[llvm][NFC] Factor out logic for getting incoming & back Loop edges

[davidxl]

llvm/include/llvm/Analysis/LoopInfo.h

524

nit about wording:

Obtain the unique incoming and back edge.  Return false if ....; otherwise return true.

[mtrofin]

• wording

[davidxl]

lgtm

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;
 
+  /// Obtain the unique incoming and back edge, and return true. Return false
+  /// if they are non-unique or the loop is dead.
+  bool getIncomingAndBackEdge(BasicBlock *&Incoming,
+                              BasicBlock *&Backedge) const;
+
   /// Return true if the Loop is in LCSSA form.
   bool isLCSSAForm(DominatorTree &DT) const;
 
   /// Return true if this Loop and all inner subloops are in LCSSA form.
   bool isRecursivelyLCSSAForm(DominatorTree &DT, const LoopInfo &LI) const;
 
   /// Return true if the Loop is in the form that the LoopSimplify form
   /// transforms loops to, which is sometimes called normal form.

llvm/lib/Analysis/LoopInfo.cpp

bool Loop::makeLoopInvariant(Instruction *I, bool &Changed,
   // condition. Conservatively strip it here so that we don't give any wrong
   // information to the optimizer.
   I->dropUnknownNonDebugMetadata();
 
   Changed = true;
   return true;
 }
 
-PHINode *Loop::getCanonicalInductionVariable() const {
+bool Loop::getIncomingAndBackEdge(BasicBlock *&Incoming,
+                                  BasicBlock *&Backedge) const {
   BasicBlock *H = getHeader();
 
-  BasicBlock *Incoming = nullptr, *Backedge = nullptr;
+  Incoming = nullptr;
+  Backedge = nullptr;
   pred_iterator PI = pred_begin(H);
   assert(PI != pred_end(H) && "Loop must have at least one backedge!");
   Backedge = *PI++;
   if (PI == pred_end(H))
-    return nullptr; // dead loop
+    return false; // dead loop
   Incoming = *PI++;
   if (PI != pred_end(H))
-    return nullptr; // multiple backedges?
+    return false; // multiple backedges?
 
   if (contains(Incoming)) {
     if (contains(Backedge))
-      return nullptr;
+      return false;
     std::swap(Incoming, Backedge);
   } else if (!contains(Backedge))
+    return false;
+
+  assert(Incoming && Backedge && "expected non-null incoming and backedges");
+  return true;
+}
+
+PHINode *Loop::getCanonicalInductionVariable() const {
+  BasicBlock *H = getHeader();
+
+  BasicBlock *Incoming = nullptr, *Backedge = nullptr;
+  if (!getIncomingAndBackEdge(Incoming, Backedge))
     return nullptr;
 
   // Loop over all of the PHI nodes, looking for a canonical indvar.
   for (BasicBlock::iterator I = H->begin(); isa<PHINode>(I); ++I) {
     PHINode *PN = cast<PHINode>(I);
     if (ConstantInt *CI =
             dyn_cast<ConstantInt>(PN->getIncomingValueForBlock(Incoming)))
       if (CI->isZero())