Codegen: Tail Duplication: Only duplicate into layout pred if it is a CFG Pred.

[iteratee]

Summary

Add a check that the layout predecessor of a block is an actual CFG
predecssor of the block as well. No current code fails this check, but
upcoming patches can trigger this, and it makes sense to separate it
out.

[echristo]

lib/CodeGen/TailDuplicator.cpp

817

Can you update and elaborate on the comment for this?

[iteratee]

Add a comment above the check.

lib/CodeGen/TailDuplicator.cpp

817

The comment now matches the code. "except for the prior block, which falls through unconditionally, move the contents of this block into the prior block."

But I'll add a comment to this line to the effect of "Layout preds are not always CFG preds", above this line.

[davidxl]

lgtm

lib/CodeGen/TailDuplicator.cpp

bool TailDuplicator::tailDuplicate(MachineFunction &MF, bool IsSimple,
   // block, which falls through unconditionally, move the contents of this
   // block into the prior block.
   MachineBasicBlock *PrevBB = &*std::prev(TailBB->getIterator());
   MachineBasicBlock *PriorTBB = nullptr, *PriorFBB = nullptr;
   SmallVector<MachineOperand, 4> PriorCond;
   // This has to check PrevBB->succ_size() because EH edges are ignored by
   // AnalyzeBranch.
   if (PrevBB->succ_size() == 1 &&
+      // Layout preds are not always CFG preds. Check.
+      *PrevBB->succ_begin() == TailBB &&
       !TII->AnalyzeBranch(*PrevBB, PriorTBB, PriorFBB, PriorCond, true) &&
       PriorCond.empty() && !PriorTBB && TailBB->pred_size() == 1 &&
       !TailBB->hasAddressTaken()) {
     DEBUG(dbgs() << "\nMerging into block: " << *PrevBB
                  << "From MBB: " << *TailBB);
     if (PreRegAlloc) {
       DenseMap<unsigned, RegSubRegPair> LocalVRMap;
       SmallVector<std::pair<unsigned, RegSubRegPair>, 4> CopyInfos;