Klasses, at which point we >> could also obsolete UseCompressedClassPointers. >> - The compressed Klass* can now be stored in the mark-word of objects. In >> order to be able to do this, we are add some changes to GC forwarding (see >> below) to protect the relevant (upper 22) bits of the mark-word. Significant >> parts of this PR deal with loading the compressed Klass* from the mark-word. >> This PR also changes some code paths (mostly in GCs) to be more careful when >> accessing Klass* (or mark-word or size) to be able to fetch it from the >> forwardee in case the object is forwarded. >> - Self-forwarding in GCs (which is used to deal with promotion failure) now >> uses a bit to indicate 'self-forwarding'. This is needed to preserve the >> crucial Klass* bits in the header. This also allows to get rid of >> preserved-header machinery in SerialGC and G1 (Parallel GC abuses >> preserved-marks to also find all other relevant oops). >> - Full GC forwarding now uses an encoding similar to compressed-oops. We >> have 40 bits for that, and can encode up to 8TB of heap. When exceeding 8TB, >> we turn off UseCompressedClassPointers (except in ZGC, which doesn't use the >> GC forwarding at all). >> - Instances can now have their base-offset (the offset where the field >> layouter starts to place fields) at offset 8 (instead of 12 or 16). >> - Arrays will now store their length at offset 8. >> - CDS can now write and read archives with the compressed header. However, >> it is not possible to read an archive that has been written with an opposite >> setting of UseCompactObjectHeaders. Some build machinery is added so that >> _co... > > Roman Kennke has updated the pull request with a new target base due to a > merge or a rebase. The pull request now contains 107 commits: > > - Merge branch 'master' into JDK-8305895-v4 > - Merge tag 'jdk-25+23' into JDK-8305895-v4 > >Added tag jdk-24+23 for changeset c0e6c3b9 > - Fix gen-ZGC removal > - Merge tag 'jdk-24+22' into JDK-8305895-v4 > >Added tag jdk-24+22 for changeset 388d44fb > - Enable riscv in CompressedClassPointersEncodingScheme test > - s390 port > - Conditionalize platform specific parts of > CompressedClassPointersEncodingScheme test > - Update copyright > - Avoid assert/endless-loop in JFR code > - Update copyright headers > - ... and 97 more: https://git.openjdk.org/jdk/compare/d3c042f9...c1a6323b test/hotspot/jtreg/runtime/FieldLayout/ArrayBaseOffsets.java line 1: > 1: /* This file too suffered the same fate; all contents were removed but the file was not deleted. - PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1843710074
Re: RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v57]
On Thu, 7 Nov 2024 17:25:40 GMT, Roman Kennke wrote: >> This is the main body of the JEP 450: Compact Object Headers (Experimental). >> >> It is also a follow-up to #20640, which now also includes (and supersedes) >> #20603 and #20605, plus the Tiny Class-Pointers parts that have been >> previously missing. >> >> Main changes: >> - Introduction of the (experimental) flag UseCompactObjectHeaders. All >> changes in this PR are protected by this flag. The purpose of the flag is to >> provide a fallback, in case that users unexpectedly observe problems with >> the new implementation. The intention is that this flag will remain >> experimental and opt-in for at least one release, then make it on-by-default >> and diagnostic (?), and eventually deprecate and obsolete it. However, there >> are a few unknowns in that plan, specifically, we may want to further >> improve compact headers to 4 bytes, we are planning to enhance the Klass* >> encoding to support virtually unlimited number of Klasses, at which point we >> could also obsolete UseCompressedClassPointers. >> - The compressed Klass* can now be stored in the mark-word of objects. In >> order to be able to do this, we are add some changes to GC forwarding (see >> below) to protect the relevant (upper 22) bits of the mark-word. Significant >> parts of this PR deal with loading the compressed Klass* from the mark-word. >> This PR also changes some code paths (mostly in GCs) to be more careful when >> accessing Klass* (or mark-word or size) to be able to fetch it from the >> forwardee in case the object is forwarded. >> - Self-forwarding in GCs (which is used to deal with promotion failure) now >> uses a bit to indicate 'self-forwarding'. This is needed to preserve the >> crucial Klass* bits in the header. This also allows to get rid of >> preserved-header machinery in SerialGC and G1 (Parallel GC abuses >> preserved-marks to also find all other relevant oops). >> - Full GC forwarding now uses an encoding similar to compressed-oops. We >> have 40 bits for that, and can encode up to 8TB of heap. When exceeding 8TB, >> we turn off UseCompressedClassPointers (except in ZGC, which doesn't use the >> GC forwarding at all). >> - Instances can now have their base-offset (the offset where the field >> layouter starts to place fields) at offset 8 (instead of 12 or 16). >> - Arrays will now store their length at offset 8. >> - CDS can now write and read archives with the compressed header. However, >> it is not possible to read an archive that has been written with an opposite >> setting of UseCompactObjectHeaders. Some build machinery is added so that >> _co... > > Roman Kennke has updated