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Rewrite the second on AnalysisUsage usage. This documents the new

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addRequiredTransitive member that Misha added, and explains the whole
concept a lot better.  Also, the document used incorrect "subsubsection"
tags instead of "doc_subsubsection" which this fixes.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@12474 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2004-03-17 21:09:55 +00:00
parent d3a533d94d
commit 892562779c
1 changed files with 115 additions and 70 deletions
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docs/WritingAnLLVMPass.html
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@ -59,7 +59,10 @@
<ul>
<li><a href="#getAnalysisUsage">The <tt>getAnalysisUsage</tt>
method</a></li>
<li><a href="#getAnalysis">The <tt>getAnalysis</tt> method</a></li>
<li><a href="#AU::addRequired">The <tt>AnalysisUsage::addRequired&lt;&gt;</tt> and <tt>AnalysisUsage::addRequiredTransitive&lt;&gt;</tt> methods</a></li>
<li><a href="#AU::addPreserved">The <tt>AnalysisUsage::addPreserved&lt;&gt;</tt> method</a></li>
<li><a href="#AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a></li>
<li><a href="#getAnalysis">The <tt>getAnalysis&lt;&gt;</tt> and <tt>getAnalysisToUpdate&lt;&gt;</tt> methods</a></li>
</ul></li>
<li><a href="#analysisgroup">Implementing Analysis Groups</a>
<ul>
@ -171,12 +174,9 @@ include $(LEVEL)/Makefile.common
<p>This makefile specifies that all of the <tt>.cpp</tt> files in the current
directory are to be compiled and linked together into a
<tt>lib/Debug/libhello.so</tt> shared object that can be dynamically loaded by
the <tt>opt</tt> or <tt>analyze</tt> tools.</p>
<p>
Note that the suffix of the shared library may differ from the example above if
your system uses a different suffix by default.
</p>
the <tt>opt</tt> or <tt>analyze</tt> tools. If your operating system uses a
suffix other than .so (such as windows of Mac OS/X), the appropriate extension
will be used.</p>
<p>Now that we have the build scripts set up, we just need to write the code for
the pass itself.</p>
@ -487,7 +487,7 @@ should return true if they modified the program, or false if they didn't.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="subsubsection">
<div class="doc_subsubsection">
<a name="doInitialization_mod">The <tt>doInitialization(Module &amp;)</tt>
method</a>
</div>
@ -516,7 +516,7 @@ free functions that it needs, adding prototypes to the module if necessary.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="subsubsection">
<div class="doc_subsubsection">
<a name="runOnFunction">The <tt>runOnFunction</tt> method</a>
</div>
@ -533,7 +533,7 @@ be returned if the function is modified.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="subsubsection">
<div class="doc_subsubsection">
<a name="doFinalization_mod">The <tt>doFinalization(Module
&amp;)</tt> method</a>
</div>
@ -581,7 +581,7 @@ href="#FunctionPass"><tt>FunctionPass</tt></a>'s have, but also have the followi
</div>
<!-- _______________________________________________________________________ -->
<div class="subsubsection">
<div class="doc_subsubsection">
<a name="doInitialization_fn">The <tt>doInitialization(Function
&amp;)</tt> method</a>
</div>
@ -603,7 +603,7 @@ fast).</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="subsubsection">
<div class="doc_subsubsection">
<a name="runOnBasicBlock">The <tt>runOnBasicBlock</tt> method</a>
</div>
@ -621,7 +621,7 @@ if the basic block is modified.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="subsubsection">
<div class="doc_subsubsection">
<a name="doFinalization_fn">The <tt>doFinalization(Function &amp;)</tt>
method</a>
</div>
@ -647,14 +647,12 @@ finalization.</p>
<div class="doc_text">
<p>A <tt>MachineFunctionPass</tt> executes on the machine-dependent
representation of each LLVM function in the program,
independent of all of the other functions in the program.
A <tt>MachineFunctionPass</tt> is also a <tt>FunctionPass</tt>, so all
<p>A <tt>MachineFunctionPass</tt> is a part of the LLVM code generator that
executes on the machine-dependent representation of each LLVM function in the
program. A <tt>MachineFunctionPass</tt> is also a <tt>FunctionPass</tt>, so all
the restrictions that apply to a <tt>FunctionPass</tt> also apply to it.
