RFR: 8340587: Optimize StackMapGenerator$Frame::checkAssignableTo

[Shaojin Wen]

On Sun, 22 Sep 2024 14:08:38 GMT, Shaojin Wen <swen at openjdk.org> wrote:

>> src/java.base/share/classes/jdk/internal/classfile/impl/StackMapGenerator.java line 1102:
>> 
>>> 1100:                 target.localsSize = localsSize;
>>> 1101:                 if (stackSize > 0) {
>>> 1102:                     target.stack = Arrays.copyOf(stack, stackSize);
>> 
>> Do you think we should use `.clone()` to avoid `getClass` checks and to avoid extra array growing in case the new stack/local immediately grows due to instructions after target?
>> 
>> Also I think you can do `localsSize > 0` check for locals assignment too.
>
> stack.length may be larger than stackSize. In this case, the length of the array copied by clone will be longer than Arrays.copyOf. However, I have no evidence that Arrays.copyOf is faster than clone.

I found through debugging that in the `target.flags == -1` branch of checkAssignableTo, `localsSize > 0 and stackSize = 0` are always, so I made this optimization.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21121#discussion_r1770564307