RFR: 8341141: Optimize DirectCodeBuilder [v24]

[Claes Redestad]

On Mon, 7 Oct 2024 22:54:24 GMT, Shaojin Wen <swen at openjdk.org> wrote:

>> Some DirectCodeBuilder related optimizations to improve startup and running performance:
>> 1. Merge calls, merge writeU1 and writeU2 into writeU3
>> 2. Merge calls, merge writeU1 and writeIndex operations
>> 3. Directly use writeU1 instead of writeBytecode
>> 4. Rewrite the implementation of load and store
>
> Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   suggestion from @liach

src/java.base/share/classes/jdk/internal/classfile/impl/DirectCodeBuilder.java line 706:

> 704:     public void writeLoadConstant(Opcode opcode, LoadableConstantEntry value) {
> 705:         // Make sure Long and Double have LDC2_W and
> 706:         // rewrite to _W if index is > 256

Pre-existing.. 
- Comment should say >= 256. 
- As we're cloning from a pre-existing pool I assume there's a (perhaps unlikely) possibility we go from a wide to a lower index? In that case the opcode could profitably be "rewritten" to `Opcode.LDC` in an else clause. (I assume `LDC_W` with an index in the 0-255 range works fine functionally; it just wastes a byte.)

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PR Review Comment: https://git.openjdk.org/jdk/pull/21243#discussion_r1791544609