Benchmark results on GOOS=linux:

GOARCH=amd64

name              old time/op  new time/op  delta
Atoi/Pos/7bit-4   20.1ns ± 2%   8.6ns ± 1%  -57.34%  (p=0.000 n=10+10)
Atoi/Pos/26bit-4  25.8ns ± 7%  11.9ns ± 0%  -53.91%  (p=0.000 n=10+8)
Atoi/Pos/31bit-4  27.3ns ± 2%  13.2ns ± 1%  -51.56%  (p=0.000 n=10+10)
Atoi/Pos/56bit-4  37.2ns ± 5%  18.2ns ± 1%  -51.26%  (p=0.000 n=10+10)
Atoi/Pos/63bit-4  38.7ns ± 1%  38.6ns ± 1%     ~     (p=0.297 n=9+10)
Atoi/Neg/7bit-4   17.6ns ± 1%   7.2ns ± 0%  -59.22%  (p=0.000 n=10+10)
Atoi/Neg/26bit-4  24.4ns ± 1%  12.4ns ± 1%  -49.28%  (p=0.000 n=10+10)
Atoi/Neg/31bit-4  26.9ns ± 0%  14.0ns ± 1%  -47.88%  (p=0.000 n=7+10)
Atoi/Neg/56bit-4  36.2ns ± 1%  19.5ns ± 0%  -46.24%  (p=0.000 n=10+9)
Atoi/Neg/63bit-4  38.9ns ± 1%  38.8ns ± 1%     ~     (p=0.385 n=9+10)

GOARCH=386

name              old time/op  new time/op  delta
Atoi/Pos/7bit-4   89.6ns ± 1%   8.2ns ± 1%  -90.84%  (p=0.000 n=9+10)
Atoi/Pos/26bit-4   187ns ± 2%    12ns ± 1%  -93.71%  (p=0.000 n=10+9)
Atoi/Pos/31bit-4   225ns ± 1%   225ns ± 1%     ~     (p=0.995 n=10+10)
Atoi/Neg/7bit-4   86.2ns ± 1%   8.5ns ± 1%  -90.14%  (p=0.000 n=10+10)
Atoi/Neg/26bit-4   183ns ± 1%    13ns ± 1%  -92.77%  (p=0.000 n=9+10)
Atoi/Neg/31bit-4   223ns ± 0%   223ns ± 0%     ~     (p=0.247 n=8+9)

Fixes #20557

Change-Id: Ib6245d88cffd4b037419e2bf8e4a71b86c6d773f
Reviewed-on: https://go-review.googlesource.com/44692Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>