bytes: add optimized Compare for arm64

Use LDP instructions to load 16 bytes per loop when the source length is long. Specially
process the 8 bytes length, 4 bytes length and 2 bytes length to get a better performance.

Benchmark result:
name                           old time/op   new time/op    delta
BytesCompare/1-8                21.0ns ± 0%    10.5ns ± 0%      ~     (p=0.079 n=4+5)
BytesCompare/2-8                11.5ns ± 0%    10.5ns ± 0%    -8.70%  (p=0.008 n=5+5)
BytesCompare/4-8                13.5ns ± 0%    10.0ns ± 0%   -25.93%  (p=0.008 n=5+5)
BytesCompare/8-8                28.8ns ± 0%     9.5ns ± 0%      ~     (p=0.079 n=4+5)
BytesCompare/16-8               40.5ns ± 0%    10.5ns ± 0%   -74.07%  (p=0.008 n=5+5)
BytesCompare/32-8               64.6ns ± 0%    12.5ns ± 0%   -80.65%  (p=0.008 n=5+5)
BytesCompare/64-8                112ns ± 0%      16ns ± 0%   -85.27%  (p=0.008 n=5+5)
BytesCompare/128-8               208ns ± 0%      24ns ± 0%   -88.22%  (p=0.008 n=5+5)
BytesCompare/256-8               400ns ± 0%      50ns ± 0%   -87.62%  (p=0.008 n=5+5)
BytesCompare/512-8               785ns ± 0%      82ns ± 0%   -89.61%  (p=0.008 n=5+5)
BytesCompare/1024-8             1.55µs ± 0%    0.14µs ± 0%      ~     (p=0.079 n=4+5)
BytesCompare/2048-8             3.09µs ± 0%    0.27µs ± 0%      ~     (p=0.079 n=4+5)
CompareBytesEqual-8             39.0ns ± 0%    12.0ns ± 0%   -69.23%  (p=0.008 n=5+5)
CompareBytesToNil-8             8.57ns ± 5%    8.23ns ± 2%    -3.99%  (p=0.016 n=5+5)
CompareBytesEmpty-8             7.37ns ± 0%    7.36ns ± 4%      ~     (p=0.690 n=5+5)
CompareBytesIdentical-8         7.39ns ± 0%    7.46ns ± 2%      ~     (p=0.667 n=5+5)
CompareBytesSameLength-8        17.0ns ± 0%    10.5ns ± 0%   -38.24%  (p=0.008 n=5+5)
CompareBytesDifferentLength-8   17.0ns ± 0%    10.5ns ± 0%   -38.24%  (p=0.008 n=5+5)
CompareBytesBigUnaligned-8      1.58ms ± 0%    0.19ms ± 0%   -88.31%  (p=0.016 n=4+5)
CompareBytesBig-8               1.59ms ± 0%    0.19ms ± 0%   -88.27%  (p=0.016 n=5+4)
CompareBytesBigIdentical-8      7.01ns ± 0%    6.60ns ± 3%    -5.91%  (p=0.008 n=5+5)

name                           old speed     new speed      delta
CompareBytesBigUnaligned-8     662MB/s ± 0%  5660MB/s ± 0%  +755.15%  (p=0.016 n=4+5)
CompareBytesBig-8              661MB/s ± 0%  5636MB/s ± 0%  +752.57%  (p=0.016 n=5+4)
CompareBytesBigIdentical-8     150TB/s ± 0%   159TB/s ± 3%    +6.27%  (p=0.008 n=5+5)

Change-Id: I70332de06f873df3bc12c4a5af1028307b670046
Reviewed-on: https://go-review.googlesource.com/90175Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/internal/bytealg/compare_arm64.s

@@ -10,7 +10,7 @@ TEXT ·Compare(SB),NOSPLIT|NOFRAME,$0-56
 	MOVD	a_len+8(FP), R0
 	MOVD	b_base+24(FP), R3
 	MOVD	b_len+32(FP), R1
-	ADD	$56, RSP, R7
+	MOVD	$ret+48(FP), R7
 	B	cmpbody<>(SB)
 
 TEXT bytes·Compare(SB),NOSPLIT|NOFRAME,$0-56
@@ -18,7 +18,7 @@ TEXT bytes·Compare(SB),NOSPLIT|NOFRAME,$0-56
 	MOVD	a_len+8(FP), R0
 	MOVD	b_base+24(FP), R3
 	MOVD	b_len+32(FP), R1
-	ADD	$56, RSP, R7
+	MOVD	$ret+48(FP), R7
 	B	cmpbody<>(SB)
 
