RFR: 8359424: Eliminate table lookup in Integer/Long toHexString

[Hannes Greule]

On Tue, 7 Jan 2025 10:39:18 GMT, Shaojin Wen <swen at openjdk.org> wrote:

> In PR #22928, UUID introduced long-based vectorized hexadecimal to string conversion, which can also be used in Integer::toHexString and Long::toHexString to eliminate table lookups. The benefit of eliminating table lookups is that the performance is better when cache misses occur.

Running it on my machine (Ryzen 9 3900X):

baseline (91db7c0877a68ad171da2b4501280fc24630ae83):
Integers.toHexString     500  avgt   15  5.717 ± 0.274  us/op
Longs.toHexString        500  avgt   15  6.851 ± 0.214  us/op

this change (1788d09787cadfe6ec23b9b10bef87a2cdc029a3):
Integers.toHexString     500  avgt   15  21.334 ± 0.268  us/op
Longs.toHexString        500  avgt   15  38.907 ± 0.589  us/op

I know that this processor has an extremely slow implementation of the `PDEP` instruction, but I'm kinda surprised you're seeing better results on aarch64. But I think Zen1 and Zen2 should considered here to avoid regressions on those architectures.

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PR Comment: https://git.openjdk.org/jdk/pull/22942#issuecomment-2577153462