1 | Keeping data small
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2 |
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3 | When many applets are compiled into busybox, all rw data and
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4 | bss for each applet are concatenated. Including those from libc,
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5 | if static busybox is built. When busybox is started, _all_ this data
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6 | is allocated, not just that one part for selected applet.
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7 |
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8 | What "allocated" exactly means, depends on arch.
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9 | On NOMMU it's probably bites the most, actually using real
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10 | RAM for rwdata and bss. On i386, bss is lazily allocated
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11 | by COWed zero pages. Not sure about rwdata - also COW?
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12 |
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13 | In order to keep busybox NOMMU and small-mem systems friendly
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14 | we should avoid large global data in our applets, and should
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15 | minimize usage of libc functions which implicitly use
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16 | such structures.
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17 |
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18 | Small experiment to measure "parasitic" bbox memory consumption:
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19 | here we start 1000 "busybox sleep 10" in parallel.
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20 | busybox binary is practically allyesconfig static one,
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21 | built against uclibc. Run on x86-64 machine with 64-bit kernel:
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22 |
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23 | bash-3.2# nmeter '%t %c %m %p %[pn]'
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24 | 23:17:28 .......... 168M 0 147
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25 | 23:17:29 .......... 168M 0 147
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26 | 23:17:30 U......... 168M 1 147
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27 | 23:17:31 SU........ 181M 244 391
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28 | 23:17:32 SSSSUUU... 223M 757 1147
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29 | 23:17:33 UUU....... 223M 0 1147
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30 | 23:17:34 U......... 223M 1 1147
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31 | 23:17:35 .......... 223M 0 1147
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32 | 23:17:36 .......... 223M 0 1147
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33 | 23:17:37 S......... 223M 0 1147
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34 | 23:17:38 .......... 223M 1 1147
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35 | 23:17:39 .......... 223M 0 1147
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36 | 23:17:40 .......... 223M 0 1147
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37 | 23:17:41 .......... 210M 0 906
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38 | 23:17:42 .......... 168M 1 147
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39 | 23:17:43 .......... 168M 0 147
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40 |
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41 | This requires 55M of memory. Thus 1 trivial busybox applet
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42 | takes 55k of memory on 64-bit x86 kernel.
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43 |
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44 | On 32-bit kernel we need ~26k per applet.
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45 |
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46 | Script:
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47 |
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48 | i=1000; while test $i != 0; do
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49 | echo -n .
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50 | busybox sleep 30 &
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51 | i=$((i - 1))
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52 | done
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53 | echo
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54 | wait
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55 |
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56 | (Data from NOMMU arches are sought. Provide 'size busybox' output too)
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57 |
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58 |
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59 | Example 1
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60 |
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61 | One example how to reduce global data usage is in
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62 | archival/libunarchive/decompress_unzip.c:
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63 |
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64 | /* This is somewhat complex-looking arrangement, but it allows
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65 | * to place decompressor state either in bss or in
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66 | * malloc'ed space simply by changing #defines below.
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67 | * Sizes on i386:
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68 | * text data bss dec hex
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69 | * 5256 0 108 5364 14f4 - bss
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70 | * 4915 0 0 4915 1333 - malloc
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71 | */
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72 | #define STATE_IN_BSS 0
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73 | #define STATE_IN_MALLOC 1
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74 |
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75 | (see the rest of the file to get the idea)
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76 |
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77 | This example completely eliminates globals in that module.
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78 | Required memory is allocated in unpack_gz_stream() [its main module]
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79 | and then passed down to all subroutines which need to access 'globals'
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80 | as a parameter.
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81 |
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82 |
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83 | Example 2
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84 |
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85 | In case you don't want to pass this additional parameter everywhere,
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86 | take a look at archival/gzip.c. Here all global data is replaced by
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87 | single global pointer (ptr_to_globals) to allocated storage.
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88 |
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89 | In order to not duplicate ptr_to_globals in every applet, you can
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90 | reuse single common one. It is defined in libbb/messages.c
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91 | as struct globals *const ptr_to_globals, but the struct globals is
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92 | NOT defined in libbb.h. You first define your own struct:
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93 |
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94 | struct globals { int a; char buf[1000]; };
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95 |
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96 | and then declare that ptr_to_globals is a pointer to it:
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97 |
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98 | #define G (*ptr_to_globals)
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99 |
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100 | ptr_to_globals is declared as constant pointer.
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101 | This helps gcc understand that it won't change, resulting in noticeably
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102 | smaller code. In order to assign it, use PTR_TO_GLOBALS macro:
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103 |
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104 | PTR_TO_GLOBALS = xzalloc(sizeof(G));
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105 |
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106 | Typically it is done in <applet>_main().
