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5222 | serge | 1 | /* ehopt.c--optimize gcc exception frame information. |
2 | Copyright 1998, 2000, 2001, 2003, 2005, 2007, 2008, 2009 |
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3 | Free Software Foundation, Inc. |
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4 | Written by Ian Lance Taylor |
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5 | |||
6 | This file is part of GAS, the GNU Assembler. |
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7 | |||
8 | GAS is free software; you can redistribute it and/or modify |
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9 | it under the terms of the GNU General Public License as published by |
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10 | the Free Software Foundation; either version 3, or (at your option) |
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11 | any later version. |
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12 | |||
13 | GAS is distributed in the hope that it will be useful, |
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14 | but WITHOUT ANY WARRANTY; without even the implied warranty of |
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15 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
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16 | GNU General Public License for more details. |
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17 | |||
18 | You should have received a copy of the GNU General Public License |
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19 | along with GAS; see the file COPYING. If not, write to the Free |
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20 | Software Foundation, 51 Franklin Street - Fifth Floor, Boston, MA |
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21 | 02110-1301, USA. */ |
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22 | |||
23 | #include "as.h" |
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24 | #include "subsegs.h" |
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25 | #include "struc-symbol.h" |
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26 | |||
27 | /* We include this ELF file, even though we may not be assembling for |
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28 | ELF, since the exception frame information is always in a format |
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29 | derived from DWARF. */ |
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30 | |||
31 | #include "dwarf2.h" |
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32 | |||
33 | /* Try to optimize gcc 2.8 exception frame information. |
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34 | |||
35 | Exception frame information is emitted for every function in the |
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36 | .eh_frame or .debug_frame sections. Simple information for a function |
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37 | with no exceptions looks like this: |
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38 | |||
39 | __FRAME_BEGIN__: |
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40 | .4byte .LLCIE1 / Length of Common Information Entry |
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41 | .LSCIE1: |
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42 | #if .eh_frame |
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43 | .4byte 0x0 / CIE Identifier Tag |
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44 | #elif .debug_frame |
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45 | .4byte 0xffffffff / CIE Identifier Tag |
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46 | #endif |
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47 | .byte 0x1 / CIE Version |
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48 | .byte 0x0 / CIE Augmentation (none) |
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49 | .byte 0x1 / ULEB128 0x1 (CIE Code Alignment Factor) |
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50 | .byte 0x7c / SLEB128 -4 (CIE Data Alignment Factor) |
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51 | .byte 0x8 / CIE RA Column |
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52 | .byte 0xc / DW_CFA_def_cfa |
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53 | .byte 0x4 / ULEB128 0x4 |
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54 | .byte 0x4 / ULEB128 0x4 |
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55 | .byte 0x88 / DW_CFA_offset, column 0x8 |
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56 | .byte 0x1 / ULEB128 0x1 |
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57 | .align 4 |
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58 | .LECIE1: |
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59 | .set .LLCIE1,.LECIE1-.LSCIE1 / CIE Length Symbol |
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60 | .4byte .LLFDE1 / FDE Length |
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61 | .LSFDE1: |
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62 | .4byte .LSFDE1-__FRAME_BEGIN__ / FDE CIE offset |
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63 | .4byte .LFB1 / FDE initial location |
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64 | .4byte .LFE1-.LFB1 / FDE address range |
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65 | .byte 0x4 / DW_CFA_advance_loc4 |
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66 | .4byte .LCFI0-.LFB1 |
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67 | .byte 0xe / DW_CFA_def_cfa_offset |
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68 | .byte 0x8 / ULEB128 0x8 |
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69 | .byte 0x85 / DW_CFA_offset, column 0x5 |
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70 | .byte 0x2 / ULEB128 0x2 |
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71 | .byte 0x4 / DW_CFA_advance_loc4 |
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72 | .4byte .LCFI1-.LCFI0 |
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73 | .byte 0xd / DW_CFA_def_cfa_register |
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74 | .byte 0x5 / ULEB128 0x5 |
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75 | .byte 0x4 / DW_CFA_advance_loc4 |
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76 | .4byte .LCFI2-.LCFI1 |
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77 | .byte 0x2e / DW_CFA_GNU_args_size |
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78 | .byte 0x4 / ULEB128 0x4 |
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79 | .byte 0x4 / DW_CFA_advance_loc4 |
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80 | .4byte .LCFI3-.LCFI2 |
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81 | .byte 0x2e / DW_CFA_GNU_args_size |
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82 | .byte 0x0 / ULEB128 0x0 |
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83 | .align 4 |
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84 | .LEFDE1: |
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85 | .set .LLFDE1,.LEFDE1-.LSFDE1 / FDE Length Symbol |
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86 | |||
87 | The immediate issue we can address in the assembler is the |
