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Add some hands-on documentation about which files to create and edit when 

adding a backend.

llvm-svn: 56783
This commit is contained in:
Matthijs Kooijman 2008-09-29 11:52:22 +00:00
parent 08d91178e9
commit eed995b796
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llvm/docs/WritingAnLLVMBackend.html
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@ -217,6 +217,56 @@ e.g.:</p>
<p>For now, just take a look at <tt>lib/Target/CBackend</tt> for an example of
how the C backend is written.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="files">Files to create/modify</a>
</div>
<div class="doc_text">
<p>To actually create your backend, you need to create and modify a few files.
Here, the absolute minimum will be discussed. To actually use LLVM's target
independent codegenerator, you must <a href="CodeGenerator.html">implement extra
things</a>.</p>
<p>First of all, you should create a subdirectory under <tt>lib/Target</tt>,
which will hold all the files related to your target. Let's assume that our
target is called, "Dummy", we would create the directory
<tt>lib/Target/Dummy</tt>.</p>
<p>In this new directory, you should put a <tt>Makefile</tt>. You can probably
copy one from another target and modify it. It should at least contain the
<tt>LEVEL</tt>, <tt>LIBRARYNAME</tt> and <tt>TARGET</tt> variables, and then
include <tt>$(LEVEL)/Makefile.common</tt>. Be careful to give the library the
correct name, it must be named <tt>LLVMDummy</tt> (see the MIPS target, for
example). Alternatively, you can split the library into
<tt>LLVMDummyCodeGen</tt> and <tt>LLVMDummyAsmPrinter</tt>, the latter of which
should be implemented in a subdirectory below <tt>lib/Target/Dummy</tt> (see the
PowerPC target, for example).</p>
<p>Note that these two naming schemes are hardcoded into llvm-config. Using any
other naming scheme will confuse llvm-config and produce lots of (seemingly
unrelated) linker errors when linking <tt>llc</tt>.</p>
<p>To make your target actually do something, you need to implement a subclass
of <tt>TargetMachine</tt>. This implementation should typically be in the file
<tt>lib/Target/DummyTargetMachine.cpp</tt>, but any file in the
<tt>lib/Target</tt> directory will be built and should work. To use LLVM's <a
href="CodeGenerator.html">target independent code generator</a>, you should
create a subclass of <tt>LLVMTargetMachine</tt>. This is what all current
machine backends do. To create a target from scratch, create a subclass of
<tt>TargetMachine</tt>. This is what the current language backends do.</p>
<p>To get LLVM to actually build and link your target, you also need to add it
to the <tt>TARGETS_TO_BUILD</tt> variable. To do this, you need to modify the
<tt>configure</tt> script to know about your target when parsing the
<tt>--enable-targets</tt> option. Search the <tt>configure</tt> script for
<tt>TARGETS_TO_BUILD</tt>, add your target to the lists there (some creativity
required) and then reconfigure. Alternatively, you can change
<tt>autotools/configure.ac</tt> and regenerate <tt>configure</tt> by running
<tt>./autoconf/AutoRegen.sh</tt>.
</div>
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