[SCEV] Adding a check on outgoing branches of a terminator instr for SCEVBackedgeConditionFolder, NFC.

[jbhateja]

Summary

For a given loop, getLoopLatch returns a non-null value
when a loop has only one latch block. In the modified
context adding an assertion to check that both the outgoing branches of
a terminator instruction (of latch) does not target same header.
+
few minor code reorganization.

[sanjoy]

llvm/trunk/lib/Analysis/ScalarEvolution.cpp

4095

Can you please add an assert here that BI->getSuccessor(0) != BI->getSuccessor(1)? That way we'll know to update this place if/when getLoopLatch() becomes smarter.

[jbhateja]

llvm/trunk/lib/Analysis/ScalarEvolution.cpp

4095

I guess we will have to look at all the places from where a call to getLoopLatch is made, adding such an assert at one place does not looks reasonable.

[sanjoy]

llvm/trunk/lib/Analysis/ScalarEvolution.cpp

4095

adding such an assert at one place does not looks reasonable

I'm not sure I agree. Here we rely on BI->getSuccessor(0) != BI->getSuccessor(1) implied from getLoopLatch() for correctness, and asserting that precondition seems like an incremental improvement, irrespective of what we do for the other places.

Secondly, I don't think asserting BI->getSuccessor(0) != BI->getSuccessor(1) everywhere we call getLoopLatch() is a good idea anyway. We should assert BI->getSuccessor(0) != BI->getSuccessor(1) where it matters to the code that called getLoopLatch(), not to all places that call getLoopLatch().

[jbhateja]

Few more minor changes , mostly NFCs.

llvm/trunk/lib/Analysis/ScalarEvolution.cpp

class SCEVBackedgeConditionFolder
     : public SCEVRewriteVisitor<SCEVBackedgeConditionFolder> {
 public:
   static const SCEV *rewrite(const SCEV *S, const Loop *L,
                              ScalarEvolution &SE) {
     bool IsPosBECond = false;
     Value *BECond = nullptr;
     if (BasicBlock *Latch = L->getLoopLatch()) {
       BranchInst *BI = dyn_cast<BranchInst>(Latch->getTerminator());
-      if (BI && BI->isConditional() &&
-          BI->getSuccessor(0) != BI->getSuccessor(1)) {
+      if (BI && BI->isConditional()) {
         BECond = BI->getCondition();
         IsPosBECond = BI->getSuccessor(0) == L->getHeader();
       } else {
         return S;
       }
     }
     SCEVBackedgeConditionFolder Rewriter(L, BECond, IsPosBECond, SE);
     return Rewriter.visit(S);