memclr_arm64.s

Documentation: runtime

     1  // Copyright 2014 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  #include "textflag.h"
     6  
     7  // See memclrNoHeapPointers Go doc for important implementation constraints.
     8  
     9  // func memclrNoHeapPointers(ptr unsafe.Pointer, n uintptr)
    10  // Also called from assembly in sys_windows_arm64.s without g (but using Go stack convention).
    11  TEXT runtime·memclrNoHeapPointers(SB),NOSPLIT,$0-16
    12  	MOVD	ptr+0(FP), R0
    13  	MOVD	n+8(FP), R1
    14  
    15  	CMP	$16, R1
    16  	// If n is equal to 16 bytes, use zero_exact_16 to zero
    17  	BEQ	zero_exact_16
    18  
    19  	// If n is greater than 16 bytes, use zero_by_16 to zero
    20  	BHI	zero_by_16
    21  
    22  	// n is less than 16 bytes
    23  	ADD	R1, R0, R7
    24  	TBZ	$3, R1, less_than_8
    25  	MOVD	ZR, (R0)
    26  	MOVD	ZR, -8(R7)
    27  	RET
    28  
    29  less_than_8:
    30  	TBZ	$2, R1, less_than_4
    31  	MOVW	ZR, (R0)
    32  	MOVW	ZR, -4(R7)
    33  	RET
    34  
    35  less_than_4:
    36  	CBZ	R1, ending
    37  	MOVB	ZR, (R0)
    38  	TBZ	$1, R1, ending
    39  	MOVH	ZR, -2(R7)
    40  
    41  ending:
    42  	RET
    43  
    44  zero_exact_16:
    45  	// n is exactly 16 bytes
    46  	STP	(ZR, ZR), (R0)
    47  	RET
    48  
    49  zero_by_16:
    50  	// n greater than 16 bytes, check if the start address is aligned
    51  	NEG	R0, R4
    52  	ANDS	$15, R4, R4
    53  	// Try zeroing using zva if the start address is aligned with 16
    54  	BEQ	try_zva
    55  
    56  	// Non-aligned store
    57  	STP	(ZR, ZR), (R0)
    58  	// Make the destination aligned
    59  	SUB	R4, R1, R1
    60  	ADD	R4, R0, R0
    61  	B	try_zva
    62  
    63  tail_maybe_long:
    64  	CMP	$64, R1
    65  	BHS	no_zva
    66  
    67  tail63:
    68  	ANDS	$48, R1, R3
    69  	BEQ	last16
    70  	CMPW	$32, R3
    71  	BEQ	last48
    72  	BLT	last32
    73  	STP.P	(ZR, ZR), 16(R0)
    74  last48:
    75  	STP.P	(ZR, ZR), 16(R0)
    76  last32:
    77  	STP.P	(ZR, ZR), 16(R0)
    78  	// The last store length is at most 16, so it is safe to use
    79  	// stp to write last 16 bytes
    80  last16:
    81  	ANDS	$15, R1, R1
    82  	CBZ	R1, last_end
    83  	ADD	R1, R0, R0
    84  	STP	(ZR, ZR), -16(R0)
    85  last_end:
    86  	RET
    87  
    88  no_zva:
    89  	SUB	$16, R0, R0
    90  	SUB	$64, R1, R1
    91  
    92  loop_64:
    93  	STP	(ZR, ZR), 16(R0)
    94  	STP	(ZR, ZR), 32(R0)
    95  	STP	(ZR, ZR), 48(R0)
    96  	STP.W	(ZR, ZR), 64(R0)
    97  	SUBS	$64, R1, R1
    98  	BGE	loop_64
    99  	ANDS	$63, R1, ZR
   100  	ADD	$16, R0, R0
   101  	BNE	tail63
   102  	RET
   103  
   104  try_zva:
   105  	// Try using the ZVA feature to zero entire cache lines
   106  	// It is not meaningful to use ZVA if the block size is less than 64,
   107  	// so make sure that n is greater than or equal to 64
   108  	CMP	$63, R1
   109  	BLE	tail63
   110  
   111  	CMP	$128, R1
   112  	// Ensure n is at least 128 bytes, so that there is enough to copy after
   113  	// alignment.
   114  	BLT	no_zva
   115  	// Check if ZVA is allowed from user code, and if so get the block size
   116  	MOVW	block_size<>(SB), R5
   117  	TBNZ	$31, R5, no_zva
   118  	CBNZ	R5, zero_by_line
   119  	// DCZID_EL0 bit assignments
   120  	// [63:5] Reserved
   121  	// [4]    DZP, if bit set DC ZVA instruction is prohibited, else permitted
   122  	// [3:0]  log2 of the block size in words, eg. if it returns 0x4 then block size is 16 words
   123  	MRS	DCZID_EL0, R3
   124  	TBZ	$4, R3, init
   125  	// ZVA not available
   126  	MOVW	$~0, R5
   127  	MOVW	R5, block_size<>(SB)
   128  	B	no_zva
   129  
   130  init:
   131  	MOVW	$4, R9
   132  	ANDW	$15, R3, R5
   133  	LSLW	R5, R9, R5
   134  	MOVW	R5, block_size<>(SB)
   135  
   136  	ANDS	$63, R5, R9
   137  	// Block size is less than 64.
   138  	BNE	no_zva
   139  
   140  zero_by_line:
   141  	CMP	R5, R1
   142  	// Not enough memory to reach alignment
   143  	BLO	no_zva
   144  	SUB	$1, R5, R6
   145  	NEG	R0, R4
   146  	ANDS	R6, R4, R4
   147  	// Already aligned
   148  	BEQ	aligned
   149  
   150  	// check there is enough to copy after alignment
   151  	SUB	R4, R1, R3
   152  
   153  	// Check that the remaining length to ZVA after alignment
   154  	// is greater than 64.
   155  	CMP	$64, R3
   156  	CCMP	GE, R3, R5, $10  // condition code GE, NZCV=0b1010
   157  	BLT	no_zva
   158  
   159  	// We now have at least 64 bytes to zero, update n
   160  	MOVD	R3, R1
   161  
   162  loop_zva_prolog:
   163  	STP	(ZR, ZR), (R0)
   164  	STP	(ZR, ZR), 16(R0)
   165  	STP	(ZR, ZR), 32(R0)
   166  	SUBS	$64, R4, R4
   167  	STP	(ZR, ZR), 48(R0)
   168  	ADD	$64, R0, R0
   169  	BGE	loop_zva_prolog
   170  
   171  	ADD	R4, R0, R0
   172  
   173  aligned:
   174  	SUB	R5, R1, R1
   175  
   176  loop_zva:
   177  	WORD	$0xd50b7420 // DC ZVA, R0
   178  	ADD	R5, R0, R0
   179  	SUBS	R5, R1, R1
   180  	BHS	loop_zva
   181  	ANDS	R6, R1, R1
   182  	BNE	tail_maybe_long
   183  	RET
   184  
   185  GLOBL block_size<>(SB), NOPTR, $8
   186
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