the pull request with a new target base due to a > merge or a rebase. The pull request now contains 107 commits: > > - Merge branch 'master' into JDK-8305895-v4 > - Merge tag 'jdk-25+23' into JDK-8305895-v4 > >Added tag jdk-24+23 for changeset c0e6c3b9 > - Fix gen-ZGC removal > - Merge tag 'jdk-24+22' into JDK-8305895-v4 > >Added tag jdk-24+22 for changeset 388d44fb > - Enable riscv in CompressedClassPointersEncodingScheme test > - s390 port > - Conditionalize platform specific parts of > CompressedClassPointersEncodingScheme test > - Update copyright > - Avoid assert/endless-loop in JFR code > - Update copyright headers > - ... and 97 more: https://git.openjdk.org/jdk/compare/d3c042f9...c1a6323b test/hotspot/jtreg/gtest/MetaspaceUtilsGtests.java line 1: This file was reduced to empty but not actually deleted. Can you fix it please. - PR Review Comment: https://git.openjdk.org/jdk/pull/20677#discussion_r1843185719
Re: RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v57]
On Thu, 7 Nov 2024 17:25:40 GMT, Roman Kennke wrote: >> This is the main body of the JEP 450: Compact Object Headers (Experimental). >> >> It is also a follow-up to #20640, which now also includes (and supersedes) >> #20603 and #20605, plus the Tiny Class-Pointers parts that have been >> previously missing. >> >> Main changes: >> - Introduction of the (experimental) flag UseCompactObjectHeaders. All >> changes in this PR are protected by this flag. The purpose of the flag is to >> provide a fallback, in case that users unexpectedly observe problems with >> the new implementation. The intention is that this flag will remain >> experimental and opt-in for at least one release, then make it on-by-default >> and diagnostic (?), and eventually deprecate and obsolete it. However, there >> are a few unknowns in that plan, specifically, we may want to further >> improve compact headers to 4 bytes, we are planning to enhance the Klass* >> encoding to support virtually unlimited number of Klasses, at which point we >> could also obsolete UseCompressedClassPointers. >> - The compressed Klass* can now be stored in the mark-word of objects. In >> order to be able to do this, we are add some changes to GC forwarding (see >> below) to protect the relevant (upper 22) bits of the mark-word. Significant >> parts of this PR deal with loading the compressed Klass* from the mark-word. >> This PR also changes some code paths (mostly in GCs) to be more careful when >> accessing Klass* (or mark-word or size) to be able to fetch it from the >> forwardee in case the object is forwarded. >> - Self-forwarding in GCs (which is used to deal with promotion failure) now >> uses a bit to indicate 'self-forwarding'. This is needed to preserve the >> crucial Klass* bits in the header. This also allows to get rid of >> preserved-header machinery in SerialGC and G1 (Parallel GC abuses >> preserved-marks to also find all other relevant oops). >> - Full GC forwarding now uses an encoding similar to compressed-oops. We >> have 40 bits for that, and can encode up to 8TB of heap. When exceeding 8TB, >> we turn off UseCompressedClassPointers (except in ZGC, which doesn't use the >> GC forwarding at all). >> - Instances can now have their base-offset (the offset where the field >> layouter starts to place fields) at offset 8 (instead of 12 or 16). >> - Arrays will now store their length at offset 8. >> - CDS can now write and read archives with the compressed header. However, >> it is not possible to read an archive that has been written with an opposite >> setting of UseCompactObjectHeaders. Some build machinery is added so that >> _co... > > Roman Kennke has updated the pull request with a new target base due to a > merge or a rebase. The pull request now contains 107 commits: > > - Merge branch 'master' into JDK-8305895-v4 > - Merge tag 'jdk-25+23' into JDK-8305895-v4 > >Added tag jdk-24+23 for changeset c0e6c3b9 > - Fix gen-ZGC removal > - Merge tag 'jdk-24+22' into JDK-8305895-v4 > >Added tag jdk-24+22 for changeset 388d44fb > - Enable riscv in CompressedClassPointersEncodingScheme test > - s390 port > - Conditionalize platform specific parts of > CompressedClassPointersEncodingScheme test > - Update copyright > - Avoid assert/endless-loop in JFR code > - Update copyright headers > - ... and 97 more: https://git.openjdk.org/jdk/compare/d3c042f9...c1a6323b Still looks good. Nice work! - Marked as reviewed by coleenp (Reviewer). PR Review: https://git.openjdk.org/jdk/pull/20677#pullrequestreview-2424274474
Re: RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v57]
