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     25 <title>Installing GCC: Building</title>
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     58 <h1 class="settitle" align="center">Installing GCC: Building</h1>
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     78 
     79 <span id="index-Installing-GCC_003a-Building"></span>
     80 
     81 <p>Now that GCC is configured, you are ready to build the compiler and
     82 runtime libraries.
     83 </p>
     84 <p>Some commands executed when making the compiler may fail (return a
     85 nonzero status) and be ignored by <code>make</code>.  These failures, which
     86 are often due to files that were not found, are expected, and can safely
     87 be ignored.
     88 </p>
     89 <p>It is normal to have compiler warnings when compiling certain files.
     90 Unless you are a GCC developer, you can generally ignore these warnings
     91 unless they cause compilation to fail.  Developers should attempt to fix
     92 any warnings encountered, however they can temporarily continue past
     93 warnings-as-errors by specifying the configure flag
     94 <samp>--disable-werror</samp>.
     95 </p>
     96 <p>On certain old systems, defining certain environment variables such as
     97 <code>CC</code> can interfere with the functioning of <code>make</code>.
     98 </p>
     99 <p>If you encounter seemingly strange errors when trying to build the
    100 compiler in a directory other than the source directory, it could be
    101 because you have previously configured the compiler in the source
    102 directory.  Make sure you have done all the necessary preparations.
    103 </p>
    104 <p>If you build GCC on a BSD system using a directory stored in an old System
    105 V file system, problems may occur in running <code>fixincludes</code> if the
    106 System V file system doesn&rsquo;t support symbolic links.  These problems
    107 result in a failure to fix the declaration of <code>size_t</code> in
    108 <samp>sys/types.h</samp>.  If you find that <code>size_t</code> is a signed type and
    109 that type mismatches occur, this could be the cause.
    110 </p>
    111 <p>The solution is not to use such a directory for building GCC.
    112 </p>
    113 <p>Similarly, when building from the source repository or snapshots, or if you modify
    114 <samp>*.l</samp> files, you need the Flex lexical analyzer generator
    115 installed.  If you do not modify <samp>*.l</samp> files, releases contain
    116 the Flex-generated files and you do not need Flex installed to build
    117 them.  There is still one Flex-based lexical analyzer (part of the
    118 build machinery, not of GCC itself) that is used even if you only
    119 build the C front end.
    120 </p>
    121 <p>When building from the source repository or snapshots, or if you modify Texinfo
    122 documentation, you need version 4.7 or later of Texinfo installed if you
    123 want Info documentation to be regenerated.  Releases contain Info
    124 documentation pre-built for the unmodified documentation in the release.
    125 </p>
    126 <div class="section" id="Building-a-native-compiler">
    127 <h3 class="section">Building a native compiler</h3>
    128 
    129 <p>For a native build, the default configuration is to perform
    130 a 3-stage bootstrap of the compiler when &lsquo;<samp>make</samp>&rsquo; is invoked.
    131 This will build the entire GCC system and ensure that it compiles
    132 itself correctly.  It can be disabled with the <samp>--disable-bootstrap</samp>
    133 parameter to &lsquo;<samp>configure</samp>&rsquo;, but bootstrapping is suggested because
    134 the compiler will be tested more completely and could also have
    135 better performance.
    136 </p>
    137 <p>The bootstrapping process will complete the following steps:
    138 </p>
    139 <ul>
    140 <li> Build tools necessary to build the compiler.
    141 
    142 </li><li> Perform a 3-stage bootstrap of the compiler.  This includes building
    143 three times the target tools for use by the compiler such as binutils
    144 (bfd, binutils, gas, gprof, ld, and opcodes) if they have been
    145 individually linked or moved into the top level GCC source tree before
    146 configuring.
    147 
    148 </li><li> Perform a comparison test of the stage2 and stage3 compilers.
    149 
    150 </li><li> Build runtime libraries using the stage3 compiler from the previous step.
    151 
    152 </li></ul>
    153 
    154 <p>If you are short on disk space you might consider &lsquo;<samp>make
    155 bootstrap-lean</samp>&rsquo; instead.  The sequence of compilation is the
    156 same described above, but object files from the stage1 and
    157 stage2 of the 3-stage bootstrap of the compiler are deleted as
    158 soon as they are no longer needed.
