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* Use the doc_code class for code listings to make it stand out from text
* Wrap text in soft, comfortable <div>s and <p>s * Wrap lines at 80 cols git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@15311 91177308-0d34-0410-b5e6-96231b3b80d8
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Misha Brukman
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08772dae1e
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294611a6e4
@ -134,16 +134,18 @@ symbolic LLVM <tt>Value*</tt>) and a static size.</p>
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important for correct Alias Analyses. For example, consider this (silly, but
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possible) C code:</p>
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<div class="doc_code">
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<pre>
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int i;
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char C[2];
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char A[10];
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/* ... */
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for (i = 0; i != 10; ++i) {
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C[0] = A[i]; /* One byte store */
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C[1] = A[9-i]; /* One byte store */
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}
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int i;
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char C[2];
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char A[10];
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/* ... */
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for (i = 0; i != 10; ++i) {
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C[0] = A[i]; /* One byte store */
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C[1] = A[9-i]; /* One byte store */
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}
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</pre>
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</div>
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<p>In this case, the <tt>basicaa</tt> pass will disambiguate the stores to
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<tt>C[0]</tt> and <tt>C[1]</tt> because they are accesses to two distinct
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@ -151,16 +153,18 @@ locations one byte apart, and the accesses are each one byte. In this case, the
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LICM pass can use store motion to remove the stores from the loop. In
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constrast, the following code:</p>
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<div class="doc_code">
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<pre>
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int i;
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char C[2];
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char A[10];
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/* ... */
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for (i = 0; i != 10; ++i) {
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((short*)C)[0] = A[i]; /* Two byte store! */
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C[1] = A[9-i]; /* One byte store */
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}
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int i;
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char C[2];
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char A[10];
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/* ... */
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for (i = 0; i != 10; ++i) {
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((short*)C)[0] = A[i]; /* Two byte store! */
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C[1] = A[9-i]; /* One byte store */
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}
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</pre>
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</div>
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<p>In this case, the two stores to C do alias each other, because the access to
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the <tt>&C[0]</tt> element is a two byte access. If size information wasn't
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@ -361,25 +365,29 @@ the <tt>AliasAnalysis</tt> base class: <tt>getAnalysisUsage</tt> and
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declaring any pass dependencies your pass has. Thus you should have something
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like this:</p>
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<div class="doc_code">
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<pre>
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void getAnalysisUsage(AnalysisUsage &AU) const {
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AliasAnalysis::getAnalysisUsage(AU);
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<i>// declare your dependencies here.</i>
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}
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void getAnalysisUsage(AnalysisUsage &AU) const {
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AliasAnalysis::getAnalysisUsage(AU);
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<i>// declare your dependencies here.</i>
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}
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</pre>
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</div>
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<p>Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method
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from your analysis run method (<tt>run</tt> for a <tt>Pass</tt>,
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<tt>runOnFunction</tt> for a <tt>FunctionPass</tt>, or <tt>InitializePass</tt>
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for an <tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):</p>
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<div class="doc_code">
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<pre>
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bool run(Module &M) {
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InitializeAliasAnalysis(this);
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<i>// Perform analysis here...</i>
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return false;
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}
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bool run(Module &M) {
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InitializeAliasAnalysis(this);
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<i>// Perform analysis here...</i>
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return false;
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}
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</pre>
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</div>
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</div>
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@ -419,17 +427,19 @@ for methods that you don't override. For methods that you do override, in code
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paths that return a conservative MayAlias or Mod/Ref result, simply return
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whatever the superclass computes. For example:</p>
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<div class="doc_code">
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<pre>
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AliasAnalysis::AliasResult alias(const Value *V1, unsigned V1Size,
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const Value *V2, unsigned V2Size) {
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if (...)
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return NoAlias;
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...
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AliasAnalysis::AliasResult alias(const Value *V1, unsigned V1Size,
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const Value *V2, unsigned V2Size) {
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if (...)
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return NoAlias;
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...