On Thu, 7 Nov 2024 17:25:40 GMT, Roman Kennke wrote: >> This is the main body of the JEP 450: Compact Object Headers (Experimental). >> >> It is also a follow-up to #20640, which now also includes (and supersedes) >> #20603 and #20605, plus the Tiny Class-Pointers parts that have been >> previously missing. >> >> Main changes: >> - Introduction of the (experimental) flag UseCompactObjectHeaders. All >> changes in this PR are protected by this flag. The purpose of the flag is to >> provide a fallback, in case that users unexpectedly observe problems with >> the new implementation. The intention is that this flag will remain >> experimental and opt-in for at least one release, then make it on-by-default >> and diagnostic (?), and eventually deprecate and obsolete it. However, there >> are a few unknowns in that plan, specifically, we may want to further >> improve compact headers to 4 bytes, we are planning to enhance the Klass* >> encoding to support virtually unlimited number of Klasses, at which point we >> could also obsolete UseCompressedClassPointers. >> - The compressed Klass* can now be stored in the mark-word of objects. In >> order to be able to do this, we are add some changes to GC forwarding (see >> below) to protect the relevant (upper 22) bits of the mark-word. Significant >> parts of this PR deal with loading the compressed Klass* from the mark-word. >> This PR also changes some code paths (mostly in GCs) to be more careful when >> accessing Klass* (or mark-word or size) to be able to fetch it from the >> forwardee in case the object is forwarded. >> - Self-forwarding in GCs (which is used to deal with promotion failure) now >> uses a bit to indicate 'self-forwarding'. This is needed to preserve the >> crucial Klass* bits in the header. This also allows to get rid of >> preserved-header machinery in SerialGC and G1 (Parallel GC abuses >> preserved-marks to also find all other relevant oops). >> - Full GC forwarding now uses an encoding similar to compressed-oops. We >> have 40 bits for that, and can encode up to 8TB of heap. When exceeding 8TB, >> we turn off UseCompressedClassPointers (except in ZGC, which doesn't use the >> GC forwarding at all). >> - Instances can now have their base-offset (the offset where the field >> layouter starts to place fields) at offset 8 (instead of 12 or 16). >> - Arrays will now store their length at offset 8. >> - CDS can now write and read archives with the compressed header. However, >> it is not possible to read an archive that has been written with an opposite >> setting of UseCompactObjectHeaders. Some build machinery is added so that >> _co... > > Roman Kennke has updated the pull request with a new target base due to a > merge or a rebase. The pull request now contains 107 commits: > > - Merge branch 'master' into JDK-8305895-v4 > - Merge tag 'jdk-25+23' into JDK-8305895-v4 > >Added tag jdk-24+23 for changeset c0e6c3b9 > - Fix gen-ZGC removal > - Merge tag 'jdk-24+22' into JDK-8305895-v4 > >Added tag jdk-24+22 for changeset 388d44fb > - Enable riscv in CompressedClassPointersEncodingScheme test > - s390 port > - Conditionalize platform specific parts of > CompressedClassPointersEncodingScheme test > - Update copyright > - Avoid assert/endless-loop in JFR code > - Update copyright headers > - ... and 97 more: https://git.openjdk.org/jdk/compare/d3c042f9...c1a6323b Marked as reviewed by stefank (Reviewer). - PR Review: https://git.openjdk.org/jdk/pull/20677#pullrequestreview-2424260100
Re: RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v57]
On Thu, 7 Nov 2024 17:25:40 GMT, Roman Kennke wrote: >> This is the main body of the JEP 450: Compact Object Headers (Experimental). >> >> It is also a follow-up to #20640, which now also includes (and supersedes) >> #20603 and #20605, plus the Tiny Class-Pointers parts that have been >> previously missing. >> >> Main changes: >> - Introduction of the (experimental) flag UseCompactObjectHeaders. All >> changes in this PR are protected by this flag. The purpose of the flag is to >> provide a fallback, in case that users unexpectedly observe problems with >> the new implementation. The intention is that this flag will remain >> experimental and opt-in for at least one release, then make it on-by-default >> and diagnostic (?), and eventually deprecate and obsolete it. However, there >> are a few unknowns in that plan, specifically, we may want to further >> improve compact headers to 4 bytes, we are planning to enhance the Klass* >> encoding to support virtually unlimited number of Klasses, at which point we >> could also obsolete UseCompressedClassPointers. >> - The compressed Klass* can now be stored in the mark-word of objects. In >> order to be able to do this, we are add some changes to GC forwarding (see >> below) to protect the relevant (upper 22) bits of the mark-word. Significant >> parts of this PR deal with loading the compressed Klass* from the mark-word. >> This PR also changes some code paths (mostly in GCs) to be more careful when >> accessing Klass* (or mark-word or size) to be able to fetch it from the >> forwardee in case the object is forwarded. >> - Self-forwarding in GCs (which is used to deal with promotion failure) now >> uses a bit to indicate 'self-forwarding'. This is needed to preserve the >> crucial Klass* bits in the header. This also allows to get rid of >> preserved-header machinery in SerialGC and G1 (Parallel GC abuses >> preserved-marks to also find all other relevant oops). >> - Full GC forwarding now uses an encoding similar to compressed-oops. We >> have 40 bits for that, and can encode up to 8TB of heap. When exceeding 8TB, >> we turn off UseCompressedClassPointers (except in ZGC, which doesn't use the >> GC forwarding at all). >> - Instances can now have their base-offset (the offset where the field >> layouter starts to place fields) at offset 8 (instead of 12 or 16). >> - Arrays will now store their length at offset 8. >> - CDS can now write and read archives with the compressed header. However, >> it is not possible to read an archive that has been written with an opposite >> setting of UseCompactObjectHeaders. Some build machinery is added so that >> _co... > > Roman Kennke has updated the pull request with a new target base due to a > merge or a rebase. The pull request now contains 107 commits: > > - Merge branch 'master' into JDK-8305895-v4 > - Merge tag 'jdk-25+23' into JDK-8305895-v4 > >Added tag jdk-24+23 for changeset c0e6c3b9 > - Fix gen-ZGC removal > - Merge tag 'jdk-24+22' into JDK-8305895-v4 > >Added tag jdk-24+22 for changeset 388d44fb > - Enable riscv in CompressedClassPointersEncodingScheme test > - s390 port > - Conditionalize platform specific parts of > CompressedClassPointersEncodingScheme test > - Update copyright > - Avoid assert/endless-loop in JFR code > - Update copyright headers > - ... and 97 more: https://git.openjdk.org/jdk/compare/d3c042f9...c1a6323b Marked as reviewed by phh (Reviewer). - PR Review: https://git.openjdk.org/jdk/pull/20677#pullrequestreview-2424210008
Re: RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v57]
On Thu, 7 Nov 2024 17:25:40 GMT, Roman Kennke wrote: >> This is the main body of the JEP 450: Compact Object Headers (Experimental). >> >> It is also a follow-up to #20640, which now also includes (and supersedes) >> #20603 and #20605, plus the Tiny Class-Pointers parts that have been >> previously missing. >> >> Main changes: >> - Introduction of the (experimental) flag UseCompactObjectHeaders. All >> changes in this PR are protected by this flag. The purpose of the flag is to >> provide a fallback, in case that users unexpectedly observe problems with >> the new implementation. The intention is that this flag will remain >> experimental and opt-in for at least one release, then make it on-by-default >> and diagnostic (?), and eventually deprecate and obsolete it. However, there >> are a few unknowns in that plan, specifically, we may want to further >> improve compact headers to 4 bytes, we are planning to enhance the Klass* >> encoding to support virtually unlimited number of Klasses, at which point we >> could also obsolete UseCompressedClassPointers. >> - The compressed Klass* can now be stored in the mark-word of objects. In >> order to be able to do this, we are add some changes to GC forwarding (see >> below) to protect the relevant (upper 22) bits of the mark-word. Significant >> parts of this PR deal with loading the compressed Klass* from the mark-word. >> This PR also changes some code paths (mostly in GCs) to be more careful when >> accessing Klass* (or mark-word or size) to be able to fetch it from the >> forwardee in case the object is forwarded. >> - Self-forwarding in GCs (which is used to deal with promotion failure) now >> uses a bit to indicate 'self-forwarding'. This is needed to preserve the >> crucial Klass* bits in the header. This also allows to get rid of >> preserved-header machinery in SerialGC and G1 (Parallel GC abuses >> preserved-marks to also find all other relevant oops). >> - Full GC forwarding now uses an encoding similar to compressed-oops. We >> have 40 bits for that, and can encode up to 8TB of heap. When exceeding 8TB, >> we turn off UseCompressedClassPointers (except in ZGC, which doesn't use the >> GC forwarding at all). >> - Instances can now have their base-offset (the offset where the field >> layouter starts to place fields) at offset 8 (instead of 12 or 16). >> - Arrays will now store their length at offset 8. >> - CDS can now write and read archives with the compressed header. However, >> it is not possible to read an archive that has been written with an opposite >> setting of UseCompactObjectHeaders. Some build machinery is added so that >> _co... > > Roman Kennke has updated the pull request with a new target base due to a > merge or a rebase. The pull request now contains 107 commits: > > - Merge branch 'master' into JDK-8305895-v4 > - Merge tag 'jdk-25+23' into JDK-8305895-v4 > >Added tag jdk-24+23 for changeset c0e6c3b9 > - Fix gen-ZGC removal > - Merge tag 'jdk-24+22' into JDK-8305895-v4 > >Added tag jdk-24+22 for changeset 388d44fb > - Enable riscv in CompressedClassPointersEncodingScheme test > - s390 port > - Conditionalize platform specific parts of > CompressedClassPointersEncodingScheme test > - Update copyright > - Avoid assert/endless-loop in JFR code > - Update copyright headers > - ... and 97 more: https://git.openjdk.org/jdk/compare/d3c042f9...c1a6323b Marked as reviewed by stuefe (Reviewer). - PR Review: https://git.openjdk.org/jdk/pull/20677#pullrequestreview-2424199289
Re: RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v57]
