[SimplifyCFG] Move a portion of an if statement that should already be implied to an assert

[craig.topper]

Summary

In this code we got to Dom by following the predecessor link of BB. So it stands to reason that BB should also show up as a successor of Dom's terminator right? There isn't a way to have the CFG connect in only one direction is there?

[mcrosier]

LGTM. My guess is that this was a very old check due to ill-formed IR, but the code has changed over the years and in it's current state your assumption seems correct.

llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp

if (SimplifyBranchOnICmpChain(BI, Builder, DL))
     return true;
 
   // If this basic block has a single dominating predecessor block and the
   // dominating block's condition implies BI's condition, we know the direction
   // of the BI branch.
   if (BasicBlock *Dom = BB->getSinglePredecessor()) {
     auto *PBI = dyn_cast_or_null<BranchInst>(Dom->getTerminator());
     if (PBI && PBI->isConditional() &&
-        PBI->getSuccessor(0) != PBI->getSuccessor(1) &&
-        (PBI->getSuccessor(0) == BB || PBI->getSuccessor(1) == BB)) {
+        PBI->getSuccessor(0) != PBI->getSuccessor(1)) {
+      assert(PBI->getSuccessor(0) == BB || PBI->getSuccessor(1) == BB);
       bool CondIsFalse = PBI->getSuccessor(1) == BB;
       Optional<bool> Implication = isImpliedCondition(
           PBI->getCondition(), BI->getCondition(), DL, CondIsFalse);
       if (Implication) {
         // Turn this into a branch on constant.
         auto *OldCond = BI->getCondition();
         ConstantInt *CI = *Implication
                               ? ConstantInt::getTrue(BB->getContext())