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Expand on the AliasSetTracker a bit, so I can remember this next time I come back to it. :)

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@10537 91177308-0d34-0410-b5e6-96231b3b80d8
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Chris Lattner 2003-12-19 08:43:07 +00:00
parent a3df8a964a
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1 changed files with 32 additions and 5 deletions
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docs/AliasAnalysis.html
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@ -366,9 +366,9 @@ simply iterate through the constructed alias sets, using the AliasSetTracker
<tt>begin()</tt>/<tt>end()</tt> methods.</p>
<p>The <tt>AliasSet</tt>s formed by the <tt>AliasSetTracker</tt> are guaranteed
to be disjoint, calculate mod/ref information for the set, and keep track of
whether or not all of the pointers in the set are Must aliases. The
AliasSetTracker also makes sure that sets are properly folded due to call
to be disjoint, calculate mod/ref information and volatility for the set, and
keep track of whether or not all of the pointers in the set are Must aliases.
The AliasSetTracker also makes sure that sets are properly folded due to call
instructions, and can provide a list of pointers in each set.</p>
<p>As an example user of this, the <a href="/doxygen/structLICM.html">Loop
@ -376,11 +376,38 @@ Invariant Code Motion</a> pass uses AliasSetTrackers to build alias information
about each loop nest. If an AliasSet in a loop is not modified, then all load
instructions from that set may be hoisted out of the loop. If any alias sets
are stored <b>and</b> are must alias sets, then the stores may be sunk to
outside of the loop. Both of these transformations obviously only apply if the
pointer argument is loop-invariant.</p>
outside of the loop, promoting the memory location to a register for the
duration of the loop nest. Both of these transformations obviously only apply
if the pointer argument is loop-invariant.</p>
</div>
<div class="doc_subsubsection">
The AliasSetTracker implementation
</div>
<div class="doc_text">
<p>The AliasSetTracker class is implemented to be as efficient as possible. It
uses the union-find algorithm to efficiently merge AliasSets when a pointer is
inserted into the AliasSetTracker that aliases multiple sets. The primary data
structure is a hash table mapping pointers to the AliasSet they are in.</p>
<p>The AliasSetTracker class must maintain a list of all of the LLVM Value*'s
that are in each AliasSet. Since the hash table already has entries for each
LLVM Value* of interest, the AliasesSets thread the linked list through these
hash-table nodes to avoid having to allocate memory unnecessarily, and to make
merging alias sets extremely efficient (the linked list merge is constant time).
</p>
<p>You shouldn't need to understand these details if you are just a client of
the AliasSetTracker, but if you look at the code, hopefully this brief
description will help make sense of why things are designed the way they
are.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="direct">Using the AliasAnalysis interface directly</a>