On Thu, 7 Nov 2024 17:25:40 GMT, Roman Kennke wrote: >> This is the main body of the JEP 450: Compact Object Headers (Experimental). >> >> It is also a follow-up to #20640, which now also includes (and supersedes) >> #20603 and #20605, plus the Tiny Class-Pointers parts that have been >> previously missing. >> >> Main changes: >> - Introduction of the (experimental) flag UseCompactObjectHeaders. All >> changes in this PR are protected by this flag. The purpose of the flag is to >> provide a fallback, in case that users unexpectedly observe problems with >> the new implementation. The intention is that this flag will remain >> experimental and opt-in for at least one release, then make it on-by-default >> and diagnostic (?), and eventually deprecate and obsolete it. However, there >> are a few unknowns in that plan, specifically, we may want to further >> improve compact headers to 4 bytes, we are planning to enhance the Klass* >> encoding to support virtually unlimited number of Klasses, at which point we >> could also obsolete UseCompressedClassPointers. >> - The compressed Klass* can now be stored in the mark-word of objects. In >> order to be able to do this, we are add some changes to GC forwarding (see >> below) to protect the relevant (upper 22) bits of the mark-word. Significant >> parts of this PR deal with loading the compressed Klass* from the mark-word. >> This PR also changes some code paths (mostly in GCs) to be more careful when >> accessing Klass* (or mark-word or size) to be able to fetch it from the >> forwardee in case the object is forwarded. >> - Self-forwarding in GCs (which is used to deal with promotion failure) now >> uses a bit to indicate 'self-forwarding'. This is needed to preserve the >> crucial Klass* bits in the header. This also allows to get rid of >> preserved-header machinery in SerialGC and G1 (Parallel GC abuses >> preserved-marks to also find all other relevant oops). >> - Full GC forwarding now uses an encoding similar to compressed-oops. We >> have 40 bits for that, and can encode up to 8TB of heap. When exceeding 8TB, >> we turn off UseCompressedClassPointers (except in ZGC, which doesn't use the >> GC forwarding at all). >> - Instances can now have their base-offset (the offset where the field >> layouter starts to place fields) at offset 8 (instead of 12 or 16). >> - Arrays will now store their length at offset 8. >> - CDS can now write and read archives with the compressed header. However, >> it is not possible to read an archive that has been written with an opposite >> setting of UseCompactObjectHeaders. Some build machinery is added so that >> _co... > > Roman Kennke has updated the pull request with a new target base due to a > merge or a rebase. The pull request now contains 107 commits: > > - Merge branch 'master' into JDK-8305895-v4 > - Merge tag 'jdk-25+23' into JDK-8305895-v4 > >Added tag jdk-24+23 for changeset c0e6c3b9 > - Fix gen-ZGC removal > - Merge tag 'jdk-24+22' into JDK-8305895-v4 > >Added tag jdk-24+22 for changeset 388d44fb > - Enable riscv in CompressedClassPointersEncodingScheme test > - s390 port > - Conditionalize platform specific parts of > CompressedClassPointersEncodingScheme test > - Update copyright > - Avoid assert/endless-loop in JFR code > - Update copyright headers > - ... and 97 more: https://git.openjdk.org/jdk/compare/d3c042f9...c1a6323b Merge looks good. build errors on MacOS unrelated. - PR Review: https://git.openjdk.org/jdk/pull/20677#pullrequestreview-2422830379
Re: RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v57]
On Thu, 7 Nov 2024 17:25:40 GMT, Roman Kennke wrote: >> This is the main body of the JEP 450: Compact Object Headers (Experimental). >> >> It is also a follow-up to #20640, which now also includes (and supersedes) >> #20603 and #20605, plus the Tiny Class-Pointers parts that have been >> previously missing. >> >> Main changes: >> - Introduction of the (experimental) flag UseCompactObjectHeaders. All >> changes in this PR are protected by this flag. The purpose of the flag is to >> provide a fallback, in case that users unexpectedly observe problems with >> the new implementation. The intention is that this flag will remain >> experimental and opt-in for at least one release, then make it on-by-default >> and diagnostic (?), and eventually deprecate and obsolete it. However, there >> are a few unknowns in that plan, specifically, we may want to further >> improve compact headers to 4 bytes, we are planning to enhance the Klass* >> encoding to support virtually unlimited number of Klasses, at which point we >> could also obsolete UseCompressedClassPointers. >> - The compressed Klass* can now be stored in the mark-word of objects. In >> order to be able to do this, we are add some changes to GC forwarding (see >> below) to protect the relevant (upper 22) bits of the mark-word. Significant >> parts of this PR deal with loading the compressed Klass* from the mark-word. >> This PR also changes some code paths (mostly in GCs) to be more careful when >> accessing Klass* (or mark-word or size) to be able to fetch it from the >> forwardee in case the object is forwarded. >> - Self-forwarding in GCs (which is used to deal with promotion failure) now >> uses a bit to indicate 'self-forwarding'. This is needed to preserve the >> crucial Klass* bits in the header. This also allows to get rid of >> preserved-header machinery in SerialGC and G1 (Parallel GC abuses >> preserved-marks to also find all other relevant oops). >> - Full GC forwarding now uses an encoding similar to compressed-oops. We >> have 40 bits for that, and can encode up to 8TB of heap. When exceeding 8TB, >> we turn off UseCompressedClassPointers (except in ZGC, which doesn't use the >> GC forwarding at all). >> - Instances can now have their base-offset (the offset where the field >> layouter starts to place fields) at offset 8 (instead of 12 or 16). >> - Arrays will now store their length at offset 8. >> - CDS can now write and read archives with the compressed header. However, >> it is not possible to read an archive that has been written with an opposite >> setting of