CL 214239 improved type printing, but introduced performance regressions:
3-5% memory increase, 1-2% CPU time increase.

There were two primary sources of the memory regression:

* allocating a bytes.Buffer for every type to print
* always printing to that buffer, even when we could return a constant string

This change addresses both of those regressions.
The sync.Pool allows buffer re-use.
String interning prevents allocation for re-used strings.

It addresses some, but not all, of the CPU time regression.


Memory performance impact vs master:

name        old alloc/op      new alloc/op      delta
Template         37.6MB ± 0%       36.3MB ± 0%  -3.30%  (p=0.008 n=5+5)
Unicode          28.7MB ± 0%       28.3MB ± 0%  -1.55%  (p=0.008 n=5+5)
GoTypes           127MB ± 0%        122MB ± 0%  -4.38%  (p=0.008 n=5+5)
Compiler          584MB ± 0%        568MB ± 0%  -2.72%  (p=0.008 n=5+5)
SSA              1.99GB ± 0%       1.95GB ± 0%  -1.97%  (p=0.008 n=5+5)
Flate            23.5MB ± 0%       22.8MB ± 0%  -2.84%  (p=0.008 n=5+5)
GoParser         29.2MB ± 0%       28.0MB ± 0%  -4.17%  (p=0.008 n=5+5)
Reflect          81.9MB ± 0%       78.6MB ± 0%  -4.09%  (p=0.008 n=5+5)
Tar              35.3MB ± 0%       34.1MB ± 0%  -3.29%  (p=0.008 n=5+5)
XML              45.5MB ± 0%       44.3MB ± 0%  -2.61%  (p=0.008 n=5+5)
[Geo mean]       82.4MB            79.9MB       -3.09%

name        old allocs/op     new allocs/op     delta
Template           394k ± 0%         363k ± 0%  -7.73%  (p=0.008 n=5+5)
Unicode            340k ± 0%         329k ± 0%  -3.25%  (p=0.008 n=5+5)
GoTypes           1.41M ± 0%        1.28M ± 0%  -9.54%  (p=0.008 n=5+5)
Compiler          5.77M ± 0%        5.39M ± 0%  -6.58%  (p=0.008 n=5+5)
SSA               19.1M ± 0%        18.1M ± 0%  -5.13%  (p=0.008 n=5+5)
Flate              247k ± 0%         228k ± 0%  -7.50%  (p=0.008 n=5+5)
GoParser           325k ± 0%         295k ± 0%  -9.24%  (p=0.008 n=5+5)
Reflect           1.04M ± 0%        0.95M ± 0%  -8.48%  (p=0.008 n=5+5)
Tar                365k ± 0%         336k ± 0%  -7.93%  (p=0.008 n=5+5)
XML                449k ± 0%         417k ± 0%  -7.10%  (p=0.008 n=5+5)
[Geo mean]         882k              818k       -7.26%


Memory performance going from 52c44884,
which is the commit preceding CL 214239, to this change:

name        old alloc/op      new alloc/op      delta
Template         36.5MB ± 0%       36.3MB ± 0%  -0.37%  (p=0.008 n=5+5)
Unicode          28.3MB ± 0%       28.3MB ± 0%  -0.06%  (p=0.008 n=5+5)
GoTypes           123MB ± 0%        122MB ± 0%  -0.64%  (p=0.008 n=5+5)
Compiler          571MB ± 0%        568MB ± 0%  -0.51%  (p=0.008 n=5+5)
SSA              1.96GB ± 0%       1.95GB ± 0%  -0.13%  (p=0.008 n=5+5)
Flate            22.8MB ± 0%       22.8MB ± 0%    ~     (p=0.421 n=5+5)
GoParser         28.1MB ± 0%       28.0MB ± 0%  -0.37%  (p=0.008 n=5+5)
Reflect          78.8MB ± 0%       78.6MB ± 0%  -0.32%  (p=0.008 n=5+5)
Tar              34.3MB ± 0%       34.1MB ± 0%  -0.35%  (p=0.008 n=5+5)
XML              44.3MB ± 0%       44.3MB ± 0%  +0.05%  (p=0.032 n=5+5)
[Geo mean]       80.1MB            79.9MB       -0.27%

