[VPlan] Update vector latch terminator edge to exit block after execution.

Instead of setting the successor to the exit using CFG.ExitBB, set it to
nullptr initially. The successor to the exit block is later set either
through createEmptyBasicBlock or after VPlan execution (because at the
moment, no block is created by VPlan for the exit block, the existing
one is reused).

This also enables BranchOnCond to be used as terminator for the exiting
block of the topmost vector region.

Depends on D126618.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D126679

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -240,7 +240,7 @@ VPBasicBlock::createEmptyBasicBlock(VPTransformState::CFGState &CFG) {
   // Hook up the new basic block to its predecessors.
   for (VPBlockBase *PredVPBlock : getHierarchicalPredecessors()) {
     VPBasicBlock *PredVPBB = PredVPBlock->getExitingBasicBlock();
-    auto &PredVPSuccessors = PredVPBB->getSuccessors();
+    auto &PredVPSuccessors = PredVPBB->getHierarchicalSuccessors();
     BasicBlock *PredBB = CFG.VPBB2IRBB[PredVPBB];
 
     assert(PredBB && "Predecessor basic-block not found building successor.");
@@ -257,25 +257,13 @@ VPBasicBlock::createEmptyBasicBlock(VPTransformState::CFGState &CFG) {
       Br->setDebugLoc(DL);
     } else if (TermBr && !TermBr->isConditional()) {
       TermBr->setSuccessor(0, NewBB);
-    } else if (PredVPSuccessors.size() == 2) {
+    } else {
+      // Set each forward successor here when it is created, excluding
+      // backedges. A backward successor is set when the branch is created.
       unsigned idx = PredVPSuccessors.front() == this ? 0 : 1;
-      assert(!PredBBTerminator->getSuccessor(idx) &&
+      assert(!TermBr->getSuccessor(idx) &&
              "Trying to reset an existing successor block.");
-      PredBBTerminator->setSuccessor(idx, NewBB);
-    } else {
-      // PredVPBB is the exiting block of a loop region. Connect its successor
-      // outside the region.
-      auto *LoopRegion = cast<VPRegionBlock>(PredVPBB->getParent());
-      assert(!LoopRegion->isReplicator() &&
-             "predecessor must be in a loop region");
-      assert(PredVPSuccessors.empty() &&
-             LoopRegion->getExitingBasicBlock() == PredVPBB &&
-             "PredVPBB must be the exiting block of its parent region");
-      assert(this == LoopRegion->getSingleSuccessor() &&
-             "the current block must be the single successor of the region");
-      PredBBTerminator->setSuccessor(0, NewBB);
-      PredBBTerminator->setSuccessor(
-          1, CFG.VPBB2IRBB[LoopRegion->getEntryBasicBlock()]);
+      TermBr->setSuccessor(idx, NewBB);
     }
   }
   return NewBB;
@@ -297,6 +285,16 @@ void VPBasicBlock::execute(VPTransformState *State) {
     // ExitBB can be re-used for the exit block of the Plan.
     NewBB = State->CFG.ExitBB;
     State->CFG.PrevBB = NewBB;
+
+    // Update the branch instruction in the predecessor to branch to ExitBB.
+    VPBlockBase *PredVPB = getSingleHierarchicalPredecessor();
+    VPBasicBlock *ExitingVPBB = PredVPB->getExitingBasicBlock();
+    assert(PredVPB->getSingleSuccessor() == this &&
+           "predecessor must have the current block as only successor");
+    BasicBlock *ExitingBB = State->CFG.VPBB2IRBB[ExitingVPBB];
+    // The Exit block of a loop is always set to be successor 0 of the Exiting
+    // block.
+    cast<BranchInst>(ExitingBB->getTerminator())->setSuccessor(0, NewBB);
   } else if (PrevVPBB && /* A */
              !((SingleHPred = getSingleHierarchicalPredecessor()) &&
                SingleHPred->getExitingBasicBlock() == PrevVPBB &&
@@ -798,11 +796,15 @@ void VPInstruction::generateInstruction(VPTransformState &State,
     auto *Plan = getParent()->getPlan();
     VPRegionBlock *TopRegion = Plan->getVectorLoopRegion();
     VPBasicBlock *Header = TopRegion->getEntry()->getEntryBasicBlock();
-    // TODO: Once the exit block is modeled in VPlan, use it instead of going
-    // through State.CFG.ExitBB.
-    BasicBlock *Exit = State.CFG.ExitBB;
 
-    Builder.CreateCondBr(Cond, Exit, State.CFG.VPBB2IRBB[Header]);
+    // Replace the temporary unreachable terminator with a new conditional
+    // branch, hooking it up to backward destination (the header) now and to the
+    // forward destination (the exit/middle block) later when it is created.
+    // Note that CreateCondBr expects a valid BB as first argument, so we need
+    // to set it to nullptr later.
+    BranchInst *CondBr = Builder.CreateCondBr(Cond, Builder.GetInsertBlock(),
+                                              State.CFG.VPBB2IRBB[Header]);
+    CondBr->setSuccessor(0, nullptr);
     Builder.GetInsertBlock()->getTerminator()->eraseFromParent();
     break;
   }