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88 | DW_CFA_advance_loc4 followed by a four byte value. The value is |
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89 | the difference of two addresses in the function. Since gcc does |
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90 | not know this value, it always uses four bytes. We will know the |
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91 | value at the end of assembly, so we can do better. */ |
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92 | |||
93 | struct cie_info |
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94 | { |
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95 | unsigned code_alignment; |
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96 | int z_augmentation; |
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97 | }; |
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98 | |||
99 | static int get_cie_info (struct cie_info *); |
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100 | |||
101 | /* Extract information from the CIE. */ |
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102 | |||
103 | static int |
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104 | get_cie_info (struct cie_info *info) |
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105 | { |
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106 | fragS *f; |
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107 | fixS *fix; |
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108 | int offset; |
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109 | char CIE_id; |
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110 | char augmentation[10]; |
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111 | int iaug; |
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112 | int code_alignment = 0; |
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113 | |||
114 | /* We should find the CIE at the start of the section. */ |
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115 | |||
116 | f = seg_info (now_seg)->frchainP->frch_root; |
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117 | fix = seg_info (now_seg)->frchainP->fix_root; |
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118 | |||
119 | /* Look through the frags of the section to find the code alignment. */ |
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120 | |||
121 | /* First make sure that the CIE Identifier Tag is 0/-1. */ |
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122 | |||
123 | if (strncmp (segment_name (now_seg), ".debug_frame", 12) == 0) |
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124 | CIE_id = (char)0xff; |
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125 | else |
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126 | CIE_id = 0; |
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127 | |||
128 | offset = 4; |
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129 | while (f != NULL && offset >= f->fr_fix) |
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130 | { |
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131 | offset -= f->fr_fix; |
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132 | f = f->fr_next; |
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133 | } |
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134 | if (f == NULL |
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135 | || f->fr_fix - offset < 4 |
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136 | || f->fr_literal[offset] != CIE_id |
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137 | || f->fr_literal[offset + 1] != CIE_id |
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138 | || f->fr_literal[offset + 2] != CIE_id |
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139 | || f->fr_literal[offset + 3] != CIE_id) |
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140 | return 0; |
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141 | |||
142 | /* Next make sure the CIE version number is 1. */ |
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143 | |||
144 | offset += 4; |
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145 | while (f != NULL && offset >= f->fr_fix) |
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146 | { |
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147 | offset -= f->fr_fix; |
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148 | f = f->fr_next; |
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149 | } |
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150 | if (f == NULL |
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151 | || f->fr_fix - offset < 1 |
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152 | || f->fr_literal[offset] != 1) |
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153 | return 0; |
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154 | |||
155 | /* Skip the augmentation (a null terminated string). */ |
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156 | |||
157 | iaug = 0; |
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158 | ++offset; |
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159 | while (1) |
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160 | { |
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161 | while (f != NULL && offset >= f->fr_fix) |
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162 | { |
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163 | offset -= f->fr_fix; |
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164 | f = f->fr_next; |
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165 | } |
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166 | if (f == NULL) |
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167 | return 0; |
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168 | |||
169 | while (offset < f->fr_fix && f->fr_literal[offset] != '\0') |
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170 | { |
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171 | if ((size_t) iaug < (sizeof augmentation) - 1) |
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172 | { |
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173 | augmentation[iaug] = f->fr_literal[offset]; |
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174 | ++iaug; |
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175 | } |
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176 | ++offset; |
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177 | } |
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178 | if (offset < f->fr_fix) |
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179 | break; |
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180 | } |
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181 | ++offset; |
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182 | while (f != NULL && offset >= f->fr_fix) |
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183 | { |
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184 | offset -= f->fr_fix; |
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185 | f = f->fr_next; |
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186 | } |
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187 | if (f == NULL) |
