[JumpThreading] Factor out code to clone a basic block (NFC)

[kazu]

Summary

This patch factors out code to clone a basic block -- partly for
readability and partly to facilitate an upcoming patch of my own.

[wmi]

llvm/lib/Transforms/Scalar/JumpThreading.cpp

1976–1978

It is a little weird for the interface to leave the insertion of BB terminator outside. Can we add SuccBB as another param, and include some processing after the call of CloneBasicBlock into this function?

1991–1996

The Block frequency update seems not precise enough. I feel it is more precise to use BB-->SuccBB branch probablity to compute the block frequency of NewBB. That can be a separated change from this refactoring, but it suggests we may want to include SuccBB as a param.

[kazu]

I've decided to scale back. I'm now simply factoring out code to
clone instructions. I thought about capturing more functionality into
CloneBasicBlock as you suggeted, but then the function would lose
generality I need for my upcoming patch.

[kazu]

llvm/lib/Transforms/Scalar/JumpThreading.cpp

1976–1978

I've decided to scale back and just copy instructions, not a basic block.

1991–1996

I'm leaving the frequency calculation code where it is for now.

[wmi]

LGTM.

llvm/include/llvm/Transforms/Scalar/JumpThreading.h

void releaseMemory() {
     BPI.reset();
   }
 
   void FindLoopHeaders(Function &F);
   bool ProcessBlock(BasicBlock *BB);
   bool MaybeMergeBasicBlockIntoOnlyPred(BasicBlock *BB);
   void UpdateSSA(BasicBlock *BB, BasicBlock *NewBB,
                  DenseMap<Instruction *, Value *> &ValueMapping);
+  std::pair<BasicBlock *, DenseMap<Instruction *, Value *>>
+      CloneBasicBlock(BasicBlock *BB, BasicBlock* PredBB);
   bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock *> &PredBBs,
                   BasicBlock *SuccBB);
   bool DuplicateCondBranchOnPHIIntoPred(
       BasicBlock *BB, const SmallVectorImpl<BasicBlock *> &PredBBs);
 
   bool ComputeValueKnownInPredecessorsImpl(
       Value *V, BasicBlock *BB, jumpthreading::PredValueInfo &Result,
       jumpthreading::ConstantPreference Preference,

llvm/lib/Transforms/Scalar/JumpThreading.cpp

for (Instruction &I : *BB) {
     SSAUpdate.AddAvailableValue(NewBB, ValueMapping[&I]);
 
     while (!UsesToRename.empty())
       SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
     LLVM_DEBUG(dbgs() << "\n");
   }
 }
 
