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1 @c Copyright (C) 1988-2020 Free Software Foundation, Inc.
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2 @c This is part of the GCC manual.
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3 @c For copying conditions, see the file gcc.texi.
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4
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5 @node Fragments
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6 @chapter Makefile Fragments
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7 @cindex makefile fragment
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8
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9 When you configure GCC using the @file{configure} script, it will
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10 construct the file @file{Makefile} from the template file
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11 @file{Makefile.in}. When it does this, it can incorporate makefile
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12 fragments from the @file{config} directory. These are used to set
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13 Makefile parameters that are not amenable to being calculated by
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14 autoconf. The list of fragments to incorporate is set by
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15 @file{config.gcc} (and occasionally @file{config.build}
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16 and @file{config.host}); @xref{System Config}.
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17
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18 Fragments are named either @file{t-@var{target}} or @file{x-@var{host}},
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19 depending on whether they are relevant to configuring GCC to produce
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20 code for a particular target, or to configuring GCC to run on a
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21 particular host. Here @var{target} and @var{host} are mnemonics
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22 which usually have some relationship to the canonical system name, but
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23 no formal connection.
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24
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25 If these files do not exist, it means nothing needs to be added for a
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26 given target or host. Most targets need a few @file{t-@var{target}}
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27 fragments, but needing @file{x-@var{host}} fragments is rare.
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28
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29 @menu
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30 * Target Fragment:: Writing @file{t-@var{target}} files.
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31 * Host Fragment:: Writing @file{x-@var{host}} files.
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32 @end menu
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33
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34 @node Target Fragment
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35 @section Target Makefile Fragments
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36 @cindex target makefile fragment
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37 @cindex @file{t-@var{target}}
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38
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39 Target makefile fragments can set these Makefile variables.
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40
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41 @table @code
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42 @findex LIBGCC2_CFLAGS
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43 @item LIBGCC2_CFLAGS
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44 Compiler flags to use when compiling @file{libgcc2.c}.
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45
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46 @findex LIB2FUNCS_EXTRA
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47 @item LIB2FUNCS_EXTRA
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48 A list of source file names to be compiled or assembled and inserted
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49 into @file{libgcc.a}.
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50
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51 @findex CRTSTUFF_T_CFLAGS
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52 @item CRTSTUFF_T_CFLAGS
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53 Special flags used when compiling @file{crtstuff.c}.
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54 @xref{Initialization}.
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55
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56 @findex CRTSTUFF_T_CFLAGS_S
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57 @item CRTSTUFF_T_CFLAGS_S
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58 Special flags used when compiling @file{crtstuff.c} for shared
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59 linking. Used if you use @file{crtbeginS.o} and @file{crtendS.o}
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60 in @code{EXTRA-PARTS}.
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61 @xref{Initialization}.
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62
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63 @findex MULTILIB_OPTIONS
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64 @item MULTILIB_OPTIONS
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65 For some targets, invoking GCC in different ways produces objects
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66 that cannot be linked together. For example, for some targets GCC
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67 produces both big and little endian code. For these targets, you must
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68 arrange for multiple versions of @file{libgcc.a} to be compiled, one for
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69 each set of incompatible options. When GCC invokes the linker, it
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70 arranges to link in the right version of @file{libgcc.a}, based on
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71 the command line options used.
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72
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73 The @code{MULTILIB_OPTIONS} macro lists the set of options for which
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74 special versions of @file{libgcc.a} must be built. Write options that
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75 are mutually incompatible side by side, separated by a slash. Write
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76 options that may be used together separated by a space. The build
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77 procedure will build all combinations of compatible options.
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78
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79 For example, if you set @code{MULTILIB_OPTIONS} to @samp{m68000/m68020
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80 msoft-float}, @file{Makefile} will build special versions of
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81 @file{libgcc.a} using the following sets of options: @option{-m68000},
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82 @option{-m68020}, @option{-msoft-float}, @samp{-m68000 -msoft-float}, and
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83 @samp{-m68020 -msoft-float}.
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84
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85 @findex MULTILIB_DIRNAMES
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86 @item MULTILIB_DIRNAMES
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87 If @code{MULTILIB_OPTIONS} is used, this variable specifies the
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88 directory names that should be used to hold the various libraries.
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89 Write one element in @code{MULTILIB_DIRNAMES} for each element in
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90 @code{MULTILIB_OPTIONS}. If @code{MULTILIB_DIRNAMES} is not used, the
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91 default value will be @code{MULTILIB_OPTIONS}, with all slashes treated
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92 as spaces.
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93
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94 @code{MULTILIB_DIRNAMES} describes the multilib directories using GCC
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95 conventions and is applied to directories that are part of the GCC
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96 installation. When multilib-enabled, the compiler will add a
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97 subdirectory of the form @var{prefix}/@var{multilib} before each
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98 directory in the search path for libraries and crt files.
