[libre-riscv-dev] [isa-dev] Re: FP reciprocal sqrt extension proposal

[MitchAlsup]

On Thursday, August 8, 2019 at 1:46:39 AM UTC-5, lkcl wrote:
>
> Just a thought: should this be merged into Ztrans? A case could be nade 
> either way.
>
> * The Libre RISCV FPDIV pipeline also computes SQRT and RSQRT
> * Some OTFC pipeline designs do RSQRT "for free"
> * However, if doing RSQRT chances are high it will be part of a design 
> that needs EXP, LOG etc anyway.
> * CORDIC can do a ton of algorithms including SQRT, RSQRT, LOG, SIN etc.
>
> Thoughts?
>
> L.
>

CORDIC is SLOW
The Moto CORDIC unit took 80-cycles or more
The Intel CORDIC was similarly slow
Modern SQRT/RSQRT hardware will deliver results in 23-30 cycles.