During map growth, buckets are evacuated in two ways.
When a value is altered, its containing bucket is evacuated.
Also, an evacuation mark is maintained and advanced every time.
Prior to this CL, the evacuation mark was always incremented,
even if the next bucket to be evacuated had already been evacuated.
This CL changes evacuation mark advancement to skip previously
evacuated buckets. This has the effect of making map evacuation both
more aggressive and more consistent.

Aggressive map evacuation is good. While the map is growing,
map accesses must check two buckets, which may be far apart in memory.
Map growth also delays garbage collection.
And if map evacuation is not aggressive enough, there is a risk that
a populate-once read-many map may be stuck permanently in map growth.
This CL does not eliminate that possibility, but it shrinks the window.

There is minimal impact on map benchmarks:

name                         old time/op    new time/op    delta
MapPop100-8                    12.4µs ±11%    12.4µs ± 7%    ~     (p=0.798 n=15+15)
MapPop1000-8                    240µs ± 8%     235µs ± 8%    ~     (p=0.217 n=15+14)
MapPop10000-8                  4.49ms ±10%    4.51ms ±15%    ~     (p=1.000 n=15+13)
MegMap-8                       11.9ns ± 2%    11.8ns ± 0%  -1.01%  (p=0.000 n=15+11)
MegOneMap-8                    9.30ns ± 1%    9.29ns ± 1%    ~     (p=0.955 n=14+14)
MegEqMap-8                     31.9µs ± 5%    31.9µs ± 3%    ~     (p=0.935 n=15+15)
MegEmptyMap-8                  2.41ns ± 2%    2.41ns ± 0%    ~     (p=0.594 n=12+14)
SmallStrMap-8                  12.8ns ± 1%    12.7ns ± 1%    ~     (p=0.569 n=14+13)
MapStringKeysEight_16-8        13.6ns ± 1%    13.7ns ± 2%    ~     (p=0.100 n=13+15)
MapStringKeysEight_32-8        12.1ns ± 1%    12.1ns ± 2%    ~     (p=0.340 n=15+15)
MapStringKeysEight_64-8        12.1ns ± 1%    12.1ns ± 2%    ~     (p=0.582 n=15+14)
MapStringKeysEight_1M-8        12.0ns ± 1%    12.1ns ± 1%    ~     (p=0.267 n=15+14)
IntMap-8                       7.96ns ± 1%    7.97ns ± 2%    ~     (p=0.991 n=15+13)
RepeatedLookupStrMapKey32-8    15.8ns ± 2%    15.8ns ± 1%    ~     (p=0.393 n=15+14)
RepeatedLookupStrMapKey1M-8    35.3µs ± 2%    35.3µs ± 1%    ~     (p=0.815 n=15+15)
NewEmptyMap-8                  36.0ns ± 4%    36.4ns ± 7%    ~     (p=0.270 n=15+15)
NewSmallMap-8                  85.5ns ± 1%    85.6ns ± 1%    ~     (p=0.674 n=14+15)
MapIter-8                      89.9ns ± 6%    90.8ns ± 6%    ~     (p=0.467 n=15+15)
MapIterEmpty-8                 10.0ns ±22%    10.0ns ±25%    ~     (p=0.846 n=15+15)
SameLengthMap-8                4.18ns ± 1%    4.17ns ± 1%    ~     (p=0.653 n=15+14)
BigKeyMap-8                    20.2ns ± 1%    20.1ns ± 1%  -0.82%  (p=0.002 n=15+15)
BigValMap-8                    22.5ns ± 8%    22.3ns ± 6%    ~     (p=0.615 n=15+15)
SmallKeyMap-8                  15.3ns ± 1%    15.3ns ± 1%    ~     (p=0.754 n=15+14)
ComplexAlgMap-8                58.4ns ± 1%    58.7ns ± 1%  +0.52%  (p=0.000 n=14+15)

There is a tiny but detectable difference in the compiler:

name       old time/op      new time/op      delta
Template        218ms ± 5%       219ms ± 4%    ~     (p=0.094 n=98+98)
Unicode        93.6ms ± 5%      93.6ms ± 4%    ~     (p=0.910 n=94+95)
GoTypes         596ms ± 5%       598ms ± 6%    ~     (p=0.533 n=98+100)
Compiler        2.72s ± 3%       2.72s ± 4%    ~     (p=0.238 n=100+99)
SSA             4.11s ± 3%       4.11s ± 3%    ~     (p=0.864 n=99+98)
Flate           129ms ± 6%       129ms ± 4%    ~     (p=0.522 n=98+96)
GoParser        151ms ± 4%       151ms ± 4%  -0.48%  (p=0.017 n=96+96)
Reflect         379ms ± 3%       376ms ± 4%  -0.57%  (p=0.011 n=99+99)
Tar             112ms ± 5%       112ms ± 6%    ~     (p=0.688 n=93+95)
XML             214ms ± 4%       214ms ± 5%    ~     (p=0.968 n=100+99)
StdCmd          16.2s ± 2%       16.2s ± 2%  -0.26%  (p=0.048 n=99+99)

name       old user-ns/op   new user-ns/op   delta
Template   252user-ms ± 4%  250user-ms ± 4%  -0.63%  (p=0.020 n=98+97)
Unicode    113user-ms ± 7%  114user-ms ± 5%    ~     (p=0.057 n=97+94)
GoTypes    776user-ms ± 5%  777user-ms ± 5%    ~     (p=0.375 n=97+96)
Compiler   3.61user-s ± 3%  3.60user-s ± 3%    ~     (p=0.445 n=98+93)
SSA        5.84user-s ± 6%  5.85user-s ± 5%    ~     (p=0.542 n=100+95)
Flate      154user-ms ± 5%  154user-ms ± 5%    ~     (p=0.699 n=99+99)
GoParser   184user-ms ± 6%  183user-ms ± 4%    ~     (p=0.557 n=98+95)
Reflect    461user-ms ± 5%  462user-ms ± 4%    ~     (p=0.853 n=97+99)
Tar        130user-ms ± 5%  129user-ms ± 6%    ~     (p=0.567 n=93+100)
XML        257user-ms ± 6%  258user-ms ± 6%    ~     (p=0.205 n=99+100)

Change-Id: Id92dd54a152904069aac415e6aaaab5c67f5f476
Reviewed-on: https://go-review.googlesource.com/37011Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>