When allocating a non-small array of buckets for a map,
also preallocate some overflow buckets.

The estimate of the number of overflow buckets
is based on a simulation of putting mid=(low+high)/2 elements
into a map, where low is the minimum number of elements
needed to reach this value of b (according to overLoadFactor),
and high is the maximum number of elements possible
to put in this value of b (according to overLoadFactor).
This estimate is surprisingly reliable and accurate.

The number of overflow buckets needed is quadratic,
for a fixed value of b.
Using this mid estimate means that we will overallocate a few
too many overflow buckets when the actual number of elements is near low,
and underallocate significantly too few overflow buckets
when the actual number of elements is near high.

The mechanism introduced in this CL can be re-used for
other overflow bucket optimizations.

For example, given an initial size hint,
we could estimate quite precisely the number of overflow buckets.
This is #19931.

We could also change from "non-nil means end-of-list"
to "pointer-to-hmap.buckets means end-of-list",
and then create a linked list of reusable overflow buckets
when they are freed by map growth.
That is #19992.

We could also use a similar mechanism to do bulk allocation
of overflow buckets.
All these uses can co-exist with only the one additional pointer
in mapextra, given a little care.

name                  old time/op    new time/op    delta
MapPopulate/1-8         60.1ns ± 2%    60.3ns ± 2%     ~     (p=0.278 n=19+20)
MapPopulate/10-8         577ns ± 1%     578ns ± 1%     ~     (p=0.140 n=20+20)
MapPopulate/100-8       8.06µs ± 1%    8.19µs ± 1%   +1.67%  (p=0.000 n=20+20)
MapPopulate/1000-8       104µs ± 1%     104µs ± 1%     ~     (p=0.317 n=20+20)
MapPopulate/10000-8      891µs ± 1%     888µs ± 1%     ~     (p=0.101 n=19+20)
MapPopulate/100000-8    8.61ms ± 1%    8.58ms ± 0%   -0.34%  (p=0.009 n=20+17)

name                  old alloc/op   new alloc/op   delta
MapPopulate/1-8          0.00B          0.00B          ~     (all equal)
MapPopulate/10-8          179B ± 0%      179B ± 0%     ~     (all equal)
MapPopulate/100-8       3.33kB ± 0%    3.38kB ± 0%   +1.48%  (p=0.000 n=20+16)
MapPopulate/1000-8      55.5kB ± 0%    53.4kB ± 0%   -3.84%  (p=0.000 n=19+20)
MapPopulate/10000-8      432kB ± 0%     428kB ± 0%   -1.06%  (p=0.000 n=19+20)
MapPopulate/100000-8    3.65MB ± 0%    3.62MB ± 0%   -0.70%  (p=0.000 n=20+20)

name                  old allocs/op  new allocs/op  delta
MapPopulate/1-8           0.00           0.00          ~     (all equal)
MapPopulate/10-8          1.00 ± 0%      1.00 ± 0%     ~     (all equal)
MapPopulate/100-8         18.0 ± 0%      17.0 ± 0%   -5.56%  (p=0.000 n=20+20)
MapPopulate/1000-8        96.0 ± 0%      72.6 ± 1%  -24.38%  (p=0.000 n=20+20)
MapPopulate/10000-8        625 ± 0%       319 ± 0%  -48.86%  (p=0.000 n=20+20)
MapPopulate/100000-8     6.23k ± 0%     4.00k ± 0%  -35.79%  (p=0.000 n=20+20)

Change-Id: I01f41cb1374bdb99ccedbc00d04fb9ae43daa204
Reviewed-on: https://go-review.googlesource.com/40979
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>