 TEXT runtime·cmpstring(SB),NOSPLIT|NOFRAME,$0-40
@@ -26,7 +26,7 @@ TEXT runtime·cmpstring(SB),NOSPLIT|NOFRAME,$0-40
 	MOVD	a_len+8(FP), R0
 	MOVD	b_base+16(FP), R3
 	MOVD	b_len+24(FP), R1
-	ADD	$40, RSP, R7
+	MOVD	$ret+32(FP), R7
 	B	cmpbody<>(SB)
 
 // On entry:
@@ -37,30 +37,98 @@ TEXT runtime·cmpstring(SB),NOSPLIT|NOFRAME,$0-40
 // R7 points to return value (-1/0/1 will be written here)
 //
 // On exit:
-// R4, R5, and R6 are clobbered
+// R4, R5, R6, R8, R9 and R10 are clobbered
 TEXT cmpbody<>(SB),NOSPLIT|NOFRAME,$0-0
 	CMP	R2, R3
-	BEQ	samebytes // same starting pointers; compare lengths
+	BEQ	samebytes         // same starting pointers; compare lengths
 	CMP	R0, R1
-	CSEL    LT, R1, R0, R6 // R6 is min(R0, R1)
+	CSEL	LT, R1, R0, R6    // R6 is min(R0, R1)
 
-	ADD	R2, R6	// R2 is current byte in a, R6 is last byte in a to compare
-loop:
-	CMP	R2, R6
-	BEQ	samebytes // all compared bytes were the same; compare lengths
-	MOVBU.P	1(R2), R4
-	MOVBU.P	1(R3), R5
+	CMP	$0, R6
+	BEQ	samebytes
+	BIC	$0xf, R6, R10
+	CBZ	R10, small        // length < 16
+	ADD	R2, R10           // end of chunk16
+	// length >= 16
+chunk16_loop:
+	LDP.P	16(R2), (R4, R8)
+	LDP.P	16(R3), (R5, R9)
 	CMP	R4, R5
-	BEQ	loop
-	// bytes differed
+	BNE	cmp
+	CMP	R8, R9
+	BNE	cmpnext
+	CMP	R10, R2
+	BNE	chunk16_loop
+	AND	$0xf, R6, R6
+	CBZ	R6, samebytes
+	SUBS	$8, R6
+	BLT	tail
+	// the length of tail > 8 bytes
+	MOVD.P	8(R2), R4
+	MOVD.P	8(R3), R5
+	CMP	R4, R5
+	BNE	cmp
+	SUB	$8, R6
+	// compare last 8 bytes
+tail:
+	MOVD	(R2)(R6), R4
+	MOVD	(R3)(R6), R5
+	CMP	R4, R5
+	BEQ	samebytes
+cmp:
+	REV	R4, R4
+	REV	R5, R5
+	CMP	R4, R5
+ret:
 	MOVD	$1, R4
-	CSNEG	LT, R4, R4, R4
+	CNEG	HI, R4, R4
 	MOVD	R4, (R7)
 	RET
+small:
+	TBZ	$3, R6, lt_8
+	MOVD	(R2), R4
+	MOVD	(R3), R5
+	CMP	R4, R5
+	BNE	cmp
+	SUBS	$8, R6
+	BEQ	samebytes
+	ADD	$8, R2
+	ADD	$8, R3
+	SUB	$8, R6
+	B	tail
+lt_8:
+	TBZ	$2, R6, lt_4
+	MOVWU	(R2), R4
+	MOVWU	(R3), R5
+	CMPW	R4, R5
+	BNE	cmp
+	SUBS	$4, R6
+	BEQ	samebytes
+	ADD	$4, R2
+	ADD	$4, R3
+lt_4:
+	TBZ	$1, R6, lt_2
+	MOVHU	(R2), R4
+	MOVHU	(R3), R5
+	CMPW	R4, R5
+	BNE	cmp
+	ADD	$2, R2
+	ADD	$2, R3
+lt_2:
+	TBZ	$0, R6, samebytes
+one:
+	MOVBU	(R2), R4
+	MOVBU	(R3), R5
+	CMPW	R4, R5
+	BNE	ret
 samebytes:
-	MOVD	$1, R4
-	CMP	R0, R1
-	CSNEG	LT, R4, R4, R4
-	CSEL	EQ, ZR, R4, R4
+	CMP	R1, R0
+	CSET	NE, R4
+	CNEG	LO, R4, R4
 	MOVD	R4, (R7)
 	RET
+cmpnext:
+	REV	R8, R4
+	REV	R9, R5
+	CMP	R4, R5
+	B	ret