internal/bytealg: optimize Count with PCALIGN in riscv64

For #63678

Benchmark on Milk-V Mars CM eMMC (Starfive/JH7110 SoC)

goos: linux
goarch: riscv64
pkg: bytes
                │ /root/bytes.old.bench │        /root/bytes.pc16.bench         │
                │        sec/op         │   sec/op     vs base                  │
Count/10                    223.9n ± 1%   220.8n ± 1%   -1.36% (p=0.001 n=10)
Count/32                    571.6n ± 0%   571.3n ± 0%        ~ (p=0.054 n=10)
Count/4K                    38.56µ ± 0%   38.55µ ± 0%   -0.01% (p=0.010 n=10)
Count/4M                    40.13m ± 0%   39.21m ± 0%   -2.28% (p=0.000 n=10)
Count/64M                   627.5m ± 0%   627.4m ± 0%   -0.01% (p=0.019 n=10)
CountEasy/10                101.3n ± 0%   101.3n ± 0%        ~ (p=1.000 n=10) ¹
CountEasy/32                139.3n ± 0%   139.3n ± 0%        ~ (p=1.000 n=10) ¹
CountEasy/4K                5.565µ ± 0%   5.564µ ± 0%   -0.02% (p=0.001 n=10)
CountEasy/4M                5.619m ± 0%   5.619m ± 0%        ~ (p=0.190 n=10)
CountEasy/64M               89.94m ± 0%   89.93m ± 0%        ~ (p=0.436 n=10)
CountSingle/10              53.80n ± 0%   46.06n ± 0%  -14.39% (p=0.000 n=10)
CountSingle/32             104.30n ± 0%   79.64n ± 0%  -23.64% (p=0.000 n=10)
CountSingle/4K             10.413µ ± 0%   7.247µ ± 0%  -30.40% (p=0.000 n=10)
CountSingle/4M             11.603m ± 0%   8.388m ± 0%  -27.71% (p=0.000 n=10)
CountSingle/64M             230.9m ± 0%   172.3m ± 0%  -25.40% (p=0.000 n=10)
CountHard1                  9.981m ± 0%   9.981m ± 0%        ~ (p=0.810 n=10)
CountHard2                  9.981m ± 0%   9.981m ± 0%        ~ (p=0.315 n=10)
CountHard3                  9.981m ± 0%   9.981m ± 0%        ~ (p=0.159 n=10)
geomean                     144.6µ        133.5µ        -7.70%
¹ all samples are equal

                │ /root/bytes.old.bench │        /root/bytes.pc16.bench         │
                │          B/s          │      B/s       vs base                │
Count/10                   42.60Mi ± 1%    43.19Mi ± 1%   +1.39% (p=0.001 n=10)
Count/32                   53.38Mi ± 0%    53.42Mi ± 0%   +0.06% (p=0.049 n=10)
Count/4K                   101.3Mi ± 0%    101.3Mi ± 0%        ~ (p=0.077 n=10)
Count/4M                   99.68Mi ± 0%   102.01Mi ± 0%   +2.34% (p=0.000 n=10)
Count/64M                  102.0Mi ± 0%    102.0Mi ± 0%        ~ (p=0.076 n=10)
CountEasy/10               94.18Mi ± 0%    94.18Mi ± 0%        ~ (p=0.054 n=10)
CountEasy/32               219.1Mi ± 0%    219.1Mi ± 0%   +0.01% (p=0.016 n=10)
CountEasy/4K               702.0Mi ± 0%    702.0Mi ± 0%   +0.00% (p=0.000 n=10)
CountEasy/4M               711.9Mi ± 0%    711.9Mi ± 0%        ~ (p=0.133 n=10)
CountEasy/64M              711.6Mi ± 0%    711.7Mi ± 0%        ~ (p=0.447 n=10)
CountSingle/10             177.2Mi ± 0%    207.0Mi ± 0%  +16.81% (p=0.000 n=10)
CountSingle/32             292.7Mi ± 0%    383.2Mi ± 0%  +30.91% (p=0.000 n=10)
CountSingle/4K             375.1Mi ± 0%    539.0Mi ± 0%  +43.70% (p=0.000 n=10)
CountSingle/4M             344.7Mi ± 0%    476.9Mi ± 0%  +38.33% (p=0.000 n=10)
CountSingle/64M            277.2Mi ± 0%    371.5Mi ± 0%  +34.05% (p=0.000 n=10)
geomean                    199.7Mi         219.8Mi       +10.10%

Change-Id: I1abf6b220b9802028f8ad5eebc8d3b7cfa3e89ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/541756
Reviewed-by: David Chase <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: Joel Sing <[email protected]>
Run-TryBot: M Zhuo <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Wang Yaduo <[email protected]>
Reviewed-by: Mark Ryan <[email protected]>

src/internal/bytealg/count_riscv64.s

@@ -14,6 +14,7 @@ TEXT ·Count<ABIInternal>(SB),NOSPLIT,$0-40
 	MOV	ZERO, X14	// count
 	ADD	X10, X11	// end
 
+	PCALIGN	$16
 loop:
 	BEQ	X10, X11, done
 	MOVBU	(X10), X15
@@ -34,6 +35,7 @@ TEXT ·CountString<ABIInternal>(SB),NOSPLIT,$0-32
 	MOV	ZERO, X14	// count
 	ADD	X10, X11	// end
 
+	PCALIGN	$16
 loop:
 	BEQ	X10, X11, done
 	MOVBU	(X10), X15