    159 </p>
    160 <p>If you wish to use non-default GCC flags when compiling the stage2
    161 and stage3 compilers, set <code>BOOT_CFLAGS</code> on the command line when
    162 doing &lsquo;<samp>make</samp>&rsquo;.  For example, if you want to save additional space
    163 during the bootstrap and in the final installation as well, you can
    164 build the compiler binaries without debugging information as in the
    165 following example.  This will save roughly 40% of disk space both for
    166 the bootstrap and the final installation.  (Libraries will still contain
    167 debugging information.)
    168 </p>
    169 <div class="example">
    170 <pre class="example">make BOOT_CFLAGS='-O' bootstrap
    171 </pre></div>
    172 
    173 <p>You can place non-default optimization flags into <code>BOOT_CFLAGS</code>; they
    174 are less well tested here than the default of &lsquo;<samp>-g -O2</samp>&rsquo;, but should
    175 still work.  In a few cases, you may find that you need to specify special
    176 flags such as <samp>-msoft-float</samp> here to complete the bootstrap; or,
    177 if the native compiler miscompiles the stage1 compiler, you may need
    178 to work around this, by choosing <code>BOOT_CFLAGS</code> to avoid the parts
    179 of the stage1 compiler that were miscompiled, or by using &lsquo;<samp>make
    180 bootstrap4</samp>&rsquo; to increase the number of stages of bootstrap.
    181 </p>
    182 <p><code>BOOT_CFLAGS</code> does not apply to bootstrapped target libraries.
    183 Since these are always compiled with the compiler currently being
    184 bootstrapped, you can use <code>CFLAGS_FOR_TARGET</code> to modify their
    185 compilation flags, as for non-bootstrapped target libraries.
    186 Again, if the native compiler miscompiles the stage1 compiler, you may
    187 need to work around this by avoiding non-working parts of the stage1
    188 compiler.  Use <code>STAGE1_TFLAGS</code> to this end.
    189 </p>
    190 <p>If you used the flag <samp>--enable-languages=&hellip;</samp> to restrict
    191 the compilers to be built, only those you&rsquo;ve actually enabled will be
    192 built.  This will of course only build those runtime libraries, for
    193 which the particular compiler has been built.  Please note,
    194 that re-defining <code>LANGUAGES</code> when calling &lsquo;<samp>make</samp>&rsquo;
    195 <strong>does not</strong> work anymore!
    196 </p>
    197 <p>If the comparison of stage2 and stage3 fails, this normally indicates
    198 that the stage2 compiler has compiled GCC incorrectly, and is therefore
    199 a potentially serious bug which you should investigate and report.  (On
    200 a few systems, meaningful comparison of object files is impossible; they
    201 always appear &ldquo;different&rdquo;.  If you encounter this problem, you will
    202 need to disable comparison in the <samp>Makefile</samp>.)
    203 </p>
    204 <p>If you do not want to bootstrap your compiler, you can configure with
    205 <samp>--disable-bootstrap</samp>.  In particular cases, you may want to
    206 bootstrap your compiler even if the target system is not the same as
    207 the one you are building on: for example, you could build a
    208 <code>powerpc-unknown-linux-gnu</code> toolchain on a
    209 <code>powerpc64-unknown-linux-gnu</code> host.  In this case, pass
    210 <samp>--enable-bootstrap</samp> to the configure script.
    211 </p>
    212 <p><code>BUILD_CONFIG</code> can be used to bring in additional customization
    213 to the build.  It can be set to a whitespace-separated list of names.
    214 For each such <code>NAME</code>, top-level <samp>config/<code>NAME</code>.mk</samp> will
    215 be included by the top-level <samp>Makefile</samp>, bringing in any settings
    216 it contains.  The default <code>BUILD_CONFIG</code> can be set using the
    217 configure option <samp>--with-build-config=<code>NAME</code>...</samp>.  Some
    218 examples of supported build configurations are:
    219 </p>
    220 <dl compact="compact">
    221 <dt><span>&lsquo;<samp>bootstrap-O1</samp>&rsquo;</span></dt>
    222 <dd><p>Removes any <samp>-O</samp>-started option from <code>BOOT_CFLAGS</code>, and adds
    223 <samp>-O1</samp> to it.  &lsquo;<samp>BUILD_CONFIG=bootstrap-O1</samp>&rsquo; is equivalent to
    224 &lsquo;<samp>BOOT_CFLAGS='-g -O1'</samp>&rsquo;.
