Refactor the identification method for FindLastIV pattern.

This patch applys range analysis. It will exclude cases where the
range of induction variable cannot be fully contained within

  [<sentinel value> + 1, <minimum value of recurrence type>)

This approach also handles truncated induction variable cases well.

Author: Mel-Chen

llvm/lib/Analysis/IVDescriptors.cpp

@@ -710,27 +710,36 @@ RecurrenceDescriptor::isFindLastIVPattern(Loop *Loop, PHINode *OrigPhi,
     return InstDesc(false, I);
 
   auto IsIncreasingLoopInduction = [&SE, &Loop](Value *V) {
-    auto *Phi = dyn_cast<PHINode>(V);
-    if (!Phi)
-      return false;
-
     if (!SE)
       return false;
 
-    InductionDescriptor ID;
-    if (!InductionDescriptor::isInductionPHI(Phi, Loop, SE, ID))
+    Type *Ty = V->getType();
+    if (!SE->isSCEVable(Ty))
       return false;
 
-    const SCEVAddRecExpr *AR = cast<SCEVAddRecExpr>(SE->getSCEV(Phi));
-    if (!AR->hasNoSignedWrap())
+    auto *AR = dyn_cast<SCEVAddRecExpr>(SE->getSCEV(V));
+    if (!AR)
       return false;
 
-    ConstantInt *IVStartValue = dyn_cast<ConstantInt>(ID.getStartValue());
-    if (!IVStartValue || IVStartValue->isMinSignedValue())
+    const SCEV *Step = AR->getStepRecurrence(*SE);
+    if (!SE->isKnownPositive(Step))
       return false;
 
-    const SCEV *Step = ID.getStep();
-    return SE->isKnownPositive(Step);
+    const ConstantRange IVRange = SE->getSignedRange(AR);
+    unsigned NumBits = Ty->getIntegerBitWidth();
+    // Keep the minmum value of the recurrence type as the sentinel value.
+    // The maximum acceptable range for the increasing induction variable,
+    // called the valid range, will be defined as
+    //   [<sentinel value> + 1, SignedMin(<recurrence type>))
+    // TODO: This range restriction can be lifted by adding an additional
+    // virtual OR reduction.
+    const APInt Sentinel = APInt::getSignedMinValue(NumBits);
+    const ConstantRange ValidRange = ConstantRange::getNonEmpty(
+        Sentinel + 1, APInt::getSignedMinValue(NumBits));
+    LLVM_DEBUG(dbgs() << "LV: FindLastIV valid range is " << ValidRange
+                      << ", and the signed range of " << *AR << " is "
+                      << IVRange << "\n");
+    return ValidRange.contains(IVRange);
   };
 
   // We are looking for selects of the form: