RFR: 8338544: Dedicated Array class descriptor implementation

[Jorn Vernee]

On Wed, 21 Aug 2024 20:25:07 GMT, Chen Liang <liach at openjdk.org> wrote:

> @cl4es discovered that Stack Map generation in ClassFile API uses `componentType` and `arrayType` for `aaload` `aastore` instructions, which are currently quite slow. We can split out array class descriptors from class or interfaces to support faster `arrayType` and `componentType` operations.
> 
> Tentative, as I currently have no way to measure the actual impact of this patch on the startup performance; however, this made the `ClassDesc` implementations much cleaner.

Nice cleanup! I'm just not really sure about the new subtype relationship (see inline comment)

src/java.base/share/classes/jdk/internal/constant/ArrayClassDescImpl.java line 116:

> 114:         sb.append(componentDesc);
> 115:         return sb.toString();
> 116:     }

Is there really that much benefit in lazily computing the descriptor? `ReferenceClassDescImpl` doesn't do this either... Maybe we can keep things simple and initialize the descriptor in the constructor?

src/java.base/share/classes/jdk/internal/constant/ArrayClassDescImpl.java line 126:

> 124:         Class<?> clazz = element.resolveConstantDesc(lookup);
> 125:         for (int i = 0; i < rank; i++)
> 126:             clazz = clazz.arrayType();

Looking at the implementation of `arrayType`, it reflectively creates an array and then returns its class. Just makes me think we need a better we to look up an array class directly in the JDK :D

src/java.base/share/classes/jdk/internal/constant/MethodTypeDescImpl.java line 89:

> 87: 
> 88:     private static ClassDesc validateArgument(ClassDesc arg) {
> 89:         if (requireNonNull(arg) == CD_void)

Is it safe to make this change? Are instances of `PrimitveClassDescImpl` canonicalized now? (not in this patch, but it looks that way in the source code).

src/java.base/share/classes/jdk/internal/constant/ReferenceClassDescImpl.java line 58:

> 56:         if (descriptor.charAt(0) == '[') {
> 57:             return ArrayClassDescImpl.ofValidatedDescriptor(descriptor);
> 58:         }

I think arrays should be handled separately by the caller, or `ArrayClassDescImpl` should be made a sub-type of  `ReferenceClassDescImpl`. This factory in `ReferenceClassDescImpl` handling a sibling seems a bit confusing.

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

PR Review: https://git.openjdk.org/jdk/pull/20665#pullrequestreview-2348740185
PR Review Comment: https://git.openjdk.org/jdk/pull/20665#discussion_r1788073751
PR Review Comment: https://git.openjdk.org/jdk/pull/20665#discussion_r1788084532
PR Review Comment: https://git.openjdk.org/jdk/pull/20665#discussion_r1788090312
PR Review Comment: https://git.openjdk.org/jdk/pull/20665#discussion_r1788110720