Reuse v.Type() and cachedTypeFields(t) when decoding maps and structs.
Always use the same data slices when in hot loops, to ensure that the
compiler generates good code. "for i < len(data) { use(d.data[i]) }"
makes it harder for the compiler.
Finally, do other minor clean-ups, such as deduplicating switch cases,
and using a switch instead of three chained ifs.
The decoder sees a noticeable speed-up, in particular when decoding
structs.
name old time/op new time/op delta
CodeDecoder-4 29.8ms ± 1% 27.5ms ± 0% -7.83% (p=0.002 n=6+6)
name old speed new speed delta
CodeDecoder-4 65.0MB/s ± 1% 70.6MB/s ± 0% +8.49% (p=0.002 n=6+6)