runtime: remove dead code and unnecessary checks for amd64

runtime: remove dead code and unnecessary checks for amd64

Use amd64 assembly header to remove unnecessary cpu flags checks
and dead code that is guaranteed to not be executed when compiling
for specific microarchitectures.

name                  old time/op  new time/op  delta
BytesCompare/1-12     3.88ns ± 1%  3.18ns ± 1%  -18.15%  (p=0.008 n=5+5)
BytesCompare/2-12     3.89ns ± 1%  3.21ns ± 2%  -17.66%  (p=0.008 n=5+5)
BytesCompare/4-12     3.89ns ± 0%  3.17ns ± 0%  -18.62%  (p=0.008 n=5+5)
BytesCompare/8-12     3.44ns ± 2%  3.39ns ± 1%   -1.36%  (p=0.008 n=5+5)
BytesCompare/16-12    3.40ns ± 1%  3.14ns ± 0%   -7.77%  (p=0.008 n=5+5)
BytesCompare/32-12    3.90ns ± 1%  3.65ns ± 0%   -6.19%  (p=0.008 n=5+5)
BytesCompare/64-12    4.96ns ± 1%  4.71ns ± 2%   -4.98%  (p=0.008 n=5+5)
BytesCompare/128-12   6.42ns ± 0%  5.99ns ± 4%   -6.75%  (p=0.008 n=5+5)
BytesCompare/256-12   9.36ns ± 0%  7.40ns ± 0%  -20.97%  (p=0.008 n=5+5)
BytesCompare/512-12   15.9ns ± 1%  11.4ns ± 1%  -28.36%  (p=0.008 n=5+5)
BytesCompare/1024-12  27.0ns ± 0%  19.3ns ± 0%  -28.36%  (p=0.008 n=5+5)
BytesCompare/2048-12  50.2ns ± 0%  43.3ns ± 0%  -13.71%  (p=0.008 n=5+5)
[Geo mean]            7.13ns       6.07ns       -14.86%

name                old speed      new speed      delta
Count/10-12          723MB/s ± 0%   704MB/s ± 1%  -2.73%  (p=0.008 n=5+5)
Count/32-12         2.21GB/s ± 0%  2.12GB/s ± 2%  -4.21%  (p=0.008 n=5+5)
Count/4K-12         1.03GB/s ± 0%  1.03GB/s ± 1%    ~     (p=1.000 n=5+5)
Count/4M-12         1.04GB/s ± 0%  1.02GB/s ± 2%    ~     (p=0.310 n=5+5)
Count/64M-12        1.02GB/s ± 0%  1.01GB/s ± 1%  -1.00%  (p=0.016 n=5+5)
CountEasy/10-12      779MB/s ± 0%   768MB/s ± 1%  -1.48%  (p=0.008 n=5+5)
CountEasy/32-12     2.15GB/s ± 0%  2.09GB/s ± 1%  -2.71%  (p=0.008 n=5+5)
CountEasy/4K-12     45.1GB/s ± 1%  45.2GB/s ± 1%    ~     (p=0.421 n=5+5)
CountEasy/4M-12     36.4GB/s ± 1%  36.5GB/s ± 1%    ~     (p=0.690 n=5+5)
CountEasy/64M-12    16.1GB/s ± 2%  16.4GB/s ± 1%    ~     (p=0.056 n=5+5)
CountSingle/10-12   2.15GB/s ± 2%  2.22GB/s ± 1%  +3.37%  (p=0.008 n=5+5)
CountSingle/32-12   5.86GB/s ± 1%  5.76GB/s ± 1%  -1.55%  (p=0.008 n=5+5)
CountSingle/4K-12   54.6GB/s ± 1%  55.0GB/s ± 1%    ~     (p=0.548 n=5+5)
CountSingle/4M-12   45.9GB/s ± 4%  46.4GB/s ± 2%    ~     (p=0.548 n=5+5)
CountSingle/64M-12  17.3GB/s ± 1%  17.2GB/s ± 2%    ~     (p=1.000 n=5+5)
[Geo mean]          5.11GB/s       5.08GB/s       -0.53%

name          old speed      new speed      delta
Equal/1-12     200MB/s ± 0%   188MB/s ± 1%   -6.11%  (p=0.008 n=5+5)
Equal/6-12    1.20GB/s ± 0%  1.13GB/s ± 1%   -6.38%  (p=0.008 n=5+5)
Equal/9-12    1.67GB/s ± 3%  1.74GB/s ± 1%   +3.83%  (p=0.008 n=5+5)
Equal/15-12   2.82GB/s ± 1%  2.89GB/s ± 1%   +2.63%  (p=0.008 n=5+5)
Equal/16-12   2.96GB/s ± 1%  3.08GB/s ± 1%   +3.95%  (p=0.008 n=5+5)
Equal/20-12   3.33GB/s ± 1%  3.54GB/s ± 1%   +6.36%  (p=0.008 n=5+5)
Equal/32-12   4.57GB/s ± 0%  5.26GB/s ± 1%  +15.09%  (p=0.008 n=5+5)
Equal/4K-12   62.0GB/s ± 1%  65.9GB/s ± 2%   +6.29%  (p=0.008 n=5+5)
Equal/4M-12   23.6GB/s ± 2%  24.8GB/s ± 4%   +5.43%  (p=0.008 n=5+5)
Equal/64M-12  11.1GB/s ± 2%  11.3GB/s ± 1%   +1.69%  (p=0.008 n=5+5)
[Geo mean]    3.91GB/s       4.03GB/s        +3.11%

