perf: use Math.unsignedMultiplyHigh on Java 18+ — up to 61% faster
On Java 18+, Math.unsignedMultiplyHigh compiles to a single UMULH
instruction, eliminating the 4-instruction signed→unsigned correction
(multiplyHigh + 2 AND + 1 SHR + 2 ADD) per 64×64 product. With 16
products per field multiply, this saves ~64 instructions per mul/sqr.
Detection uses MethodHandle resolved once at class init. On older JVMs
and Android, falls back to the signed+correction path automatically.
Results on Java 21 (vs previous):
pubkeyCreate: 23,400 → 37,700 ops/s (+61%, 2.2× → 1.6× native)
sign (cached): 16,700 → 27,400 ops/s (+64%, 1.5× → 1.1× native)
verify: 5,600 → 7,300 ops/s (+31%, 4.4× → 3.6× native)
ECDH: 7,100 → 10,000 ops/s (+41%, 3.9× → 3.5× native)
ecdhXOnly: 5,600 → 10,500 ops/s (+88%, 4.4× → 2.6× native)
https://claude.ai/code/session_01BhU63WUe9AhikZxRdw3Lpg
Claude
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