Checkin new loop-preheader insertion pass.

llvm-svn: 3942

lib/Transforms/Scalar/LoopSimplify.cpp

@@ -0,0 +1,189 @@
+//===- LoopPreheaders.cpp - Loop Preheader Insertion Pass -----------------===//
+//
+// Insert Loop pre-headers into the CFG for each function in the module.  This
+// pass updates loop information and dominator information.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Analysis/Dominators.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Function.h"
+#include "llvm/iTerminators.h"
+#include "llvm/iPHINode.h"
+#include "llvm/Constant.h"
+#include "llvm/Support/CFG.h"
+#include "Support/StatisticReporter.h"
+
+namespace {
+  Statistic<> NumInserted("preheaders\t- Number of pre-header nodes inserted");
+
+  struct Preheaders : public FunctionPass {
+    virtual bool runOnFunction(Function &F);
+    
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+      // We need loop information to identify the loops...
+      AU.addRequired<LoopInfo>();
+
+      AU.addPreserved<LoopInfo>();
+      AU.addPreserved<DominatorSet>();
+      AU.addPreserved<ImmediateDominators>();
+      AU.addPreserved<DominatorTree>();
+      AU.addPreservedID(BreakCriticalEdgesID);  // No crit edges added....
+    }
+  private:
+    bool ProcessLoop(Loop *L);
+    void InsertPreheaderForLoop(Loop *L);
+  };
+
+  RegisterOpt<Preheaders> X("preheaders",
+                         "Insert a pre-header node for every loop in the CFG");
+}
+
+// Publically exposed interface to pass...
+const PassInfo *LoopPreheadersID = X.getPassInfo();
+Pass *createLoopPreheaderInsertionPass() { return new Preheaders(); }
+
+
+/// runOnFunction - Run down all loops in the CFG (recursively, but we could do
+/// it in any convenient order) inserting preheaders...
+///
+bool Preheaders::runOnFunction(Function &F) {
+  bool Changed = false;
+  LoopInfo &LI = getAnalysis<LoopInfo>();
+
+  for (unsigned i = 0, e = LI.getTopLevelLoops().size(); i != e; ++i)
+    Changed |= ProcessLoop(LI.getTopLevelLoops()[i]);
+
+  return Changed;
+}
+
+
+/// ProcessLoop - Walk the loop structure in depth first order, ensuring that
+/// all loops have preheaders.
+///
+bool Preheaders::ProcessLoop(Loop *L) {
+  bool Changed = false;
+
+  // Does the loop already have a preheader?  If so, don't modify the loop...
+  if (L->getLoopPreheader() == 0) {
+    InsertPreheaderForLoop(L);
+    NumInserted++;
+    Changed = true;
+  }
+
+  const std::vector<Loop*> &SubLoops = L->getSubLoops();
+  for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
+    Changed |= ProcessLoop(SubLoops[i]);
+  return Changed;
+}
+
+
+/// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
+/// preheader, this method is called to insert one.  This method has two phases:
+/// preheader insertion and analysis updating.
+///
+void Preheaders::InsertPreheaderForLoop(Loop *L) {
+  BasicBlock *Header = L->getHeader();
+
+  // Compute the set of predecessors of the loop that are not in the loop.
+  std::vector<BasicBlock*> OutsideBlocks;
+  for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
+       PI != PE; ++PI)
+      if (!L->contains(*PI))           // Coming in from outside the loop?
+        OutsideBlocks.push_back(*PI);  // Keep track of it...
+  
+  assert(OutsideBlocks.size() != 1 && "Loop already has a preheader!");
+  
+  // Create new basic block, insert right before the header of the loop...
+  BasicBlock *NewBB = new BasicBlock(Header->getName()+".preheader", Header);
+
+  // The preheader first gets an unconditional branch to the loop header...
+  BranchInst *BI = new BranchInst(Header);
+  NewBB->getInstList().push_back(BI);
+  
