[DA] getBackedgeTakenCount in isKnownLessThan can return CouldNotCompute

llvm/lib/Analysis/DependenceAnalysis.cpp

@@ -1183,7 +1183,12 @@ bool DependenceInfo::isKnownLessThan(const SCEV *S, const SCEV *Size) const {
   // Special check for addrecs using BE taken count
   if (const SCEVAddRecExpr *AddRec = dyn_cast<SCEVAddRecExpr>(S))
     if (AddRec->isAffine() && AddRec->hasNoSignedWrap()) {
-      const SCEV *BECount = SE->getBackedgeTakenCount(AddRec->getLoop());
+      const SCEV *BECount = collectUpperBound(AddRec->getLoop(), MaxType);
+      // If the BTC cannot be computed, check the base case for S.
+      if (!BECount)
+        return false;
+      if (isa<SCEVCouldNotCompute>(BECount))
+        return SE->isKnownNegative(SE->getMinusSCEV(S, Size));
       const SCEV *Start = AddRec->getStart();
       const SCEV *Step = AddRec->getStepRecurrence(*SE);
       const SCEV *End = AddRec->evaluateAtIteration(BECount, *SE);

llvm/test/Analysis/DependenceAnalysis/becount-couldnotcompute.ll

@@ -0,0 +1,22 @@
+; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py UTC_ARGS: --version 6
+; RUN: opt < %s -disable-output "-passes=print<da>" -aa-pipeline=basic-aa 2>&1 | FileCheck %s
+
+; Test for function isKnownLessThan that calculates a back-edge taken count,
+; which can return a CouldNotCompute SCEV.
+
+define void @test(i64 %conv, ptr %a) {
+; CHECK-LABEL: 'test'
+; CHECK-NEXT:  Src: %ld = load i32, ptr %arrayidx12, align 4 --> Dst: %ld = load i32, ptr %arrayidx12, align 4
+; CHECK-NEXT:    da analyze - none!
+;
+entry:
+  %sub = add i64 %conv, 1
+  br label %loop
+
+loop:
+  %i = phi i64 [ %add26, %loop ], [ 0, %entry ]
+  %arrayidx12 = getelementptr i32, ptr %a, i64 %i
+  %ld = load i32, ptr %arrayidx12, align 4
+  %add26 = add nsw i64 %sub, %i
+  br label %loop
+}