lfs-book/chapter05/gcc-pass2.xml

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<sect1 id="ch-tools-gcc-pass2" role="wrap">
  <?dbhtml filename="gcc-pass2.html"?>

  <sect1info condition="script">
    <productname>gcc</productname>
    <productnumber>&gcc-version;</productnumber>
    <address>&gcc-url;</address>
  </sect1info>

  <title>GCC-&gcc-version; - Pass 2</title>

  <indexterm zone="ch-tools-gcc-pass2">
    <primary sortas="a-GCC">GCC</primary>
    <secondary>tools, pass 2</secondary>
  </indexterm>

  <sect2 role="package">
    <title/>

    <xi:include xmlns:xi="http://www.w3.org/2003/XInclude"
    href="../chapter06/gcc.xml"
    xpointer="xpointer(/sect1/sect2[1]/para[1])"/>

    <segmentedlist>
      <segtitle>&buildtime;</segtitle>
      <segtitle>&diskspace;</segtitle>

      <seglistitem>
        <seg>&gcc-ch5p2-sbu;</seg>
        <seg>&gcc-ch5p2-du;</seg>
      </seglistitem>
    </segmentedlist>

  </sect2>

  <sect2 role="installation">
    <title>Installation of GCC</title>

    <para>Versions of GCC later than 4.3 will treat this build as if
    it were a relocated compiler and disallow searching for startfiles in
    the location specified by <parameter>--prefix</parameter>. Since this
    will not be a relocated compiler, and the startfiles in
    <filename class="directory">/tools</filename> are crucial to building
    a working compiler linked to the libs in <filename class="directory">/tools</filename>,
    apply the following patch which partially reverts GCC to its old behavior:</para>

<screen><userinput remap="pre">patch -Np1 -i ../&gcc-startfiles-patch;</userinput></screen>

    <para>Under normal circumstances the GCC <command>fixincludes</command> script
    is run in order to fix potentially broken header files. As GCC-&gcc-version;
    and Glibc-&glibc-version; have already been installed at this point, and
    their respective header files are known to not require fixing, the
    <command>fixincludes</command> script is not required. In fact, running
    this script may actually pollute the build environment by
    installing fixed headers from the host system into GCC's private include
    directory. The running of the <command>fixincludes</command> script can
    be suppressed by issuing the following commands:</para>

<screen><userinput remap="pre">cp -v gcc/Makefile.in{,.orig}
sed 's@\./fixinc\.sh@-c true@' gcc/Makefile.in.orig &gt; gcc/Makefile.in</userinput></screen>

    <para>For x86 machines, a bootstrap build of GCC uses the
    <option>-fomit-frame-pointer</option> compiler flag.  Non-bootstrap builds
    omit this flag by default, and the goal should be to produce a compiler
    that is exactly the same as if it were bootstrapped.  Apply the following
    <command>sed</command> command to force the build to use the flag:</para> 

<screen><userinput remap="pre">cp -v gcc/Makefile.in{,.tmp}
sed 's/^XCFLAGS =$/&amp; -fomit-frame-pointer/' gcc/Makefile.in.tmp \
  &gt; gcc/Makefile.in</userinput></screen>

    <para>The following command will change the location of GCC's default
    dynamic linker to use the one installed in
    <filename class="directory">/tools</filename>. It also removes <filename
    class="directory">/usr/include</filename> from GCC's include search path.
    Doing this now rather than adjusting the specs file after installation
    ensures that the new dynamic linker is used during the actual build of
    GCC. That is, all of the binaries created during the build will link
    against the new Glibc. Issue:</para>

<screen><userinput remap="pre">for file in \
 $(find gcc/config -name linux64.h -o -name linux.h -o -name sysv4.h)
do
  cp -uv $file{,.orig}
  sed -e 's@/lib\(64\)\?\(32\)\?/ld@/tools&amp;@g' \
  -e 's@/usr@/tools@g' $file.orig &gt; $file
  echo '
#undef STANDARD_INCLUDE_DIR
#define STANDARD_INCLUDE_DIR 0
#define STANDARD_STARTFILE_PREFIX_1 ""
#define STANDARD_STARTFILE_PREFIX_2 ""' &gt;&gt; $file
  touch $file.orig
done</userinput></screen>

    <para>In case the above seems hard to follow, let's break it down a bit.
    First we find all the files under the gcc/config directory that are named
    either <filename>linux.h</filename>, <filename>linux64.h</filename> or
    <filename>sysv4.h</filename>.
    For each file found, we copy it to a file of the same name but with an added
    suffix of <quote>.orig</quote>. Then the first sed expression prepends
    <quote>/tools</quote> to every instance of <quote>/lib/ld</quote>,
    <quote>/lib64/ld</quote> or <quote>/lib32/ld</quote>, while the second one
    replaces hard-coded instances of <quote>/usr</quote>. Then we add our define
    statements which alter the include search path and the default startfile prefix
    to the end of the file.
    Finally, we use <command>touch</command> to update the timestamp on the copied files.
    When used in conjunction with <command>cp -u</command>, this prevents unexpected
    changes to the original files in case the command is inadvertently run twice.
    </para>

    <para>On x86_64, unsetting the multilib spec for GCC ensures that it
    won't attempt to link against libraries on the host:</para>

<screen><userinput remap="pre">case $(uname -m) in
  x86_64)
    for file in $(find gcc/config -name t-linux64) ; do \
      cp -v $file{,.orig}
      sed '/MULTILIB_OSDIRNAMES/d' $file.orig &gt; $file
    done
  ;;
esac</userinput></screen>

    <para>As in the first build of GCC it requires the GMP and MPFR packages.
    Unpack the tarballs and move them into the required directory names:</para>

