Replace MULQ with MULX in the inline assembly for fe_mul on x86_64.
MULX advantages over MULQ:
- Uses RDX as implicit input (not RAX), outputs to two arbitrary regs
- Does NOT clobber flags, enabling better instruction scheduling
- Enables the compiler to interleave multiplies with carries
Requires BMI2 (available on Haswell+ / Zen+). Added -mbmi2 to
CMakeLists.txt for x86_64 builds.
Note: ADCX/ADOX (ADX extension) for dual carry chains was investigated
but the compiler doesn't auto-generate them from __int128 code, and
hand-encoding in inline ASM requires restructuring the entire multiply
to express two independent carry chains. The MULX-only approach still
gives a measurable improvement.
fe_mul: 17.2ns → 15.5ns (10% faster)
pubkeyCreate: 16.7µs → 15.5µs (7% faster)
verifyFast: 36.5µs → 36.1µs (1% faster, 27,700 ops/s)
https://claude.ai/code/session_011KVZhDcV2G7idNWEBz12GY
Claude
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