The fused fieldMulReduceWith + crossinline lambda was designed for ART
which struggles with deep call chains. On HotSpot C2, it creates a
2351-bytecode method (exceeding FreqInlineSize=325) that can't be
inlined into FieldP.mul, and wastes 180 bytecodes on lambda param
shuffling that C2 must clean up.
The unfused path (U256.mulWide + FieldP.reduceWide) produces a tiny
40-bytecode fieldMulReduce that HotSpot easily inlines. HotSpot's 8+
level inlining depth handles the full chain down to the
Math.unsignedMultiplyHigh intrinsic (single MULQ on x86-64).
Benchmark on JVM 21 x86-64: equivalent performance (1.5× verify,
0.9× sign-cached vs native C). Cleaner bytecode with no lambda waste.
https://claude.ai/code/session_01EMY5RnXb9rnsyU2KbXrSaY
Claude
41e54ca33e