www: Adding webpage to track memory access transformation
llvm-svn: 133354
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Raghesh Aloor
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<head>
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<meta content="text/html; charset=ISO-8859-1"
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http-equiv="Content-Type">
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<title>memaccess.html</title>
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</head>
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<body>
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<div style="margin-left: 320px;"><small style="font-weight: bold;"><small><a><big><big><big>Support
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for
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memory access
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transformations in Polly</big></big></big></a></small></small><big><span
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style="font-weight: bold;"></span></big><br>
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<big><span style="font-weight: bold;"></span></big><br>
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<div style="text-align: left;"><strong></strong>This project adds
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memory access transformations to Polly. In many cases<br>
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changing the memory access pattern yields to better data
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locality or removes<br>
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dependences that would otherwise block
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transformations. They may also<br>
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allow LLVM to use registers to store
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certain values.<br>
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</div>
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<br>
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An examples which uses this feature is given below<br>
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</div>
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<div style="margin-left: 320px;"><br>
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Consider the following loop
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<ul>
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<ul>
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</ul>
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</ul>
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<div style="margin-left: 40px;"><small style="font-style: italic;">for
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(i = 0; i < 8; i++)</small><br>
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<small style="font-style: italic;">sum += A[i];</small><br
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style="font-style: italic;">
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</div>
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<br>
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With support for memory access transformation this loop can be executed<br>
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in parallel. It can be
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transformed to<small style="font-style: italic;"><br>
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<br>
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</small>
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<div style="margin-left: 40px;"><small style="font-style: italic;"><create
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and
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initialize an array 'tmp'
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with size 4></small><br>
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<small style="font-style: italic;">for (i = 0; i < 8; i++) {</small><br>
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<small style="font-style: italic;">tmp[i % 4] += A[i];</small><br>
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<small style="font-style: italic;">}</small><small><span
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style="font-style: italic;"></span></small><br>
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<small><span style="font-style: italic;">sum = tmp[0] + tmp[1] + tmp[2]
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+ tmp[3];</span></small><br>
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</div>
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<br>
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With the help of some optimizer (like
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PluTo) the following code can be <br>
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generated, where the outer loop is
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parallel.
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<p style="padding-left: 30px; font-style: italic;"><small>parfor (ii =
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0; ii < 4; ii++) {<br>
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tmp[ii] = 0;<br>
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for (i = ii * 2; i < (ii+1) * 2; i++)<br>
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tmp[ii] += A[i];</small></p>
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<p style="padding-left: 30px; font-style: italic; font-weight: bold;"><small><span
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style="font-weight: normal;">}</span><br style="font-weight: normal;">
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<span style="font-weight: normal;">sum = tmp[0] + tmp[1] + tmp[2] +
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tmp[3];</span><br>
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<strong></strong></small></p>
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<p><strong><span style="text-decoration: underline;">TODO<br>
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</span></strong></p>
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<p><strong><span style="text-decoration: underline;"><small>Step 1</small><br>
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</span></strong></p>
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Polly exports its polyhedral description in a JSCoP file. Define how
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memory <small><br>
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</small>layout transformations are going to be expressed in Polly and
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in
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the JSCOP file. <br>
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A simple example is given below.<br>
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<br>
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Consider the following loop.<br>
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<br>
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<div style="margin-left: 40px;"><small><span style="font-style: italic;">for
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(i
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= 0; i < 12; i++)</span><br style="font-style: italic;">
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<span style="font-style: italic;"> A[i] = 0;</span><br
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style="font-style: italic;">
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</small></div>
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<br>
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In the JSCOP file the memory is represented as follows.<br>
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<br>
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<div style="margin-left: 40px;"><small><span style="font-style: italic;">
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"accesses":
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[{</span></small><br style="font-style: italic;">
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<small><span style="font-style: italic;">
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"kind":
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"write",</span></small><br style="font-style: italic;">
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<small><span style="font-style: italic;">
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"relation":
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"{
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Stmt[i] -> </span><strong
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style="font-style: italic; font-weight: bold;">A</strong><span
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style="font-style: italic;"><span style="font-weight: bold;">[i]</span>
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}"</span></small><br style="font-style: italic;">
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<small><span style="font-style: italic;"> }]</span></small><br
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style="font-style: italic;">
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</div>
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<br>
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Suppose
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we want to perform a transformation such that the following<br>
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code is generated<br>
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<br>
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<div style="margin-left: 40px;"><small><span style="font-style: italic;">for
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(i
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= 0; i < 12; i++)</span><br style="font-style: italic;">
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<span style="font-style: italic;">
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A[0] = i;</span><br style="font-style: italic;">
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</small></div>
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<br>
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The corresponding JSCOP file represenation would be<br>
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<br>
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<div style="margin-left: 40px;"><small><span style="font-style: italic;">
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"accesses":
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[{</span><br style="font-style: italic;">
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<span style="font-style: italic;">
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"kind":
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"read",</span><br style="font-style: italic;">
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<span style="font-style: italic;">
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"relation":
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"{
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Stmt[i] -> </span><strong
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style="font-style: italic; font-weight: bold;">A</strong><span
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style="font-style: italic;"><span style="font-weight: bold;">[0]</span>
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}"</span><br style="font-style: italic;">
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<span style="font-style: italic;"> }]</span><br
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style="font-style: italic;">
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</small></div>
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<br>
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We need to detect this access function change.<br>
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</div>
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<span id="q_12f99e5de7fd0932_1" class="h4"></span>
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</body>
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</html>
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