src/internal/bytealg/compare_amd64.s

// Copyright 2018 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

#include "go_asm.h"
#include "asm_amd64.h"
#include "textflag.h"

TEXT ·Compare<ABIInternal>(SB),NOSPLIT,$0-56
	// AX = a_base (want in SI)
	// BX = a_len  (want in BX)
	// CX = a_cap  (unused)
	// DI = b_base (want in DI)
	// SI = b_len  (want in DX)
	// R8 = b_cap  (unused)
	MOVQ	SI, DX
	MOVQ	AX, SI
	JMP	cmpbody<>(SB)

TEXT runtime·cmpstring<ABIInternal>(SB),NOSPLIT,$0-40
	// AX = a_base (want in SI)
	// BX = a_len  (want in BX)
	// CX = b_base (want in DI)
	// DI = b_len  (want in DX)
	MOVQ	AX, SI
	MOVQ	DI, DX
	MOVQ	CX, DI
	JMP	cmpbody<>(SB)

// input:
//   SI = a
//   DI = b
//   BX = alen
//   DX = blen
// output:
//   AX = output (-1/0/1)
TEXT cmpbody<>(SB),NOSPLIT,$0-0
	CMPQ	SI, DI
	JEQ	allsame
	CMPQ	BX, DX
	MOVQ	DX, R8
	CMOVQLT	BX, R8 // R8 = min(alen, blen) = # of bytes to compare
	CMPQ	R8, $8
	JB	small

	CMPQ	R8, $63
	JBE	loop
#ifndef hasAVX2
	CMPB	internal∕cpu·X86+const_offsetX86HasAVX2(SB), $1
	JEQ     big_loop_avx2
	JMP	big_loop
#else
	JMP	big_loop_avx2
#endif
loop:
	CMPQ	R8, $16
	JBE	_0through16
	MOVOU	(SI), X0
	MOVOU	(DI), X1
	PCMPEQB X0, X1
	PMOVMSKB X1, AX
	XORQ	$0xffff, AX	// convert EQ to NE
	JNE	diff16	// branch if at least one byte is not equal
	ADDQ	$16, SI
	ADDQ	$16, DI
	SUBQ	$16, R8
	JMP	loop

diff64:
	ADDQ	$48, SI
	ADDQ	$48, DI
	JMP	diff16
diff48:
	ADDQ	$32, SI
	ADDQ	$32, DI
	JMP	diff16
diff32:
	ADDQ	$16, SI
	ADDQ	$16, DI
	// AX = bit mask of differences
diff16:
	BSFQ	AX, BX	// index of first byte that differs
	XORQ	AX, AX
	MOVB	(SI)(BX*1), CX
	CMPB	CX, (DI)(BX*1)
	SETHI	AX
	LEAQ	-1(AX*2), AX	// convert 1/0 to +1/-1
	RET

	// 0 through 16 bytes left, alen>=8, blen>=8
_0through16:
	CMPQ	R8, $8
	JBE	_0through8
	MOVQ	(SI), AX
	MOVQ	(DI), CX
	CMPQ	AX, CX
	JNE	diff8
_0through8:
	MOVQ	-8(SI)(R8*1), AX
	MOVQ	-8(DI)(R8*1), CX
	CMPQ	AX, CX
	JEQ	allsame

	// AX and CX contain parts of a and b that differ.
diff8:
	BSWAPQ	AX	// reverse order of bytes
	BSWAPQ	CX
	XORQ	AX, CX
	BSRQ	CX, CX	// index of highest bit difference
	SHRQ	CX, AX	// move a's bit to bottom
	ANDQ	$1, AX	// mask bit
	LEAQ	-1(AX*2), AX // 1/0 => +1/-1
	RET

	// 0-7 bytes in common
small:
	LEAQ	(R8*8), CX	// bytes left -> bits left
	NEGQ	CX		//  - bits lift (== 64 - bits left mod 64)
	JEQ	allsame

	// load bytes of a into high bytes of AX
	CMPB	SI, $0xf8
	JA	si_high
	MOVQ	(SI), SI
	JMP	si_finish
si_high:
	MOVQ	-8(SI)(R8*1), SI
	SHRQ	CX, SI
si_finish:
	SHLQ	CX, SI

	// load bytes of b in to high bytes of BX
	CMPB	DI, $0xf8
	JA	di_high
	MOVQ	(DI), DI
	JMP	di_finish
di_high:
	MOVQ	-8(DI)(R8*1), DI
	SHRQ	CX, DI
di_finish:
	SHLQ	CX, DI

	BSWAPQ	SI	// reverse order of bytes
	BSWAPQ	DI
	XORQ	SI, DI	// find bit differences
	JEQ	allsame
	BSRQ	DI, CX	// index of highest bit difference
	SHRQ	CX, SI	// move a's bit to bottom
	ANDQ	$1, SI	// mask bit
	LEAQ	-1(SI*2), AX // 1/0 => +1/-1
	RET

allsame:
	XORQ	AX, AX
	XORQ	CX, CX
	CMPQ	BX, DX
	SETGT	AX	// 1 if alen > blen
	SETEQ	CX	// 1 if alen == blen
	LEAQ	-1(CX)(AX*2), AX	// 1,0,-1 result
	RET

	// this works for >= 64 bytes of data.
#ifndef hasAVX2
big_loop:
	MOVOU	(SI), X0
	MOVOU	(DI), X1
	PCMPEQB X0, X1
	PMOVMSKB X1, AX
	XORQ	$0xffff, AX
	JNE	diff16

	MOVOU	16(SI), X0
	MOVOU	16(DI), X1
	PCMPEQB X0, X1
	PMOVMSKB X1, AX
	XORQ	$0xffff, AX
	JNE	diff32

	MOVOU	32(SI), X0
	MOVOU	32(DI), X1
	PCMPEQB X0, X1
	PMOVMSKB X1, AX
	XORQ	$0xffff, AX
	JNE	diff48

	MOVOU	48(SI), X0
	MOVOU	48(DI), X1
	PCMPEQB X0, X1
	PMOVMSKB X1, AX
	XORQ	$0xffff, AX
	JNE	diff64

	ADDQ	$64, SI
	ADDQ	$64, DI
	SUBQ	$64, R8
	CMPQ	R8, $64
	JBE	loop
	JMP	big_loop
#endif

	// Compare 64-bytes per loop iteration.
	// Loop is unrolled and uses AVX2.
big_loop_avx2:
	VMOVDQU	(SI), Y2
	VMOVDQU	(DI), Y3
	VMOVDQU	32(SI), Y4
	VMOVDQU	32(DI), Y5
	VPCMPEQB Y2, Y3, Y0
	VPMOVMSKB Y0, AX
	XORL	$0xffffffff, AX
	JNE	diff32_avx2
	VPCMPEQB Y4, Y5, Y6
	VPMOVMSKB Y6, AX
	XORL	$0xffffffff, AX
	JNE	diff64_avx2

	ADDQ	$64, SI
	ADDQ	$64, DI
	SUBQ	$64, R8
	CMPQ	R8, $64
	JB	big_loop_avx2_exit
	JMP	big_loop_avx2

	// Avoid AVX->SSE transition penalty and search first 32 bytes of 64 byte chunk.
diff32_avx2:
	VZEROUPPER
	JMP diff16

	// Same as diff32_avx2, but for last 32 bytes.
diff64_avx2:
	VZEROUPPER
	JMP diff48

	// For <64 bytes remainder jump to normal loop.
big_loop_avx2_exit:
	VZEROUPPER
	JMP loop