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406 lines
20 KiB
XML
406 lines
20 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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<!ENTITY % general-entities SYSTEM "../general.ent">
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%general-entities;
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]>
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<sect1 id="ch-tools-toolchaintechnotes" xreflabel="Toolchain Technical Notes">
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<?dbhtml filename="toolchaintechnotes.html"?>
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<title>Toolchain Technical Notes</title>
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<para>This section explains some of the rationale and technical details
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behind the overall build method. Don't try to immediately
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understand everything in this section. Most of this information will be
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clearer after performing an actual build. Come back and re-read this chapter
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at any time during the build process.</para>
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<para>The overall goal of <xref linkend="chapter-cross-tools"/> and <xref
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linkend="chapter-temporary-tools"/> is to produce a temporary area
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containing a set of tools that are known to be good, and that are isolated from the host system.
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By using the <command>chroot</command> command, the compilations in the remaining chapters
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will be isolated within that environment, ensuring a clean, trouble-free
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build of the target LFS system. The build process has been designed to
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minimize the risks for new readers, and to provide the most educational value
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at the same time.</para>
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<para>This build process is based on
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<emphasis>cross-compilation</emphasis>. Cross-compilation is normally used
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to build a compiler and its associated toolchain for a machine different from
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the one that is used for the build. This is not strictly necessary for LFS,
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since the machine where the new system will run is the same as the one
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used for the build. But cross-compilation has one great advantage:
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anything that is cross-compiled cannot depend on the host environment.</para>
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<sect2 id="cross-compile" xreflabel="About Cross-Compilation">
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<title>About Cross-Compilation</title>
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<note>
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<para>
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The LFS book is not (and does not contain) a general tutorial to
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build a cross- (or native) toolchain. Don't use the commands in the
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book for a cross-toolchain for some purpose other
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than building LFS, unless you really understand what you are doing.
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</para>
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</note>
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<para>Cross-compilation involves some concepts that deserve a section of
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their own. Although this section may be omitted on a first reading,
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coming back to it later will help you gain a fuller understanding of
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the process.</para>
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<para>Let us first define some terms used in this context.</para>
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<variablelist>
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<varlistentry><term>The build</term><listitem>
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<para>is the machine where we build programs. Note that this machine
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is also referred to as the <quote>host.</quote></para></listitem>
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</varlistentry>
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<varlistentry><term>The host</term><listitem>
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<para>is the machine/system where the built programs will run. Note
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that this use of <quote>host</quote> is not the same as in other
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sections.</para></listitem>
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</varlistentry>
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<varlistentry><term>The target</term><listitem>
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<para>is only used for compilers. It is the machine the compiler
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produces code for. It may be different from both the build and
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the host.</para></listitem>
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</varlistentry>
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</variablelist>
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<para>As an example, let us imagine the following scenario (sometimes
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referred to as <quote>Canadian Cross</quote>). We have a
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compiler on a slow machine only, let's call it machine A, and the compiler
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ccA. We also have a fast machine (B), but no compiler for (B), and we
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want to produce code for a third, slow machine (C). We will build a
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compiler for machine C in three stages.</para>
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<informaltable align="center">
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<tgroup cols="5">
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<colspec colnum="1" align="center"/>
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<colspec colnum="2" align="center"/>
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<colspec colnum="3" align="center"/>
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<colspec colnum="4" align="center"/>
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<colspec colnum="5" align="left"/>
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<thead>
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<row><entry>Stage</entry><entry>Build</entry><entry>Host</entry>
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<entry>Target</entry><entry>Action</entry></row>
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</thead>
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<tbody>
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<row>
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<entry>1</entry><entry>A</entry><entry>A</entry><entry>B</entry>
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<entry>Build cross-compiler cc1 using ccA on machine A.</entry>
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</row>
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<row>
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<entry>2</entry><entry>A</entry><entry>B</entry><entry>C</entry>
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<entry>Build cross-compiler cc2 using cc1 on machine A.</entry>
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</row>
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<row>
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<entry>3</entry><entry>B</entry><entry>C</entry><entry>C</entry>
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<entry>Build compiler ccC using cc2 on machine B.</entry>
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</row>
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</tbody>
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</tgroup>
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</informaltable>
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<para>Then, all the programs needed by machine C can be compiled
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using cc2 on the fast machine B. Note that unless B can run programs
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produced for C, there is no way to test the newly built programs until machine
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C itself is running. For example, to run a test suite on ccC, we may want to add a
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fourth stage:</para>
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<informaltable align="center">
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<tgroup cols="5">
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<colspec colnum="1" align="center"/>
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<colspec colnum="2" align="center"/>
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<colspec colnum="3" align="center"/>
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<colspec colnum="4" align="center"/>
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<colspec colnum="5" align="left"/>
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<thead>
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<row><entry>Stage</entry><entry>Build</entry><entry>Host</entry>
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<entry>Target</entry><entry>Action</entry></row>
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</thead>
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<tbody>
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<row>
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<entry>4</entry><entry>C</entry><entry>C</entry><entry>C</entry>
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<entry>Rebuild and test ccC using ccC on machine C.</entry>
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</row>
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</tbody>
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</tgroup>
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</informaltable>
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<para>In the example above, only cc1 and cc2 are cross-compilers, that is,
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they produce code for a machine different from the one they are run on.
