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Rev | Author | Line No. | Line |
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5222 | serge | 1 | /* ehopt.c--optimize gcc exception frame information. |
6324 | serge | 2 | Copyright (C) 1998-2015 Free Software Foundation, Inc. |
5222 | serge | 3 | Written by Ian Lance Taylor |
4 | |||
5 | This file is part of GAS, the GNU Assembler. |
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6 | |||
7 | GAS is free software; you can redistribute it and/or modify |
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8 | it under the terms of the GNU General Public License as published by |
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9 | the Free Software Foundation; either version 3, or (at your option) |
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10 | any later version. |
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11 | |||
12 | GAS is distributed in the hope that it will be useful, |
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13 | but WITHOUT ANY WARRANTY; without even the implied warranty of |
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14 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
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15 | GNU General Public License for more details. |
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16 | |||
17 | You should have received a copy of the GNU General Public License |
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18 | along with GAS; see the file COPYING. If not, write to the Free |
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19 | Software Foundation, 51 Franklin Street - Fifth Floor, Boston, MA |
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20 | 02110-1301, USA. */ |
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21 | |||
22 | #include "as.h" |
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23 | #include "subsegs.h" |
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24 | #include "struc-symbol.h" |
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25 | |||
26 | /* We include this ELF file, even though we may not be assembling for |
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27 | ELF, since the exception frame information is always in a format |
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28 | derived from DWARF. */ |
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29 | |||
30 | #include "dwarf2.h" |
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31 | |||
32 | /* Try to optimize gcc 2.8 exception frame information. |
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33 | |||
34 | Exception frame information is emitted for every function in the |
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35 | .eh_frame or .debug_frame sections. Simple information for a function |
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36 | with no exceptions looks like this: |
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37 | |||
38 | __FRAME_BEGIN__: |
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39 | .4byte .LLCIE1 / Length of Common Information Entry |
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40 | .LSCIE1: |
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41 | #if .eh_frame |
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42 | .4byte 0x0 / CIE Identifier Tag |
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43 | #elif .debug_frame |
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44 | .4byte 0xffffffff / CIE Identifier Tag |
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45 | #endif |
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46 | .byte 0x1 / CIE Version |
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47 | .byte 0x0 / CIE Augmentation (none) |
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48 | .byte 0x1 / ULEB128 0x1 (CIE Code Alignment Factor) |
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49 | .byte 0x7c / SLEB128 -4 (CIE Data Alignment Factor) |
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50 | .byte 0x8 / CIE RA Column |
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51 | .byte 0xc / DW_CFA_def_cfa |
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52 | .byte 0x4 / ULEB128 0x4 |
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53 | .byte 0x4 / ULEB128 0x4 |
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54 | .byte 0x88 / DW_CFA_offset, column 0x8 |
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55 | .byte 0x1 / ULEB128 0x1 |
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56 | .align 4 |
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57 | .LECIE1: |
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58 | .set .LLCIE1,.LECIE1-.LSCIE1 / CIE Length Symbol |
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59 | .4byte .LLFDE1 / FDE Length |
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60 | .LSFDE1: |
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61 | .4byte .LSFDE1-__FRAME_BEGIN__ / FDE CIE offset |
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62 | .4byte .LFB1 / FDE initial location |
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63 | .4byte .LFE1-.LFB1 / FDE address range |
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64 | .byte 0x4 / DW_CFA_advance_loc4 |
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65 | .4byte .LCFI0-.LFB1 |
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66 | .byte 0xe / DW_CFA_def_cfa_offset |
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67 | .byte 0x8 / ULEB128 0x8 |
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68 | .byte 0x85 / DW_CFA_offset, column 0x5 |
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69 | .byte 0x2 / ULEB128 0x2 |
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70 | .byte 0x4 / DW_CFA_advance_loc4 |
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71 | .4byte .LCFI1-.LCFI0 |
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72 | .byte 0xd / DW_CFA_def_cfa_register |
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73 | .byte 0x5 / ULEB128 0x5 |
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74 | .byte 0x4 / DW_CFA_advance_loc4 |
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75 | .4byte .LCFI2-.LCFI1 |
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76 | .byte 0x2e / DW_CFA_GNU_args_size |
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77 | .byte 0x4 / ULEB128 0x4 |
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78 | .byte 0x4 / DW_CFA_advance_loc4 |
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79 | .4byte .LCFI3-.LCFI2 |
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80 | .byte 0x2e / DW_CFA_GNU_args_size |
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81 | .byte 0x0 / ULEB128 0x0 |
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82 | .align 4 |
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83 | .LEFDE1: |
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84 | .set .LLFDE1,.LEFDE1-.LSFDE1 / FDE Length Symbol |
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85 | |||
86 | The immediate issue we can address in the assembler is the |