UseCompactObjectHeaders. Some build machinery is added so that >> _co... > > Roman Kennke has updated the pull request with a new target base due to a > merge or a rebase. The pull request now contains 107 commits: > > - Merge branch 'master' into JDK-8305895-v4 > - Merge tag 'jdk-25+23' into JDK-8305895-v4 > >Added tag jdk-24+23 for changeset c0e6c3b9 > - Fix gen-ZGC removal > - Merge tag 'jdk-24+22' into JDK-8305895-v4 > >Added tag jdk-24+22 for changeset 388d44fb > - Enable riscv in CompressedClassPointersEncodingScheme test > - s390 port > - Conditionalize platform specific parts of > CompressedClassPointersEncodingScheme test > - Update copyright > - Avoid assert/endless-loop in JFR code > - Update copyright headers > - ... and 97 more: https://git.openjdk.org/jdk/compare/d3c042f9...c1a6323b GHA failures look like one unrelated timeout and one unrelated infra problem. Please confirm. I also run tier1 on x86_64 x aarch64 x -UCOH x + UCOH, with nothing sticking out (same timeout observed, though). - PR Comment: https://git.openjdk.org/jdk/pull/20677#issuecomment-2463245179
Re: RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v57]
On Thu, 7 Nov 2024 17:25:40 GMT, Roman Kennke wrote: >> This is the main body of the JEP 450: Compact Object Headers (Experimental). >> >> It is also a follow-up to #20640, which now also includes (and supersedes) >> #20603 and #20605, plus the Tiny Class-Pointers parts that have been >> previously missing. >> >> Main changes: >> - Introduction of the (experimental) flag UseCompactObjectHeaders. All >> changes in this PR are protected by this flag. The purpose of the flag is to >> provide a fallback, in case that users unexpectedly observe problems with >> the new implementation. The intention is that this flag will remain >> experimental and opt-in for at least one release, then make it on-by-default >> and diagnostic (?), and eventually deprecate and obsolete it. However, there >> are a few unknowns in that plan, specifically, we may want to further >> improve compact headers to 4 bytes, we are planning to enhance the Klass* >> encoding to support virtually unlimited number of Klasses, at which point we >> could also obsolete UseCompressedClassPointers. >> - The compressed Klass* can now be stored in the mark-word of objects. In >> order to be able to do this, we are add some changes to GC forwarding (see >> below) to protect the relevant (upper 22) bits of the mark-word. Significant >> parts of this PR deal with loading the compressed Klass* from the mark-word. >> This PR also changes some code paths (mostly in GCs) to be more careful when >> accessing Klass* (or mark-word or size) to be able to fetch it from the >> forwardee in case the object is forwarded. >> - Self-forwarding in GCs (which is used to deal with promotion failure) now >> uses a bit to indicate 'self-forwarding'. This is needed to preserve the >> crucial Klass* bits in the header. This also allows to get rid of >> preserved-header machinery in SerialGC and G1 (Parallel GC abuses >> preserved-marks to also find all other relevant oops). >> - Full GC forwarding now uses an encoding similar to compressed-oops. We >> have 40 bits for that, and can encode up to 8TB of heap. When exceeding 8TB, >> we turn off UseCompressedClassPointers (except in ZGC, which doesn't use the >> GC forwarding at all). >> - Instances can now have their base-offset (the offset where the field >> layouter starts to place fields) at offset 8 (instead of 12 or 16). >> - Arrays will now store their length at offset 8. >> - CDS can now write and read archives with the compressed header. However, >> it is not possible to read an archive that has been written with an opposite >> setting of UseCompactObjectHeaders. Some build machinery is added so that >> _co... > > Roman Kennke has updated the pull request with a new target base due to a > merge or a rebase. The pull request now contains 107 commits: > > - Merge branch 'master' into JDK-8305895-v4 > - Merge tag 'jdk-25+23' into JDK-8305895-v4 > >Added tag jdk-24+23 for changeset c0e6c3b9 > - Fix gen-ZGC removal > - Merge tag 'jdk-24+22' into JDK-8305895-v4 > >Added tag jdk-24+22 for changeset 388d44fb > - Enable riscv in CompressedClassPointersEncodingScheme test > - s390 port > - Conditionalize platform specific parts of > CompressedClassPointersEncodingScheme test > - Update copyright > - Avoid assert/endless-loop in JFR code > - Update copyright headers > - ... and 97 more: https://git.openjdk.org/jdk/compare/d3c042f9...c1a6323b Marked as reviewed by stefank (Reviewer). Marked as reviewed by stefank (Reviewer). - PR Review: https://git.openjdk.org/jdk/pull/20677#pullrequestreview-2417620293 PR Review: https://git.openjdk.org/jdk/pull/20677#pullrequestreview-2421753879
Re: RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v57]