name                              old speed      new speed      delta
IndexByte/10-12                   2.64GB/s ± 0%  2.69GB/s ± 0%   +1.67%  (p=0.008 n=5+5)
IndexByte/32-12                   6.79GB/s ± 0%  6.27GB/s ± 0%   -7.57%  (p=0.008 n=5+5)
IndexByte/4K-12                   56.2GB/s ± 0%  56.9GB/s ± 0%   +1.27%  (p=0.008 n=5+5)
IndexByte/4M-12                   40.1GB/s ± 1%  41.7GB/s ± 1%   +4.05%  (p=0.008 n=5+5)
IndexByte/64M-12                  17.5GB/s ± 0%  17.7GB/s ± 1%     ~     (p=0.095 n=5+5)
IndexBytePortable/10-12           2.06GB/s ± 1%  2.16GB/s ± 1%   +5.08%  (p=0.008 n=5+5)
IndexBytePortable/32-12           1.40GB/s ± 1%  1.54GB/s ± 1%  +10.05%  (p=0.008 n=5+5)
IndexBytePortable/4K-12           3.99GB/s ± 0%  4.08GB/s ± 0%   +2.16%  (p=0.008 n=5+5)
IndexBytePortable/4M-12           4.05GB/s ± 1%  4.08GB/s ± 2%     ~     (p=0.095 n=5+5)
IndexBytePortable/64M-12          3.80GB/s ± 1%  3.81GB/s ± 0%     ~     (p=0.421 n=5+5)
IndexRune/10-12                    746MB/s ± 1%   752MB/s ± 0%   +0.85%  (p=0.008 n=5+5)
IndexRune/32-12                   2.33GB/s ± 0%  2.42GB/s ± 0%   +3.66%  (p=0.008 n=5+5)
IndexRune/4K-12                   44.4GB/s ± 0%  44.2GB/s ± 0%     ~     (p=0.095 n=5+5)
IndexRune/4M-12                   36.2GB/s ± 1%  36.3GB/s ± 2%     ~     (p=0.841 n=5+5)
IndexRune/64M-12                  16.2GB/s ± 2%  16.3GB/s ± 2%     ~     (p=0.548 n=5+5)
IndexRuneASCII/10-12              2.57GB/s ± 0%  2.58GB/s ± 0%   +0.63%  (p=0.008 n=5+5)
IndexRuneASCII/32-12              6.00GB/s ± 0%  6.30GB/s ± 1%   +4.98%  (p=0.008 n=5+5)
IndexRuneASCII/4K-12              56.7GB/s ± 0%  56.8GB/s ± 1%     ~     (p=0.151 n=5+5)
IndexRuneASCII/4M-12              41.6GB/s ± 1%  41.7GB/s ± 2%     ~     (p=0.151 n=5+5)
IndexRuneASCII/64M-12             17.7GB/s ± 1%  17.6GB/s ± 1%     ~     (p=0.222 n=5+5)
Index/10-12                       1.06GB/s ± 1%  1.06GB/s ± 0%     ~     (p=0.310 n=5+5)
Index/32-12                       3.57GB/s ± 0%  3.56GB/s ± 1%     ~     (p=0.056 n=5+5)
Index/4K-12                       1.02GB/s ± 2%  1.03GB/s ± 0%     ~     (p=0.690 n=5+5)
Index/4M-12                       1.04GB/s ± 0%  1.03GB/s ± 1%     ~     (p=1.000 n=4+5)
Index/64M-12                      1.02GB/s ± 0%  1.02GB/s ± 0%     ~     (p=0.905 n=5+4)
IndexEasy/10-12                   1.12GB/s ± 2%  1.15GB/s ± 1%   +3.10%  (p=0.008 n=5+5)
IndexEasy/32-12                   3.14GB/s ± 2%  3.13GB/s ± 1%     ~     (p=0.310 n=5+5)
IndexEasy/4K-12                   47.6GB/s ± 1%  47.7GB/s ± 2%     ~     (p=0.310 n=5+5)
IndexEasy/4M-12                   36.4GB/s ± 1%  36.3GB/s ± 2%     ~     (p=0.690 n=5+5)
IndexEasy/64M-12                  16.1GB/s ± 1%  16.4GB/s ± 5%     ~     (p=0.151 n=5+5)
[Geo mean]                        6.39GB/s       6.46GB/s        +1.11%

Change-Id: Ic1ca62f5cc719d87e2c4aeff25ad73507facff82
Reviewed-on: https://go-review.googlesource.com/c/go/+/397576
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>