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87 | DW_CFA_advance_loc4 followed by a four byte value. The value is |
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88 | the difference of two addresses in the function. Since gcc does |
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89 | not know this value, it always uses four bytes. We will know the |
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90 | value at the end of assembly, so we can do better. */ |
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91 | |||
92 | struct cie_info |
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93 | { |
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94 | unsigned code_alignment; |
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95 | int z_augmentation; |
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96 | }; |
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97 | |||
98 | static int get_cie_info (struct cie_info *); |
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99 | |||
100 | /* Extract information from the CIE. */ |
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101 | |||
102 | static int |
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103 | get_cie_info (struct cie_info *info) |
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104 | { |
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105 | fragS *f; |
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106 | fixS *fix; |
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107 | int offset; |
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108 | char CIE_id; |
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109 | char augmentation[10]; |
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110 | int iaug; |
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111 | int code_alignment = 0; |
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112 | |||
113 | /* We should find the CIE at the start of the section. */ |
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114 | |||
115 | f = seg_info (now_seg)->frchainP->frch_root; |
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116 | fix = seg_info (now_seg)->frchainP->fix_root; |
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117 | |||
118 | /* Look through the frags of the section to find the code alignment. */ |
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119 | |||
120 | /* First make sure that the CIE Identifier Tag is 0/-1. */ |
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121 | |||
122 | if (strncmp (segment_name (now_seg), ".debug_frame", 12) == 0) |
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123 | CIE_id = (char)0xff; |
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124 | else |
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125 | CIE_id = 0; |
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126 | |||
127 | offset = 4; |
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128 | while (f != NULL && offset >= f->fr_fix) |
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129 | { |
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130 | offset -= f->fr_fix; |
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131 | f = f->fr_next; |
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132 | } |
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133 | if (f == NULL |
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134 | || f->fr_fix - offset < 4 |
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135 | || f->fr_literal[offset] != CIE_id |
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136 | || f->fr_literal[offset + 1] != CIE_id |
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137 | || f->fr_literal[offset + 2] != CIE_id |
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138 | || f->fr_literal[offset + 3] != CIE_id) |
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139 | return 0; |
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140 | |||
141 | /* Next make sure the CIE version number is 1. */ |
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142 | |||
143 | offset += 4; |
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144 | while (f != NULL && offset >= f->fr_fix) |
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145 | { |
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146 | offset -= f->fr_fix; |
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147 | f = f->fr_next; |
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148 | } |
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149 | if (f == NULL |
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150 | || f->fr_fix - offset < 1 |
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151 | || f->fr_literal[offset] != 1) |
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152 | return 0; |
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153 | |||
154 | /* Skip the augmentation (a null terminated string). */ |
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155 | |||
156 | iaug = 0; |
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157 | ++offset; |
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158 | while (1) |
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159 | { |
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160 | while (f != NULL && offset >= f->fr_fix) |
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161 | { |
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162 | offset -= f->fr_fix; |
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163 | f = f->fr_next; |
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164 | } |
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165 | if (f == NULL) |
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166 | return 0; |
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167 | |||
168 | while (offset < f->fr_fix && f->fr_literal[offset] != '\0') |
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169 | { |
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170 | if ((size_t) iaug < (sizeof augmentation) - 1) |
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171 | { |
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172 | augmentation[iaug] = f->fr_literal[offset]; |
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173 | ++iaug; |
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174 | } |
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175 | ++offset; |
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176 | } |
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177 | if (offset < f->fr_fix) |
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178 | break; |
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179 | } |
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180 | ++offset; |
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181 | while (f != NULL && offset >= f->fr_fix) |
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182 | { |
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183 | offset -= f->fr_fix; |
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184 | f = f->fr_next; |
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185 | } |
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186 | if (f == NULL) |