<screen><userinput remap="pre">tar -jxf ../mpfr-&mpfr-version;.tar.bz2
mv -v mpfr-&mpfr-version; mpfr
tar -jxf ../gmp-&gmp-version;.tar.bz2
mv -v gmp-&gmp-version; gmp</userinput></screen>

    <para>Create a separate build directory again:</para>

<screen><userinput remap="pre">mkdir -v ../gcc-build
cd ../gcc-build</userinput></screen>

    <para>Before starting to build GCC, remember to unset any environment
    variables that override the default optimization flags.</para>

    <para>Now prepare GCC for compilation:</para>

<screen><userinput remap="configure">CC="$LFS_TGT-gcc -B/tools/lib/" \
    AR=$LFS_TGT-ar RANLIB=$LFS_TGT-ranlib \
    ../gcc-&gcc-version;/configure --prefix=/tools \
    --with-local-prefix=/tools --enable-clocale=gnu \
    --enable-shared --enable-threads=posix \
    --enable-__cxa_atexit --enable-languages=c,c++ \
    --disable-libstdcxx-pch --disable-multilib \
    --disable-bootstrap</userinput></screen>

    <variablelist>
      <title>The meaning of the new configure options:</title>

      <varlistentry>
        <term><parameter>--enable-clocale=gnu</parameter></term>
        <listitem>
          <para>This option ensures the correct locale model is selected
          for the C++ libraries under all circumstances. If the configure
          script finds the <emphasis>de_DE</emphasis> locale installed,
          it will select the correct gnu locale model. However, if the
          <emphasis>de_DE</emphasis> locale is not installed, there is the
          risk of building Application Binary Interface (ABI)-incompatible
          C++ libraries because the incorrect generic locale model may be
          selected.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><parameter>--enable-threads=posix</parameter></term>
        <listitem>
          <para>This enables C++ exception handling for multi-threaded code.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><parameter>--enable-__cxa_atexit</parameter></term>
        <listitem>
          <para>This option allows use of <function>__cxa_atexit</function>,
          rather than <function>atexit</function>, to register C++ destructors
          for local statics and global objects. This option is essential for
          fully standards-compliant handling of destructors. It also affects
          the C++ ABI, and therefore results in C++ shared libraries and C++
          programs that are interoperable with other Linux distributions.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><parameter>--enable-languages=c,c++</parameter></term>
        <listitem>
          <para>This option ensures that both the C and C++ compilers are
          built.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><parameter>--disable-libstdcxx-pch</parameter></term>
        <listitem>
          <para>Do not build the pre-compiled header (PCH) for
          <filename class="libraryfile">libstdc++</filename>. It takes up a
          lot of space, and we have no use for it.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term><parameter>--disable-bootstrap</parameter></term>
        <listitem>
          <para>For native builds of GCC, the default is to a "bootstrap" build.
          This does not just compile GCC, but compiles it several times. It
          uses the programs compiled in a first round to compile itself a
          second time, and then again a third time.  The second and third
          iterations are compared to make sure it can reproduce itself
          flawlessly. This also implies that it was compiled correctly.
          However, the LFS build method should provide a solid compiler
          without the need to bootstrap each time.</para> 
        </listitem>
      </varlistentry>

    </variablelist>

    <para>Compile the package:</para>

<screen><userinput remap="make">make</userinput></screen>

    <para>Install the package:</para>

<screen><userinput remap="install">make install</userinput></screen>

    <para>As a finishing touch, create a symlink. Many programs and scripts
    run <command>cc</command> instead of <command>gcc</command>, which is
    used to keep programs generic and therefore usable on all kinds of UNIX
    systems where the GNU C compiler is not always installed. Running
    <command>cc</command> leaves the system administrator free to decide
    which C compiler to install:</para>

<screen><userinput remap="install">ln -vs gcc /tools/bin/cc</userinput></screen>

  <caution>
    <para>At this point, it is imperative to stop and ensure that the basic
    functions (compiling and linking) of the new toolchain are working as
    expected. To perform a sanity check, run the following commands:</para>

<screen><userinput>echo 'main(){}' &gt; dummy.c
cc dummy.c
readelf -l a.out | grep ': /tools'</userinput></screen>

    <para>If everything is working correctly, there should be no errors,
    and the output of the last command will be of the form:</para>

<screen><computeroutput>[Requesting program interpreter:
    /tools/lib/ld-linux.so.2]</computeroutput></screen>

    <para>Note that <filename class="directory">/tools/lib</filename>, or
    <filename class="directory">/tools/lib64</filename> for 64-bit machines
    appears as the prefix of the dynamic linker.</para>

    <para>If the output is not shown as above or there was no output at all,
    then something is wrong. Investigate and retrace the steps to find out
    where the problem is and correct it. This issue must be resolved before
    continuing on. First, perform the sanity check again, using
    <command>gcc</command> instead of <command>cc</command>. If this works,
    then the <filename class="symlink">/tools/bin/cc</filename> symlink is
    missing. Install the symlink as per above.
    Next, ensure that the <envar>PATH</envar> is correct. This
    can be checked by running <command>echo $PATH</command> and verifying that
    <filename class="directory">/tools/bin</filename> is at the head of the
    list. If the <envar>PATH</envar> is wrong it could mean that you are not
    logged in as user <systemitem class="username">lfs</systemitem> or that
    something went wrong back in <xref linkend="ch-tools-settingenviron"
    role="."/></para>

    <para>Once all is well, clean up the test files:</para>

<screen><userinput>rm -v dummy.c a.out</userinput></screen>

  </caution>

  </sect2>

  <sect2 role="content">
    <title/>

    <para>Details on this package are located in
    <xref linkend="contents-gcc" role="."/></para>

  </sect2>

</sect1>