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107 |
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108 | Now you can reference "globals" by G.a, G.buf and so on, in any function.
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109 |
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110 |
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111 | bb_common_bufsiz1
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112 |
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113 | There is one big common buffer in bss - bb_common_bufsiz1. It is a much
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114 | earlier mechanism to reduce bss usage. Each applet can use it for
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115 | its needs. Library functions are prohibited from using it.
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116 |
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117 | 'G.' trick can be done using bb_common_bufsiz1 instead of malloced buffer:
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118 |
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119 | #define G (*(struct globals*)&bb_common_bufsiz1)
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120 |
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121 | Be careful, though, and use it only if globals fit into bb_common_bufsiz1.
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122 | Since bb_common_bufsiz1 is BUFSIZ + 1 bytes long and BUFSIZ can change
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123 | from one libc to another, you have to add compile-time check for it:
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124 |
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125 | if (sizeof(struct globals) > sizeof(bb_common_bufsiz1))
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126 | BUG_<applet>_globals_too_big();
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127 |
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128 |
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129 | Drawbacks
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130 |
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131 | You have to initialize it by hand. xzalloc() can be helpful in clearing
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132 | allocated storage to 0, but anything more must be done by hand.
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133 |
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134 | All global variables are prefixed by 'G.' now. If this makes code
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135 | less readable, use #defines:
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136 |
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137 | #define dev_fd (G.dev_fd)
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138 | #define sector (G.sector)
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139 |
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140 |
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141 | Word of caution
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142 |
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143 | If applet doesn't use much of global data, converting it to use
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144 | one of above methods is not worth the resulting code obfuscation.
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145 | If you have less than ~300 bytes of global data - don't bother.
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146 |
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147 |
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148 | gcc's data alignment problem
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149 |
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150 | The following attribute added in vi.c:
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151 |
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152 | static int tabstop;
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153 | static struct termios term_orig __attribute__ ((aligned (4)));
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154 | static struct termios term_vi __attribute__ ((aligned (4)));
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155 |
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156 | reduces bss size by 32 bytes, because gcc sometimes aligns structures to
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157 | ridiculously large values. asm output diff for above example:
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158 |
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159 | tabstop:
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160 | .zero 4
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161 | .section .bss.term_orig,"aw",@nobits
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162 | - .align 32
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163 | + .align 4
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164 | .type term_orig, @object
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165 | .size term_orig, 60
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166 | term_orig:
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167 | .zero 60
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168 | .section .bss.term_vi,"aw",@nobits
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169 | - .align 32
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170 | + .align 4
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171 | .type term_vi, @object
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172 | .size term_vi, 60
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173 |
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174 | gcc doesn't seem to have options for altering this behaviour.
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175 |
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176 | gcc 3.4.3 and 4.1.1 tested:
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177 | char c = 1;
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178 | // gcc aligns to 32 bytes if sizeof(struct) >= 32
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179 | struct {
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180 | int a,b,c,d;
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181 | int i1,i2,i3;
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182 | } s28 = { 1 }; // struct will be aligned to 4 bytes
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183 | struct {
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184 | int a,b,c,d;
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185 | int i1,i2,i3,i4;
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186 | } s32 = { 1 }; // struct will be aligned to 32 bytes
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187 | // same for arrays
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188 | char vc31[31] = { 1 }; // unaligned
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189 | char vc32[32] = { 1 }; // aligned to 32 bytes
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190 |
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191 | -fpack-struct=1 reduces alignment of s28 to 1 (but probably
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192 | will break layout of many libc structs) but s32 and vc32
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193 | are still aligned to 32 bytes.
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194 |
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195 | I will try to cook up a patch to add a gcc option for disabling it.
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196 | Meanwhile, this is where it can be disabled in gcc source:
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197 |
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198 | gcc/config/i386/i386.c
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199 | int
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200 | ix86_data_alignment (tree type, int align)
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201 | {
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202 | #if 0
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203 | if (AGGREGATE_TYPE_P (type)
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204 | && TYPE_SIZE (type)
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205 | && TREE_CODE (TYPE_SIZE (type)) == INTEGER_CST
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206 | && (TREE_INT_CST_LOW (TYPE_SIZE (type)) >= 256
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207 | || TREE_INT_CST_HIGH (TYPE_SIZE (type))) && align < 256)
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208 | return 256;
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209 | #endif
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210 |
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211 | Result (non-static busybox built against glibc):
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212 |
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213 | # size /usr/srcdevel/bbox/fix/busybox.t0/busybox busybox
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214 | text data bss dec hex filename
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215 | 634416 2736 23856 661008 a1610 busybox
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216 | 632580 2672 22944 658196 a0b14 busybox_noalign
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