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<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white">Hi,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white">This is a proposal about epilog loop vectorization.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white">Currently Loop Vectorizer inserts an epilogue loop for handling loops that don’t have known iteration counts.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white">The Loop Vectorizer supports loops with an unknown trip count, unknown trip count may not be a multiple of the vector width, and the vectorizer has
to execute the last few iterations as scalar code. It keeps a scalar copy of the loop for the remaining iterations.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white">Loop with the large width has a high possibility of executing many scalar iterations.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white">i.e. i8 data type with 256bits target register can vectorize with vector width 32, with that maximum trip count possibility for scalar(epilog) loop
is 31, which is significant & worth vectorizing.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white">Large vector factor has following challenges:<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l1 level1 lfo1"><![if !supportLists]><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black"><span style="mso-list:Ignore">1)<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white">Possibility of remainder iteration is substantial.<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l1 level1 lfo1"><![if !supportLists]><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black"><span style="mso-list:Ignore">2)<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white">Actual trip count at runtime is substantial but not meeting minimum trip count to execute vector loop.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white">These challenges can be addressed by mask instructions, but these instructions are limited and may not be available to all targets.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white">By epilog vectorization our aim to vectorize epilog loop where original loop is vectorized with large vector factor and has a high possibility of executing
scalar iterations.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white">This require following changes:<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l0 level1 lfo2"><![if !supportLists]><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black"><span style="mso-list:Ignore">1)<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white">Costing: Preserve all profitable vector factor.<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-.25in;mso-list:l0 level1 lfo2"><![if !supportLists]><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black"><span style="mso-list:Ignore">2)<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white">Transform: Create an additional vector loop with next profitable vector factor.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white">Please refer attached file (BlockLayout.png) for the details about transformed block layout.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white">Patch is available at:
<a href="https://reviews.llvm.org/D30247">https://reviews.llvm.org/D30247</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white">Regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Sans Unicode",sans-serif;color:black;background:white">Ashutosh<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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