On Android (ART), the unsignedMultiplyHigh wrapper function has per-call
branching for API level detection that prevents ART's JIT from inlining
the intrinsic into the hot loop. This creates ~10,000 extra function
calls per signature verify (20 wrapper calls × 500 field muls).
This commit introduces fieldMulReduce/fieldSqrReduce as expect/actual
functions that fuse U256.mulWide + FieldP.reduceWide into a single
compilation unit. The multiply-high intrinsic is passed as a crossinline
lambda and inlined at each call site, producing platform-specific code
with zero wrapper overhead:
- Android API 35+: Math.unsignedMultiplyHigh inlined directly (UMULH)
- Android API 31-34: Math.multiplyHigh + correction inlined (SMULH)
- Android API <31: pure-Kotlin fallback inlined
- JVM: Math.unsignedMultiplyHigh inlined directly
- Native: pure-Kotlin fallback inlined
The API level check happens ONCE per fieldMulReduce call (outermost
branch) rather than per multiply-high call (innermost loop), so ART
profiles and JIT-compiles only the hot path.
https://claude.ai/code/session_01EMY5RnXb9rnsyU2KbXrSaY
Claude
084230937b