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Recommend writing new instruction selectors using pattern-based mechanisms

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@22098 91177308-0d34-0410-b5e6-96231b3b80d8
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Misha Brukman 2005-05-17 02:27:30 +00:00
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docs/WritingAnLLVMBackend.html
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@ -109,15 +109,14 @@ described the instruction set with the assembly printer generator in mind, that
step can be almost automated.</li>
</ul>
<p>Now, for static code generation you also need to write an instruction
selector for your platform: see <tt>lib/Target/*/*ISelSimple.cpp</tt> which
is no longer "simple" but it gives you the idea: you have to be able to create
MachineInstrs for any given LLVM instruction using the <tt>InstVisitor</tt>
pattern, and produce a <tt>MachineFunction</tt> with
<tt>MachineBasicBlock</tt>s full of <tt><a
href="CodeGenerator.html#machineinstr">MachineInstr</a></tt>s for a
corresponding LLVM Function. Creating an instruction selector is perhaps the
most time-consuming part of creating a back-end.</p>
<p>You also need to write an instruction selector for your platform. The
recommended method is the pattern-matching instruction selector. You can see
examples in other targets: <tt>lib/Target/*/*ISelPattern.cpp</tt>. The former
method for writing instruction selectors (<b>not</b> recommended) is
encapsulated in <tt>lib/Target/*/*ISelSimple.cpp</tt>, which are
<tt>InstVisitor</tt>-based translators, generating code for an LLVM instruction
at a time. Creating an instruction selector is perhaps the most time-consuming
part of creating a back-end.</p>
<p>To create a JIT for your platform:</p>