8357220: Introduce a BSMAttributeEntry struct #25298

jdksjolen · 2025-05-19T07:35:16Z

Hi,

The constant pool currently has a lot of methods specific to extracting parts of the operands array. What this array actually is, is a sequence of bootstrap method attribute entries, where each entry has the following components:

struct BSMAE {
  u2 bootstrap_method_index;
  u2 argument_count;
  u2 arguments[argument_count];
}

We can removes some of these operands array specific methods, and instead allows you to extract BSMAttributeEntrys which you can then use to extract its piece wise components. This makes for a nicer interface, and a bit easier to come into as a reader of the code, as it more closely mirrors the JVMS.

Please consider!

Testing: Currently GHA, running tier1-tier3

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JDK-8357220: Introduce a BSMAttributeEntry struct (Enhancement - P4)

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John R Rose <[email protected]>

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bridgekeeper · 2025-05-19T07:36:06Z

👋 Welcome back jsjolen! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2025-05-19T07:36:10Z

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

openjdk · 2025-05-19T07:36:41Z

@jdksjolen The following labels will be automatically applied to this pull request:

hotspot
serviceability

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing lists. If you would like to change these labels, use the /label pull request command.

jdksjolen · 2025-05-19T07:36:44Z

/contributor /contributor add @rose00

openjdk · 2025-05-19T07:37:47Z

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jdksjolen · 2025-05-19T07:43:19Z

/contributor add @rose00

openjdk · 2025-05-19T07:43:45Z

@jdksjolen
Contributor John R Rose <[email protected]> successfully added.

mlbridge · 2025-05-19T10:08:03Z

Webrevs

01: Full - Incremental (13e27259)
00: Full (e10d91a5)

plummercj · 2025-05-19T17:49:59Z

src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/oops/ConstantPool.java

+  private static int INDY_BSM_OFFSET = 0;
+  private static int INDY_ARGC_OFFSET = 1;
+  private static int INDY_ARGV_OFFSET = 2;


Is the SA code that references these fields still correct? It seems the references are from the ClassWriter, which we don't have very good test coverage for. Probably this is a bug that is not being detected.

The references are still correct, as we haven't changed the struct layout. We're also not planning on doing so. I don't think that there is a bug here.

Ok. It's just unusual to see constants like these that are needed by SA but serve no purpose in hotspot.

It does seem like you want to mirror these constants from the vmStructs side like it was rather than defining them here in the SA. I see why you did this but maybe you can add the constants to BSMAttributeEntry and declare them constants using declare_constant(BSMAttributeEntry::_indy...).
In case you change them at some later time.

jdksjolen · 2025-05-20T06:33:38Z

src/hotspot/share/oops/constantPool.hpp

-  enum {
-         _indy_bsm_offset  = 0,  // CONSTANT_MethodHandle bsm
-         _indy_argc_offset = 1,  // u2 argc
-         _indy_argv_offset = 2   // u2 argv[argc]
-  };
-


@plummercj , this is where the enum constants come from.

jdksjolen · 2025-05-20T06:34:20Z

src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/oops/ConstantPool.java

+  private static int INDY_BSM_OFFSET = 0;
+  private static int INDY_ARGC_OFFSET = 1;
+  private static int INDY_ARGV_OFFSET = 2;


The references are still correct, as we haven't changed the struct layout. We're also not planning on doing so. I don't think that there is a bug here.

lfoltan

Overall looks good, a couple of comments below.

lfoltan · 2025-05-21T17:28:12Z

src/hotspot/share/oops/constantPool.hpp

@@ -77,6 +77,36 @@ class CPKlassSlot {
  }
 };

+class BSMAttributeEntry {


Please consider moving the description comment from lines #569-#572 ahead of this structure

This?

// The first part of the operands array consists of an index into the second part. // Extract a 32-bit index value from the first part.

Wouldn't it be better to leave it where it is, but add a comment ahead of the structure?

lfoltan · 2025-05-21T17:28:27Z

src/hotspot/share/oops/constantPool.hpp

+  u2* argument_indexes() {
+    return reinterpret_cast<u2*>(this + 1);
+  }
+


Remove blank lines at #92, #95, #103

We typically keep a space before we change access modifier, let's keep that pattern in here. We can tighten up the rest of the code according to your suggestions, however.

lfoltan · 2025-05-21T17:36:03Z

src/hotspot/share/oops/constantPool.hpp

+  }
+
+  int argument_index(int n) const {
+    assert(checked_cast<u2>(n) < _argument_count, "oob");


Should the assert contain a check that _argument_count is >= 0 as well?