    225 </p>
    226 </dd>
    227 <dt><span>&lsquo;<samp>bootstrap-O3</samp>&rsquo;</span></dt>
    228 <dt><span>&lsquo;<samp>bootstrap-Og</samp>&rsquo;</span></dt>
    229 <dd><p>Analogous to <code>bootstrap-O1</code>.
    230 </p>
    231 </dd>
    232 <dt><span>&lsquo;<samp>bootstrap-lto</samp>&rsquo;</span></dt>
    233 <dd><p>Enables Link-Time Optimization for host tools during bootstrapping.
    234 &lsquo;<samp>BUILD_CONFIG=bootstrap-lto</samp>&rsquo; is equivalent to adding
    235 <samp>-flto</samp> to &lsquo;<samp>BOOT_CFLAGS</samp>&rsquo;.  This option assumes that the host
    236 supports the linker plugin (e.g. GNU ld version 2.21 or later or GNU gold
    237 version 2.21 or later).
    238 </p>
    239 </dd>
    240 <dt><span>&lsquo;<samp>bootstrap-lto-noplugin</samp>&rsquo;</span></dt>
    241 <dd><p>This option is similar to <code>bootstrap-lto</code>, but is intended for
    242 hosts that do not support the linker plugin.  Without the linker plugin 
    243 static libraries are not compiled with link-time optimizations.  Since 
    244 the GCC middle end and back end are in <samp>libbackend.a</samp> this means
    245 that only the front end is actually LTO optimized.
    246 </p>
    247 </dd>
    248 <dt><span>&lsquo;<samp>bootstrap-lto-lean</samp>&rsquo;</span></dt>
    249 <dd><p>This option is similar to <code>bootstrap-lto</code>, but is intended for
    250 faster build by only using LTO in the final bootstrap stage.
    251 With &lsquo;<samp>make profiledbootstrap</samp>&rsquo; the LTO frontend
    252 is trained only on generator files.
    253 </p>
    254 </dd>
    255 <dt><span>&lsquo;<samp>bootstrap-debug</samp>&rsquo;</span></dt>
    256 <dd><p>Verifies that the compiler generates the same executable code, whether
    257 or not it is asked to emit debug information.  To this end, this
    258 option builds stage2 host programs without debug information, and uses
    259 <samp>contrib/compare-debug</samp> to compare them with the stripped stage3
    260 object files.  If <code>BOOT_CFLAGS</code> is overridden so as to not enable
    261 debug information, stage2 will have it, and stage3 won&rsquo;t.  This option
    262 is enabled by default when GCC bootstrapping is enabled, if
    263 <code>strip</code> can turn object files compiled with and without debug
    264 info into identical object files.  In addition to better test
    265 coverage, this option makes default bootstraps faster and leaner.
    266 </p>
    267 </dd>
    268 <dt><span>&lsquo;<samp>bootstrap-debug-big</samp>&rsquo;</span></dt>
    269 <dd><p>Rather than comparing stripped object files, as in
    270 <code>bootstrap-debug</code>, this option saves internal compiler dumps
    271 during stage2 and stage3 and compares them as well, which helps catch
    272 additional potential problems, but at a great cost in terms of disk
    273 space.  It can be specified in addition to &lsquo;<samp>bootstrap-debug</samp>&rsquo;.
    274 </p>
    275 </dd>
    276 <dt><span>&lsquo;<samp>bootstrap-debug-lean</samp>&rsquo;</span></dt>
    277 <dd><p>This option saves disk space compared with <code>bootstrap-debug-big</code>,
    278 but at the expense of some recompilation.  Instead of saving the dumps
    279 of stage2 and stage3 until the final compare, it uses
    280 <samp>-fcompare-debug</samp> to generate, compare and remove the dumps
    281 during stage3, repeating the compilation that already took place in
    282 stage2, whose dumps were not saved.
    283 </p>
    284 </dd>
    285 <dt><span>&lsquo;<samp>bootstrap-debug-lib</samp>&rsquo;</span></dt>
    286 <dd><p>This option tests executable code invariance over debug information
    287 generation on target libraries, just like <code>bootstrap-debug-lean</code>
    288 tests it on host programs.  It builds stage3 libraries with
    289 <samp>-fcompare-debug</samp>, and it can be used along with any of the
    290 <code>bootstrap-debug</code> options above.
