RFR: 8354899: Reduce overhead associated with type switches

[Chen Liang]

On Thu, 17 Apr 2025 00:08:55 GMT, Chen Liang <liach at openjdk.org> wrote:

> The downcallHandle method in Linker has a complex implementation, and type switches is triggered in multiple places in that handle.
> 
> After bytestacks analysis of a simple program that links the native implementation of Class::forName0, it turns out that:
> 1. there are a few missed usages of type switch in ClassFile API
> 2. type switch can speed up generation by using explicit stack maps, as stack map gen is over 50% of the generation instructions because the switch is branch-heavy, but the frames are actually simple.
> 
> These two combined are not that significant in the overall overhead of downcallHandle, but would be helpful for other workloads.

Testing: tier 1-3

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PR Comment: https://git.openjdk.org/jdk/pull/24705#issuecomment-2814146206