RFR: 8338546: Speed up ConstantPoolBuilder::classEntry(ClassDesc) [v2]

[Chen Liang]

On Mon, 16 Sep 2024 19:11:23 GMT, Claes Redestad <redestad at openjdk.org> wrote:

>> I measured in bytestacks and the clinit instructions reduced by about 2/3
>
> Yes, you have to pick and evaluate between two trade-offs here: a compact routine to compute the array, or a larger initializer which does less compute. Number of instructions executed in the interpreter is a diagnostic tool since if often hides costs that come from having to store, read, parse and manage bytecode - sometimes twice if this hits the default CDS archive. There's no definite answer, but generally the trend is towards compute getting cheaper and IO/memory getting (relatively) more expensive. So picking a larger representation to avoid some one-off compute might be the wrong trade-off.
> 
> (Note that this particular instance is probably negligible either way, I just don't want bytestacks instrumentation to put us down a path where we might ultimately end up worse off. Diagnostics needs to be verified with realistic wall clock measurements. When uncertain: go for simplicity.)

I will leave this hash computation mechanism whole as is; table initialization and small table optimizations can come in other patches later.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20667#discussion_r1767398487