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187 | return 0; |
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188 | |||
189 | augmentation[iaug] = '\0'; |
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190 | if (augmentation[0] == '\0') |
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191 | { |
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192 | /* No augmentation. */ |
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193 | } |
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194 | else if (strcmp (augmentation, "eh") == 0) |
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195 | { |
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196 | /* We have to skip a pointer. Unfortunately, we don't know how |
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197 | large it is. We find out by looking for a matching fixup. */ |
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198 | while (fix != NULL |
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199 | && (fix->fx_frag != f || fix->fx_where != offset)) |
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200 | fix = fix->fx_next; |
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201 | if (fix == NULL) |
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202 | offset += 4; |
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203 | else |
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204 | offset += fix->fx_size; |
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205 | while (f != NULL && offset >= f->fr_fix) |
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206 | { |
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207 | offset -= f->fr_fix; |
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208 | f = f->fr_next; |
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209 | } |
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210 | if (f == NULL) |
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211 | return 0; |
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212 | } |
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213 | else if (augmentation[0] != 'z') |
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214 | return 0; |
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215 | |||
216 | /* We're now at the code alignment factor, which is a ULEB128. If |
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217 | it isn't a single byte, forget it. */ |
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218 | |||
219 | code_alignment = f->fr_literal[offset] & 0xff; |
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220 | if ((code_alignment & 0x80) != 0) |
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221 | code_alignment = 0; |
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222 | |||
223 | info->code_alignment = code_alignment; |
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224 | info->z_augmentation = (augmentation[0] == 'z'); |
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225 | |||
226 | return 1; |
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227 | } |
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228 | |||
229 | enum frame_state |
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230 | { |
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231 | state_idle, |
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232 | state_saw_size, |
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233 | state_saw_cie_offset, |
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234 | state_saw_pc_begin, |
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235 | state_seeing_aug_size, |
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236 | state_skipping_aug, |
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237 | state_wait_loc4, |
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238 | state_saw_loc4, |
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239 | state_error, |
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240 | }; |
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241 | |||
242 | /* This function is called from emit_expr. It looks for cases which |
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243 | we can optimize. |
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244 | |||
245 | Rather than try to parse all this information as we read it, we |
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246 | look for a single byte DW_CFA_advance_loc4 followed by a 4 byte |
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247 | difference. We turn that into a rs_cfa_advance frag, and handle |
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248 | those frags at the end of the assembly. If the gcc output changes |
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249 | somewhat, this optimization may stop working. |
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250 | |||
251 | This function returns non-zero if it handled the expression and |
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252 | emit_expr should not do anything, or zero otherwise. It can also |
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253 | change *EXP and *PNBYTES. */ |
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254 | |||
255 | int |
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256 | check_eh_frame (expressionS *exp, unsigned int *pnbytes) |
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257 | { |
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258 | struct frame_data |
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259 | { |
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260 | enum frame_state state; |
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261 | |||
262 | int cie_info_ok; |
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263 | struct cie_info cie_info; |
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264 | |||
265 | symbolS *size_end_sym; |
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266 | fragS *loc4_frag; |
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267 | int loc4_fix; |
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268 | |||
269 | int aug_size; |
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270 | int aug_shift; |
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271 | }; |
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272 | |||
273 | static struct frame_data eh_frame_data; |
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274 | static struct frame_data debug_frame_data; |
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275 | struct frame_data *d; |
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276 | |||
277 | /* Don't optimize. */ |
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278 | if (flag_traditional_format) |
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279 | return 0; |
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280 | |||
281 | #ifdef md_allow_eh_opt |
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282 | if (! md_allow_eh_opt) |
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283 | return 0; |
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284 | #endif |
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285 | |||