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188 | return 0; |
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189 | |||
190 | augmentation[iaug] = '\0'; |
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191 | if (augmentation[0] == '\0') |
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192 | { |
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193 | /* No augmentation. */ |
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194 | } |
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195 | else if (strcmp (augmentation, "eh") == 0) |
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196 | { |
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197 | /* We have to skip a pointer. Unfortunately, we don't know how |
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198 | large it is. We find out by looking for a matching fixup. */ |
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199 | while (fix != NULL |
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200 | && (fix->fx_frag != f || fix->fx_where != offset)) |
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201 | fix = fix->fx_next; |
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202 | if (fix == NULL) |
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203 | offset += 4; |
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204 | else |
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205 | offset += fix->fx_size; |
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206 | while (f != NULL && offset >= f->fr_fix) |
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207 | { |
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208 | offset -= f->fr_fix; |
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209 | f = f->fr_next; |
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210 | } |
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211 | if (f == NULL) |
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212 | return 0; |
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213 | } |
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214 | else if (augmentation[0] != 'z') |
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215 | return 0; |
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216 | |||
217 | /* We're now at the code alignment factor, which is a ULEB128. If |
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218 | it isn't a single byte, forget it. */ |
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219 | |||
220 | code_alignment = f->fr_literal[offset] & 0xff; |
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221 | if ((code_alignment & 0x80) != 0) |
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222 | code_alignment = 0; |
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223 | |||
224 | info->code_alignment = code_alignment; |
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225 | info->z_augmentation = (augmentation[0] == 'z'); |
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226 | |||
227 | return 1; |
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228 | } |
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229 | |||
230 | enum frame_state |
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231 | { |
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232 | state_idle, |
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233 | state_saw_size, |
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234 | state_saw_cie_offset, |
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235 | state_saw_pc_begin, |
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236 | state_seeing_aug_size, |
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237 | state_skipping_aug, |
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238 | state_wait_loc4, |
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239 | state_saw_loc4, |
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240 | state_error, |
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241 | }; |
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242 | |||
243 | /* This function is called from emit_expr. It looks for cases which |
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244 | we can optimize. |
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245 | |||
246 | Rather than try to parse all this information as we read it, we |
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247 | look for a single byte DW_CFA_advance_loc4 followed by a 4 byte |
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248 | difference. We turn that into a rs_cfa_advance frag, and handle |
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249 | those frags at the end of the assembly. If the gcc output changes |
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250 | somewhat, this optimization may stop working. |
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251 | |||
252 | This function returns non-zero if it handled the expression and |
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253 | emit_expr should not do anything, or zero otherwise. It can also |
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254 | change *EXP and *PNBYTES. */ |
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255 | |||
256 | int |
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257 | check_eh_frame (expressionS *exp, unsigned int *pnbytes) |
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258 | { |
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259 | struct frame_data |
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260 | { |
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261 | enum frame_state state; |
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262 | |||
263 | int cie_info_ok; |
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264 | struct cie_info cie_info; |
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265 | |||
266 | symbolS *size_end_sym; |
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267 | fragS *loc4_frag; |
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268 | int loc4_fix; |
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269 | |||
270 | int aug_size; |
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271 | int aug_shift; |
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272 | }; |
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273 | |||
274 | static struct frame_data eh_frame_data; |
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275 | static struct frame_data debug_frame_data; |
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276 | struct frame_data *d; |
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277 | |||
278 | /* Don't optimize. */ |
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279 | if (flag_traditional_format) |
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280 | return 0; |
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281 | |||
282 | #ifdef md_allow_eh_opt |
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283 | if (! md_allow_eh_opt) |
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284 | return 0; |
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285 | #endif |
