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Minor modifications to make the Hello World example resemble the Hello World

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pass in the tree. Also some minor formatting changes.
PR9413

llvm-svn: 141655
This commit is contained in:
Bill Wendling 2011-10-11 07:03:52 +00:00
parent db2d702bff
commit aeb63260c7
1 changed files with 69 additions and 49 deletions
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118
docs/WritingAnLLVMPass.html
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@ -227,11 +227,13 @@ the pass itself.</p>
<p>Now that we have a way to compile our new pass, we just have to write it.
Start out with:</p>
<div class="doc_code"><pre>
<div class="doc_code">
<pre>
<b>#include</b> "<a href="http://llvm.org/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
<b>#include</b> "<a href="http://llvm.org/doxygen/Function_8h-source.html">llvm/Function.h</a>"
<b>#include</b> "<a href="http://llvm.org/doxygen/raw__ostream_8h.html">llvm/Support/raw_ostream.h</a>"
</pre></div>
</pre>
</div>
<p>Which are needed because we are writing a <tt><a
href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass</a></tt>,
@ -240,53 +242,66 @@ href="http://llvm.org/doxygen/classllvm_1_1Function.html">Function</a></tt>'s,
and we will be doing some printing.</p>
<p>Next we have:</p>
<div class="doc_code"><pre>
<div class="doc_code">
<pre>
<b>using namespace llvm;</b>
</pre></div>
</pre>
</div>
<p>... which is required because the functions from the include files
live in the llvm namespace.
</p>
live in the llvm namespace.</p>
<p>Next we have:</p>
<div class="doc_code"><pre>
<div class="doc_code">
<pre>
<b>namespace</b> {
</pre></div>
</pre>
</div>
<p>... which starts out an anonymous namespace. Anonymous namespaces are to C++
what the "<tt>static</tt>" keyword is to C (at global scope). It makes the
things declared inside of the anonymous namespace only visible to the current
things declared inside of the anonymous namespace visible only to the current
file. If you're not familiar with them, consult a decent C++ book for more
information.</p>
<p>Next, we declare our pass itself:</p>
<div class="doc_code"><pre>
<div class="doc_code">
<pre>
<b>struct</b> Hello : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
</pre></div><p>
</pre>
</div>
<p>This declares a "<tt>Hello</tt>" class that is a subclass of <tt><a
href="http://llvm.org/doxygen/classllvm_1_1FunctionPass.html">FunctionPass</a></tt>.
The different builtin pass subclasses are described in detail <a
href="#passtype">later</a>, but for now, know that <a
href="#FunctionPass"><tt>FunctionPass</tt></a>'s operate a function at a
href="#FunctionPass"><tt>FunctionPass</tt></a>'s operate on a function at a
time.</p>
<div class="doc_code"><pre>
static char ID;
Hello() : FunctionPass(ID) {}
</pre></div><p>
<div class="doc_code">
<pre>
static char ID;
Hello() : FunctionPass(ID) {}
</pre>
</div>
<p> This declares pass identifier used by LLVM to identify pass. This allows LLVM to
avoid using expensive C++ runtime information.</p>
<p>This declares pass identifier used by LLVM to identify pass. This allows LLVM
to avoid using expensive C++ runtime information.</p>
<div class="doc_code"><pre>
<div class="doc_code">
<pre>
<b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &amp;F) {
errs() &lt;&lt; "<i>Hello: </i>" &lt;&lt; F.getName() &lt;&lt; "\n";
errs() &lt;&lt; "<i>Hello: </i>";
errs().write_escaped(F.getName()) &lt;&lt; "\n";
<b>return false</b>;
}
}; <i>// end of struct Hello</i>
</pre></div>
} <i>// end of anonymous namespace</i>
</pre>
</div>
<p>We declare a "<a href="#runOnFunction"><tt>runOnFunction</tt></a>" method,
which overloads an abstract virtual method inherited from <a
@ -294,31 +309,34 @@ href="#FunctionPass"><tt>FunctionPass</tt></a>. This is where we are supposed
to do our thing, so we just print out our message with the name of each
function.</p>
<div class="doc_code"><pre>
char Hello::ID = 0;
</pre></div>
<div class="doc_code">
<pre>
char Hello::ID = 0;
</pre>
</div>
<p> We initialize pass ID here. LLVM uses ID's address to identify pass so
<p>We initialize pass ID here. LLVM uses ID's address to identify a pass, so
initialization value is not important.</p>
<div class="doc_code"><pre>
static RegisterPass&lt;Hello&gt; X("<i>hello</i>", "<i>Hello World Pass</i>",
false /* Only looks at CFG */,
false /* Analysis Pass */);
} <i>// end of anonymous namespace</i>
</pre></div>
<div class="doc_code">
<pre>
static RegisterPass&lt;Hello&gt; X("<i>hello</i>", "<i>Hello World Pass</i>",
false /* Only looks at CFG */,
false /* Analysis Pass */);
</pre>
</div>
<p>Lastly, we <a href="#registration">register our class</a> <tt>Hello</tt>,
giving it a command line
argument "<tt>hello</tt>", and a name "<tt>Hello World Pass</tt>".
Last two arguments describe its behavior.
If a pass walks CFG without modifying it then third argument is set to true.
If a pass is an analysis pass, for example dominator tree pass, then true
is supplied as fourth argument. </p>
<p>Lastly, we <a href="#registration">register our class</a> <tt>Hello</tt>,
giving it a command line argument "<tt>hello</tt>", and a name "<tt>Hello World
Pass</tt>". The last two arguments describe its behavior: if a pass walks CFG
without modifying it then the third argument is set to <tt>true</tt>; if a pass
is an analysis pass, for example dominator tree pass, then <tt>true</tt> is
supplied as the fourth argument.</p>
<p>As a whole, the <tt>.cpp</tt> file looks like:</p>
<div class="doc_code"><pre>
<div class="doc_code">
<pre>
<b>#include</b> "<a href="http://llvm.org/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
<b>#include</b> "<a href="http://llvm.org/doxygen/Function_8h-source.html">llvm/Function.h</a>"
<b>#include</b> "<a href="http://llvm.org/doxygen/raw__ostream_8h.html">llvm/Support/raw_ostream.h</a>"
@ -332,24 +350,26 @@ is supplied as fourth argument. </p>
Hello() : FunctionPass(ID) {}
<b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &amp;F) {
errs() &lt;&lt; "<i>Hello: </i>" &lt;&lt; F.getName() &lt;&lt; "\n";
errs() &lt;&lt; "<i>Hello: </i>";
errs().write_escaped(F.getName()) &lt;&lt; '\n';
<b>return false</b>;
}
};
char Hello::ID = 0;
static RegisterPass&lt;Hello&gt; X("hello", "Hello World Pass", false, false);
}
</pre></div>
};
}
char Hello::ID = 0;
static RegisterPass&lt;Hello&gt; X("hello", "Hello World Pass", false, false);
</pre>
</div>
<p>Now that it's all together, compile the file with a simple "<tt>gmake</tt>"
command in the local directory and you should get a new file
"<tt>Debug+Asserts/lib/Hello.so</tt>" under the top level directory of the LLVM
source tree (not in the local directory). Note that everything in this file is
contained in an anonymous namespace: this reflects the fact that passes are self
contained units that do not need external interfaces (although they can have
them) to be useful.</p>
contained in an anonymous namespace &mdash; this reflects the fact that passes
are self contained units that do not need external interfaces (although they can
have them) to be useful.</p>
</div>