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The other compilers ccA and ccC produce code for the machine they are run
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on. Such compilers are called <emphasis>native</emphasis> compilers.</para>
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</sect2>
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<sect2 id="lfs-cross">
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<title>Implementation of Cross-Compilation for LFS</title>
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<note>
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<para>All the cross-compiled packages in this book use an
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autoconf-based building system. The autoconf-based building system
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accepts system types in the form cpu-vendor-kernel-os,
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referred to as the system triplet. Since the vendor field is often
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irrelevant, autoconf lets you omit it.</para>
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<para>An astute reader may wonder
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why a <quote>triplet</quote> refers to a four component name. The
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kernel field and the os field began as a single
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<quote>system</quote> field. Such a three-field form is still valid
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today for some systems, for example,
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<literal>x86_64-unknown-freebsd</literal>. But
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two systems can share the same kernel and still be too different to
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use the same triplet to describe them. For example, Android running on a
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mobile phone is completely different from Ubuntu running on an ARM64
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server, even though they are both running on the same type of CPU (ARM64) and
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using the same kernel (Linux).</para>
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<para>Without an emulation layer, you cannot run an
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executable for a server on a mobile phone or vice versa. So the
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<quote>system</quote> field has been divided into kernel and os fields, to
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designate these systems unambiguously. In our example, the Android
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system is designated <literal>aarch64-unknown-linux-android</literal>,
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and the Ubuntu system is designated
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<literal>aarch64-unknown-linux-gnu</literal>.</para>
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<para>The word <quote>triplet</quote> remains embedded in the lexicon. A simple way to determine your
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system triplet is to run the <command>config.guess</command>
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script that comes with the source for many packages. Unpack the binutils
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sources, run the script <userinput>./config.guess</userinput>, and note
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the output. For example, for a 32-bit Intel processor the
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output will be <emphasis>i686-pc-linux-gnu</emphasis>. On a 64-bit
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system it will be <emphasis>x86_64-pc-linux-gnu</emphasis>. On most
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Linux systems the even simpler <command>gcc -dumpmachine</command> command
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will give you similar information.</para>
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<para>You should also be aware of the name of the platform's dynamic linker, often
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referred to as the dynamic loader (not to be confused with the standard
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linker <command>ld</command> that is part of binutils). The dynamic linker
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provided by package glibc finds and loads the shared libraries needed by a
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program, prepares the program to run, and then runs it. The name of the
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dynamic linker for a 32-bit Intel machine is <filename
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class="libraryfile">ld-linux.so.2</filename>; it's <filename
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class="libraryfile">ld-linux-x86-64.so.2</filename> on 64-bit systems. A
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sure-fire way to determine the name of the dynamic linker is to inspect a
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random binary from the host system by running: <userinput>readelf -l
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<name of binary> | grep interpreter</userinput> and noting the
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output. The authoritative reference covering all platforms is in
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<ulink url='https://sourceware.org/glibc/wiki/ABIList'>a Glibc wiki
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page</ulink>.</para>
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</note>
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<para>In order to fake a cross-compilation in LFS, the name of the host triplet
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is slightly adjusted by changing the "vendor" field in the
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<envar>LFS_TGT</envar> variable so it says "lfs". We also use the
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<parameter>--with-sysroot</parameter> option when building the cross-linker and
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cross-compiler, to tell them where to find the needed host files. This
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ensures that none of the other programs built in <xref
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linkend="chapter-temporary-tools"/> can link to libraries on the build
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machine. Only two stages are mandatory, plus one more for tests.</para>
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<informaltable align="center">
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<tgroup cols="5">
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<colspec colnum="1" align="center"/>
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<colspec colnum="2" align="center"/>
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<colspec colnum="3" align="center"/>
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<colspec colnum="4" align="center"/>
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<colspec colnum="5" align="left"/>
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<thead>
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<row><entry>Stage</entry><entry>Build</entry><entry>Host</entry>
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<entry>Target</entry><entry>Action</entry></row>
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</thead>
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<tbody>
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<row>
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<entry>1</entry><entry>pc</entry><entry>pc</entry><entry>lfs</entry>
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<entry>Build cross-compiler cc1 using cc-pc on pc.</entry>
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</row>
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<row>
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<entry>2</entry><entry>pc</entry><entry>lfs</entry><entry>lfs</entry>
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<entry>Build compiler cc-lfs using cc1 on pc.</entry>
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</row>
