math/big: regression in BitLen #57014
Description
CL 450055 replaced big.Int.BitLen with a portable constant-time operation for fear of bits.LeadingZeros using a lookup table on some architectures (notably not on x86, which uses a hardware instruction). @drchase's benchmarks found https://perf.golang.org/search?q=upload%3A20221130.13+pkg%3Agithub.com%2Fericlagergren%2Fdecimal%2Fbenchmarks, non-crypto code with a significant performance hit for a property (constant-time) it didn't need.
Talked to @FiloSottile, and he is going to send a CL removing use of big.Int.BitLen from crypto code and restoring the old implementation.
Filing issue so we remember to submit the CL before the release.