[Utils] Cleanup similar cases to MergeBlockIntoPredecessor.

alinas · alinas · commit 4eb1a573fab3 · 2019-10-16T22:23:20.000Z
Summary: There are two cases where a block is merged into its predecessor and the MergeBlockIntoPredecessor API is not used. Update the API so it can be reused in the other cases, in order to avoid code duplication. Cleanup motivated by D68659. Reviewers: chandlerc, sanjoy.google, george.burgess.iv Subscribers: llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D68670 llvm-svn: 375050
diff --git a/llvm/include/llvm/Transforms/Utils/BasicBlockUtils.h b/llvm/include/llvm/Transforms/Utils/BasicBlockUtils.h
@@ -83,10 +83,16 @@ bool DeleteDeadPHIs(BasicBlock *BB, const TargetLibraryInfo *TLI = nullptr);
 
 /// Attempts to merge a block into its predecessor, if possible. The return
 /// value indicates success or failure.
+/// By default do not merge blocks if BB's predecessor has multiple successors.
+/// If PredecessorWithTwoSuccessors = true, the blocks can only be merged
+/// if BB's Pred has a branch to BB and to AnotherBB, and BB has a single
+/// successor Sing. In this case the branch will be updated with Sing instead of
+/// BB, and BB will still be merged into its predecessor and removed.
 bool MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU = nullptr,
                                LoopInfo *LI = nullptr,
                                MemorySSAUpdater *MSSAU = nullptr,
-                               MemoryDependenceResults *MemDep = nullptr);
+                               MemoryDependenceResults *MemDep = nullptr,
+                               bool PredecessorWithTwoSuccessors = false);
 
 /// Replace all uses of an instruction (specified by BI) with a value, then
 /// remove and delete the original instruction.
diff --git a/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp b/llvm/lib/Transforms/Scalar/LoopUnswitch.cpp
@@ -1621,43 +1621,24 @@ void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) {
         if (!SinglePred) continue;  // Nothing to do.
         assert(SinglePred == Pred && "CFG broken");
 
-        LLVM_DEBUG(dbgs() << "Merging blocks: " << Pred->getName() << " <- "
-                          << Succ->getName() << "\n");
-
-        // Resolve any single entry PHI nodes in Succ.
-        while (PHINode *PN = dyn_cast<PHINode>(Succ->begin()))
-          ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist, L, LPM,
-                            MSSAU.get());
-
-        Instruction *STI = Succ->getTerminator();
-        Instruction *Start = &*Succ->begin();
-        // If there's nothing to move, mark the starting instruction as the last
-        // instruction in the block.
-        if (Start == STI)
-          Start = BI;
-
-        // Move all of the successor contents from Succ to Pred.
-        Pred->getInstList().splice(BI->getIterator(), Succ->getInstList(),
-                                   Succ->begin(), STI->getIterator());
-        if (MSSAU)
-          MSSAU->moveAllAfterMergeBlocks(Succ, Pred, Start);
-
-        // Move terminator instruction from Succ now, we're deleting BI below.
-        // FIXME: remove BI first might be more intuitive.
-        Pred->getInstList().splice(Pred->end(), Succ->getInstList());
-
-        // If Succ has any successors with PHI nodes, update them to have
-        // entries coming from Pred instead of Succ.
-        Succ->replaceAllUsesWith(Pred);
-
+        // Make the LPM and Worklist updates specific to LoopUnswitch.
         LPM->deleteSimpleAnalysisValue(BI, L);
         RemoveFromWorklist(BI, Worklist);
-        BI->eraseFromParent();
-
-        // Remove Succ from the loop tree.
-        LI->removeBlock(Succ);
         LPM->deleteSimpleAnalysisValue(Succ, L);
-        Succ->eraseFromParent();
+        auto SuccIt = Succ->begin();
+        while (PHINode *PN = dyn_cast<PHINode>(SuccIt++)) {
+          for (unsigned It = 0, E = PN->getNumOperands(); It != E; ++It)
+            if (Instruction *Use = dyn_cast<Instruction>(PN->getOperand(It)))
+              Worklist.push_back(Use);
+          for (User *U : PN->users())
+            Worklist.push_back(cast<Instruction>(U));
+          LPM->deleteSimpleAnalysisValue(PN, L);
+          RemoveFromWorklist(PN, Worklist);
+          ++NumSimplify;
+        }
+        // Merge the block and make the remaining analyses updates.
+        DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
+        MergeBlockIntoPredecessor(Succ, &DTU, LI, MSSAU.get());
         ++NumSimplify;
         continue;
       }
diff --git a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -170,7 +170,8 @@ bool llvm::DeleteDeadPHIs(BasicBlock *BB, const TargetLibraryInfo *TLI) {
 
 bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU,
                                      LoopInfo *LI, MemorySSAUpdater *MSSAU,
-                                     MemoryDependenceResults *MemDep) {
+                                     MemoryDependenceResults *MemDep,
+                                     bool PredecessorWithTwoSuccessors) {
   if (BB->hasAddressTaken())
     return false;
 