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99
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100 For example, if @code{MULTILIB_OPTIONS} is set to @samp{m68000/m68020
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101 msoft-float}, then the default value of @code{MULTILIB_DIRNAMES} is
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102 @samp{m68000 m68020 msoft-float}. You may specify a different value if
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103 you desire a different set of directory names.
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104
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105 @findex MULTILIB_MATCHES
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106 @item MULTILIB_MATCHES
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107 Sometimes the same option may be written in two different ways. If an
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108 option is listed in @code{MULTILIB_OPTIONS}, GCC needs to know about
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109 any synonyms. In that case, set @code{MULTILIB_MATCHES} to a list of
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110 items of the form @samp{option=option} to describe all relevant
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111 synonyms. For example, @samp{m68000=mc68000 m68020=mc68020}.
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112
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113 @findex MULTILIB_EXCEPTIONS
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114 @item MULTILIB_EXCEPTIONS
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115 Sometimes when there are multiple sets of @code{MULTILIB_OPTIONS} being
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116 specified, there are combinations that should not be built. In that
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117 case, set @code{MULTILIB_EXCEPTIONS} to be all of the switch exceptions
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118 in shell case syntax that should not be built.
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119
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120 For example the ARM processor cannot execute both hardware floating
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121 point instructions and the reduced size THUMB instructions at the same
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122 time, so there is no need to build libraries with both of these
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123 options enabled. Therefore @code{MULTILIB_EXCEPTIONS} is set to:
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124 @smallexample
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125 *mthumb/*mhard-float*
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126 @end smallexample
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127
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128 @findex MULTILIB_REQUIRED
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129 @item MULTILIB_REQUIRED
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130 Sometimes when there are only a few combinations are required, it would
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131 be a big effort to come up with a @code{MULTILIB_EXCEPTIONS} list to
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132 cover all undesired ones. In such a case, just listing all the required
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133 combinations in @code{MULTILIB_REQUIRED} would be more straightforward.
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134
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135 The way to specify the entries in @code{MULTILIB_REQUIRED} is same with
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136 the way used for @code{MULTILIB_EXCEPTIONS}, only this time what are
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137 required will be specified. Suppose there are multiple sets of
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138 @code{MULTILIB_OPTIONS} and only two combinations are required, one
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139 for ARMv7-M and one for ARMv7-R with hard floating-point ABI and FPU, the
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140 @code{MULTILIB_REQUIRED} can be set to:
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141 @smallexample
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142 @code{MULTILIB_REQUIRED} = mthumb/march=armv7-m
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143 @code{MULTILIB_REQUIRED} += march=armv7-r/mfloat-abi=hard/mfpu=vfpv3-d16
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144 @end smallexample
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145
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146 The @code{MULTILIB_REQUIRED} can be used together with
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147 @code{MULTILIB_EXCEPTIONS}. The option combinations generated from
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148 @code{MULTILIB_OPTIONS} will be filtered by @code{MULTILIB_EXCEPTIONS}
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149 and then by @code{MULTILIB_REQUIRED}.
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150
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151 @findex MULTILIB_REUSE
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152 @item MULTILIB_REUSE
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153 Sometimes it is desirable to reuse one existing multilib for different
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154 sets of options. Such kind of reuse can minimize the number of multilib
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155 variants. And for some targets it is better to reuse an existing multilib
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156 than to fall back to default multilib when there is no corresponding multilib.
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157 This can be done by adding reuse rules to @code{MULTILIB_REUSE}.
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158
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159 A reuse rule is comprised of two parts connected by equality sign. The left
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160 part is the option set used to build multilib and the right part is the option
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161 set that will reuse this multilib. Both parts should only use options
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162 specified in @code{MULTILIB_OPTIONS} and the equality signs found in options
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163 name should be replaced with periods. An explicit period in the rule can be
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164 escaped by preceding it with a backslash. The order of options in the left
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165 part matters and should be same with those specified in
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166 @code{MULTILIB_REQUIRED} or aligned with the order in @code{MULTILIB_OPTIONS}.
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167 There is no such limitation for options in the right part as we don't build
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168 multilib from them.
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169
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170 @code{MULTILIB_REUSE} is different from @code{MULTILIB_MATCHES} in that it
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171 sets up relations between two option sets rather than two options. Here is an
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172 example to demo how we reuse libraries built in Thumb mode for applications built
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173 in ARM mode:
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174 @smallexample
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175 @code{MULTILIB_REUSE} = mthumb/march.armv7-r=marm/march.armv7-r
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176 @end smallexample
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177
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178 Before the advent of @code{MULTILIB_REUSE}, GCC select multilib by comparing command
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179 line options with options used to build multilib. The @code{MULTILIB_REUSE} is
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180 complementary to that way. Only when the original comparison matches nothing it will
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181 work to see if it is OK to reuse some existing multilib.