name        old allocs/op     new allocs/op     delta
Template           372k ± 0%         363k ± 0%  -2.46%  (p=0.008 n=5+5)
Unicode            333k ± 0%         329k ± 0%  -0.97%  (p=0.008 n=5+5)
GoTypes           1.33M ± 0%        1.28M ± 0%  -3.71%  (p=0.008 n=5+5)
Compiler          5.53M ± 0%        5.39M ± 0%  -2.50%  (p=0.008 n=5+5)
SSA               18.3M ± 0%        18.1M ± 0%  -1.22%  (p=0.008 n=5+5)
Flate              234k ± 0%         228k ± 0%  -2.44%  (p=0.008 n=5+5)
GoParser           305k ± 0%         295k ± 0%  -3.23%  (p=0.008 n=5+5)
Reflect            980k ± 0%         949k ± 0%  -3.12%  (p=0.008 n=5+5)
Tar                345k ± 0%         336k ± 0%  -2.69%  (p=0.008 n=5+5)
XML                425k ± 0%         417k ± 0%  -1.72%  (p=0.008 n=5+5)
[Geo mean]         838k              818k       -2.41%


Remaining CPU time regression, that is,
the change from before CL 214239 to this change:

name        old time/op       new time/op       delta
Template          208ms ± 2%        209ms ± 1%    ~     (p=0.181 n=47+46)
Unicode          82.9ms ± 2%       81.9ms ± 2%  -1.25%  (p=0.000 n=50+48)
GoTypes           709ms ± 3%        714ms ± 3%  +0.77%  (p=0.003 n=48+49)
Compiler          3.31s ± 2%        3.32s ± 2%    ~     (p=0.271 n=48+48)
SSA               10.8s ± 1%        10.9s ± 1%  +0.61%  (p=0.000 n=46+47)
Flate             134ms ± 2%        134ms ± 1%  +0.41%  (p=0.002 n=48+46)
GoParser          166ms ± 2%        167ms ± 2%  +0.41%  (p=0.010 n=46+48)
Reflect           440ms ± 4%        444ms ± 4%  +1.05%  (p=0.002 n=50+49)
Tar               183ms ± 2%        184ms ± 2%    ~     (p=0.074 n=45+45)
XML               247ms ± 2%        248ms ± 2%  +0.67%  (p=0.001 n=49+48)
[Geo mean]        425ms             427ms       +0.34%

name        old user-time/op  new user-time/op  delta
Template          271ms ± 2%        271ms ± 2%    ~     (p=0.654 n=48+48)
Unicode           117ms ± 2%        116ms ± 3%    ~     (p=0.458 n=47+45)
GoTypes           952ms ± 3%        963ms ± 2%  +1.11%  (p=0.000 n=48+49)
Compiler          4.50s ± 5%        4.49s ± 7%    ~     (p=0.894 n=50+50)
SSA               15.0s ± 2%        15.1s ± 2%  +0.46%  (p=0.015 n=50+49)
Flate             166ms ± 2%        167ms ± 2%  +0.40%  (p=0.005 n=49+48)
GoParser          202ms ± 2%        203ms ± 2%  +0.60%  (p=0.002 n=49+47)
Reflect           583ms ± 3%        588ms ± 3%  +0.82%  (p=0.001 n=49+46)
Tar               223ms ± 2%        224ms ± 2%  +0.37%  (p=0.046 n=48+46)
XML               310ms ± 2%        311ms ± 2%  +0.46%  (p=0.009 n=50+49)
[Geo mean]        554ms             556ms       +0.36%


Change-Id: I85951a6538373ef4309a2cc366cc1ebaf1f4582d
Reviewed-on: https://go-review.googlesource.com/c/go/+/214818
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>