@@ -185,9 +186,24 @@ bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU,
     return false;
 
   // Can't merge if there are multiple distinct successors.
-  if (PredBB->getUniqueSuccessor() != BB)
+  if (!PredecessorWithTwoSuccessors && PredBB->getUniqueSuccessor() != BB)
     return false;
 
+  // Currently only allow PredBB to have two predecessors, one being BB.
+  // Update BI to branch to BB's only successor instead of BB.
+  BranchInst *PredBB_BI;
+  BasicBlock *NewSucc = nullptr;
+  unsigned FallThruPath;
+  if (PredecessorWithTwoSuccessors) {
+    if (!(PredBB_BI = dyn_cast<BranchInst>(PredBB->getTerminator())))
+      return false;
+    BranchInst *BB_JmpI = dyn_cast<BranchInst>(BB->getTerminator());
+    if (!BB_JmpI || !BB_JmpI->isUnconditional())
+      return false;
+    NewSucc = BB_JmpI->getSuccessor(0);
+    FallThruPath = PredBB_BI->getSuccessor(0) == BB ? 0 : 1;
+  }
+
   // Can't merge if there is PHI loop.
   for (PHINode &PN : BB->phis())
     for (Value *IncValue : PN.incoming_values())
@@ -246,11 +262,20 @@ bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU,
   // source...
   BB->replaceAllUsesWith(PredBB);
 
-  // Delete the unconditional branch from the predecessor...
-  PredBB->getInstList().pop_back();
+  if (PredecessorWithTwoSuccessors) {
+    // Delete the unconditional branch from BB.
+    BB->getInstList().pop_back();
 
-  // Move terminator instruction and add unreachable to now empty BB.
-  PredBB->getInstList().splice(PredBB->end(), BB->getInstList());
+    // Update branch in the predecessor.
+    PredBB_BI->setSuccessor(FallThruPath, NewSucc);
+  } else {
+    // Delete the unconditional branch from the predecessor.
+    PredBB->getInstList().pop_back();
+
+    // Move terminator instruction.
+    PredBB->getInstList().splice(PredBB->end(), BB->getInstList());
+  }
+  // Add unreachable to now empty BB.
   new UnreachableInst(BB->getContext(), BB);
 
   // Eliminate duplicate dbg.values describing the entry PHI node post-splice.
diff --git a/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp b/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp
@@ -615,35 +615,9 @@ bool LoopRotate::simplifyLoopLatch(Loop *L) {
   LLVM_DEBUG(dbgs() << "Folding loop latch " << Latch->getName() << " into "
                     << LastExit->getName() << "\n");
 
-  Instruction *FirstLatchInst = &*Latch->begin();
-  // If there's nothing to move, mark the starting instruction as the last
-  // instruction in the block.
-  if (FirstLatchInst == Jmp)
-    FirstLatchInst = BI;
-
-  // Hoist the instructions from Latch into LastExit.
-  LastExit->getInstList().splice(BI->getIterator(), Latch->getInstList(),
-                                 Latch->begin(), Jmp->getIterator());
-
-  // Update MemorySSA
-  if (MSSAU)
-    MSSAU->moveAllAfterMergeBlocks(Latch, LastExit, FirstLatchInst);
-
-  unsigned FallThruPath = BI->getSuccessor(0) == Latch ? 0 : 1;
-  BasicBlock *Header = Jmp->getSuccessor(0);
-  assert(Header == L->getHeader() && "expected a backward branch");
-
-  // Remove Latch from the CFG so that LastExit becomes the new Latch.
-  BI->setSuccessor(FallThruPath, Header);
-  Latch->replaceSuccessorsPhiUsesWith(LastExit);
-  Jmp->eraseFromParent();
-
-  // Nuke the Latch block.
-  assert(Latch->empty() && "unable to evacuate Latch");
-  LI->removeBlock(Latch);
-  if (DT)
-    DT->eraseNode(Latch);
-  Latch->eraseFromParent();
+  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
+  MergeBlockIntoPredecessor(Latch, &DTU, LI, MSSAU, nullptr,
+                            /*PredecessorWithTwoSuccessors=*/true);
 
   if (MSSAU && VerifyMemorySSA)
     MSSAU->getMemorySSA()->verifyMemorySSA();
