runtime: use ERMS in memclr_amd64

runtime: use ERMS in memclr_amd64

This patch adds support for REP STOSB in memclr(). The current
implementation uses REP STOSB when 1) ERMS is supported
2) size is bigger than 2kb and less than 32mb.

The threshold of 2kb is chosen based on benchmark results and is
close to what Intel mentioned in their comparison of ERMSB and AVX
(Table 3-4. Relative Performance of Memcpy() Using ERMSB Vs. 128-bit
AVX in the Intel Optimization Guide).

While REP STOS uses a no-RFO write protocol, ERMS could show the
same or slower performance comparing to Non-Temporal Stores when the
size is bigger than LLC depending on hardware.

Benchmarks (including MemclrRange from CL373362)
goos: darwin
goarch: amd64
pkg: runtime
cpu: Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
name                           old time/op    new time/op    delta
Memclr/5-12                      1.90ns ± 2%    2.13ns ± 2%  +11.72%  (p=0.001 n=7+7)
Memclr/16-12                     2.33ns ± 4%    2.36ns ± 4%     ~     (p=0.259 n=7+7)
Memclr/64-12                     2.58ns ± 2%    2.61ns ± 3%     ~     (p=0.091 n=7+7)
Memclr/256-12                    4.89ns ± 4%    4.94ns ± 3%     ~     (p=0.620 n=7+7)
Memclr/4096-12                   38.4ns ± 2%    39.5ns ± 5%     ~     (p=0.078 n=7+7)
Memclr/65536-12                   929ns ± 2%    1040ns ±19%     ~     (p=0.268 n=5+7)
Memclr/1M-12                     24.2µs ± 5%    19.0µs ± 9%  -21.62%  (p=0.001 n=7+7)
Memclr/4M-12                     93.3µs ± 3%    73.2µs ± 4%  -21.50%  (p=0.001 n=7+7)
Memclr/8M-12                      209µs ± 6%     164µs ± 3%  -21.55%  (p=0.001 n=7+7)
Memclr/16M-12                     731µs ± 4%     507µs ± 6%  -30.71%  (p=0.001 n=7+7)
Memclr/64M-12                    1.79ms ± 1%    1.83ms ± 3%   +2.47%  (p=0.041 n=6+6)
MemclrRange/1_2_47K-12            873ns ± 3%     899ns ± 5%     ~     (p=0.053 n=7+7)
MemclrRange/2_8_166K-12          2.98µs ± 4%    2.90µs ± 5%     ~     (p=0.165 n=7+7)
MemclrRange/4_16_315K-12         6.81µs ± 4%    5.31µs ± 9%  -22.01%  (p=0.001 n=7+7)
MemclrRange/128_256_1623K-12     37.5µs ± 4%    28.1µs ± 4%  -25.19%  (p=0.001 n=7+6)
[Geo mean]                       1.56µs         1.43µs        -8.43%

name                           old speed      new speed      delta
Memclr/5-12                    2.63GB/s ± 2%  2.35GB/s ± 2%  -10.50%  (p=0.001 n=7+7)
Memclr/16-12                   6.86GB/s ± 4%  6.79GB/s ± 4%     ~     (p=0.259 n=7+7)
Memclr/64-12                   24.8GB/s ± 2%  24.5GB/s ± 3%     ~     (p=0.097 n=7+7)
Memclr/256-12                  52.4GB/s ± 4%  51.9GB/s ± 3%     ~     (p=0.620 n=7+7)
Memclr/4096-12                  107GB/s ± 2%   104GB/s ± 5%     ~     (p=0.073 n=7+7)
Memclr/65536-12                70.6GB/s ± 2%  64.2GB/s ±21%     ~     (p=0.268 n=5+7)
Memclr/1M-12                   43.4GB/s ± 5%  55.5GB/s ±10%  +28.04%  (p=0.001 n=7+7)
Memclr/4M-12                   45.0GB/s ± 4%  57.3GB/s ± 4%  +27.38%  (p=0.001 n=7+7)
Memclr/8M-12                   40.1GB/s ± 5%  51.1GB/s ± 3%  +27.37%  (p=0.001 n=7+7)
Memclr/16M-12                  23.0GB/s ± 4%  33.1GB/s ± 6%  +44.39%  (p=0.001 n=7+7)
Memclr/64M-12                  37.6GB/s ± 1%  36.7GB/s ± 3%   -2.38%  (p=0.041 n=6+6)
MemclrRange/1_2_47K-12         55.9GB/s ± 3%  54.3GB/s ± 5%     ~     (p=0.053 n=7+7)
MemclrRange/2_8_166K-12        57.4GB/s ± 5%  58.9GB/s ± 5%     ~     (p=0.165 n=7+7)
MemclrRange/4_16_315K-12       47.4GB/s ± 4%  60.9GB/s ± 9%  +28.40%  (p=0.001 n=7+7)
MemclrRange/128_256_1623K-12   44.3GB/s ± 4%  58.4GB/s ± 9%  +31.73%  (p=0.001 n=7+7)
[Geo mean]                     33.6GB/s       36.8GB/s        +9.27%