I don't think so, it's a u2 so that's just part of its type.

lfoltan · 2025-05-21T17:37:04Z

src/hotspot/share/oops/constantPool.hpp

-               int next_offset = bootstrap_operand_limit(cp_index));
-    assert(end_offset == next_offset, "matched ending");
-    return argc;
+    int bsmai = bootstrap_methods_attribute_index(cp_index);


The "matched ending" assert was the only code that used bootstrap_operand_limit(), so that method could be removed as well. This comment applies to bootstrap_operand_base() as well.

lfoltan · 2025-05-21T17:46:54Z

src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/oops/ConstantPool.java

-  private static int INDY_BSM_OFFSET;
-  private static int INDY_ARGC_OFFSET;
-  private static int INDY_ARGV_OFFSET;
+  private static int INDY_BSM_OFFSET = 0;


Please update the copyright of this file.

jdksjolen

Thank you for the review Lois! I've addressed most of your comments, but I didn't add any documentation for the BSMAttributeEntry (yet).

jdksjolen · 2025-05-22T17:46:15Z

src/hotspot/share/oops/constantPool.hpp

+  u2* argument_indexes() {
+    return reinterpret_cast<u2*>(this + 1);
+  }
+


We typically keep a space before we change access modifier, let's keep that pattern in here. We can tighten up the rest of the code according to your suggestions, however.

jdksjolen · 2025-05-22T17:47:19Z

src/hotspot/share/oops/constantPool.hpp

+  }
+
+  int argument_index(int n) const {
+    assert(checked_cast<u2>(n) < _argument_count, "oob");


I don't think so, it's a u2 so that's just part of its type.

jdksjolen · 2025-05-22T17:47:56Z

src/hotspot/share/oops/constantPool.hpp

-               int next_offset = bootstrap_operand_limit(cp_index));
-    assert(end_offset == next_offset, "matched ending");
-    return argc;
+    int bsmai = bootstrap_methods_attribute_index(cp_index);


jdksjolen · 2025-05-22T17:49:57Z

src/hotspot/share/oops/constantPool.hpp

@@ -77,6 +77,36 @@ class CPKlassSlot {
  }
 };

+class BSMAttributeEntry {


This?

// The first part of the operands array consists of an index into the second part. // Extract a 32-bit index value from the first part.

Wouldn't it be better to leave it where it is, but add a comment ahead of the structure?

coleenp

I think this looks like a nice improvement but I thought operands was going to turn into two arrays? Also there's probably a better name than 'operands' maybe bsm_operands?

coleenp · 2025-05-29T11:38:09Z

src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/oops/ConstantPool.java

+  private static int INDY_BSM_OFFSET = 0;
+  private static int INDY_ARGC_OFFSET = 1;
+  private static int INDY_ARGV_OFFSET = 2;


It does seem like you want to mirror these constants from the vmStructs side like it was rather than defining them here in the SA. I see why you did this but maybe you can add the constants to BSMAttributeEntry and declare them constants using declare_constant(BSMAttributeEntry::_indy...).
In case you change them at some later time.

Introduce BSMAttributeEntry

e10d91a

openjdk bot added the serviceability [email protected] label May 19, 2025

openjdk bot added the hotspot [email protected] label May 19, 2025

jdksjolen changed the title ~~Introduce BSMAttributeEntry~~ 8357220 May 19, 2025

openjdk bot changed the title ~~8357220~~ 8357220: Introduce a BSMAttributeEntry struct May 19, 2025

jdksjolen marked this pull request as ready for review May 19, 2025 10:03

openjdk bot added the rfr Pull request is ready for review label May 19, 2025

plummercj reviewed May 19, 2025

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jdksjolen commented May 20, 2025

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lfoltan reviewed May 21, 2025

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Lois's comments

13e2725

jdksjolen commented May 22, 2025

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coleenp reviewed May 29, 2025

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8357220: Introduce a BSMAttributeEntry struct #25298

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8357220: Introduce a BSMAttributeEntry struct #25298

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