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<p class="MsoNormal"><span style="font-size:11.0pt">Hello Everyone, <o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">I am still looking for some insight regarding the below issue. Please reach out if you have any advice!<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">-Adel<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="color:black">From: </span></b><span style="color:black">"Ejjeh, Adel" <aejjeh@illinois.edu><br>
<b>Date: </b>Thursday, April 23, 2020 at 5:21 PM<br>
<b>To: </b>LLVM Dev <llvm-dev@lists.llvm.org><br>
<b>Subject: </b>Incorrect behavior in the LLVM dependence analyzer<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">Hi all,</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt">I am trying to use the dependence analyzer in a pass that I am writing and I was surprised to see an incorrect behavior when I try to query DependenceInfo for dependences between instructions. Specifically,
if the two instructions are loads/stores accessing an array in a loop, the depend() method would return a dependence regardless of the order of instructions specified. (i.e. if the two instructions where L1 and S1, both depend(L1, S1) and depend(S1,L1) would
return a dependence), even though only one of the two dependences is valid when the chronological order of the instructions is considered. To illustrate consider this example:</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt">for(int i = 0; i < n; i++) {</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt">S0: A[i] = …;</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt">S1: … = A[i+1];</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt">}</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt">In the example, there is an antidependence between S1 and S0 with distance 1. However, when querying DependenceInfo it also returns a flow dependence from S0 to S1. Is this the expected behavior of DependenceInfo
or Is it a known problem? If it is the expected behavior, is there a way to check if the dependence you are getting back is a correct one or not? Does anyone have any ideas about possible workarounds or solutions?</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Regards,</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt">- Adel</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt">-- </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:24.0pt;font-family:"Apple Chancery";color:#203864">Adel Ejjeh</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt;color:black">PhD Candidate – Computer Science</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt;color:black">University of Illinois at Urbana Champaign</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt;color:black">201 N Goodwin Ave, Urbana, IL 61801</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt;color:black"><a href="mailto:aejjeh@illinois.edu"><span style="color:#0563C1">aejjeh@illinois.edu</span></a> |
<a href="mailto:adel.ejjeh@gmail.com"><span style="color:#0563C1">adel.ejjeh@gmail.com</span></a></span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt"> </span><o:p></o:p></p>
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