fuseBlockPlain was accidentally quadratic.
If you had plain blocks b1 -> b2 -> b3 -> b4,
each containing single values v1, v2, v3, and v4 respectively,
fuseBlockPlain would move v1 from b1 to b2 to b3 to b4,
then v2 from b2 to b3 to b4, etc.

There are two obvious fixes.

* Look for runs of blocks in fuseBlockPlain
  and handle them in a single go.
* Fuse from end to beginning; any given value in a run
  of blocks to fuse then moves only once.

The latter is much simpler, so that's what this CL does.

Somewhat surprisingly, this change does not pass toolstash-check.

The resulting set of blocks is the same,
and the values in them are the same,
but the order of values in them differ,
and that order of values (while arbitrary)
is enough to change the compiler's output.
This may be due to #20178; deadstore is the next pass after fuse.

Adding basic sorting to the beginning of deadstore
is enough to make this CL pass toolstash-check:

	for _, b := range f.Blocks {
		obj.SortSlice(b.Values, func(i, j int) bool { return b.Values[i].ID < b.Values[j].ID })
	}

Happily, this CL appears to result in better code on average,
if only by accident. It cuts 4k off of cmd/go; go1 benchmarks
are noisy as always but don't regress (numbers below).

No impact on the standard compilebench benchmarks.
For the code in #13554, this speeds up compilation dramatically:

name  old time/op       new time/op       delta
Pkg         53.1s ± 2%        12.8s ± 3%  -75.92%  (p=0.008 n=5+5)

name  old user-time/op  new user-time/op  delta
Pkg         55.0s ± 2%        14.9s ± 3%  -73.00%  (p=0.008 n=5+5)

name  old alloc/op      new alloc/op      delta
Pkg        2.04GB ± 0%       2.04GB ± 0%   +0.18%  (p=0.008 n=5+5)

name  old allocs/op     new allocs/op     delta
Pkg         6.21M ± 0%        6.21M ± 0%     ~     (p=0.222 n=5+5)

name  old object-bytes  new object-bytes  delta
Pkg         28.4M ± 0%        28.4M ± 0%   +0.00%  (p=0.008 n=5+5)

name  old export-bytes  new export-bytes  delta
Pkg           208 ± 0%          208 ± 0%     ~     (all equal)


Updates #13554


go1 benchmarks:

name                     old time/op    new time/op    delta
BinaryTree17-8              2.29s ± 2%     2.26s ± 2%  -1.43%  (p=0.000 n=48+50)
Fannkuch11-8                2.74s ± 2%     2.79s ± 2%  +1.63%  (p=0.000 n=50+49)
FmtFprintfEmpty-8          36.6ns ± 3%    34.6ns ± 4%  -5.29%  (p=0.000 n=49+50)
FmtFprintfString-8         58.3ns ± 3%    59.1ns ± 3%  +1.35%  (p=0.000 n=50+49)
FmtFprintfInt-8            62.4ns ± 2%    63.2ns ± 3%  +1.19%  (p=0.000 n=49+49)
FmtFprintfIntInt-8         95.1ns ± 2%    96.7ns ± 3%  +1.61%  (p=0.000 n=49+50)
FmtFprintfPrefixedInt-8     118ns ± 3%     113ns ± 2%  -4.00%  (p=0.000 n=50+49)
FmtFprintfFloat-8           191ns ± 2%     192ns ± 2%  +0.40%  (p=0.034 n=50+50)
FmtManyArgs-8               419ns ± 2%     420ns ± 2%    ~     (p=0.228 n=49+49)
GobDecode-8                5.26ms ± 3%    5.19ms ± 2%  -1.33%  (p=0.000 n=50+49)
GobEncode-8                4.12ms ± 2%    4.15ms ± 3%  +0.68%  (p=0.007 n=49+50)
Gzip-8                      198ms ± 2%     197ms ± 2%  -0.50%  (p=0.018 n=48+48)
Gunzip-8                   31.9ms ± 3%    31.8ms ± 3%  -0.47%  (p=0.024 n=50+50)
HTTPClientServer-8         64.4µs ± 0%    64.0µs ± 0%  -0.55%  (p=0.000 n=43+46)
JSONEncode-8               10.6ms ± 2%    10.6ms ± 3%    ~     (p=0.543 n=49+49)
JSONDecode-8               43.3ms ± 3%    43.1ms ± 2%    ~     (p=0.079 n=50+50)
Mandelbrot200-8            3.70ms ± 2%    3.70ms ± 2%    ~     (p=0.553 n=47+50)
GoParse-8                  2.70ms ± 2%    2.71ms ± 3%    ~     (p=0.843 n=49+50)
RegexpMatchEasy0_32-8      70.5ns ± 4%    70.4ns ± 4%    ~     (p=0.867 n=48+50)
RegexpMatchEasy0_1K-8       162ns ± 3%     162ns ± 2%    ~     (p=0.739 n=48+48)
RegexpMatchEasy1_32-8      66.1ns ± 5%    66.2ns ± 4%    ~     (p=0.970 n=50+50)
RegexpMatchEasy1_1K-8       297ns ± 7%     296ns ± 7%    ~     (p=0.406 n=50+50)
RegexpMatchMedium_32-8      105ns ± 5%     105ns ± 5%    ~     (p=0.702 n=50+50)
RegexpMatchMedium_1K-8     32.3µs ± 4%    32.2µs ± 3%    ~     (p=0.614 n=49+49)
RegexpMatchHard_32-8       1.75µs ±18%    1.74µs ±12%    ~     (p=0.738 n=50+48)
RegexpMatchHard_1K-8       52.2µs ±14%    51.3µs ±13%    ~     (p=0.230 n=50+50)
Revcomp-8                   366ms ± 3%     367ms ± 3%    ~     (p=0.745 n=49+49)
Template-8                 48.5ms ± 4%    48.5ms ± 4%    ~     (p=0.824 n=50+48)
TimeParse-8                 263ns ± 2%     256ns ± 2%  -2.98%  (p=0.000 n=48+49)
TimeFormat-8                265ns ± 3%     262ns ± 3%  -1.35%  (p=0.000 n=48+49)
[Geo mean]                 41.1µs         40.9µs       -0.48%


Change-Id: Ib35fa15b54282abb39c077d150beee27f610891a
Reviewed-on: https://go-review.googlesource.com/43570
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>