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<?xml version="1.0" encoding="iso-8859-1"?>
<sect1 id="code-generators">
<title>GHC Backends</title>
<indexterm><primary>ghc backends</primary></indexterm>
<indexterm><primary>ghc code generators</primary></indexterm>
<para>GHC supports multiple backend code generators. This is the part
of the compiler responsible for taking the last intermediate
representation that GHC uses (a form called Cmm that is a simple, C like
language) and compiling it to executable code. The backends that GHC
support are described below.
</para>
<sect2 id="native-code-gen">
<title>Native code Generator (<option>-fasm</option>)</title>
<indexterm><primary>native code generator</primary></indexterm>
The default backend for GHC. It is a native code generator, compiling Cmm
all the way to assembly code. It is the fastest backend and generally
produces good performance code. It has the best support for compiling
shared libraries. Select it with the <option>-fasm</option> flag.
</sect2>
<sect2 id="llvm-code-gen">
<title>LLVM Code Generator (<option>-fllvm</option>)</title>
<indexterm><primary>LLVM code generator</primary></indexterm>
<para>This is an alternative backend that uses the
<ulink url="http://llvm.org">LLVM</ulink> compiler to produce
executable code. It generally produces code as with performance as
good as the native code generator but for some cases can produce
much faster code. This is especially true for numeric, array heavy
code using packages like vector. The penalty is a significant increase in
compilation times. Select the LLVM backend with the
<option>-fllvm</option> flag. Currently <emphasis>LLVM 2.8</emphasis> and
later are supported.
</para>
<para>You must install and have LLVM available on your PATH for the LLVM
code generator to work. Specifically GHC needs to be able to call the
<command>opt</command> and <command>llc</command> tools. Secondly, if you
are running Mac OS X with LLVM 3.0 or greater then
you also need the <ulink url="http://clang.llvm.org">Clang c
compiler</ulink> compiler available on your PATH.
</para>
<para>To install LLVM and Clang:
<itemizedlist>
<listitem><para><emphasis>Linux</emphasis>: Use your package management tool.</para>
</listitem>
<listitem><para><emphasis>Mac OS X</emphasis>: Clang is included by
default on recent OS X machines when XCode is installed (from
<literal>10.6</literal> and later). LLVM is not included. In
order to use the LLVM based code generator, you should install
the <ulink
url="http://mxcl.github.com/homebrew/">Homebrew</ulink>
package manager for OS X. Alternatively you can download
binaries for LLVM and Clang from <ulink
url="http://llvm.org/releases/download.html">here</ulink>.</para>
</listitem>
<listitem><para><emphasis>Windows</emphasis>: You should download binaries for
LLVM and clang from
<ulink url="http://llvm.org/releases/download.html">here</ulink>.</para>
</listitem>
</itemizedlist>
</para>
</sect2>
<sect2 id="c-code-gen">
<title>C Code Generator (<option>-fvia-C</option>)</title>
<indexterm><primary>C code generator</primary></indexterm>
<indexterm><primary>-fvia-C</primary></indexterm>
<para>This is the oldest code generator in GHC and is generally not included
any more having been deprecated around GHC 7.0. Select it with the
<option>-fvia-C</option> flag.
</para>
<para>The C code generator is only supported when GHC is built in
unregisterised mode, a mode where GHC produces 'portable' C code as
output to facilitate porting GHC itself to a new platform. This mode
produces much slower code though so it's unlikely your version of
GHC was built this way. If it has then the native code generator
probably won't be available. You can check this information by calling
<link linkend="ghc-info"><literal>ghc --info</literal></link>.
</para>
</sect2>
<sect2 id="unreg">
<title>Unregisterised compilation</title>
<indexterm><primary>unregisterised compilation</primary></indexterm>
<para>The term "unregisterised" really means "compile via vanilla C",
disabling some of the platform-specific tricks that GHC normally uses to
make programs go faster. When compiling unregisterised, GHC simply
generates a C file which is compiled via gcc.</para>
<para>When GHC is build in unregisterised mode only the LLVM and C code
generators will be available. The native code generator won't be. LLVM
usually offers a substantial performance benefit over the C backend in
unregisterised mode.</para>
<para>Unregisterised compilation can be useful when porting GHC to a new
machine, since it reduces the prerequisite tools to
<command>gcc</command>, <command>as</command>, and <command>ld</command>
and nothing more, and furthermore the amount of platform-specific code
that needs to be written in order to get
unregisterised compilation going is usually fairly small.</para>
<para>Unregisterised compilation cannot be selected at compile-time; you
have to build GHC with the appropriate options set. Consult the GHC
Building Guide for details.</para>
<para>You can check if your GHC is unregisterised by calling
<link linkend="ghc-info"><literal>ghc --info</literal></link>.</para>
</sect2>
</sect1>
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