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286 | |||
287 | /* Select the proper section data. */ |
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288 | if (strncmp (segment_name (now_seg), ".eh_frame", 9) == 0 |
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289 | && segment_name (now_seg)[9] != '_') |
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290 | d = &eh_frame_data; |
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291 | else if (strncmp (segment_name (now_seg), ".debug_frame", 12) == 0) |
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292 | d = &debug_frame_data; |
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293 | else |
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294 | return 0; |
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295 | |||
296 | if (d->state >= state_saw_size && S_IS_DEFINED (d->size_end_sym)) |
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297 | { |
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298 | /* We have come to the end of the CIE or FDE. See below where |
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299 | we set saw_size. We must check this first because we may now |
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300 | be looking at the next size. */ |
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301 | d->state = state_idle; |
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302 | } |
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303 | |||
304 | switch (d->state) |
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305 | { |
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306 | case state_idle: |
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307 | if (*pnbytes == 4) |
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308 | { |
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309 | /* This might be the size of the CIE or FDE. We want to know |
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310 | the size so that we don't accidentally optimize across an FDE |
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311 | boundary. We recognize the size in one of two forms: a |
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312 | symbol which will later be defined as a difference, or a |
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313 | subtraction of two symbols. Either way, we can tell when we |
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314 | are at the end of the FDE because the symbol becomes defined |
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315 | (in the case of a subtraction, the end symbol, from which the |
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316 | start symbol is being subtracted). Other ways of describing |
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317 | the size will not be optimized. */ |
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318 | if ((exp->X_op == O_symbol || exp->X_op == O_subtract) |
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319 | && ! S_IS_DEFINED (exp->X_add_symbol)) |
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320 | { |
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321 | d->state = state_saw_size; |
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322 | d->size_end_sym = exp->X_add_symbol; |
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323 | } |
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324 | } |
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325 | break; |
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326 | |||
327 | case state_saw_size: |
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328 | case state_saw_cie_offset: |
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329 | /* Assume whatever form it appears in, it appears atomically. */ |
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330 | d->state = (enum frame_state) (d->state + 1); |
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331 | break; |
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332 | |||
333 | case state_saw_pc_begin: |
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334 | /* Decide whether we should see an augmentation. */ |
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335 | if (! d->cie_info_ok |
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336 | && ! (d->cie_info_ok = get_cie_info (&d->cie_info))) |
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337 | d->state = state_error; |
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338 | else if (d->cie_info.z_augmentation) |
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339 | { |
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340 | d->state = state_seeing_aug_size; |
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341 | d->aug_size = 0; |
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342 | d->aug_shift = 0; |
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343 | } |
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344 | else |
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345 | d->state = state_wait_loc4; |
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346 | break; |
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347 | |||
348 | case state_seeing_aug_size: |
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349 | /* Bytes == -1 means this comes from an leb128 directive. */ |
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350 | if ((int)*pnbytes == -1 && exp->X_op == O_constant) |
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351 | { |
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352 | d->aug_size = exp->X_add_number; |
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353 | d->state = state_skipping_aug; |
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354 | } |
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355 | else if (*pnbytes == 1 && exp->X_op == O_constant) |
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356 | { |
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357 | unsigned char byte = exp->X_add_number; |
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358 | d->aug_size |= (byte & 0x7f) << d->aug_shift; |
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359 | d->aug_shift += 7; |
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360 | if ((byte & 0x80) == 0) |
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361 | d->state = state_skipping_aug; |
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362 | } |
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363 | else |
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364 | d->state = state_error; |
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365 | if (d->state == state_skipping_aug && d->aug_size == 0) |
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366 | d->state = state_wait_loc4; |
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367 | break; |
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368 | |||
369 | case state_skipping_aug: |
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370 | if ((int)*pnbytes < 0) |
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371 | d->state = state_error; |
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372 | else |