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<i>// Couldn't determine a must or no-alias result.</i>
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return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
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}
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<i>// Couldn't determine a must or no-alias result.</i>
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return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
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}
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</pre>
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</div>
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<p>In addition to analysis queries, you must make sure to unconditionally pass
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LLVM <a href="#updating">update notification</a> methods to the superclass as
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@ -473,7 +483,6 @@ for each value in the program. When this method is called, they should remove
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any entries for the specified value, if they exist.
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection">The <tt>copyValue</tt> method</div>
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@ -485,7 +494,6 @@ this is the only way to introduce a new value. This method indicates that the
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new value has exactly the same properties as the value being copied.
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection">The <tt>replaceWithNewValue</tt> method</div>
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@ -549,8 +557,8 @@ just use the <tt>load-vn</tt> pass, which uses alias analysis.</p>
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<p>Many transformations need information about alias <b>sets</b> that are active
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in some scope, rather than information about pairwise aliasing. The <tt><a
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href="/doxygen/classllvm_1_1AliasSetTracker.html">AliasSetTracker</a></tt> class is used
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to efficiently build these Alias Sets from the pairwise alias analysis
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href="/doxygen/classllvm_1_1AliasSetTracker.html">AliasSetTracker</a></tt> class
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is used to efficiently build these Alias Sets from the pairwise alias analysis
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information provided by the <tt>AliasAnalysis</tt> interface.</p>
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<p>First you initialize the AliasSetTracker by using the "<tt>add</tt>" methods
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@ -602,7 +610,6 @@ are.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="direct">Using the <tt>AliasAnalysis</tt> interface directly</a>
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@ -662,7 +669,6 @@ problem.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection">
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<a name="basic-aa">The <tt>-basicaa</tt> pass</a>
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@ -845,8 +851,8 @@ pointer.</p>
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</div>
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<div class="doc_text">
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<p>
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The <tt>-load-vn</tt> pass uses alias analysis to "<a href="#loadvn">value
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<p>The <tt>-load-vn</tt> pass uses alias analysis to "<a href="#loadvn">value
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number</a>" loads and pointers values, which is used by the GCSE pass to
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eliminate instructions. The <tt>-load-vn</tt> pass relies on alias information
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and must-alias information. This combination of passes can make the following
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@ -864,18 +870,19 @@ calls into direct calls.</li>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="aliasanalysis-debug">Clients for debugging and evaluation of implementations</a>
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<a name="aliasanalysis-debug">Clients for debugging and evaluation of
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implementations</a>
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</div>
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These passes are useful for evaluating the various alias analysis
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implementations. You can use them with commands like '<tt>opt -anders-aa -ds-aa
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-aa-eval foo.bc -disable-output -stats</tt>'.
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<div class="doc_text">
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<p>These passes are useful for evaluating the various alias analysis
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implementations. You can use them with commands like '<tt>opt -anders-aa -ds-aa
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-aa-eval foo.bc -disable-output -stats</tt>'.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection">
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@ -900,17 +907,19 @@ the <tt>AliasSetTracker</tt> class.</p>
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<div class="doc_text">
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<p>The <tt>-count-aa</tt> pass is useful to see how many queries a particular
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pass is making and what responses are returned by the alias analysis. An
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example usage is:</p>
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pass is making and what responses are returned by the alias analysis. As an
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example,</p>
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<div class="doc_code">
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<pre>
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$ opt -basicaa -count-aa -ds-aa -count-aa -licm
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% opt -basicaa -count-aa -ds-aa -count-aa -licm
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</pre>
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</div>
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<p>Which will print out how many queries (and what responses are returned) by
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the <tt>-licm</tt> pass (of the <tt>-ds-aa</tt> pass) and how many queries are
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made of the <tt>-basicaa</tt> pass by the <tt>-ds-aa</tt> pass. This can be
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useful when debugging a transformation or an alias analysis implementation.</p>
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<p>will print out how many queries (and what responses are returned) by the
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<tt>-licm</tt> pass (of the <tt>-ds-aa</tt> pass) and how many queries are made
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of the <tt>-basicaa</tt> pass by the <tt>-ds-aa</tt> pass. This can be useful
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when debugging a transformation or an alias analysis implementation.</p>
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</div>
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