Old loops look like this:
   loop:
     CMPQ ...
     JGE exit
     ...
     JMP loop
   exit:

New loops look like this:
    JMP entry
  loop:
    ...
  entry:
    CMPQ ...
    JLT loop

This removes one instruction (the unconditional jump) from
the inner loop.
Kinda surprisingly, it matters.

This is a bit different than the peeling that the old obj
library did in that we don't duplicate the loop exit test.
We just jump to the test.  I'm not sure if it is better or
worse to do that (peeling gets rid of the JMP but means more
code duplication), but this CL is certainly a much simpler
compiler change, so I'll try this way first.

The obj library used to do peeling before
CL https://go-review.googlesource.com/c/36205 turned it off.

Fixes #15837 (remove obj instruction reordering)
The reordering is already removed, this CL implements the only
part of that reordering that we'd like to keep.

Fixes #14758 (append loop)
name    old time/op    new time/op    delta
Foo-12     817ns ± 4%     538ns ± 0%  -34.08%   (p=0.000 n=10+9)
Bar-12     850ns ±11%     570ns ±13%  -32.88%  (p=0.000 n=10+10)

Update #19595 (BLAS slowdown)
name                       old time/op  new time/op  delta
DgemvMedMedNoTransIncN-12  13.2µs ± 9%  10.2µs ± 1%  -22.26%  (p=0.000 n=9+9)

Fixes #19633 (append loop)
name    old time/op    new time/op    delta
Foo-12     810ns ± 1%     540ns ± 0%  -33.30%   (p=0.000 n=8+9)

Update #18977 (Fannkuch11 regression)
name         old time/op    new time/op    delta
Fannkuch11-8                2.80s ± 0%     3.01s ± 0%  +7.47%   (p=0.000 n=9+10)
This one makes no sense.  There's strictly 1 less instruction in the
inner loop (17 instead of 18).  They are exactly the same instructions
except for the JMP that has been elided.

go1 benchmarks generally don't look very impressive.  But the gains for the
specific issues above make this CL still probably worth it.
name                      old time/op    new time/op    delta
BinaryTree17-8              2.32s ± 0%     2.34s ± 0%  +1.14%    (p=0.000 n=9+7)
Fannkuch11-8                2.80s ± 0%     3.01s ± 0%  +7.47%   (p=0.000 n=9+10)
FmtFprintfEmpty-8          44.1ns ± 1%    46.1ns ± 1%  +4.53%  (p=0.000 n=10+10)
FmtFprintfString-8         67.8ns ± 0%    74.4ns ± 1%  +9.80%   (p=0.000 n=10+9)
FmtFprintfInt-8            74.9ns ± 0%    78.4ns ± 0%  +4.67%   (p=0.000 n=8+10)
FmtFprintfIntInt-8          117ns ± 1%     123ns ± 1%  +4.69%   (p=0.000 n=9+10)
FmtFprintfPrefixedInt-8     160ns ± 1%     146ns ± 0%  -8.22%   (p=0.000 n=8+10)
FmtFprintfFloat-8           214ns ± 0%     206ns ± 0%  -3.91%    (p=0.000 n=8+8)
FmtManyArgs-8               468ns ± 0%     497ns ± 1%  +6.09%   (p=0.000 n=8+10)
GobDecode-8                6.16ms ± 0%    6.21ms ± 1%  +0.76%   (p=0.000 n=9+10)
GobEncode-8                4.90ms ± 0%    4.92ms ± 1%  +0.37%   (p=0.028 n=9+10)
Gzip-8                      209ms ± 0%     212ms ± 0%  +1.33%  (p=0.000 n=10+10)
Gunzip-8                   36.6ms ± 0%    38.0ms ± 1%  +4.03%    (p=0.000 n=9+9)
HTTPClientServer-8         84.2µs ± 0%    86.0µs ± 1%  +2.14%    (p=0.000 n=9+9)
JSONEncode-8               13.6ms ± 3%    13.8ms ± 1%  +1.55%   (p=0.003 n=9+10)
JSONDecode-8               53.2ms ± 5%    52.9ms ± 0%    ~     (p=0.280 n=10+10)
Mandelbrot200-8            3.78ms ± 0%    3.78ms ± 1%    ~      (p=0.661 n=10+9)
GoParse-8                  2.89ms ± 0%    2.94ms ± 2%  +1.50%  (p=0.000 n=10+10)
RegexpMatchEasy0_32-8      68.5ns ± 2%    68.9ns ± 1%    ~     (p=0.136 n=10+10)
RegexpMatchEasy0_1K-8       220ns ± 1%     225ns ± 1%  +2.41%  (p=0.000 n=10+10)
RegexpMatchEasy1_32-8      64.7ns ± 0%    64.5ns ± 0%  -0.28%  (p=0.042 n=10+10)
RegexpMatchEasy1_1K-8       348ns ± 1%     355ns ± 0%  +1.90%  (p=0.000 n=10+10)
RegexpMatchMedium_32-8      102ns ± 1%     105ns ± 1%  +2.95%  (p=0.000 n=10+10)
RegexpMatchMedium_1K-8     33.1µs ± 3%    32.5µs ± 0%  -1.75%  (p=0.000 n=10+10)
RegexpMatchHard_32-8       1.71µs ± 1%    1.70µs ± 1%  -0.84%   (p=0.002 n=10+9)
RegexpMatchHard_1K-8       51.1µs ± 0%    50.8µs ± 1%  -0.48%  (p=0.004 n=10+10)
Revcomp-8                   411ms ± 1%     402ms ± 0%  -2.22%   (p=0.000 n=10+9)
Template-8                 61.8ms ± 1%    59.7ms ± 0%  -3.44%    (p=0.000 n=9+9)
TimeParse-8                 306ns ± 0%     318ns ± 0%  +3.83%  (p=0.000 n=10+10)
TimeFormat-8                320ns ± 0%     318ns ± 1%  -0.53%   (p=0.012 n=7+10)

Change-Id: Ifaf29abbe5874e437048e411ba8f7cfbc9e1c94b
Reviewed-on: https://go-review.googlesource.com/38431
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>