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373 | { |
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374 | int left = (d->aug_size -= *pnbytes); |
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375 | if (left == 0) |
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376 | d->state = state_wait_loc4; |
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377 | else if (left < 0) |
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378 | d->state = state_error; |
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379 | } |
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380 | break; |
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381 | |||
382 | case state_wait_loc4: |
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383 | if (*pnbytes == 1 |
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384 | && exp->X_op == O_constant |
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385 | && exp->X_add_number == DW_CFA_advance_loc4) |
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386 | { |
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387 | /* This might be a DW_CFA_advance_loc4. Record the frag and the |
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388 | position within the frag, so that we can change it later. */ |
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389 | frag_grow (1); |
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390 | d->state = state_saw_loc4; |
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391 | d->loc4_frag = frag_now; |
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392 | d->loc4_fix = frag_now_fix (); |
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393 | } |
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394 | break; |
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395 | |||
396 | case state_saw_loc4: |
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397 | d->state = state_wait_loc4; |
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398 | if (*pnbytes != 4) |
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399 | break; |
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400 | if (exp->X_op == O_constant) |
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401 | { |
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402 | /* This is a case which we can optimize. The two symbols being |
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403 | subtracted were in the same frag and the expression was |
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404 | reduced to a constant. We can do the optimization entirely |
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405 | in this function. */ |
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406 | if (exp->X_add_number < 0x40) |
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407 | { |
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408 | d->loc4_frag->fr_literal[d->loc4_fix] |
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409 | = DW_CFA_advance_loc | exp->X_add_number; |
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410 | /* No more bytes needed. */ |
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411 | return 1; |
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412 | } |
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413 | else if (exp->X_add_number < 0x100) |
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414 | { |
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415 | d->loc4_frag->fr_literal[d->loc4_fix] = DW_CFA_advance_loc1; |
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416 | *pnbytes = 1; |
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417 | } |
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418 | else if (exp->X_add_number < 0x10000) |
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419 | { |
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420 | d->loc4_frag->fr_literal[d->loc4_fix] = DW_CFA_advance_loc2; |
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421 | *pnbytes = 2; |
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422 | } |
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423 | } |
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424 | else if (exp->X_op == O_subtract && d->cie_info.code_alignment == 1) |
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425 | { |
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426 | /* This is a case we can optimize. The expression was not |
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427 | reduced, so we can not finish the optimization until the end |
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428 | of the assembly. We set up a variant frag which we handle |
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429 | later. */ |
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430 | frag_var (rs_cfa, 4, 0, 1 << 3, make_expr_symbol (exp), |
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431 | d->loc4_fix, (char *) d->loc4_frag); |
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432 | return 1; |
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433 | } |
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434 | else if ((exp->X_op == O_divide |
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435 | || exp->X_op == O_right_shift) |
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436 | && d->cie_info.code_alignment > 1) |
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437 | { |
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438 | if (exp->X_add_symbol->bsym |
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439 | && exp->X_op_symbol->bsym |
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440 | && exp->X_add_symbol->sy_value.X_op == O_subtract |
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441 | && exp->X_op_symbol->sy_value.X_op == O_constant |
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442 | && ((exp->X_op == O_divide |
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443 | ? exp->X_op_symbol->sy_value.X_add_number |
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444 | : (offsetT) 1 << exp->X_op_symbol->sy_value.X_add_number) |
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445 | == (offsetT) d->cie_info.code_alignment)) |
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446 | { |
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447 | /* This is a case we can optimize as well. The expression was |
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448 | not reduced, so we can not finish the optimization until the |
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449 | end of the assembly. We set up a variant frag which we |
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450 | handle later. */ |
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451 | frag_var (rs_cfa, 4, 0, d->cie_info.code_alignment << 3, |
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452 | make_expr_symbol (&exp->X_add_symbol->sy_value), |
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453 | d->loc4_fix, (char *) d->loc4_frag); |
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454 | return 1; |
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455 | } |