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<row>
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<entry>3</entry><entry>lfs</entry><entry>lfs</entry><entry>lfs</entry>
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<entry>Rebuild and test cc-lfs using cc-lfs on lfs.</entry>
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</row>
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</tbody>
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</tgroup>
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</informaltable>
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<para>In the preceding table, <quote>on pc</quote> means the commands are run
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on a machine using the already installed distribution. <quote>On
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lfs</quote> means the commands are run in a chrooted environment.</para>
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<para>This is not yet the end of the story. The C language is not
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merely a compiler; it also defines a standard library. In this book, the
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GNU C library, named glibc, is used (there is an alternative, "musl"). This library must
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be compiled for the LFS machine; that is, using the cross-compiler cc1.
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But the compiler itself uses an internal library providing complex
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subroutines for functions not available in the assembler instruction set. This
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internal library is named libgcc, and it must be linked to the glibc
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library to be fully functional. Furthermore, the standard library for
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C++ (libstdc++) must also be linked with glibc. The solution to this
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chicken and egg problem is first to build a degraded cc1-based libgcc,
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lacking some functionalities such as threads and exception handling, and then
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to build glibc using this degraded compiler (glibc itself is not
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degraded), and also to build libstdc++. This last library will lack some of the
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functionality of libgcc.</para>
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<para>The upshot of the preceding paragraph is that cc1 is unable to
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build a fully functional libstdc++ with the degraded libgcc, but cc1
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is the only compiler available for building the C/C++ libraries
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during stage 2. There are two reasons we don't immediately use the
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compiler built in stage 2, cc-lfs, to build those libraries.</para>
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<itemizedlist>
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<listitem>
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<para>
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Generally speaking, cc-lfs cannot run on pc (the host system). Even though the
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triplets for pc and lfs are compatible with each other, an executable
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for lfs must depend on glibc-&glibc-version;; the host distro
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may utilize either a different implementation of libc (for example, musl), or
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a previous release of glibc (for example, glibc-2.13).
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</para>
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</listitem>
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<listitem>
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<para>
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Even if cc-lfs can run on pc, using it on pc would create
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a risk of linking to the pc libraries, since cc-lfs is a native
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compiler.
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</para>
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</listitem>
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</itemizedlist>
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<para>So when we build gcc stage 2, we instruct the building system to
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rebuild libgcc and libstdc++ with cc1, but we link libstdc++ to the newly
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rebuilt libgcc instead of the old, degraded build. This makes the rebuilt
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libstdc++ fully functional.</para>
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<para>In &ch-final; (or <quote>stage 3</quote>), all the packages needed for
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the LFS system are built. Even if a package has already been installed into
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the LFS system in a previous chapter, we still rebuild the package. The main reason for
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rebuilding these packages is to make them stable: if we reinstall an LFS
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package on a completed LFS system, the reinstalled content of the package
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should be the same as the content of the same package when first installed in
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&ch-final;. The temporary packages installed in &ch-tmp-cross; or
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&ch-tmp-chroot; cannot satisfy this requirement, because some of them
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are built without optional dependencies, and autoconf cannot
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perform some feature checks in &ch-tmp-cross; because of cross-compilation,
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causing the temporary packages to lack optional features,
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or use suboptimal code routines. Additionally, a minor reason for
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rebuilding the packages is to run the test suites.</para>
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</sect2>
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<sect2 id="other-details">
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<title>Other Procedural Details</title>
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<para>The cross-compiler will be installed in a separate <filename
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class="directory">$LFS/tools</filename> directory, since it will not
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be part of the final system.</para>
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<para>Binutils is installed first because the <command>configure</command>
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runs of both gcc and glibc perform various feature tests on the assembler
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and linker to determine which software features to enable or disable. This
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is more important than one might realize at first. An incorrectly configured
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gcc or glibc can result in a subtly broken toolchain, where the impact of
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such breakage might not show up until near the end of the build of an
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entire distribution. A test suite failure will usually highlight this error
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before too much additional work is performed.</para>
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<para>Binutils installs its assembler and linker in two locations,