On Thu, 7 Nov 2024 17:25:40 GMT, Roman Kennke wrote: >> This is the main body of the JEP 450: Compact Object Headers (Experimental). >> >> It is also a follow-up to #20640, which now also includes (and supersedes) >> #20603 and #20605, plus the Tiny Class-Pointers parts that have been >> previously missing. >> >> Main changes: >> - Introduction of the (experimental) flag UseCompactObjectHeaders. All >> changes in this PR are protected by this flag. The purpose of the flag is to >> provide a fallback, in case that users unexpectedly observe problems with >> the new implementation. The intention is that this flag will remain >> experimental and opt-in for at least one release, then make it on-by-default >> and diagnostic (?), and eventually deprecate and obsolete it. However, there >> are a few unknowns in that plan, specifically, we may want to further >> improve compact headers to 4 bytes, we are planning to enhance the Klass* >> encoding to support virtually unlimited number of Klasses, at which point we >> could also obsolete UseCompressedClassPointers. >> - The compressed Klass* can now be stored in the mark-word of objects. In >> order to be able to do this, we are add some changes to GC forwarding (see >> below) to protect the relevant (upper 22) bits of the mark-word. Significant >> parts of this PR deal with loading the compressed Klass* from the mark-word. >> This PR also changes some code paths (mostly in GCs) to be more careful when >> accessing Klass* (or mark-word or size) to be able to fetch it from the >> forwardee in case the object is forwarded. >> - Self-forwarding in GCs (which is used to deal with promotion failure) now >> uses a bit to indicate 'self-forwarding'. This is needed to preserve the >> crucial Klass* bits in the header. This also allows to get rid of >> preserved-header machinery in SerialGC and G1 (Parallel GC abuses >> preserved-marks to also find all other relevant oops). >> - Full GC forwarding now uses an encoding similar to compressed-oops. We >> have 40 bits for that, and can encode up to 8TB of heap. When exceeding 8TB, >> we turn off UseCompressedClassPointers (except in ZGC, which doesn't use the >> GC forwarding at all). >> - Instances can now have their base-offset (the offset where the field >> layouter starts to place fields) at offset 8 (instead of 12 or 16). >> - Arrays will now store their length at offset 8. >> - CDS can now write and read archives with the compressed header. However, >> it is not possible to read an archive that has been written with an opposite >> setting of UseCompactObjectHeaders. Some build machinery is added so that >> _co... > > Roman Kennke has updated the pull request with a new target base due to a > merge or a rebase. The pull request now contains 107 commits: > > - Merge branch 'master' into JDK-8305895-v4 > - Merge tag 'jdk-25+23' into JDK-8305895-v4 > >Added tag jdk-24+23 for changeset c0e6c3b9 > - Fix gen-ZGC removal > - Merge tag 'jdk-24+22' into JDK-8305895-v4 > >Added tag jdk-24+22 for changeset 388d44fb > - Enable riscv in CompressedClassPointersEncodingScheme test > - s390 port > - Conditionalize platform specific parts of > CompressedClassPointersEncodingScheme test > - Update copyright > - Avoid assert/endless-loop in JFR code > - Update copyright headers > - ... and 97 more: https://git.openjdk.org/jdk/compare/d3c042f9...c1a6323b Reapproving. Please wait for GHA to complete, when JEP is targeted to integrate. Thanks! - Marked as reviewed by coleenp (Reviewer). PR Review: https://git.openjdk.org/jdk/pull/20677#pullrequestreview-2421741026
Re: RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v57]
> This is the main body of the JEP 450: Compact Object Headers (Experimental). > > It is also a follow-up to #20640, which now also includes (and supersedes) > #20603 and #20605, plus the Tiny Class-Pointers parts that have been > previously missing. > > Main changes: > - Introduction of the (experimental) flag UseCompactObjectHeaders. All > changes in this PR are protected by this flag. The purpose of the flag is to > provide a fallback, in case that users unexpectedly observe problems with the > new implementation. The intention is that this flag will remain experimental > and opt-in for at least one release, then make it on-by-default and > diagnostic (?), and eventually deprecate and obsolete it. However, there are > a few unknowns in that plan, specifically, we may want to further improve > compact headers to 4 bytes, we are planning to enhance the Klass* encoding to > support virtually unlimited number of Klasses, at which point we could also > obsolete UseCompressedClassPointers. > - The compressed Klass* can now be stored in the mark-word of objects. In > order to be able to do this, we are add some changes to GC forwarding (see > below) to protect the relevant (upper 22) bits of the mark-word. Significant > parts of this PR deal with loading the compressed Klass* from the mark-word. > This PR also changes some code paths (mostly in GCs) to be more careful when > accessing Klass* (or mark-word or size) to be able to fetch it from the > forwardee in case the object is forwarded. > - Self-forwarding in GCs (which is used to deal with promotion failure) now > uses a bit to indicate 'self-forwarding'. This is needed to preserve the > crucial Klass* bits in the header. This also allows to get rid of > preserved-header machinery in SerialGC and G1 (Parallel GC abuses > preserved-marks to also find all other relevant oops). > - Full GC forwarding now uses an encoding similar to compressed-oops. We > have 40 bits for that, and can encode up to 8TB of heap. When exceeding 8TB, > we turn off UseCompressedClassPointers (except in ZGC, which doesn't use the > GC forwarding at all). > - Instances can now have their base-offset (the offset where the field > layouter starts to place