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456 | } |
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457 | break; |
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458 | |||
459 | case state_error: |
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460 | /* Just skipping everything. */ |
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461 | break; |
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462 | } |
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463 | |||
464 | return 0; |
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465 | } |
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466 | |||
467 | /* The function estimates the size of a rs_cfa variant frag based on |
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468 | the current values of the symbols. It is called before the |
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469 | relaxation loop. We set fr_subtype{0:2} to the expected length. */ |
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470 | |||
471 | int |
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472 | eh_frame_estimate_size_before_relax (fragS *frag) |
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473 | { |
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474 | offsetT diff; |
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475 | int ca = frag->fr_subtype >> 3; |
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476 | int ret; |
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477 | |||
478 | diff = resolve_symbol_value (frag->fr_symbol); |
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479 | |||
480 | gas_assert (ca > 0); |
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481 | diff /= ca; |
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482 | if (diff < 0x40) |
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483 | ret = 0; |
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484 | else if (diff < 0x100) |
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485 | ret = 1; |
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486 | else if (diff < 0x10000) |
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487 | ret = 2; |
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488 | else |
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489 | ret = 4; |
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490 | |||
491 | frag->fr_subtype = (frag->fr_subtype & ~7) | ret; |
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492 | |||
493 | return ret; |
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494 | } |
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495 | |||
496 | /* This function relaxes a rs_cfa variant frag based on the current |
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497 | values of the symbols. fr_subtype{0:2} is the current length of |
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498 | the frag. This returns the change in frag length. */ |
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499 | |||
500 | int |
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501 | eh_frame_relax_frag (fragS *frag) |
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502 | { |
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503 | int oldsize, newsize; |
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504 | |||
505 | oldsize = frag->fr_subtype & 7; |
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506 | newsize = eh_frame_estimate_size_before_relax (frag); |
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507 | return newsize - oldsize; |
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508 | } |
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509 | |||
510 | /* This function converts a rs_cfa variant frag into a normal fill |
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511 | frag. This is called after all relaxation has been done. |
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512 | fr_subtype{0:2} will be the desired length of the frag. */ |
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513 | |||
514 | void |
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515 | eh_frame_convert_frag (fragS *frag) |
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516 | { |
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517 | offsetT diff; |
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518 | fragS *loc4_frag; |
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519 | int loc4_fix, ca; |
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520 | |||
521 | loc4_frag = (fragS *) frag->fr_opcode; |
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522 | loc4_fix = (int) frag->fr_offset; |
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523 | |||
524 | diff = resolve_symbol_value (frag->fr_symbol); |
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525 | |||
526 | ca = frag->fr_subtype >> 3; |
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527 | gas_assert (ca > 0); |
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528 | diff /= ca; |
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529 | switch (frag->fr_subtype & 7) |
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530 | { |
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531 | case 0: |
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532 | gas_assert (diff < 0x40); |
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533 | loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc | diff; |
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534 | break; |
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535 | |||
536 | case 1: |
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537 | gas_assert (diff < 0x100); |
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538 | loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc1; |
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539 | frag->fr_literal[frag->fr_fix] = diff; |
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540 | break; |
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541 | |||
542 | case 2: |
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543 | gas_assert (diff < 0x10000); |
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544 | loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc2; |
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545 | md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 2); |
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546 | break; |
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547 | |||
548 | default: |
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549 | md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 4); |
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550 | break; |
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551 | } |
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552 | |||
553 | frag->fr_fix += frag->fr_subtype & 7; |
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554 | frag->fr_type = rs_fill; |
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555 | frag->fr_subtype = 0; |
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556 | frag->fr_offset = 0; |
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557 | }>>>>>>><>><>><>>>>>>><>>>>>> |