fmt.Printf("float32 %f\n", float32(1234.56789))
fmt.Printf("float64 %f\n", float64(1234.56789))
->
float32 1234.567871
float64 1234.567890

this is a snapshot. extended instruction support, corner cases
and fixes coming in subseuent cls.

R=rsc
CC=dpx, golang-dev
https://golang.org/cl/876045