+  // For every PHI node in the loop body, insert a PHI node into NewBB where
+  // the incoming values from the out of loop edges are moved to NewBB.  We
+  // have two possible cases here.  If the loop is dead, we just insert dummy
+  // entries into the PHI nodes for the new edge.  If the loop is not dead, we
+  // move the incoming edges in Header into new PHI nodes in NewBB.
+  //
+  if (!OutsideBlocks.empty()) {  // Is the loop not obviously dead?
+    for (BasicBlock::iterator I = Header->begin();
+         PHINode *PN = dyn_cast<PHINode>(&*I); ++I) {
+      
+      // Create the new PHI node, insert it into NewBB at the end of the block
+      PHINode *NewPHI = new PHINode(PN->getType(), PN->getName()+".ph", BI);
+        
+      // Move all of the edges from blocks outside the loop to the new PHI
+      for (unsigned i = 0, e = OutsideBlocks.size(); i != e; ++i) {
+        Value *V = PN->removeIncomingValue(OutsideBlocks[i]);
+        NewPHI->addIncoming(V, OutsideBlocks[i]);
+      }
+      
+      // Add an incoming value to the PHI node in the loop for the preheader
+      // edge
+      PN->addIncoming(NewPHI, NewBB);
+    }
+    
+    // Now that the PHI nodes are updated, actually move the edges from
+    // OutsideBlocks to point to NewBB instead of Header.
+    //
+    for (unsigned i = 0, e = OutsideBlocks.size(); i != e; ++i) {
+      TerminatorInst *TI = OutsideBlocks[i]->getTerminator();
+      for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s)
+        if (TI->getSuccessor(s) == Header)
+          TI->setSuccessor(s, NewBB);
+    }
+    
+  } else {                       // Otherwise the loop is dead...
+    for (BasicBlock::iterator I = Header->begin();
+         PHINode *PN = dyn_cast<PHINode>(&*I); ++I)
+      // Insert dummy values as the incoming value...
+      PN->addIncoming(Constant::getNullValue(PN->getType()), NewBB);
+  }
+
+
+  //===--------------------------------------------------------------------===//
+  //  Update analysis results now that we have preformed the transformation
+  //
+  
+  // We know that we have loop information to update... update it now.
+  if (Loop *Parent = L->getParentLoop())
+    Parent->addBasicBlockToLoop(NewBB, getAnalysis<LoopInfo>());
+  
+  // Update dominator information if it is around...
+  if (DominatorSet *DS = getAnalysisToUpdate<DominatorSet>()) {
+    // We need to add information about the fact that NewBB dominates Header.
+    DS->addDominator(Header, NewBB);
+    
+    // The blocks that dominate NewBB are the blocks that dominate Header,
+    // minus Header, plus NewBB.
+    DominatorSet::DomSetType DomSet = DS->getDominators(Header);
+    DomSet.erase(Header);  // Header does not dominate us...
+    DS->addBasicBlock(NewBB, DomSet);
+  }
+  
+  // Update immediate dominator information if we have it...
+  if (ImmediateDominators *ID = getAnalysisToUpdate<ImmediateDominators>()) {
+    // Whatever i-dominated the header node now immediately dominates NewBB
+    ID->addNewBlock(NewBB, ID->get(Header));
+    
+    // The preheader now is the immediate dominator for the header node...
+    ID->setImmediateDominator(Header, NewBB);
+  }
+  
+  // Update DominatorTree information if it is active.
+  if (DominatorTree *DT = getAnalysisToUpdate<DominatorTree>()) {
+    // The immediate dominator of the preheader is the immediate dominator of
+    // the old header.
+    //
+    DominatorTree::Node *HeaderNode = DT->getNode(Header);
+    DominatorTree::Node *PHNode = DT->createNewNode(NewBB, HeaderNode);
+    
+    // Change the header node so that PNHode is the new immediate dominator
+    DT->changeImmediateDominator(HeaderNode, PHNode);
+  }
+}
+
+