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<filename class="directory">$LFS/tools/bin</filename> and <filename
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class="directory">$LFS/tools/$LFS_TGT/bin</filename>. The tools in one
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location are hard linked to the other. An important facet of the linker is
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its library search order. Detailed information can be obtained from
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<command>ld</command> by passing it the <parameter>--verbose</parameter>
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flag. For example, <command>$LFS_TGT-ld --verbose | grep SEARCH</command>
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will illustrate the current search paths and their order. (Note that this
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example can be run as shown only while logged in as user
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<systemitem class="username">lfs</systemitem>. If you come back to this
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page later, replace <command>$LFS_TGT-ld</command> with
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<command>ld</command>).</para>
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<para>The next package installed is gcc. An example of what can be
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seen during its run of <command>configure</command> is:</para>
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<screen><computeroutput>checking what assembler to use... /mnt/lfs/tools/i686-lfs-linux-gnu/bin/as
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checking what linker to use... /mnt/lfs/tools/i686-lfs-linux-gnu/bin/ld</computeroutput></screen>
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<para>This is important for the reasons mentioned above. It also
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demonstrates that gcc's configure script does not search the PATH
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directories to find which tools to use. However, during the actual
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operation of <command>gcc</command> itself, the same search paths are not
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necessarily used. To find out which standard linker <command>gcc</command>
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will use, run: <command>$LFS_TGT-gcc -print-prog-name=ld</command>. (Again,
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remove the <command>$LFS_TGT-</command> prefix if coming back to this
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later.)</para>
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<para>Detailed information can be obtained from <command>gcc</command> by
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passing it the <parameter>-v</parameter> command line option while compiling
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a program. For example, <command>$LFS_TGT-gcc -v
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<replaceable>example.c</replaceable></command> (or without <command>
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$LFS_TGT-</command> if coming back later) will show
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detailed information about the preprocessor, compilation, and assembly
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stages, including <command>gcc</command>'s search paths for included
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headers and their order.</para>
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<para>Next up: sanitized Linux API headers. These allow the
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standard C library (glibc) to interface with features that the Linux
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kernel will provide.</para>
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<para>Next comes glibc. The most important
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considerations for building glibc are the compiler, binary tools, and
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kernel headers. The compiler and binary tools are generally not an issue
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since glibc will always use those relating to the <parameter>--host</parameter>
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parameter passed to its configure script; e.g., in our case, the compiler
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will be <command>$LFS_TGT-gcc</command> and the <command>readelf</command>
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tool will be <command>$LFS_TGT-readelf</command>. The kernel headers can
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be a bit more complicated. Therefore, we take no risks and use
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the available configure switch to enforce the correct selection. After
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the run of <command>configure</command>, check the contents of the
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<filename>config.make</filename> file in the <filename
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class="directory">build</filename> directory for all important details.
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These items highlight an important aspect of the glibc
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package—it is very self-sufficient in terms of its build machinery,
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and generally does not rely on toolchain defaults.</para>
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<para>As mentioned above, the standard C++ library is compiled next, followed in
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<xref linkend="chapter-temporary-tools"/> by other programs that must
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be cross-compiled to break circular dependencies at build time.
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The install step of all those packages uses the
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<envar>DESTDIR</envar> variable to force installation
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in the LFS filesystem.</para>
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<para>At the end of <xref linkend="chapter-temporary-tools"/> the native
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LFS compiler is installed. First binutils-pass2 is built,
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in the same <envar>DESTDIR</envar> directory as the other programs,
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then the second pass of gcc is constructed, omitting some
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non-critical libraries. Due to some weird logic in gcc's
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configure script, <envar>CC_FOR_TARGET</envar> ends up as
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<command>cc</command> when the host is the same as the target, but
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different from the build system. This is why
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<parameter>CC_FOR_TARGET=$LFS_TGT-gcc</parameter> is declared explicitly
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as one of the configuration options.</para>
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<para>Upon entering the chroot environment in <xref
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linkend="chapter-chroot-temporary-tools"/>,
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|
the temporary installations of programs needed for the proper
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operation of the toolchain are performed. From this point onwards, the
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core toolchain is self-contained and self-hosted. In
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<xref linkend="chapter-building-system"/>, final versions of all the
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packages needed for a fully functional system are built, tested, and
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installed.</para>
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</sect2>
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</sect1>
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