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