runtime: optimise memclrNoHeapPointers on riscv64

runtime: optimise memclrNoHeapPointers on riscv64

Implement a more optimised memclrNoHeapPointers on riscv64, where up
to 64 bytes are zeroed per loop after achieving alignment.

name                     old time/op    new time/op     delta
Memclr/5-4                 53.1ns _ 0%     37.0ns _ 4%   -30.37%  (p=0.002 n=3+3)
Memclr/16-4                42.3ns _ 0%     34.8ns _ 0%   -17.81%  (p=0.000 n=3+3)
Memclr/64-4                67.8ns _ 0%     44.0ns _ 1%   -35.09%  (p=0.000 n=3+3)
Memclr/256-4                185ns _ 0%       78ns _ 1%   -57.52%  (p=0.000 n=3+3)
Memclr/4096-4              2.50_s _ 1%     0.76_s _ 1%   -69.72%  (p=0.000 n=3+3)
Memclr/65536-4             55.6_s _ 0%     31.4_s _ 0%   -43.56%  (p=0.000 n=3+3)
Memclr/1M-4                1.15ms _ 0%     0.72ms _ 1%   -37.14%  (p=0.000 n=3+3)
Memclr/4M-4                12.1ms _ 3%     10.1ms _ 0%   -16.51%  (p=0.007 n=3+3)
Memclr/8M-4                25.2ms _ 1%     21.5ms _ 0%   -14.75%  (p=0.001 n=3+3)
Memclr/16M-4               52.8ms _ 5%     43.4ms _ 0%   -17.95%  (p=0.019 n=3+3)
Memclr/64M-4                202ms _ 1%      173ms _ 0%   -14.42%  (p=0.001 n=3+3)
MemclrRange/1K_2K-4        54.0_s _ 1%     22.0_s _ 0%   -59.26%  (p=0.000 n=3+3)
MemclrRange/2K_8K-4         176_s _ 1%       64_s _ 1%   -63.77%  (p=0.000 n=3+3)
MemclrRange/4K_16K-4        172_s _ 6%       60_s _ 0%   -65.38%  (p=0.002 n=3+3)
MemclrRange/160K_228K-4    1.54ms _ 1%     0.91ms _ 0%   -40.60%  (p=0.000 n=3+3)

name                     old speed      new speed       delta
Memclr/5-4               94.1MB/s _ 0%  135.3MB/s _ 4%   +43.70%  (p=0.004 n=3+3)
Memclr/16-4               378MB/s _ 0%    460MB/s _ 0%   +21.67%  (p=0.000 n=3+3)
Memclr/64-4               943MB/s _ 0%   1454MB/s _ 1%   +54.07%  (p=0.000 n=3+3)
Memclr/256-4             1.39GB/s _ 0%   3.26GB/s _ 1%  +135.38%  (p=0.000 n=3+3)
Memclr/4096-4            1.64GB/s _ 1%   5.41GB/s _ 1%  +230.28%  (p=0.000 n=3+3)
Memclr/65536-4           1.18GB/s _ 0%   2.09GB/s _ 0%   +77.19%  (p=0.000 n=3+3)
Memclr/1M-4               914MB/s _ 0%   1454MB/s _ 1%   +59.08%  (p=0.000 n=3+3)
Memclr/4M-4               346MB/s _ 3%    415MB/s _ 0%   +19.73%  (p=0.004 n=3+3)
Memclr/8M-4               332MB/s _ 1%    390MB/s _ 0%   +17.29%  (p=0.000 n=3+3)
Memclr/16M-4              318MB/s _ 5%    387MB/s _ 0%   +21.72%  (p=0.013 n=3+3)
Memclr/64M-4              333MB/s _ 1%    389MB/s _ 0%   +16.84%  (p=0.001 n=3+3)
MemclrRange/1K_2K-4      1.78GB/s _ 1%   4.36GB/s _ 0%  +145.48%  (p=0.000 n=3+3)
MemclrRange/2K_8K-4      1.89GB/s _ 1%   5.22GB/s _ 1%  +176.03%  (p=0.000 n=3+3)
MemclrRange/4K_16K-4     1.88GB/s _ 5%   5.42GB/s _ 0%  +188.40%  (p=0.000 n=3+3)
MemclrRange/160K_228K-4  1.08GB/s _ 1%   1.82GB/s _ 0%   +68.35%  (p=0.000 n=3+3)

Change-Id: Ide566f148f890f70217ed08fd472ee5893d3511c
Reviewed-on: https://go-review.googlesource.com/c/go/+/426255
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>