+/// Clone basic block BB except its terminator just for predecessor PredBB.
+/// Return the newly created basic block and the map from the variables in BB to
+/// the variables in the newly created basic block.
+std::pair<BasicBlock *, DenseMap<Instruction *, Value *>>
+JumpThreadingPass::CloneBasicBlock(BasicBlock *BB, BasicBlock* PredBB) {
+  // We are going to have to map operands from the original BB block to the new
+  // copy of the block 'NewBB'.  If there are PHI nodes in BB, evaluate them to
+  // account for entry from PredBB.
+  DenseMap<Instruction*, Value*> ValueMapping;
+
+  BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
+                                         BB->getName()+".thread",
+                                         BB->getParent(), BB);
+  NewBB->moveAfter(PredBB);
+
+  // Set the block frequency of NewBB.
+  if (HasProfileData) {
+    auto NewBBFreq =
+        BFI->getBlockFreq(PredBB) * BPI->getEdgeProbability(PredBB, BB);
+    BFI->setBlockFreq(NewBB, NewBBFreq.getFrequency());
+  }
+
+  BasicBlock::iterator BI = BB->begin();
+  // Clone the phi nodes of BB into NewBB. The resulting phi nodes are trivial,
+  // since NewBB only has one predecessor, but SSAUpdater might need to rewrite
+  // the operand of the cloned phi.
+  for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) {
+    PHINode *NewPN = PHINode::Create(PN->getType(), 1, PN->getName(), NewBB);
+    NewPN->addIncoming(PN->getIncomingValueForBlock(PredBB), PredBB);
+    ValueMapping[PN] = NewPN;
+  }
+
+  // Clone the non-phi instructions of BB into NewBB, keeping track of the
+  // mapping and using it to remap operands in the cloned instructions.
+  for (; !BI->isTerminator(); ++BI) {
+    Instruction *New = BI->clone();
+    New->setName(BI->getName());
+    NewBB->getInstList().push_back(New);
+    ValueMapping[&*BI] = New;
+
+    // Remap operands to patch up intra-block references.
+    for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
+      if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
+        DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
+        if (I != ValueMapping.end())
+          New->setOperand(i, I->second);
+      }
+  }
+
+  return {NewBB, ValueMapping};
+}
+
 /// ThreadEdge - We have decided that it is safe and profitable to factor the
 /// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
 /// across BB.  Transform the IR to reflect this change.
 bool JumpThreadingPass::ThreadEdge(BasicBlock *BB,
                                    const SmallVectorImpl<BasicBlock *> &PredBBs,
                                    BasicBlock *SuccBB) {
   // If threading to the same block as we come from, we would infinite loop.
   if (SuccBB == BB) {
LLVM_DEBUG(dbgs() << "  Threading edge from '" << PredBB->getName()
                     << ", across block:\n    " << *BB << "\n");
 
   if (DTU->hasPendingDomTreeUpdates())
     LVI->disableDT();
   else
     LVI->enableDT();
   LVI->threadEdge(PredBB, BB, SuccBB);
 
-  // We are going to have to map operands from the original BB block to the new
-  // copy of the block 'NewBB'.  If there are PHI nodes in BB, evaluate them to
-  // account for entry from PredBB.
   DenseMap<Instruction*, Value*> ValueMapping;
-
-  BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
-                                         BB->getName()+".thread",
-                                         BB->getParent(), BB);
-  NewBB->moveAfter(PredBB);
-
-  // Set the block frequency of NewBB.
-  if (HasProfileData) {
-    auto NewBBFreq =
-        BFI->getBlockFreq(PredBB) * BPI->getEdgeProbability(PredBB, BB);
-    BFI->setBlockFreq(NewBB, NewBBFreq.getFrequency());
-  }
-
-  BasicBlock::iterator BI = BB->begin();
-  // Clone the phi nodes of BB into NewBB. The resulting phi nodes are trivial,
-  // since NewBB only has one predecessor, but SSAUpdater might need to rewrite
-  // the operand of the cloned phi.
-  for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) {
-    PHINode *NewPN = PHINode::Create(PN->getType(), 1, PN->getName(), NewBB);
-    NewPN->addIncoming(PN->getIncomingValueForBlock(PredBB), PredBB);
-    ValueMapping[PN] = NewPN;
-  }
-
-  // Clone the non-phi instructions of BB into NewBB, keeping track of the
-  // mapping and using it to remap operands in the cloned instructions.
-  for (; !BI->isTerminator(); ++BI) {
-    Instruction *New = BI->clone();
-    New->setName(BI->getName());
-    NewBB->getInstList().push_back(New);
-    ValueMapping[&*BI] = New;
-
-    // Remap operands to patch up intra-block references.
-    for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
-      if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
-        DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
-        if (I != ValueMapping.end())
-          New->setOperand(i, I->second);
-      }
-  }
+  BasicBlock *NewBB;
+  std::tie(NewBB, ValueMapping) = CloneBasicBlock(BB, PredBB);
 
   // We didn't copy the terminator from BB over to NewBB, because there is now
   // an unconditional jump to SuccBB.  Insert the unconditional jump.
   BranchInst *NewBI = BranchInst::Create(SuccBB, NewBB);
   NewBI->setDebugLoc(BB->getTerminator()->getDebugLoc());
 
   // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
   // PHI nodes for NewBB now.