What's the purpose of vfrsqrt and vfrec7.v?

riscv / riscv-v-spec

Working draft of the proposed RISC-V V vector extension

https://jira.riscv.org/browse/RVG-122

Creative Commons Attribution 4.0 International

950 stars 270 forks source link

What's the purpose of vfrsqrt and vfrec7.v? #882

Open lygstate opened 1 year ago

dkravitz-sifive commented 1 year ago

To provide a seed value for a Newton-Raphson routine to quickly calculate a sufficiently-accurate value. Per notes in the RVV spec:

The 7 bit accuracy was chosen as it requires 0,1,2,3 Newton-Raphson iterations to converge to close to bfloat16, FP16, FP32, FP64 accuracy respectively.