    291 </p>
    292 <p>There aren&rsquo;t <code>-lean</code> or <code>-big</code> counterparts to this option
    293 because most libraries are only build in stage3, so bootstrap compares
    294 would not get significant coverage.  Moreover, the few libraries built
    295 in stage2 are used in stage3 host programs, so we wouldn&rsquo;t want to
    296 compile stage2 libraries with different options for comparison purposes.
    297 </p>
    298 </dd>
    299 <dt><span>&lsquo;<samp>bootstrap-debug-ckovw</samp>&rsquo;</span></dt>
    300 <dd><p>Arranges for error messages to be issued if the compiler built on any
    301 stage is run without the option <samp>-fcompare-debug</samp>.  This is
    302 useful to verify the full <samp>-fcompare-debug</samp> testing coverage.  It
    303 must be used along with <code>bootstrap-debug-lean</code> and
    304 <code>bootstrap-debug-lib</code>.
    305 </p>
    306 </dd>
    307 <dt><span>&lsquo;<samp>bootstrap-cet</samp>&rsquo;</span></dt>
    308 <dd><p>This option enables Intel CET for host tools during bootstrapping.
    309 &lsquo;<samp>BUILD_CONFIG=bootstrap-cet</samp>&rsquo; is equivalent to adding
    310 <samp>-fcf-protection</samp> to &lsquo;<samp>BOOT_CFLAGS</samp>&rsquo;.  This option
    311 assumes that the host supports Intel CET (e.g. GNU assembler version
    312 2.30 or later).
    313 </p>
    314 </dd>
    315 <dt><span>&lsquo;<samp>bootstrap-time</samp>&rsquo;</span></dt>
    316 <dd><p>Arranges for the run time of each program started by the GCC driver,
    317 built in any stage, to be logged to <samp>time.log</samp>, in the top level of
    318 the build tree.
    319 </p>
    320 </dd>
    321 <dt><span>&lsquo;<samp>bootstrap-asan</samp>&rsquo;</span></dt>
    322 <dd><p>Compiles GCC itself using Address Sanitization in order to catch invalid memory
    323 accesses within the GCC code.
    324 </p>
    325 </dd>
    326 </dl>
    327 
    328 </div>
    329 <div class="section" id="Building-a-cross-compiler">
    330 <h3 class="section">Building a cross compiler</h3>
    331 
    332 <p>When building a cross compiler, it is not generally possible to do a
    333 3-stage bootstrap of the compiler.  This makes for an interesting problem
    334 as parts of GCC can only be built with GCC.
    335 </p>
    336 <p>To build a cross compiler, we recommend first building and installing a
    337 native compiler.  You can then use the native GCC compiler to build the
    338 cross compiler.  The installed native compiler needs to be GCC version
    339 2.95 or later.
    340 </p>
    341 <p>Assuming you have already installed a native copy of GCC and configured
    342 your cross compiler, issue the command <code>make</code>, which performs the
    343 following steps:
    344 </p>
    345 <ul>
    346 <li> Build host tools necessary to build the compiler.
    347 
    348 </li><li> Build target tools for use by the compiler such as binutils (bfd,
    349 binutils, gas, gprof, ld, and opcodes)
    350 if they have been individually linked or moved into the top level GCC source
    351 tree before configuring.
    352 
    353 </li><li> Build the compiler (single stage only).
    354 
    355 </li><li> Build runtime libraries using the compiler from the previous step.
    356 </li></ul>
    357 
    358 <p>Note that if an error occurs in any step the make process will exit.
    359 </p>
    360 <p>If you are not building GNU binutils in the same source tree as GCC,
    361 you will need a cross-assembler and cross-linker installed before
    362 configuring GCC.  Put them in the directory
    363 <samp><var>prefix</var>/<var>target</var>/bin</samp>.  Here is a table of the tools
    364 you should put in this directory:
    365 </p>
    366 <dl compact="compact">
    367 <dt><span><samp>as</samp></span></dt>
    368 <dd><p>This should be the cross-assembler.
    369 </p>
    370 </dd>
    371 <dt><span><samp>ld</samp></span></dt>
    372 <dd><p>This should be the cross-linker.
    373 </p>
    374 </dd>
    375 <dt><span><samp>ar</samp></span></dt>
    376 <dd><p>This should be the cross-archiver: a program which can manipulate
    377 archive files (linker libraries) in the target machine&rsquo;s format.