<tt>MachineFunctionPass</tt>es also have additional restrictions. In
particular, <tt>MachineFunctionPass</tt>es are not allowed to do any of
the following:</p>
<tt>MachineFunctionPass</tt>es also have additional restrictions. In particular,
<tt>MachineFunctionPass</tt>es are not allowed to do any of the following:</p>
<ol>
<li>Modify any LLVM Instructions, BasicBlocks or Functions.</li>
@ -669,7 +667,7 @@ data)</li>
</div>
<!-- _______________________________________________________________________ -->
<div class="subsubsection">
<div class="doc_subsubsection">
<a name="runOnMachineFunction">The <tt>runOnMachineFunction(MachineFunction
&amp;MF)</tt> method</a>
</div>
@ -718,11 +716,7 @@ registering a pass that logically should be available for use in the
<li><b><tt>RegisterAnalysis</tt></b> - This template should be used when you are
registering a pass that logically should be available for use in the
'<tt>analysis</tt>' utility.</li>
<li><b><tt>RegisterLLC</tt></b> - This template should be used when you are
registering a pass that logically should be available for use in the
'<tt>llc</tt>' utility.</li>
'<tt>analyze</tt>' utility.</li>
<li><b><tt>RegisterPass</tt></b> - This is the generic form of the
<tt>Register*</tt> templates that should be used if you want your pass listed by
@ -740,14 +734,6 @@ example <tt>opt</tt> or <tt>analyze</tt>). The second argument is the name of
the pass, which is to be used for the <tt>--help</tt> output of programs, as
well as for debug output generated by the <tt>--debug-pass</tt> option.</p>
<p>If you pass is constructed by its default constructor, you only ever have to
pass these two arguments. If, on the other hand, you require other information
(like target specific information), you must pass an additional argument. This
argument is a pointer to a function used to create the pass. For an example of
how this works, look at the <a
href="http://llvm.cs.uiuc.edu/doxygen/LowerAllocations_8cpp-source.html">LowerAllocations.cpp</a>
file.</p>
<p>If a pass is registered to be used by the <tt>analyze</tt> utility, you
should implement the virtual <tt>print</tt> method:</p>
@ -819,34 +805,77 @@ invalidated sets may be specified for your transformation. The implementation
should fill in the <tt><a
href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AnalysisUsage.html">AnalysisUsage</a></tt>
object with information about which passes are required and not invalidated. To
do this, the following set methods are provided by the <tt><a
href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AnalysisUsage.html">AnalysisUsage</a></tt>
class:</p>
do this, a pass may call any of the following methods on the AnalysisUsage
object:</p>
</div>
<pre>
<i>// addRequires - Add the specified pass to the required set for your pass.</i>
<b>template</b>&lt;<b>class</b> PassClass&gt;
AnalysisUsage &amp;AnalysisUsage::addRequired();
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="AU::addRequired">The <tt>AnalysisUsage::addRequired&lt;&gt;</tt> and <tt>AnalysisUsage::addRequiredTransitive&lt;&gt;</tt> methods</a>
</div>
<i>// addPreserved - Add the specified pass to the set of analyses preserved by
// this pass</i>
<b>template</b>&lt;<b>class</b> PassClass&gt;
AnalysisUsage &amp;AnalysisUsage::addPreserved();
<div class="doc_text">
<p>
If you pass requires a previous pass to be executed (an analysis for example),
it can use one of these methods to arrange for it to be run before your pass.
LLVM has many different types of analyses and passes that can be required,
spaning the range from <tt>DominatorSet</tt> to <tt>BreakCriticalEdges</tt>.
requiring <tt>BreakCriticalEdges</tt>, for example, guarantees that there will
be no critical edges in the CFG when your pass has been run.
</p>
<i>// setPreservesAll - Call this if the pass does not modify its input at all</i>
<b>void</b> AnalysisUsage::setPreservesAll();
<p>
Some analyses chain to other analyses to do their job. For example, an <a
href="AliasAnalysis.html">AliasAnalysis</a> implementation is required to <a
href="AliasAnalysis.html#chaining">chain</a> to other alias analysis passes. In
cases where analyses chain, the <tt>addRequiredTransitive</tt> method should be
used instead of the <tt>addRequired</tt> method. This informs the PassManager
that the transitively required pass should be alive as long as the requiring
pass is.
</p>
</div>
<i>// setPreservesCFG - This function should be called by the pass, iff they do not:
//
// 1. Add or remove basic blocks from the function
// 2. Modify terminator instructions in any way.
