When a map is small, it's not worth putting
the contents in an array and then looping over the array.
Just generate code instead.

This makes smaller binaries.
It might also be better for cache lines.

It also can avoids adding control flow in the middle
of the init function, which can be very large.
Eliminating this source of extra blocks
makes phi insertion easier for temp-heavy init functions.
This reduces the time required for compiler to
panic while compiling the code in #19751
from 15 minutes to 45 seconds.

The cutoff of 25 was chosen fairly unscientifically
by looking at the size of cmd/go.

Cutoff of   0: 10689604
Cutoff of   5: 10683572
Cutoff of  15: 10682324
Cutoff of  25: 10681700
Cutoff of  50: 10685476
Cutoff of 100: 10689412

There are probably more sophisticated mechanisms available.
For example, the smaller the key/value sizes, the better
generated code will be vs a table.
Nevertheless this is simple and seems like a good start.

Updates #19751

name       old time/op     new time/op     delta
Template       204ms ± 6%      202ms ± 5%  -0.78%  (p=0.027 n=47+45)
Unicode       84.8ms ± 6%     85.2ms ± 7%    ~     (p=0.146 n=46+45)
GoTypes        551ms ± 2%      556ms ± 3%  +0.76%  (p=0.004 n=43+45)
SSA            3.93s ± 3%      3.95s ± 4%    ~     (p=0.179 n=50+49)
Flate          123ms ± 4%      123ms ± 5%    ~     (p=0.201 n=47+49)
GoParser       145ms ± 3%      145ms ± 4%    ~     (p=0.937 n=50+50)
Reflect        356ms ± 3%      354ms ± 5%  -0.44%  (p=0.048 n=46+50)
Tar            107ms ± 6%      106ms ± 6%    ~     (p=0.188 n=50+49)
XML            201ms ± 4%      200ms ± 4%    ~     (p=0.085 n=50+49)

name       old user-ns/op  new user-ns/op  delta
Template        252M ± 9%       250M ± 7%    ~     (p=0.206 n=49+47)
Unicode         106M ± 7%       106M ± 9%    ~     (p=0.331 n=47+46)
GoTypes         724M ± 5%       729M ± 5%    ~     (p=0.160 n=47+49)
SSA            5.64G ± 2%      5.62G ± 4%    ~     (p=0.148 n=47+50)
Flate           147M ± 6%       147M ± 5%    ~     (p=0.466 n=50+49)
GoParser        179M ± 5%       179M ± 6%    ~     (p=0.584 n=50+49)
Reflect         448M ± 6%       441M ± 8%  -1.39%  (p=0.027 n=50+49)
Tar             124M ± 6%       123M ± 5%    ~     (p=0.221 n=50+47)
XML             244M ± 5%       243M ± 4%    ~     (p=0.275 n=49+49)

name       old alloc/op    new alloc/op    delta
Template      39.9MB ± 0%     39.4MB ± 0%  -1.28%  (p=0.008 n=5+5)
Unicode       29.8MB ± 0%     29.8MB ± 0%    ~     (p=0.310 n=5+5)
GoTypes        113MB ± 0%      113MB ± 0%    ~     (p=0.421 n=5+5)
SSA            854MB ± 0%      854MB ± 0%    ~     (p=0.151 n=5+5)
Flate         25.3MB ± 0%     25.3MB ± 0%    ~     (p=1.000 n=5+5)
GoParser      31.8MB ± 0%     31.8MB ± 0%    ~     (p=0.222 n=5+5)
Reflect       78.2MB ± 0%     78.2MB ± 0%    ~     (p=1.000 n=5+5)
Tar           26.7MB ± 0%     26.7MB ± 0%    ~     (p=0.841 n=5+5)
XML           42.3MB ± 0%     42.3MB ± 0%  -0.15%  (p=0.008 n=5+5)

name       old allocs/op   new allocs/op   delta
Template        390k ± 1%       386k ± 1%  -1.05%  (p=0.016 n=5+5)
Unicode         319k ± 0%       320k ± 0%    ~     (p=0.310 n=5+5)
GoTypes        1.14M ± 0%      1.14M ± 0%    ~     (p=0.421 n=5+5)
SSA            7.60M ± 0%      7.59M ± 0%    ~     (p=0.310 n=5+5)
Flate           234k ± 0%       235k ± 1%    ~     (p=1.000 n=5+5)
GoParser        315k ± 1%       317k ± 0%    ~     (p=0.151 n=5+5)
Reflect         978k ± 0%       978k ± 0%    ~     (p=0.841 n=5+5)
Tar             251k ± 1%       251k ± 1%    ~     (p=0.690 n=5+5)
XML             394k ± 0%       392k ± 0%    ~     (p=0.056 n=5+5)


Change-Id: Ic53a18627082abe075a1cbc33330ce015e50850a
Reviewed-on: https://go-review.googlesource.com/39354
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>