8353273: Reduce number of oop map entries in instances

[tstuefe]

In preparation for planned GC performance improvements (KLUT), I would like to reduce the average number of oop map entries.

For details, please see JBS issue text.

---

Patch results:

The patch brings a positive change of oop map size, reducing the likelihood of lengthy oop maps. Here the oop map size distribution over all JDK classes in the JDK image:

Before:

   5395  - non-static oop maps (0 entries)
   9330  - non-static oop maps (1 entries)
   1449  - non-static oop maps (2 entries)
    274  - non-static oop maps (3 entries)
    218  - non-static oop maps (4 entries)
     75  - non-static oop maps (5 entries)
      7  - non-static oop maps (6 entries)
      4  - non-static oop maps (7 entries)

Now:

   5395  - non-static oop maps (0 entries)
  10178  - non-static oop maps (1 entries)
    933  - non-static oop maps (2 entries)
    229  - non-static oop maps (3 entries)
     16  - non-static oop maps (4 entries)
      1  - non-static oop maps (5 entries)

For example, java.util.concurrent.ConcurrentHashMap$TreeNode is changed from having 2 entries to having just one entry, which is nice for a class that may be instantiated a lot:

Before:

java.util.concurrent.ConcurrentHashMap$TreeNode {0x000000000d1dddc0}
 - ---- non-static fields (9 words):
 - final 'hash' 'I' @12
 - final 'key' 'Ljava/lang/Object;' @16 
 - volatile 'val' 'Ljava/lang/Object;' @20 
 - volatile 'next' 'Ljava/util/concurrent/ConcurrentHashMap$Node;' @24       << last field of base class
 - 'red' 'Z' @28                                                             << derived class starts here, non-oops lead
 - 'parent' 'Ljava/util/concurrent/ConcurrentHashMap$TreeNode;' @32 
 - 'left' 'Ljava/util/concurrent/ConcurrentHashMap$TreeNode;' @36 
 - 'right' 'Ljava/util/concurrent/ConcurrentHashMap$TreeNode;' @40 
 - 'prev' 'Ljava/util/concurrent/ConcurrentHashMap$TreeNode;' @44 
 - non-static oop maps (2 entries): 16-24 32-44  

Now:

java.util.concurrent.ConcurrentHashMap$TreeNode {0x000000007e1de450}
 - ---- non-static fields (9 words):
 - final 'hash' 'I' @12 
 - final 'key' 'Ljava/lang/Object;' @16 
 - volatile 'val' 'Ljava/lang/Object;' @20 
 - volatile 'next' 'Ljava/util/concurrent/ConcurrentHashMap$Node;' @24       << last field of base class
 - 'parent' 'Ljava/util/concurrent/ConcurrentHashMap$TreeNode;' @28          << class starts here, oops lead
 - 'left' 'Ljava/util/concurrent/ConcurrentHashMap$TreeNode;' @32 
 - 'right' 'Ljava/util/concurrent/ConcurrentHashMap$TreeNode;' @36 
 - 'prev' 'Ljava/util/concurrent/ConcurrentHashMap$TreeNode;' @40 
 - 'red' 'Z' @44 
 - non-static oop maps (1 entries): 16-40 

Note how the sole primitive field of the derived class, "red", changed position to let oops lead.

Here a contrived example to demonstrate how reordering works across several inheritance levels:

Before:

GeneratedClass9730 {0x000000008f93ea50}
 - ---- non-static fields (11 words):
 - public 'derived0_I_0' 'I' @12 
 - public 'derived0_o_0' 'Ljava/lang/Object;' @16                            << last field of base class
 - public 'derived1_I_0' 'I' @20 
 - public 'derived1_o_0' 'Ljava/lang/Object;' @24                            << last field of derived class 1
 - public 'derived2_I_0' 'I' @28 
 - public 'derived2_o_0' 'Ljava/lang/Object;' @32                            << last field of derived class 2
 - public 'derived3_I_0' 'I' @36 
 - public 'derived3_o_0' 'Ljava/lang/Object;' @40                            << last field of derived class 3
 - public 'derived4_I_0' 'I' @44 
 - public 'derived4_o_0' 'Ljava/lang/Object;' @48                            << last field of derived class 4
 - public 'o0' 'Ljava/lang/Object;' @52                                      << this class starts here
 - non-static oop maps (5 entries): 16-16 24-24 32-32 40-40 48-52 

