[LoopFlatten][LoopInfo] Use Loop to identify latch compare instruction

[RosieSumpter]

Summary

Make getLatchCmpInst non-static and use it in LoopFlatten as a more
robust way of identifying the compare.

[SjoerdMeijer]

llvm/lib/Analysis/LoopInfo.cpp

175

Nit: do we need the this->?

300

Same

llvm/lib/Transforms/Scalar/LoopFlatten.cpp

113–114

I was expecting we could perhaps get rid of a bit more code. This check is performed in Loop::getLatchCmpInst, so do we need it here?

[RosieSumpter]

• Remove this->
• Remove checks on back branch

[SjoerdMeijer]

I think exposing getLatchCmpInst is useful and makes sense.
The refactoring that this enables also LGTM.

Please wait with committing until tomorrow in case someone else want to have a look.

llvm/include/llvm/Analysis/LoopInfo.h

public:
   /// recurrence that starts at 0 and increments by one each time through the
   /// loop. If so, return the phi node that corresponds to it.
   ///
   /// The IndVarSimplify pass transforms loops to have a canonical induction
   /// variable.
   ///
   PHINode *getCanonicalInductionVariable() const;
 
+  /// Get the latch condition instruction.
+  ICmpInst *getLatchCmpInst() const;
+
   /// Obtain the unique incoming and back edge. Return false if they are
   /// non-unique or the loop is dead; otherwise, return true.
   bool getIncomingAndBackEdge(BasicBlock *&Incoming,
                               BasicBlock *&Backedge) const;
 
   /// Below are some utilities to get the loop guard, loop bounds and induction
   /// variable, and to check if a given phinode is an auxiliary induction
   /// variable, if the loop is guarded, and if the loop is canonical.

llvm/lib/Analysis/LoopInfo.cpp

if (ConstantInt *CI =
             if (ConstantInt *CI = dyn_cast<ConstantInt>(Inc->getOperand(1)))
               if (CI->isOne())
                 return PN;
   }
   return nullptr;
 }
 
 /// Get the latch condition instruction.
-static ICmpInst *getLatchCmpInst(const Loop &L) {
-  if (BasicBlock *Latch = L.getLoopLatch())
+ICmpInst *Loop::getLatchCmpInst() const {
+  if (BasicBlock *Latch = getLoopLatch())
     if (BranchInst *BI = dyn_cast_or_null<BranchInst>(Latch->getTerminator()))
       if (BI->isConditional())
         return dyn_cast<ICmpInst>(BI->getCondition());
 
   return nullptr;
 }
 
 /// Return the final value of the loop induction variable if found.
 static Value *findFinalIVValue(const Loop &L, const PHINode &IndVar,
                                const Instruction &StepInst) {
-  ICmpInst *LatchCmpInst = getLatchCmpInst(L);
+  ICmpInst *LatchCmpInst = L.getLatchCmpInst();
   if (!LatchCmpInst)
     return nullptr;
 
   Value *Op0 = LatchCmpInst->getOperand(0);
   Value *Op1 = LatchCmpInst->getOperand(1);
   if (Op0 == &IndVar || Op0 == &StepInst)
     return Op1;
 

 }
 
 PHINode *Loop::getInductionVariable(ScalarEvolution &SE) const {
   if (!isLoopSimplifyForm())
     return nullptr;
 
   BasicBlock *Header = getHeader();
   assert(Header && "Expected a valid loop header");
-  ICmpInst *CmpInst = getLatchCmpInst(*this);
+  ICmpInst *CmpInst = getLatchCmpInst();
   if (!CmpInst)
     return nullptr;
 
   Instruction *LatchCmpOp0 = dyn_cast<Instruction>(CmpInst->getOperand(0));
   Instruction *LatchCmpOp1 = dyn_cast<Instruction>(CmpInst->getOperand(1));
 
   for (PHINode &IndVar : Header->phis()) {
     InductionDescriptor IndDesc;

llvm/lib/Transforms/Scalar/LoopFlatten.cpp

static bool findLoopComponents(
   }
 
   // There must be exactly one exiting block, and it must be the same at the
   // latch.
   BasicBlock *Latch = L->getLoopLatch();
   if (L->getExitingBlock() != Latch) {
     LLVM_DEBUG(dbgs() << "Exiting and latch block are different\n");
     return false;
   }
-  // Latch block must end in a conditional branch.
-  BackBranch = dyn_cast<BranchInst>(Latch->getTerminator());
-  if (!BackBranch || !BackBranch->isConditional()) {
-    LLVM_DEBUG(dbgs() << "Could not find back-branch\n");
-    return false;
-  }
-  IterationInstructions.insert(BackBranch);
-  LLVM_DEBUG(dbgs() << "Found back branch: "; BackBranch->dump());
-  bool ContinueOnTrue = L->contains(BackBranch->getSuccessor(0));
 
   // Find the induction PHI. If there is no induction PHI, we can't do the
   // transformation. TODO: could other variables trigger this? Do we have to
   // search for the best one?
   InductionPHI = L->getInductionVariable(*SE);
   if (!InductionPHI) {
     LLVM_DEBUG(dbgs() << "Could not find induction PHI\n");
     return false;
   }
   LLVM_DEBUG(dbgs() << "Found induction PHI: "; InductionPHI->dump());
 
+  bool ContinueOnTrue = L->contains(Latch->getTerminator()->getSuccessor(0));
   auto IsValidPredicate = [&](ICmpInst::Predicate Pred) {
     if (ContinueOnTrue)
       return Pred == CmpInst::ICMP_NE || Pred == CmpInst::ICMP_ULT;
     else
       return Pred == CmpInst::ICMP_EQ;
   };
 
-  // Find Compare and make sure it is valid
-  ICmpInst *Compare = dyn_cast<ICmpInst>(BackBranch->getCondition());
+  // Find Compare and make sure it is valid. getLatchCmpInst checks that the
+  // back branch of the latch is conditional.
+  ICmpInst *Compare = L->getLatchCmpInst();
   if (!Compare || !IsValidPredicate(Compare->getUnsignedPredicate()) ||
       Compare->hasNUsesOrMore(2)) {
     LLVM_DEBUG(dbgs() << "Could not find valid comparison\n");
     return false;
   }
+  BackBranch = cast<BranchInst>(Latch->getTerminator());
+  IterationInstructions.insert(BackBranch);
+  LLVM_DEBUG(dbgs() << "Found back branch: "; BackBranch->dump());
   IterationInstructions.insert(Compare);
   LLVM_DEBUG(dbgs() << "Found comparison: "; Compare->dump());
 
   // Find increment and limit from the compare
   Increment = nullptr;
   if (match(Compare->getOperand(0),
             m_c_Add(m_Specific(InductionPHI), m_ConstantInt<1>()))) {
     Increment = dyn_cast<BinaryOperator>(Compare->getOperand(0));