goos: linux
goarch: amd64
pkg: runtime
cpu: Intel(R) Xeon(R) Gold 6230N CPU @ 2.30GHz
name                     old time/op    new time/op     delta
Memclr/5-2                 2.53ns ± 0%     2.52ns ± 0%   -0.25%  (p=0.001 n=7+7)
Memclr/16-2                2.77ns ± 0%     2.55ns ± 0%   -7.97%  (p=0.000 n=5+7)
Memclr/64-2                3.16ns ± 0%     3.16ns ± 0%     ~     (p=0.432 n=7+7)
Memclr/256-2               7.26ns ± 0%     7.26ns ± 0%     ~     (p=0.220 n=7+7)
Memclr/4096-2              49.3ns ± 0%     43.5ns ± 0%  -11.80%  (p=0.001 n=7+7)
Memclr/65536-2             1.32µs ± 1%     1.24µs ± 0%   -6.31%  (p=0.001 n=7+7)
Memclr/1M-2                27.3µs ± 0%     26.6µs ± 5%     ~     (p=0.195 n=7+7)
Memclr/4M-2                 195µs ± 0%      148µs ± 4%  -24.22%  (p=0.001 n=7+7)
Memclr/8M-2                 391µs ± 0%      308µs ± 0%  -21.09%  (p=0.001 n=7+6)
Memclr/16M-2                782µs ± 0%      639µs ± 1%  -18.31%  (p=0.001 n=7+7)
Memclr/64M-2               2.83ms ± 1%     2.84ms ± 1%     ~     (p=0.620 n=7+7)
MemclrRange/1K_2K-2        1.24µs ± 0%     1.24µs ± 0%     ~     (p=1.000 n=7+6)
MemclrRange/2K_8K-2        3.89µs ± 0%     3.11µs ± 0%  -20.00%  (p=0.001 n=6+7)
MemclrRange/4K_16K-2       3.63µs ± 0%     2.37µs ± 0%  -34.61%  (p=0.001 n=7+7)
MemclrRange/160K_228K-2    31.0µs ± 0%     30.6µs ± 1%   -1.50%  (p=0.001 n=7+7)
[Geo mean]                 1.97µs          1.76µs       -10.59%

name                     old speed      new speed       delta
Memclr/5-2               1.98GB/s ± 0%   1.98GB/s ± 0%   +0.27%  (p=0.001 n=7+7)
Memclr/16-2              5.78GB/s ± 0%   6.28GB/s ± 0%   +8.67%  (p=0.001 n=7+7)
Memclr/64-2              20.2GB/s ± 0%   20.3GB/s ± 0%     ~     (p=0.535 n=7+7)
Memclr/256-2             35.3GB/s ± 0%   35.2GB/s ± 0%     ~     (p=0.259 n=7+7)
Memclr/4096-2            83.1GB/s ± 0%   94.2GB/s ± 0%  +13.39%  (p=0.001 n=7+7)
Memclr/65536-2           49.7GB/s ± 1%   53.0GB/s ± 0%   +6.73%  (p=0.001 n=7+7)
Memclr/1M-2              38.4GB/s ± 0%   39.4GB/s ± 4%     ~     (p=0.209 n=7+7)
Memclr/4M-2              21.5GB/s ± 0%   28.4GB/s ± 4%  +32.02%  (p=0.001 n=7+7)
Memclr/8M-2              21.5GB/s ± 0%   27.2GB/s ± 0%  +26.73%  (p=0.001 n=7+6)
Memclr/16M-2             21.4GB/s ± 0%   26.2GB/s ± 1%  +22.42%  (p=0.001 n=7+7)
Memclr/64M-2             23.7GB/s ± 1%   23.7GB/s ± 1%     ~     (p=0.620 n=7+7)
MemclrRange/1K_2K-2      77.3GB/s ± 0%   77.3GB/s ± 0%     ~     (p=0.710 n=7+7)
MemclrRange/2K_8K-2      85.7GB/s ± 0%  107.1GB/s ± 0%  +25.00%  (p=0.001 n=6+7)
MemclrRange/4K_16K-2     89.0GB/s ± 0%  136.1GB/s ± 0%  +52.92%  (p=0.001 n=7+7)
MemclrRange/160K_228K-2  53.6GB/s ± 0%   54.4GB/s ± 1%   +1.52%  (p=0.001 n=7+7)
[Geo mean]               29.2GB/s        32.7GB/s       +11.86%

Change-Id: I8f3533f88ebd303ae1666a77391fec304bea9724
Reviewed-on: https://go-review.googlesource.com/c/go/+/374396
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
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