286 | /* Select the proper section data. */ |
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287 | if (strncmp (segment_name (now_seg), ".eh_frame", 9) == 0 |
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288 | && segment_name (now_seg)[9] != '_') |
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289 | d = &eh_frame_data; |
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290 | else if (strncmp (segment_name (now_seg), ".debug_frame", 12) == 0) |
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291 | d = &debug_frame_data; |
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292 | else |
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293 | return 0; |
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294 | |||
295 | if (d->state >= state_saw_size && S_IS_DEFINED (d->size_end_sym)) |
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296 | { |
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297 | /* We have come to the end of the CIE or FDE. See below where |
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298 | we set saw_size. We must check this first because we may now |
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299 | be looking at the next size. */ |
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300 | d->state = state_idle; |
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301 | } |
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302 | |||
303 | switch (d->state) |
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304 | { |
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305 | case state_idle: |
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306 | if (*pnbytes == 4) |
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307 | { |
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308 | /* This might be the size of the CIE or FDE. We want to know |
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309 | the size so that we don't accidentally optimize across an FDE |
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310 | boundary. We recognize the size in one of two forms: a |
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311 | symbol which will later be defined as a difference, or a |
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312 | subtraction of two symbols. Either way, we can tell when we |
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313 | are at the end of the FDE because the symbol becomes defined |
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314 | (in the case of a subtraction, the end symbol, from which the |
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315 | start symbol is being subtracted). Other ways of describing |
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316 | the size will not be optimized. */ |
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317 | if ((exp->X_op == O_symbol || exp->X_op == O_subtract) |
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318 | && ! S_IS_DEFINED (exp->X_add_symbol)) |
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319 | { |
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320 | d->state = state_saw_size; |
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321 | d->size_end_sym = exp->X_add_symbol; |
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322 | } |
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323 | } |
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324 | break; |
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325 | |||
326 | case state_saw_size: |
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327 | case state_saw_cie_offset: |
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328 | /* Assume whatever form it appears in, it appears atomically. */ |
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329 | d->state = (enum frame_state) (d->state + 1); |
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330 | break; |
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331 | |||
332 | case state_saw_pc_begin: |
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333 | /* Decide whether we should see an augmentation. */ |
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334 | if (! d->cie_info_ok |
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335 | && ! (d->cie_info_ok = get_cie_info (&d->cie_info))) |
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336 | d->state = state_error; |
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337 | else if (d->cie_info.z_augmentation) |
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338 | { |
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339 | d->state = state_seeing_aug_size; |
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340 | d->aug_size = 0; |
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341 | d->aug_shift = 0; |
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342 | } |
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343 | else |
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344 | d->state = state_wait_loc4; |
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345 | break; |
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346 | |||
347 | case state_seeing_aug_size: |
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348 | /* Bytes == -1 means this comes from an leb128 directive. */ |
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349 | if ((int)*pnbytes == -1 && exp->X_op == O_constant) |
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350 | { |
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351 | d->aug_size = exp->X_add_number; |
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352 | d->state = state_skipping_aug; |
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353 | } |
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354 | else if (*pnbytes == 1 && exp->X_op == O_constant) |
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355 | { |
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356 | unsigned char byte = exp->X_add_number; |
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357 | d->aug_size |= (byte & 0x7f) << d->aug_shift; |
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358 | d->aug_shift += 7; |
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359 | if ((byte & 0x80) == 0) |
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360 | d->state = state_skipping_aug; |
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361 | } |
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362 | else |
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363 | d->state = state_error; |
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364 | if (d->state == state_skipping_aug && d->aug_size == 0) |
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365 | d->state = state_wait_loc4; |
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366 | break; |
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367 | |||
368 | case state_skipping_aug: |
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369 | if ((int)*pnbytes < 0) |
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370 | d->state = state_error; |
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371 | else |
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372 | { |
