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<p class="MsoNormal">My understanding is that alias analysis returns results in the function scope, not in loop scope.
<o:p></o:p></p>
<p class="MsoNormal">Since both the phis access both global arrays, that should results in BasicAA conservatively returning MayAlias.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I debugged this a little bit and narrowed it down to the section of the code in BasicAAResult::aliasPHI() which has this comment-<o:p></o:p></p>
<p class="MsoNormal"><i> // Analyse the PHIs' inputs under the assumption that the PHIs are<o:p></o:p></i></p>
<p class="MsoNormal"><i> // NoAlias.<o:p></o:p></i></p>
<p class="MsoNormal"><i> // If the PHIs are May/MustAlias there must be (recursively) an input<o:p></o:p></i></p>
<p class="MsoNormal"><i> // operand from outside the PHIs' cycle that is MayAlias/MustAlias or<o:p></o:p></i></p>
<p class="MsoNormal"><i> // there must be an operation on the PHIs within the PHIs' value cycle<o:p></o:p></i></p>
<p class="MsoNormal"><i> // that causes a MayAlias.<o:p></o:p></i></p>
<p class="MsoNormal"><i> // Pretend the phis do not alias.<o:p></o:p></i></p>
<p class="MsoNormal"><i><o:p> </o:p></i></p>
<p class="MsoNormal">It seems to be analyzing corresponding phi operands assuming the PHIs to be ‘Noalias’ to begin with.
<o:p></o:p></p>
<p class="MsoNormal">IMHO, this setup does not work correctly for loop header phis.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><b>From:</b> Hiroshi Yamauchi <yamauchi@google.com> <br>
<b>Sent:</b> Tuesday, March 17, 2020 8:38 AM<br>
<b>To:</b> Chawla, Pankaj <pankaj.chawla@intel.com><br>
<b>Cc:</b> llvm-dev@lists.llvm.org<br>
<b>Subject:</b> Re: [llvm-dev] valid BasicAA behavior?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Perhaps BasicAA is telling that A and B don't alias during one particular iteration of the loop even though they are swapped?<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">1: ; preds = %0, %35<br>
%2 = phi double* [ getelementptr inbounds ([1000 x double], [1000 x double]* @Ag, i64 0, i64 0), %0 ], [ %4, %35 ]<br>
%3 = phi i32 [ 0, %0 ], [ %36, %35 ]<br>
%4 = phi double* [ getelementptr inbounds ([1000 x double], [1000 x double]* @Bg, i64 0, i64 0), %0 ], [ %2, %35 ]<br>
br label %5<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><a href="https://godbolt.org/z/vHJmL5">https://godbolt.org/z/vHJmL5</a><o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Mon, Mar 16, 2020 at 10:28 PM Chawla, Pankaj via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Hi all,<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I have this test case-<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">#define N 1000<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">extern double Ag[N];<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">extern double Bg[N];<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">void consume(double *A, double *B);<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">void swap_deps() {<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> double *A = Ag;<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> double *B = Bg;<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> for (int i = 0; i < 97; ++i) {<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> for (int j = 0; j < N; ++j) {<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> B[j] = A[j] + 1;<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> }<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> double *tmp = A;<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> A = B;<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> B = tmp;<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> }<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> consume(A, B);<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">}<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">BasicAA is returning ‘NoAlias’ when queried for phis created in the i-loop for A and B.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I was expecting it to return MayAlias since A and B are being swapped in the outer loop and so they access same locations in alternate iterations of the i-loop.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Is BasicAA returning the correct result in this case?<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Thanks,<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Pankaj<o:p></o:p></p>
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<p class="MsoNormal">_______________________________________________<br>
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<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a><br>
<a href="https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev" target="_blank">https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev</a><o:p></o:p></p>
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