1 | ; $Id: ASMBitNextClear.asm 98103 2023-01-17 14:15:46Z vboxsync $
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2 | ;; @file
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3 | ; IPRT - ASMBitNextClear().
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4 | ;
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5 |
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6 | ;
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7 | ; Copyright (C) 2006-2023 Oracle and/or its affiliates.
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8 | ;
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9 | ; This file is part of VirtualBox base platform packages, as
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10 | ; available from https://www.virtualbox.org.
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11 | ;
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12 | ; This program is free software; you can redistribute it and/or
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13 | ; modify it under the terms of the GNU General Public License
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14 | ; as published by the Free Software Foundation, in version 3 of the
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15 | ; License.
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16 | ;
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17 | ; This program is distributed in the hope that it will be useful, but
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18 | ; WITHOUT ANY WARRANTY; without even the implied warranty of
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19 | ; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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20 | ; General Public License for more details.
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21 | ;
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22 | ; You should have received a copy of the GNU General Public License
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23 | ; along with this program; if not, see <https://www.gnu.org/licenses>.
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24 | ;
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25 | ; The contents of this file may alternatively be used under the terms
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26 | ; of the Common Development and Distribution License Version 1.0
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27 | ; (CDDL), a copy of it is provided in the "COPYING.CDDL" file included
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28 | ; in the VirtualBox distribution, in which case the provisions of the
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29 | ; CDDL are applicable instead of those of the GPL.
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30 | ;
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31 | ; You may elect to license modified versions of this file under the
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32 | ; terms and conditions of either the GPL or the CDDL or both.
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33 | ;
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34 | ; SPDX-License-Identifier: GPL-3.0-only OR CDDL-1.0
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35 | ;
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36 |
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37 |
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38 | ;*******************************************************************************
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39 | ;* Header Files *
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40 | ;*******************************************************************************
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41 | %include "iprt/asmdefs.mac"
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42 |
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43 | BEGINCODE
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44 |
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45 | ;;
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46 | ; Finds the next clear bit in a bitmap.
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47 | ;
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48 | ; @returns (32/64:eax, 16:ax+dx) Index of the first zero bit.
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49 | ; @returns (32/64:eax, 16:ax+dx) -1 if no clear bit was found.
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50 | ; @param msc:rcx gcc:rdi pvBitmap Pointer to the bitmap.
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51 | ; @param msc:edx gcc:rsi cBits The number of bits in the bitmap. Multiple of 32.
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52 | ; @param msc:r8d gcc:rcx iBitPrev The previous bit, start searching after it.
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53 | ;
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54 | ; @remarks Not quite sure how much sense it makes to do this in assembly, but
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55 | ; it started out with the ASMBit* API, so that's why we still have it.
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56 | ;
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57 | RT_BEGINPROC ASMBitNextClear
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58 | %if ARCH_BITS == 16
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59 | push bp
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60 | mov bp, sp
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61 | %endif
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62 | push xDI
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63 |
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64 | ;
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65 | ; Align input registers: rdi=pvBitmap, ecx=iPrevBit
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66 | ;
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67 | %if ARCH_BITS == 64
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68 | %ifdef ASM_CALL64_GCC
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69 | ; ecx = iBitPrev param, rdi=pvBitmap param.
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70 | %else
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71 | mov rdi, rcx ; rdi=pvBits
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72 | mov ecx, r8d ; ecx=iPrevBit
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73 | mov r8d, edx ; r8d=cBits (saved for .scan_dwords)
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74 | %endif
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75 | mov r9, rdi ; Save rdi for bit calculation.
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76 | %elif ARCH_BITS == 32
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77 | mov edi, [esp + 8] ; edi=pvBits
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78 | mov ecx, [esp + 8 + 8] ; ecx=iPrevBit
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79 | %elif ARCH_BITS == 16
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80 | les di, [bp + 4] ; es:di=pvBits
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81 | mov ecx, [bp + 4 + 8] ; ecx=iPrevBit
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82 | %endif
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83 |
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84 | ;
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85 | ; If iPrevBit and iPrevBit + 1 are in the same dword, inspect it for further bits.
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86 | ;
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87 | inc ecx
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88 | mov eax, ecx
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89 | shr eax, 5
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90 | shl eax, 2 ; eax=byte offset into bitmap of current dword.
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91 | add xDI, xAX ; xDI=current dword address (of iPrevBit + 1).
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92 | and ecx, 31
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93 | jz .scan_dwords
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94 |
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95 | %if ARCH_BITS == 16
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96 | mov edx, [es:di]
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97 | %else
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98 | mov edx, [xDI]
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99 | %endif
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100 | not edx ; edx = inverted current dword
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101 | shr edx, cl ; Shift out bits that we have searched.
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102 | jz .next_dword ; If zero, nothing to find. Go rep scasd.
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103 | shl edx, cl ; Shift it back so bsf will return the right index.
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104 |
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105 | bsf edx, edx ; edx=index of first clear bit
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106 |
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107 | shl eax, 3 ; Turn eax back into a bit offset of the current dword.
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108 | add eax, edx ; eax=bit offset
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109 |
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110 | .return:
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111 | pop xDI
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112 | %if ARCH_BITS == 16
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113 | mov edx, eax
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114 | shr edx, 16
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115 | leave
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116 | %endif
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117 | ret
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118 |
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119 | ;
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120 | ; Do dword scan.
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121 | ;
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122 |
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123 | ; Skip empty dword.
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124 | .next_dword:
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125 | add xDI, 4 ; Skip the empty dword.
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126 | add eax, 4
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127 |
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128 | .scan_dwords:
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129 | ; First load and align bit count.
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130 | %if ARCH_BITS == 64
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131 | %ifdef ASM_CALL64_GCC
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132 | mov ecx, esi
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133 | %else
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134 | mov ecx, r8d
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135 | %endif
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136 | %elif ARCH_BITS == 32
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137 | mov ecx, [esp + 8 + 4]
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138 | %elif ARCH_BITS == 16
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139 | mov ecx, [bp + 4 + 4]
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140 | %endif
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141 | add ecx, 31
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142 | shr ecx, 5 ; ecx=bitmap size in dwords.
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143 |
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144 | ; Adjust ecx to how many dwords there are left to scan. (eax = current byte offset)
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145 | shr eax, 2 ; eax=current dword offset.
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146 | sub ecx, eax
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147 | jbe .return_failure
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148 |
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149 | ; Do the scanning.
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150 | cld
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151 | mov eax, 0ffffffffh
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152 | repe scasd
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153 | je .return_failure
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154 |
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155 | ; Find the bit in question.
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156 | sub xDI, 4 ; One step back.
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157 | %if ARCH_BITS == 16
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158 | movzx edi, di
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159 | xor eax, [es:xDI]
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160 | %else
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161 | xor eax, [xDI]
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162 | %endif
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163 | bsf eax, eax
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164 | jz .return_failure ; race paranoia
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165 |
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166 | ; Calc the bit offset.
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167 | %if ARCH_BITS == 16
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168 | sub di, [bp + 4]
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169 | movzx edi, di
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170 | %elif ARCH_BITS == 32
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171 | sub xDI, [esp + 8]
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172 | %elif ARCH_BITS == 64
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173 | sub xDI, r9
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174 | %endif
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175 | shl edi, 3 ; edi=bit offset of current dword.
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176 | add eax, edi
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177 | jmp .return
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178 |
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179 | .return_failure:
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180 | mov eax, 0ffffffffh
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181 | jmp .return
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182 | ENDPROC ASMBitNextClear
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183 |
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