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373 | int left = (d->aug_size -= *pnbytes); |
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374 | if (left == 0) |
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375 | d->state = state_wait_loc4; |
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376 | else if (left < 0) |
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377 | d->state = state_error; |
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378 | } |
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379 | break; |
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380 | |||
381 | case state_wait_loc4: |
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382 | if (*pnbytes == 1 |
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383 | && exp->X_op == O_constant |
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384 | && exp->X_add_number == DW_CFA_advance_loc4) |
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385 | { |
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386 | /* This might be a DW_CFA_advance_loc4. Record the frag and the |
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387 | position within the frag, so that we can change it later. */ |
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388 | frag_grow (1); |
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389 | d->state = state_saw_loc4; |
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390 | d->loc4_frag = frag_now; |
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391 | d->loc4_fix = frag_now_fix (); |
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392 | } |
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393 | break; |
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394 | |||
395 | case state_saw_loc4: |
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396 | d->state = state_wait_loc4; |
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397 | if (*pnbytes != 4) |
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398 | break; |
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399 | if (exp->X_op == O_constant) |
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400 | { |
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401 | /* This is a case which we can optimize. The two symbols being |
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402 | subtracted were in the same frag and the expression was |
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403 | reduced to a constant. We can do the optimization entirely |
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404 | in this function. */ |
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405 | if (exp->X_add_number < 0x40) |
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406 | { |
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407 | d->loc4_frag->fr_literal[d->loc4_fix] |
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408 | = DW_CFA_advance_loc | exp->X_add_number; |
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409 | /* No more bytes needed. */ |
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410 | return 1; |
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411 | } |
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412 | else if (exp->X_add_number < 0x100) |
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413 | { |
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414 | d->loc4_frag->fr_literal[d->loc4_fix] = DW_CFA_advance_loc1; |
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415 | *pnbytes = 1; |
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416 | } |
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417 | else if (exp->X_add_number < 0x10000) |
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418 | { |
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419 | d->loc4_frag->fr_literal[d->loc4_fix] = DW_CFA_advance_loc2; |
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420 | *pnbytes = 2; |
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421 | } |
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422 | } |
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423 | else if (exp->X_op == O_subtract && d->cie_info.code_alignment == 1) |
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424 | { |
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425 | /* This is a case we can optimize. The expression was not |
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426 | reduced, so we can not finish the optimization until the end |
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427 | of the assembly. We set up a variant frag which we handle |
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428 | later. */ |
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429 | frag_var (rs_cfa, 4, 0, 1 << 3, make_expr_symbol (exp), |
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430 | d->loc4_fix, (char *) d->loc4_frag); |
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431 | return 1; |
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432 | } |
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433 | else if ((exp->X_op == O_divide |
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434 | || exp->X_op == O_right_shift) |
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435 | && d->cie_info.code_alignment > 1) |
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436 | { |
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437 | if (exp->X_add_symbol->bsym |
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438 | && exp->X_op_symbol->bsym |
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439 | && exp->X_add_symbol->sy_value.X_op == O_subtract |
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440 | && exp->X_op_symbol->sy_value.X_op == O_constant |
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441 | && ((exp->X_op == O_divide |
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442 | ? exp->X_op_symbol->sy_value.X_add_number |
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443 | : (offsetT) 1 << exp->X_op_symbol->sy_value.X_add_number) |
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444 | == (offsetT) d->cie_info.code_alignment)) |
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445 | { |
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446 | /* This is a case we can optimize as well. The expression was |
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447 | not reduced, so we can not finish the optimization until the |
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448 | end of the assembly. We set up a variant frag which we |
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449 | handle later. */ |
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450 | frag_var (rs_cfa, 4, 0, d->cie_info.code_alignment << 3, |
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451 | make_expr_symbol (&exp->X_add_symbol->sy_value), |
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452 | d->loc4_fix, (char *) d->loc4_frag); |
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453 | return 1; |
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454 | } |
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455 | } |
