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Editing: memclr_amd64.s

// Copyright 2014 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.

//go:build !plan9

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

// See memclrNoHeapPointers Go doc for important implementation constraints.

// func memclrNoHeapPointers(ptr unsafe.Pointer, n uintptr)
// ABIInternal for performance.
TEXT runtime·memclrNoHeapPointers<ABIInternal>(SB), NOSPLIT, $0-16
	// AX = ptr
	// BX = n
	MOVQ	AX, DI	// DI = ptr
	XORQ	AX, AX

// MOVOU seems always faster than REP STOSQ when Enhanced REP STOSQ is not available.
tail:
	// BSR+branch table make almost all memmove/memclr benchmarks worse. Not worth doing.
	TESTQ	BX, BX
	JEQ	_0
	CMPQ	BX, $2
	JBE	_1or2
	CMPQ	BX, $4
	JBE	_3or4
	CMPQ	BX, $8
	JB	_5through7
	JE	_8
	CMPQ	BX, $16
	JBE	_9through16
	CMPQ	BX, $32
	JBE	_17through32
	CMPQ	BX, $64
	JBE	_33through64
	CMPQ	BX, $128
	JBE	_65through128
	CMPQ	BX, $256
	JBE	_129through256

CMPB	internal∕cpu·X86+const_offsetX86HasERMS(SB), $1 // enhanced REP MOVSB/STOSB
	JNE	skip_erms

// If the size is less than 2kb, do not use ERMS as it has a big start-up cost.
	// Table 3-4. Relative Performance of Memcpy() Using ERMSB Vs. 128-bit AVX
	// in the Intel Optimization Guide shows better performance for ERMSB starting
	// from 2KB. Benchmarks show the similar threshold for REP STOS vs AVX.
	CMPQ    BX, $2048
	JAE	loop_preheader_erms

skip_erms:
#ifndef hasAVX2
	CMPB	internal∕cpu·X86+const_offsetX86HasAVX2(SB), $1
	JE	loop_preheader_avx2
	// TODO: for really big clears, use MOVNTDQ, even without AVX2.

loop:
	MOVOU	X15, 0(DI)
	MOVOU	X15, 16(DI)
	MOVOU	X15, 32(DI)
	MOVOU	X15, 48(DI)
	MOVOU	X15, 64(DI)
	MOVOU	X15, 80(DI)
	MOVOU	X15, 96(DI)
	MOVOU	X15, 112(DI)
	MOVOU	X15, 128(DI)
	MOVOU	X15, 144(DI)
	MOVOU	X15, 160(DI)
	MOVOU	X15, 176(DI)
	MOVOU	X15, 192(DI)
	MOVOU	X15, 208(DI)
	MOVOU	X15, 224(DI)
	MOVOU	X15, 240(DI)
	SUBQ	$256, BX
	ADDQ	$256, DI
	CMPQ	BX, $256
	JAE	loop
	JMP	tail
#endif

loop_preheader_avx2:
	VPXOR X0, X0, X0
	// For smaller sizes MOVNTDQ may be faster or slower depending on hardware.
	// For larger sizes it is always faster, even on dual Xeons with 30M cache.
	// TODO take into account actual LLC size. E. g. glibc uses LLC size/2.
	CMPQ    BX, $0x2000000
	JAE	loop_preheader_avx2_huge

loop_avx2:
	VMOVDQU	Y0, 0(DI)
	VMOVDQU	Y0, 32(DI)
	VMOVDQU	Y0, 64(DI)
	VMOVDQU	Y0, 96(DI)
	SUBQ	$128, BX
	ADDQ	$128, DI
	CMPQ	BX, $128
	JAE	loop_avx2
	VMOVDQU  Y0, -32(DI)(BX*1)
	VMOVDQU  Y0, -64(DI)(BX*1)
	VMOVDQU  Y0, -96(DI)(BX*1)
	VMOVDQU  Y0, -128(DI)(BX*1)
	VZEROUPPER
	RET

loop_preheader_erms:
#ifndef hasAVX2
	CMPB	internal∕cpu·X86+const_offsetX86HasAVX2(SB), $1
	JNE	loop_erms
#endif