After

GeneratedClass9730 {0x00000000a793e5f8}
 - ---- non-static fields (11 words):
 - public 'derived0_I_0' 'I' @12 
 - public 'derived0_o_0' 'Ljava/lang/Object;' @16                            << last field of base class
 - public 'derived1_o_0' 'Ljava/lang/Object;' @20 
 - public 'derived1_I_0' 'I' @24                                             << last field of derived class 1
 - public 'derived2_I_0' 'I' @28 
 - public 'derived2_o_0' 'Ljava/lang/Object;' @32                            << last field of derived class 2
 - public 'derived3_o_0' 'Ljava/lang/Object;' @36 
 - public 'derived3_I_0' 'I' @40                                             << last field of derived class 3
 - public 'derived4_I_0' 'I' @44 
 - public 'derived4_o_0' 'Ljava/lang/Object;' @48                            << last field of derived class 4
 - public 'o0' 'Ljava/lang/Object;' @52                                      << this class starts here
 - non-static oop maps (3 entries): 16-20 32-36 48-52  

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue

Issue

• JDK-8353273: Reduce number of oop map entries in instances (Enhancement - P4)

Reviewers

• Frederic Parain (@fparain - Reviewer) Review applies to dfbe4859
• Johan Sjölen (@jdksjolen - Reviewer) Review applies to dfbe4859
• Leonid Mesnik (@lmesnik - Reviewer)

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[openjdk]

@tstuefe This change now passes all automated pre-integration checks.

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After integration, the commit message for the final commit will be:

8353273: Reduce number of oop map entries in instances

Reviewed-by: lmesnik, fparain, jsjolen

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At the time when this comment was updated there had been 17 new commits pushed to the master branch:

• 3cbe686: 8348967: Deprecate security permission classes for removal
• 4645ddb: 8341976: C2: use_mem_state != load->find_exact_control(load->in(0)) assert failure
• 58ff36f: 8350705: [JMH] test security.SSLHandshake failed for 2 threads configuration
• ... and 14 more: https://git.openjdk.org/jdk/compare/80ff7b9c9406c7845ecb3bc40910e92ccdd23ff2...master

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[mlbridge]

Webrevs

• 02: Full - Incremental (94c55bde)
• 01: Full - Incremental (dfbe4859)
• 00: Full (f4933c1d)

[tstuefe]

Ping @fparain ?

[fparain]

Looks good to me.
Thank you for improving instances' layouts.

Fred

[tstuefe]

Looks good to me. Thank you for improving instances' layouts.

Fred

Thanks Frederic :)

[fparain]

A possible improvement to this code would be to compute if the super class' layout ends with oops during the reconstruction (reconstruct_layout()), to avoid having to iterate over the fields a second time.

[tstuefe]

A possible improvement to this code would be to compute if the super class' layout ends with oops during the reconstruction (reconstruct_layout()), to avoid having to iterate over the fields a second time.

That is a good idea. A prior version of this stored the last field information after constructing the layout in the InstanceKlass (as a "ends_with_oop" bool) but I did not like that, since I did not wanted to add a new member to IK. But if we reconstruct the layout for the super class anyway, I can do the same there. Thansk!

[tstuefe]

@fparain I adapted your idea. I also added a regression test.
Thank you.

[fparain]

This version looks better.
Thank you for the changes.

Fred

[tstuefe]

Thanks @fparain !

May I have a second review?

[jdksjolen]

Hi,

Neat change. I have read up on how field layouts work, and this seems correct to me. Let me run this in our CI for a bit and see what comes out before I approve it, thank you.

[jdksjolen]

Passes tests, approved.

[tstuefe]

Thank you all.

/integrate

[openjdk]

Going to push as commit 743d1c6.
Since your change was applied there have been 31 commits pushed to the master branch:

• 2a0cf83: 8354088: [BACKOUT] Run jtreg in the work dir
• 73bb647: 8353829: RISC-V: Auto-enable several more extensions for debug builds
• c26c575: 8353014: Exclude AOT tooling classes from AOT cache
• ... and 28 more: https://git.openjdk.org/jdk/compare/80ff7b9c9406c7845ecb3bc40910e92ccdd23ff2...master

Your commit was automatically rebased without conflicts.

[openjdk]

@tstuefe Pushed as commit 743d1c6.

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