The existing implementation has poor performance for inserting large
number of keys because it chooses to append the new key to the sorted
keys array and then sort the new array again (time complexity is
O(nlogn)).

The improvement tries to utilize the sorted keys array by searching the
index and doing an insertion if any (time complexity is O(logn)).

Benchmarked on 4-core machine with `go test -cpu 1,2,4,8 -count 5
-benchmem -bench=.`(the equal results are omitted):

name                          old time/op    new time/op    delta
MapAddDifferentRandom-8          408µs ±11%      69µs ± 3%  -82.95%  (p=0.008 n=5+5)
MapAddDifferentRandom-4          389µs ±19%      69µs ± 2%  -82.28%  (p=0.008 n=5+5)
MapAddDifferentRandom-2          365µs ± 4%      75µs ± 6%  -79.51%  (p=0.008 n=5+5)
MapSetDifferentRandom-8          365µs ± 4%      76µs ±40%  -79.07%  (p=0.008 n=5+5)
MapAddDifferentRandom            366µs ± 3%      78µs ± 6%  -78.66%  (p=0.008 n=5+5)
MapSetDifferentRandom            369µs ± 2%      81µs ±34%  -77.99%  (p=0.008 n=5+5)
MapSetDifferentRandom-2          378µs ±10%     100µs ±32%  -73.47%  (p=0.008 n=5+5)
MapSetDifferentRandom-4          352µs ± 4%     108µs ± 7%  -69.40%  (p=0.008 n=5+5)
IntAdd-2                        23.1ns ±21%    15.5ns ±23%  -32.79%  (p=0.032 n=5+5)
IntSet-2                        21.4ns ±14%    16.7ns ±17%  -22.00%  (p=0.016 n=5+5)
FloatAdd-8                      88.8ns ± 9%    70.8ns ±25%  -20.23%  (p=0.024 n=5+5)
FloatSet-2                      22.3ns ±15%    17.8ns ±14%  -20.14%  (p=0.008 n=5+5)
IntAdd-8                        21.7ns ± 3%    18.7ns ± 4%  -14.00%  (p=0.008 n=5+5)
MapAddDifferent-8               1.58µs ± 7%    1.42µs ± 6%  -10.06%  (p=0.016 n=5+5)
StringSet-2                     42.4ns ± 1%    43.7ns ± 5%   +3.07%  (p=0.048 n=4+5)
FloatSet                        8.27ns ± 2%    8.60ns ± 1%   +3.94%  (p=0.008 n=5+5)
FloatAdd                        12.5ns ± 2%    13.0ns ± 4%   +4.33%  (p=0.032 n=5+5)
MapSetString-4                  94.6ns ± 0%   101.7ns ± 4%   +7.55%  (p=0.016 n=4+5)
MapAddSameSteadyState-2         34.9ns ± 3%    37.7ns ± 5%   +8.14%  (p=0.008 n=5+5)
StringSet-4                     34.5ns ± 0%    37.6ns ± 9%   +9.02%  (p=0.016 n=4+5)
MapSetDifferent-8                377ns ± 3%     411ns ± 7%   +9.07%  (p=0.008 n=5+5)
MapAddSameSteadyState           39.1ns ± 2%    42.8ns ± 6%   +9.36%  (p=0.008 n=5+5)
MapAddDifferentSteadyState       172ns ± 4%     190ns ± 9%  +10.96%  (p=0.016 n=5+5)
MapSet                           143ns ± 4%     159ns ± 2%  +11.06%  (p=0.008 n=5+5)
MapSet-4                        96.9ns ± 5%   107.8ns ± 6%  +11.25%  (p=0.008 n=5+5)
MapSet-2                         102ns ± 6%     114ns ± 8%  +11.94%  (p=0.008 n=5+5)
IntSet                          8.18ns ± 1%   12.78ns ±13%  +56.31%  (p=0.008 n=5+5)