VPXOR X0, X0, X0
	// At this point both ERMS and AVX2 is supported. While REP STOS can use a no-RFO
	// write protocol, ERMS could show the same or slower performance comparing to
	// Non-Temporal Stores when the size is bigger than LLC depending on hardware.
	CMPQ	BX, $0x2000000
	JAE	loop_preheader_avx2_huge

loop_erms:
	// STOSQ is used to guarantee that the whole zeroed pointer-sized word is visible
	// for a memory subsystem as the GC requires this.
	MOVQ	BX, CX
	SHRQ	$3, CX
	ANDQ	$7, BX
	REP;	STOSQ
	JMP	tail

loop_preheader_avx2_huge:
	// Align to 32 byte boundary
	VMOVDQU  Y0, 0(DI)
	MOVQ	DI, SI
	ADDQ	$32, DI
	ANDQ	$~31, DI
	SUBQ	DI, SI
	ADDQ	SI, BX
loop_avx2_huge:
	VMOVNTDQ	Y0, 0(DI)
	VMOVNTDQ	Y0, 32(DI)
	VMOVNTDQ	Y0, 64(DI)
	VMOVNTDQ	Y0, 96(DI)
	SUBQ	$128, BX
	ADDQ	$128, DI
	CMPQ	BX, $128
	JAE	loop_avx2_huge
	// In the description of MOVNTDQ in [1]
	// "... fencing operation implemented with the SFENCE or MFENCE instruction
	// should be used in conjunction with MOVNTDQ instructions..."
	// [1] 64-ia-32-architectures-software-developer-manual-325462.pdf
	SFENCE
	VMOVDQU  Y0, -32(DI)(BX*1)
	VMOVDQU  Y0, -64(DI)(BX*1)
	VMOVDQU  Y0, -96(DI)(BX*1)
	VMOVDQU  Y0, -128(DI)(BX*1)
	VZEROUPPER
	RET

_1or2:
	MOVB	AX, (DI)
	MOVB	AX, -1(DI)(BX*1)
	RET
_0:
	RET
_3or4:
	MOVW	AX, (DI)
	MOVW	AX, -2(DI)(BX*1)
	RET
_5through7:
	MOVL	AX, (DI)
	MOVL	AX, -4(DI)(BX*1)
	RET
_8:
	// We need a separate case for 8 to make sure we clear pointers atomically.
	MOVQ	AX, (DI)
	RET
_9through16:
	MOVQ	AX, (DI)
	MOVQ	AX, -8(DI)(BX*1)
	RET
_17through32:
	MOVOU	X15, (DI)
	MOVOU	X15, -16(DI)(BX*1)
	RET
_33through64:
	MOVOU	X15, (DI)
	MOVOU	X15, 16(DI)
	MOVOU	X15, -32(DI)(BX*1)
	MOVOU	X15, -16(DI)(BX*1)
	RET
_65through128:
	MOVOU	X15, (DI)
	MOVOU	X15, 16(DI)
	MOVOU	X15, 32(DI)
	MOVOU	X15, 48(DI)
	MOVOU	X15, -64(DI)(BX*1)
	MOVOU	X15, -48(DI)(BX*1)
	MOVOU	X15, -32(DI)(BX*1)
	MOVOU	X15, -16(DI)(BX*1)
	RET
_129through256:
	MOVOU	X15, (DI)
	MOVOU	X15, 16(DI)
	MOVOU	X15, 32(DI)
	MOVOU	X15, 48(DI)
	MOVOU	X15, 64(DI)
	MOVOU	X15, 80(DI)
	MOVOU	X15, 96(DI)
	MOVOU	X15, 112(DI)
	MOVOU	X15, -128(DI)(BX*1)
	MOVOU	X15, -112(DI)(BX*1)
	MOVOU	X15, -96(DI)(BX*1)
	MOVOU	X15, -80(DI)(BX*1)
	MOVOU	X15, -64(DI)(BX*1)
	MOVOU	X15, -48(DI)(BX*1)
	MOVOU	X15, -32(DI)(BX*1)
	MOVOU	X15, -16(DI)(BX*1)
	RET

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