RFR: 8332528: Generate code in SwitchBootstraps.generateTypeSwitch that require fewer adaptations

[Claes Redestad]

On Mon, 20 May 2024 18:06:32 GMT, Chen Liang <liach at openjdk.org> wrote:

>> We can fold the call to `Objects.checkIndex` into the code generated in generateTypeSwitchSkeleton instead of doing so by filtering the MH argument. This loads 9 less classes (of which 8 generated LFs and Species classes) on a minimal test, while being neutral on a throughput sanity test:
>> 
>> 
>> Name                   Cnt  Base   Error   Test   Error  Unit  Change
>> SwitchSanity.switchSum  15 8,162 ± 0,117  8,152 ± 0,131 ns/op   1,00x (p = 0,800 )
>>   * = significant
>>   ```
>> 
>> A few additional optimizations includes generating the switch method using the precise type (to avoid the need for an explicitCast adaptation), and moving some seldom used `findStatic` calls to a holder. All in all this means a reduction by 33-34M cycles to bootstrap a trivial switch expression on my M1.
>
> src/java.base/share/classes/java/lang/runtime/SwitchBootstraps.java line 406:
> 
>> 404:             cb.iload(RESTART_IDX);
>> 405:             cb.loadConstant(labelConstants.length + 1);
>> 406:             cb.invokestatic(CD_Objects, "checkIndex", MethodTypeDesc.of(ConstantDescs.CD_int, ConstantDescs.CD_int, ConstantDescs.CD_int));
> 
> We should cache this MethodTypeDesc too.

`MethodTypeDesc.of` is actually quite cheap when inputs are `ClassDesc`s. Besides I think the main focus in these bootstraps should be improving the overhead of the first few calls - reduce dependencies, reduce runtime code generation - not treat this code as something that will be run many times over in a hot loop.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19307#discussion_r1607207823