internal/bytealg: correct alignment checks for compare/memequal on ri…

…scv64

On riscv64 we need 8 byte alignment for 8 byte loads - the existing check
was only ensuring 4 byte alignment, which potentially results in unaligned
loads being performed. Unaligned loads incur a significant performance penality
due to the resulting kernel traps and fix ups.

Adjust BenchmarkCompareBytesBigUnaligned so that this issue would have been
more readily visible.

Updates #50615

name                                 old time/op    new time/op      delta
CompareBytesBigUnaligned/offset=1-4    6.98ms _ 5%      6.84ms _ 3%       ~     (p=0.319 n=5+5)
CompareBytesBigUnaligned/offset=2-4    6.75ms _ 1%      6.99ms _ 4%       ~     (p=0.063 n=5+5)
CompareBytesBigUnaligned/offset=3-4    6.84ms _ 1%      6.74ms _ 1%     -1.48%  (p=0.003 n=5+5)
CompareBytesBigUnaligned/offset=4-4     146ms _ 1%         7ms _ 6%    -95.08%  (p=0.000 n=5+5)
CompareBytesBigUnaligned/offset=5-4    7.05ms _ 5%      6.75ms _ 1%       ~     (p=0.079 n=5+5)
CompareBytesBigUnaligned/offset=6-4    7.11ms _ 5%      6.89ms _ 5%       ~     (p=0.177 n=5+5)
CompareBytesBigUnaligned/offset=7-4    7.14ms _ 5%      6.91ms _ 6%       ~     (p=0.165 n=5+5)

name                                 old speed      new speed        delta
CompareBytesBigUnaligned/offset=1-4   150MB/s _ 5%     153MB/s _ 3%       ~     (p=0.336 n=5+5)
CompareBytesBigUnaligned/offset=2-4   155MB/s _ 1%     150MB/s _ 4%       ~     (p=0.058 n=5+5)
CompareBytesBigUnaligned/offset=3-4   153MB/s _ 1%     156MB/s _ 1%     +1.51%  (p=0.004 n=5+5)
CompareBytesBigUnaligned/offset=4-4  7.16MB/s _ 1%  145.79MB/s _ 6%  +1936.23%  (p=0.000 n=5+5)
CompareBytesBigUnaligned/offset=5-4   149MB/s _ 5%     155MB/s _ 1%       ~     (p=0.078 n=5+5)
CompareBytesBigUnaligned/offset=6-4   148MB/s _ 5%     152MB/s _ 5%       ~     (p=0.175 n=5+5)
CompareBytesBigUnaligned/offset=7-4   147MB/s _ 5%     152MB/s _ 6%       ~     (p=0.160 n=5+5)

Change-Id: I2c859e061919db482318ce63b85b808aa973a9ba
Reviewed-on: https://go-review.googlesource.com/c/go/+/431099
Reviewed-by: Meng Zhuo <mzh@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/bytes/compare_test.go

@@ -6,6 +6,7 @@ package bytes_test
 
 import (
 	. "bytes"
+	"fmt"
 	"internal/testenv"
 	"testing"
 )
@@ -213,22 +214,30 @@ func BenchmarkCompareBytesDifferentLength(b *testing.B) {
 	}
 }
 
-func BenchmarkCompareBytesBigUnaligned(b *testing.B) {
+func benchmarkCompareBytesBigUnaligned(b *testing.B, offset int) {
 	b.StopTimer()
 	b1 := make([]byte, 0, 1<<20)
 	for len(b1) < 1<<20 {
 		b1 = append(b1, "Hello Gophers!"...)
 	}
-	b2 := append([]byte("hello"), b1...)
+	b2 := append([]byte("12345678")[:offset], b1...)
 	b.StartTimer()
-	for i := 0; i < b.N; i++ {
-		if Compare(b1, b2[len("hello"):]) != 0 {
+	for j := 0; j < b.N; j++ {
+		if Compare(b1, b2[offset:]) != 0 {
 			b.Fatal("b1 != b2")
 		}
 	}
 	b.SetBytes(int64(len(b1)))
 }
 
+func BenchmarkCompareBytesBigUnaligned(b *testing.B) {
+	for i := 1; i < 8; i++ {
+		b.Run(fmt.Sprintf("offset=%d", i), func(b *testing.B) {
+			benchmarkCompareBytesBigUnaligned(b, i)
+		})
+	}
+}
+
 func BenchmarkCompareBytesBig(b *testing.B) {
 	b.StopTimer()
 	b1 := make([]byte, 0, 1<<20)

src/internal/bytealg/compare_riscv64.s

@@ -58,8 +58,8 @@ use_a_len:
 	BLT	X5, X6, loop4_check
 
 	// Check alignment - if alignment differs we have to do one byte at a time.
-	AND	$3, X10, X7
-	AND	$3, X12, X8
+	AND	$7, X10, X7
+	AND	$7, X12, X8
 	BNE	X7, X8, loop4_check
 	BEQZ	X7, loop32_check
 

src/internal/bytealg/equal_riscv64.s

@@ -42,8 +42,8 @@ TEXT memequal<>(SB),NOSPLIT|NOFRAME,$0
 	BLT	X12, X23, loop4_check
 
 	// Check alignment - if alignment differs we have to do one byte at a time.
-	AND	$3, X10, X9
-	AND	$3, X11, X19
+	AND	$7, X10, X9
+	AND	$7, X11, X19
 	BNE	X9, X19, loop4_check
 	BEQZ	X9, loop32_check