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182
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183 @findex MULTILIB_EXTRA_OPTS
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184 @item MULTILIB_EXTRA_OPTS
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185 Sometimes it is desirable that when building multiple versions of
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186 @file{libgcc.a} certain options should always be passed on to the
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187 compiler. In that case, set @code{MULTILIB_EXTRA_OPTS} to be the list
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188 of options to be used for all builds. If you set this, you should
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189 probably set @code{CRTSTUFF_T_CFLAGS} to a dash followed by it.
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190
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191 @findex MULTILIB_OSDIRNAMES
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192 @item MULTILIB_OSDIRNAMES
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193 If @code{MULTILIB_OPTIONS} is used, this variable specifies
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194 a list of subdirectory names, that are used to modify the search
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195 path depending on the chosen multilib. Unlike @code{MULTILIB_DIRNAMES},
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196 @code{MULTILIB_OSDIRNAMES} describes the multilib directories using
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197 operating systems conventions, and is applied to the directories such as
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198 @code{lib} or those in the @env{LIBRARY_PATH} environment variable.
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199 The format is either the same as of
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200 @code{MULTILIB_DIRNAMES}, or a set of mappings. When it is the same
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201 as @code{MULTILIB_DIRNAMES}, it describes the multilib directories
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202 using operating system conventions, rather than GCC conventions. When it is a set
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203 of mappings of the form @var{gccdir}=@var{osdir}, the left side gives
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204 the GCC convention and the right gives the equivalent OS defined
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205 location. If the @var{osdir} part begins with a @samp{!},
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206 GCC will not search in the non-multilib directory and use
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207 exclusively the multilib directory. Otherwise, the compiler will
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208 examine the search path for libraries and crt files twice; the first
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209 time it will add @var{multilib} to each directory in the search path,
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210 the second it will not.
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211
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212 For configurations that support both multilib and multiarch,
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213 @code{MULTILIB_OSDIRNAMES} also encodes the multiarch name, thus
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214 subsuming @code{MULTIARCH_DIRNAME}. The multiarch name is appended to
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215 each directory name, separated by a colon (e.g.@:
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216 @samp{../lib32:i386-linux-gnu}).
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217
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218 Each multiarch subdirectory will be searched before the corresponding OS
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219 multilib directory, for example @samp{/lib/i386-linux-gnu} before
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220 @samp{/lib/../lib32}. The multiarch name will also be used to modify the
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221 system header search path, as explained for @code{MULTIARCH_DIRNAME}.
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222
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223 @findex MULTIARCH_DIRNAME
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224 @item MULTIARCH_DIRNAME
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225 This variable specifies the multiarch name for configurations that are
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226 multiarch-enabled but not multilibbed configurations.
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227
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228 The multiarch name is used to augment the search path for libraries, crt
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229 files and system header files with additional locations. The compiler
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230 will add a multiarch subdirectory of the form
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231 @var{prefix}/@var{multiarch} before each directory in the library and
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232 crt search path. It will also add two directories
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233 @code{LOCAL_INCLUDE_DIR}/@var{multiarch} and
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234 @code{NATIVE_SYSTEM_HEADER_DIR}/@var{multiarch}) to the system header
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235 search path, respectively before @code{LOCAL_INCLUDE_DIR} and
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236 @code{NATIVE_SYSTEM_HEADER_DIR}.
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237
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238 @code{MULTIARCH_DIRNAME} is not used for configurations that support
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239 both multilib and multiarch. In that case, multiarch names are encoded
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240 in @code{MULTILIB_OSDIRNAMES} instead.
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241
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242 More documentation about multiarch can be found at
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243 @uref{https://wiki.debian.org/Multiarch}.
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244
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245 @findex SPECS
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246 @item SPECS
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247 Unfortunately, setting @code{MULTILIB_EXTRA_OPTS} is not enough, since
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248 it does not affect the build of target libraries, at least not the
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249 build of the default multilib. One possible work-around is to use
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250 @code{DRIVER_SELF_SPECS} to bring options from the @file{specs} file
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251 as if they had been passed in the compiler driver command line.
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252 However, you don't want to be adding these options after the toolchain
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253 is installed, so you can instead tweak the @file{specs} file that will
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254 be used during the toolchain build, while you still install the
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255 original, built-in @file{specs}. The trick is to set @code{SPECS} to
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256 some other filename (say @file{specs.install}), that will then be
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257 created out of the built-in specs, and introduce a @file{Makefile}
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258 rule to generate the @file{specs} file that's going to be used at
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259 build time out of your @file{specs.install}.
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260
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261 @item T_CFLAGS
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262 These are extra flags to pass to the C compiler. They are used both
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263 when building GCC, and when compiling things with the just-built GCC@.
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264 This variable is deprecated and should not be used.
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265 @end table
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266
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267 @node Host Fragment
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268 @section Host Makefile Fragments
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269 @cindex host makefile fragment
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270 @cindex @file{x-@var{host}}
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271
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272 The use of @file{x-@var{host}} fragments is discouraged. You should only
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273 use it for makefile dependencies.
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