fields) at offset 8 (instead of 12 or 16). > - Arrays will now store their length at offset 8. > - CDS can now write and read archives with the compressed header. However, > it is not possible to read an archive that has been written with an opposite > setting of UseCompactObjectHeaders. Some build machinery is added so that > _coh variants of CDS archiv... Roman Kennke has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 107 commits: - Merge branch 'master' into JDK-8305895-v4 - Merge tag 'jdk-25+23' into JDK-8305895-v4 Added tag jdk-24+23 for changeset c0e6c3b9 - Fix gen-ZGC removal - Merge tag 'jdk-24+22' into JDK-8305895-v4 Added tag jdk-24+22 for changeset 388d44fb - Enable riscv in CompressedClassPointersEncodingScheme test - s390 port - Conditionalize platform specific parts of CompressedClassPointersEncodingScheme test - Update copyright - Avoid assert/endless-loop in JFR code - Update copyright headers - ... and 97 more: https://git.openjdk.org/jdk/compare/d3c042f9...c1a6323b - Changes: https://git.openjdk.org/jdk/pull/20677/files Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20677&range=56 Stats: 5212 lines in 218 files changed: 3585 ins; 864 del; 763 mod Patch: https://git.openjdk.org/jdk/pull/20677.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/20677/head:pull/20677 PR: https://git.openjdk.org/jdk/pull/20677
Re: RFR: 8305895: Implement JEP 450: Compact Object Headers (Experimental) [v57]
On Thu, 7 Nov 2024 17:25:40 GMT, Roman Kennke wrote: >> This is the main body of the JEP 450: Compact Object Headers (Experimental). >> >> It is also a follow-up to #20640, which now also includes (and supersedes) >> #20603 and #20605, plus the Tiny Class-Pointers parts that have been >> previously missing. >> >> Main changes: >> - Introduction of the (experimental) flag UseCompactObjectHeaders. All >> changes in this PR are protected by this flag. The purpose of the flag is to >> provide a fallback, in case that users unexpectedly observe problems with >> the new implementation. The intention is that this flag will remain >> experimental and opt-in for at least one release, then make it on-by-default >> and diagnostic (?), and eventually deprecate and obsolete it. However, there >> are a few unknowns in that plan, specifically, we may want to further >> improve compact headers to 4 bytes, we are planning to enhance the Klass* >> encoding to support virtually unlimited number of Klasses, at which point we >> could also obsolete UseCompressedClassPointers. >> - The compressed Klass* can now be stored in the mark-word of objects. In >> order to be able to do this, we are add some changes to GC forwarding (see >> below) to protect the relevant (upper 22) bits of the mark-word. Significant >> parts of this PR deal with loading the compressed Klass* from the mark-word. >> This PR also changes some code paths (mostly in GCs) to be more careful when >> accessing Klass* (or mark-word or size) to be able to fetch it from the >> forwardee in case the object is forwarded. >> - Self-forwarding in GCs (which is used to deal with promotion failure) now >> uses a bit to indicate 'self-forwarding'. This is needed to preserve the >> crucial Klass* bits in the header. This also allows to get rid of >> preserved-header machinery in SerialGC and G1 (Parallel GC abuses >> preserved-marks to also find all other relevant oops). >> - Full GC forwarding now uses an encoding similar to compressed-oops. We >> have 40 bits for that, and can encode up to 8TB of heap. When exceeding 8TB, >> we turn off UseCompressedClassPointers (except in ZGC, which doesn't use the >> GC forwarding at all). >> - Instances can now have their base-offset (the offset where the field >> layouter starts to place fields) at offset 8 (instead of 12 or 16). >> - Arrays will now store their length at offset 8. >> - CDS can now write and read archives with the compressed header. However, >> it is not possible to read an archive that has been written with an opposite >> setting of UseCompactObjectHeaders. Some build machinery is added so that >> _co... > > Roman Kennke has updated the pull request with a new target base due to a > merge or a rebase. The pull request now contains 107 commits: > > - Merge branch 'master' into JDK-8305895-v4 > - Merge tag 'jdk-25+23' into JDK-8305895-v4 > >Added tag jdk-24+23 for changeset c0e6c3b9 > - Fix gen-ZGC removal > - Merge tag 'jdk-24+22' into JDK-8305895-v4 > >Added tag jdk-24+22 for changeset 388d44fb > - Enable riscv in CompressedClassPointersEncodingScheme test > - s390 port > - Conditionalize platform specific parts of > CompressedClassPointersEncodingScheme test > - Update copyright > - Avoid assert/endless-loop in JFR code > - Update copyright headers > - ... and 97 more: https://git.openjdk.org/jdk/compare/d3c042f9...c1a6323b I'd like to prepare for integration now. I merged from master and resolved some conflicts. I am now running at least tier1 on aarch64 x x86_64 x -UCOH x +UCOH, possibly tier2 .. 4, too (time permitting). In the meantime, could you please re-approve the PR? I hope it doesn't catch any more conflicts until we're ready for intergration. As soon as the JEP is targeted (sometime today, I think), tests are clean and approvals are there, I would like to integrate. - PR Comment: https://git.openjdk.org/jdk/pull/20677#issuecomment-2462834035