    378 </p>
    379 </dd>
    380 <dt><span><samp>ranlib</samp></span></dt>
    381 <dd><p>This should be a program to construct a symbol table in an archive file.
    382 </p></dd>
    383 </dl>
    384 
    385 <p>The installation of GCC will find these programs in that directory,
    386 and copy or link them to the proper place to for the cross-compiler to
    387 find them when run later.
    388 </p>
    389 <p>The easiest way to provide these files is to build the Binutils package.
    390 Configure it with the same <samp>--host</samp> and <samp>--target</samp>
    391 options that you use for configuring GCC, then build and install
    392 them.  They install their executables automatically into the proper
    393 directory.  Alas, they do not support all the targets that GCC
    394 supports.
    395 </p>
    396 <p>If you are not building a C library in the same source tree as GCC,
    397 you should also provide the target libraries and headers before
    398 configuring GCC, specifying the directories with
    399 <samp>--with-sysroot</samp> or <samp>--with-headers</samp> and
    400 <samp>--with-libs</samp>.  Many targets also require &ldquo;start files&rdquo; such
    401 as <samp>crt0.o</samp> and
    402 <samp>crtn.o</samp> which are linked into each executable.  There may be several
    403 alternatives for <samp>crt0.o</samp>, for use with profiling or other
    404 compilation options.  Check your target&rsquo;s definition of
    405 <code>STARTFILE_SPEC</code> to find out what start files it uses.
    406 </p>
    407 </div>
    408 <div class="section" id="Building-in-parallel">
    409 <h3 class="section">Building in parallel</h3>
    410 
    411 <p>GNU Make 3.80 and above, which is necessary to build GCC, support
    412 building in parallel.  To activate this, you can use &lsquo;<samp>make -j 2</samp>&rsquo;
    413 instead of &lsquo;<samp>make</samp>&rsquo;.  You can also specify a bigger number, and
    414 in most cases using a value greater than the number of processors in
    415 your machine will result in fewer and shorter I/O latency hits, thus
    416 improving overall throughput; this is especially true for slow drives
    417 and network filesystems.
    418 </p>
    419 </div>
    420 <div class="section" id="Building-the-Ada-compiler">
    421 <h3 class="section">Building the Ada compiler</h3>
    422 
    423 <p><a href="prerequisites.html#GNAT-prerequisite">GNAT prerequisites</a>.
    424 </p>
    425 </div>
    426 <div class="section" id="Building-with-profile-feedback">
    427 <h3 class="section">Building with profile feedback</h3>
    428 
    429 <p>It is possible to use profile feedback to optimize the compiler itself.  This
    430 should result in a faster compiler binary.  Experiments done on x86 using gcc
    431 3.3 showed approximately 7 percent speedup on compiling C programs.  To
    432 bootstrap the compiler with profile feedback, use <code>make profiledbootstrap</code>.
    433 </p>
    434 <p>When &lsquo;<samp>make profiledbootstrap</samp>&rsquo; is run, it will first build a <code>stage1</code>
    435 compiler.  This compiler is used to build a <code>stageprofile</code> compiler
    436 instrumented to collect execution counts of instruction and branch
    437 probabilities.  Training run is done by building <code>stagetrain</code>
    438 compiler.  Finally a <code>stagefeedback</code> compiler is built
    439 using the information collected.
    440 </p>
    441 <p>Unlike standard bootstrap, several additional restrictions apply.  The
    442 compiler used to build <code>stage1</code> needs to support a 64-bit integral type.
    443 It is recommended to only use GCC for this.
    444 </p>
    445 <p>On Linux/x86_64 hosts with some restrictions (no virtualization) it is
    446 also possible to do autofdo build with &lsquo;<samp>make
    447 autoprofiledback</samp>&rsquo;. This uses Linux perf to sample branches in the
    448 binary and then rebuild it with feedback derived from the profile.
    449 Linux perf and the <code>autofdo</code> toolkit needs to be installed for
    450 this.
    451 </p>
    452 <p>Only the profile from the current build is used, so when an error
    453 occurs it is recommended to clean before restarting. Otherwise
    454 the code quality may be much worse.
    455 </p>
    456 <hr />
    457 <p>
    458 <p><a href="./index.html">Return to the GCC Installation page</a>
    459 </p>
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