name                          old alloc/op   new alloc/op   delta
MapSetDifferentRandom-4         19.8kB ± 0%    16.6kB ± 0%  -16.21%  (p=0.008 n=5+5)
MapSetDifferentRandom           19.8kB ± 0%    16.6kB ± 0%  -16.21%  (p=0.008 n=5+5)
MapSetDifferentRandom-8         19.8kB ± 0%    16.6kB ± 0%  -16.20%  (p=0.008 n=5+5)
MapSetDifferentRandom-2         19.8kB ± 0%    16.6kB ± 0%  -16.20%  (p=0.008 n=5+5)
MapAddDifferentRandom           20.6kB ± 0%    17.4kB ± 0%  -15.57%  (p=0.008 n=5+5)
MapAddDifferentRandom-8         20.6kB ± 0%    17.4kB ± 0%  -15.57%  (p=0.008 n=5+5)
MapAddDifferentRandom-2         20.6kB ± 0%    17.4kB ± 0%  -15.56%  (p=0.008 n=5+5)
MapAddDifferentRandom-4         20.6kB ± 0%    17.4kB ± 0%  -15.56%  (p=0.008 n=5+5)
MapAddDifferent                 1.09kB ± 0%    0.96kB ± 0%  -11.76%  (p=0.008 n=5+5)
MapAddDifferent-2               1.09kB ± 0%    0.96kB ± 0%  -11.76%  (p=0.008 n=5+5)
MapAddDifferent-4               1.09kB ± 0%    0.96kB ± 0%  -11.76%  (p=0.008 n=5+5)
MapAddDifferent-8               1.09kB ± 0%    0.96kB ± 0%  -11.76%  (p=0.008 n=5+5)
MapAddSame                        480B ± 0%      448B ± 0%   -6.67%  (p=0.008 n=5+5)
MapAddSame-2                      480B ± 0%      448B ± 0%   -6.67%  (p=0.008 n=5+5)
MapAddSame-4                      480B ± 0%      448B ± 0%   -6.67%  (p=0.008 n=5+5)
MapAddSame-8                      480B ± 0%      448B ± 0%   -6.67%  (p=0.008 n=5+5)

name                          old allocs/op  new allocs/op  delta
MapSetDifferentRandom              423 ± 0%       323 ± 0%  -23.64%  (p=0.008 n=5+5)
MapSetDifferentRandom-2            423 ± 0%       323 ± 0%  -23.64%  (p=0.008 n=5+5)
MapSetDifferentRandom-4            423 ± 0%       323 ± 0%  -23.64%  (p=0.008 n=5+5)
MapSetDifferentRandom-8            423 ± 0%       323 ± 0%  -23.64%  (p=0.008 n=5+5)
MapAddDifferentRandom              523 ± 0%       423 ± 0%  -19.12%  (p=0.008 n=5+5)
MapAddDifferentRandom-2            523 ± 0%       423 ± 0%  -19.12%  (p=0.008 n=5+5)
MapAddDifferentRandom-4            523 ± 0%       423 ± 0%  -19.12%  (p=0.008 n=5+5)
MapAddDifferentRandom-8            523 ± 0%       423 ± 0%  -19.12%  (p=0.008 n=5+5)
MapAddDifferent                   31.0 ± 0%      27.0 ± 0%  -12.90%  (p=0.008 n=5+5)
MapAddDifferent-2                 31.0 ± 0%      27.0 ± 0%  -12.90%  (p=0.008 n=5+5)
MapAddDifferent-4                 31.0 ± 0%      27.0 ± 0%  -12.90%  (p=0.008 n=5+5)
MapAddDifferent-8                 31.0 ± 0%      27.0 ± 0%  -12.90%  (p=0.008 n=5+5)
MapAddSame                        11.0 ± 0%      10.0 ± 0%   -9.09%  (p=0.008 n=5+5)
MapAddSame-2                      11.0 ± 0%      10.0 ± 0%   -9.09%  (p=0.008 n=5+5)
MapAddSame-4                      11.0 ± 0%      10.0 ± 0%   -9.09%  (p=0.008 n=5+5)
MapAddSame-8                      11.0 ± 0%      10.0 ± 0%   -9.09%  (p=0.008 n=5+5)

Fixes #31414

Change-Id: I2dd655dec9dbbf8d7881a0f3f8edcb3d29199a48
GitHub-Last-Rev: a816fe3f62498481500a0ce9695de9fd3aa9f7b7
GitHub-Pull-Request: golang/go#31418
Reviewed-on: https://go-review.googlesource.com/c/go/+/171718
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>