//
// This is automatically implied for <a href="#BasicBlockPass">BasicBlockPass</a>'s
//</i>
<b>void</b> AnalysisUsage::setPreservesCFG();
</pre>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="AU::addPreserved">The <tt>AnalysisUsage::addPreserved&lt;&gt;</tt> method</a>
</div>
<p>Some examples of how to use these methods are:</p>
<div class="doc_text">
<p>
One of the jobs of the PassManager is to optimize how and when analyses are run.
In particular, it attempts to avoid recomputing data unless it needs to. For
this reason, passes are allowed to declare that they preserve (i.e., they don't
invalidate) an existing analysis if it's available. For example, a simple
constant folding pass would not modify the CFG, so it can't possible effect the
results of dominator analysis. By default, all passes are assumed to invalidate
all others.
</p>
<p>
The <tt>AnalysisUsage</tt> class provides several methods which are useful in
certain circumstances that are related to <tt>addPreserved</tt>. In particular,
the <tt>setPreservesAll</tt> method can be called to indicate that the pass does
not modify the LLVM program at all (which is true for analyses), and the
<tt>setPreservesCFG</tt> method can be used by transformations that change
instructions in the program but do not modify the CFG or terminator instructions
(note that this property is implicitly set for <a
href="#BasicBlockPass">BasicBlockPass</a>'s).
</p>
<p>
<tt>addPreserved</tt> is particularly useful for transformations like
<tt>BreakCriticalEdges</tt>. This pass knows how to update a small set of loop
and dominator related analyses if they exist, so it can preserve them, despite
the fact that it hacks on the CFG.
</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a>
</div>
<div class="doc_text">
<pre>
<i>// This is an example implementation from an analysis, which does not modify
@ -871,20 +900,22 @@ class:</p>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="getAnalysis">The <tt>getAnalysis&lt;&gt;</tt> method</a>
<a name="getAnalysis">The <tt>getAnalysis&lt;&gt;</tt> and <tt>getAnalysisToUpdate&lt;&gt;</tt> methods</a>
</div>
<div class="doc_text">
<p>The <tt>Pass::getAnalysis&lt;&gt;</tt> method is inherited by your class,
providing you with access to the passes that you declared that you required with
the <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method. It takes
a single template argument that specifies which pass class you want, and returns
a reference to that pass.</p>
<p>The <tt>Pass::getAnalysis&lt;&gt;</tt> method is automatically inherited by
your class, providing you with access to the passes that you declared that you
required with the <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a>
method. It takes a single template argument that specifies which pass class you
want, and returns a reference to that pass. For example:</p>
<pre>
<b>template</b>&lt;<b>typename</b> PassClass&gt;
AnalysisType &amp;getAnalysis();
bool LICM::runOnFunction(Function &F) {
LoopInfo &amp;LI = getAnalysis&lt;LoopInfo&gt;();
...
}
</pre>
<p>This method call returns a reference to the pass desired. You may get a
@ -894,6 +925,20 @@ href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> implementation. This
method can be called by your <tt>run*</tt> method implementation, or by any
other local method invoked by your <tt>run*</tt> method.</p>
<p>
If your pass is capable of updating analyses if they exist (e.g.,
<tt>BreakCriticalEdges</tt>, as described above), you can use the
<tt>getAnalysisToUpdate</tt> method, which returns a pointer to the analysis if
it is active. For example:</p>
<pre>
...
if (DominatorSet *DS = getAnalysisToUpdate&lt;DominatorSet&gt;()) {
<i>// A DominatorSet is active. This code will update it.</i>
}
...
</pre>
</div>
<!-- *********************************************************************** -->
@ -924,7 +969,7 @@ Analysis Groups.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="subsubsection">
<div class="doc_subsubsection">
<a name="agconcepts">Analysis Group Concepts</a>
</div>
@ -975,7 +1020,7 @@ hypothetical example) instead.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="subsubsection">
<div class="doc_subsubsection">
<a name="registerag">Using <tt>RegisterAnalysisGroup</tt></a>
</div>
@ -1345,7 +1390,7 @@ where we are going:</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="subsubsection">
<div class="doc_subsubsection">
<a name="SMP">Multithreaded LLVM</a>
</div>
@ -1368,7 +1413,7 @@ Despite that, we have kept the LLVM passes SMP ready, and you should too.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="subsubsection">
<div class="doc_subsubsection">
<a name="ModuleSource">A new <tt>ModuleSource</tt> interface</a>
</div>
@ -1395,7 +1440,7 @@ capable of this, the loader just needs to be reworked a bit.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="subsubsection">
<div class="doc_subsubsection">
<a name="PassFunctionPass"><tt>Pass</tt>es requiring <tt>FunctionPass</tt>es</a>
</div>