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456 | break; |
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457 | |||
458 | case state_error: |
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459 | /* Just skipping everything. */ |
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460 | break; |
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461 | } |
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462 | |||
463 | return 0; |
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464 | } |
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465 | |||
466 | /* The function estimates the size of a rs_cfa variant frag based on |
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467 | the current values of the symbols. It is called before the |
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468 | relaxation loop. We set fr_subtype{0:2} to the expected length. */ |
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469 | |||
470 | int |
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471 | eh_frame_estimate_size_before_relax (fragS *frag) |
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472 | { |
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473 | offsetT diff; |
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474 | int ca = frag->fr_subtype >> 3; |
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475 | int ret; |
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476 | |||
477 | diff = resolve_symbol_value (frag->fr_symbol); |
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478 | |||
479 | gas_assert (ca > 0); |
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480 | diff /= ca; |
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481 | if (diff < 0x40) |
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482 | ret = 0; |
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483 | else if (diff < 0x100) |
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484 | ret = 1; |
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485 | else if (diff < 0x10000) |
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486 | ret = 2; |
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487 | else |
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488 | ret = 4; |
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489 | |||
490 | frag->fr_subtype = (frag->fr_subtype & ~7) | ret; |
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491 | |||
492 | return ret; |
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493 | } |
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494 | |||
495 | /* This function relaxes a rs_cfa variant frag based on the current |
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496 | values of the symbols. fr_subtype{0:2} is the current length of |
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497 | the frag. This returns the change in frag length. */ |
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498 | |||
499 | int |
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500 | eh_frame_relax_frag (fragS *frag) |
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501 | { |
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502 | int oldsize, newsize; |
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503 | |||
504 | oldsize = frag->fr_subtype & 7; |
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505 | newsize = eh_frame_estimate_size_before_relax (frag); |
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506 | return newsize - oldsize; |
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507 | } |
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508 | |||
509 | /* This function converts a rs_cfa variant frag into a normal fill |
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510 | frag. This is called after all relaxation has been done. |
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511 | fr_subtype{0:2} will be the desired length of the frag. */ |
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512 | |||
513 | void |
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514 | eh_frame_convert_frag (fragS *frag) |
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515 | { |
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516 | offsetT diff; |
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517 | fragS *loc4_frag; |
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518 | int loc4_fix, ca; |
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519 | |||
520 | loc4_frag = (fragS *) frag->fr_opcode; |
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521 | loc4_fix = (int) frag->fr_offset; |
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522 | |||
523 | diff = resolve_symbol_value (frag->fr_symbol); |
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524 | |||
525 | ca = frag->fr_subtype >> 3; |
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526 | gas_assert (ca > 0); |
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527 | diff /= ca; |
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528 | switch (frag->fr_subtype & 7) |
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529 | { |
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530 | case 0: |
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531 | gas_assert (diff < 0x40); |
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532 | loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc | diff; |
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533 | break; |
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534 | |||
535 | case 1: |
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536 | gas_assert (diff < 0x100); |
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537 | loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc1; |
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538 | frag->fr_literal[frag->fr_fix] = diff; |
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539 | break; |
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540 | |||
541 | case 2: |
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542 | gas_assert (diff < 0x10000); |
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543 | loc4_frag->fr_literal[loc4_fix] = DW_CFA_advance_loc2; |
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544 | md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 2); |
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545 | break; |
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546 | |||
547 | default: |
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548 | md_number_to_chars (frag->fr_literal + frag->fr_fix, diff, 4); |
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549 | break; |
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550 | } |
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551 | |||
552 | frag->fr_fix += frag->fr_subtype & 7; |
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553 | frag->fr_type = rs_fill; |
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554 | frag->fr_subtype = 0; |
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555 | frag->fr_offset = 0; |
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556 | }>>>>>>><>><>><>>>>>>><>>>>>> |