Add loop rotation pass.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@35714 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 431 insertions, 0 deletions

diff --git a/lib/Transforms/Scalar/LoopRotation.cpp b/lib/Transforms/Scalar/LoopRotation.cpp
new file mode 100644
index 0000000..edcfe7e
--- /dev/null
+++ b/lib/Transforms/Scalar/LoopRotation.cpp
@@ -0,0 +1,431 @@
+//===- LoopRotation.cpp - Loop Rotation Pass ------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file was developed by Devang Patel and is distributed under
+// the University of Illinois Open Source License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements Loop Rotation Pass.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "loop-rotation"
+
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Function.h"
+#include "llvm/Instructions.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/LoopPass.h"
+#include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/ADT/SmallVector.h"
+#include <map>
+
+using namespace llvm;
+
+#define MAX_HEADER_SIZE 16
+
+STATISTIC(NumRotated, "Number of loops rotated");
+namespace {
+
+  cl::opt<unsigned>
+  RotateThreshold("rotate-threshold", cl::init(200), cl::Hidden,
+                  cl::desc("The cut-off point for loop rotating"));
+
+  class VISIBILITY_HIDDEN InsnReplacementData {
+  public:
+    InsnReplacementData(Instruction *O, Instruction *P, Instruction *H) 
+      : Original(O), PreHeader(P), Header(H) {}
+  public:
+    Instruction *Original; // Original instruction
+    Instruction *PreHeader; // New pre-header replacement
+    Instruction *Header; // New header replacement
+  };
+
+  class VISIBILITY_HIDDEN LoopRotate : public LoopPass {
+
+  public:
+    bool runOnLoop(Loop *L, LPPassManager &LPM);
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+      AU.addRequiredID(LCSSAID);
+      AU.addPreservedID(LCSSAID);
+      //AU.addRequired<LoopInfo>();
+      //AU.addPreserved<LoopInfo>();
+    }
+
+    // Helper functions
+
+    /// Do actual work
+    bool rotateLoop(Loop *L, LPPassManager &LPM);
+    
+    /// Initialize local data
+    void initialize();
+
+    /// Make sure all Exit block PHINodes have required incoming values.
+    /// If incoming value is constant or defined outside the loop then
+    /// PHINode may not have an entry for new pre-header. 
+    void  updateExitBlock();
+
+    /// Return true if this instruction is used outside original header.
+    bool usedOutsideOriginalHeader(Instruction *In);
+
+    /// Find Replacement information for instruction. Return NULL if it is
+    /// not available.
+    InsnReplacementData *findReplacementData(Instruction *I);
+
+  private:
+
+    Loop *L;
+    BasicBlock *OrigHeader;
+    BasicBlock *OrigPreHeader;
+    BasicBlock *OrigLatch;
+    BasicBlock *NewHeader;
+    BasicBlock *NewPreHeader;
+    BasicBlock *Exit;
+
+    SmallVector<InsnReplacementData, MAX_HEADER_SIZE> RD;
+  };
+  
+  RegisterPass<LoopRotate> X ("loop-rotate", "Rotate Loops");
+}
+
+LoopPass *llvm::createLoopRotatePass() { return new LoopRotate(); }
+
+bool LoopRotate::runOnLoop(Loop *Lp, LPPassManager &LPM) {
+  
+  bool RotatedOneLoop = false;
+  initialize();
+
+  // One loop can be rotated multiple times.
+  while (rotateLoop(Lp,LPM)) {
+    RotatedOneLoop = true;
+    initialize();
+  }
+
+  return RotatedOneLoop;
+}
+
+bool LoopRotate::rotateLoop(Loop *Lp, LPPassManager &LPM) {
+
+  L = Lp;
+  if ( NumRotated >= RotateThreshold) 
+    return false;
+
+  OrigHeader =  L->getHeader();
+  OrigPreHeader = L->getLoopPreheader();
+  OrigLatch = L->getLoopLatch();
+
+  // If loop has only one block then there is not much to rotate.
+  if (L->getBlocks().size() <= 1)
+    return false;
+
+  if (!OrigHeader || !OrigLatch || !OrigPreHeader)
+    return false;
+
+  // If loop header is not one of the loop exit block then
+  // either this loop is already rotated or it is not 
+  // suitable for loop rotation transformations.
+  if (!L->isLoopExit(OrigHeader))
+    return false;
+
+  BranchInst *BI = dyn_cast<BranchInst>(OrigHeader->getTerminator());
+  if (!BI)
+    return false;
+
+  std::vector<BasicBlock *> ExitBlocks;
+  L->getExitBlocks(ExitBlocks);
+  if (ExitBlocks.size() > 1)
+    return false;
+
+  // Find new Loop header. NewHeader is a Header's one and only successor
+  // that is inside loop.  Header's all other successors are out side the
+  // loop. Otherwise loop is not suitable for rotation.
+  for (unsigned index = 0; index < BI->getNumSuccessors(); ++index) {
+    BasicBlock *S = BI->getSuccessor(index);
+    if (L->contains(S)) {
+      if (!NewHeader) 
+        NewHeader = S;
+      else
+        // Loop Header has two successors inside loop. This loop is
+        // not suitable for rotation.
+        return false;
+    } else {
+      if (!Exit)
+        Exit = S;
+      else
+        // Loop has multiple exits.
+        return false;
+    }
+  }
+  assert (NewHeader && "Unable to determine new loop header");
+
+  // Check size of original header and reject
+  // loop if it is very big.
+  if (OrigHeader->getInstList().size() > MAX_HEADER_SIZE)
+    return false;
+
+  // Now, this loop is suitable for rotation.
+
+  // Copy Prepare PHI nodes and other instructions from original header
+  // into new pre-header. Unlike original header, new pre-header is
+  // not a member of loop. New pre-header has only one predecessor,
+  // that is original loop pre-header.
+  //
+  // New loop header is one and only successor of original header that 
+  // is inside the loop. All other original header successors are outside 
+  // the loop. Copy PHI Nodes from original header into new loop header. 
+  // Add second incoming value, from new loop pre-header into these phi 
+  // nodes. If a value defined in original header is used outside original 
+  // header then new loop header will need new phi nodes with two incoming 
+  // values, one definition from original header and second definition is 
+  // from new loop pre-header (which is a clone of original header definition).
+
+  NewPreHeader = new BasicBlock("bb.nph", OrigHeader->getParent(), OrigHeader);
+  for (BasicBlock::iterator I = OrigHeader->begin(), E = OrigHeader->end();
+       I != E; ++I) {
+    Instruction *In = I;
+
+    if (PHINode *PN = dyn_cast<PHINode>(I)) {
+
+      // Create new PHI node with one value incoming from OrigPreHeader.
+      // NewPreHeader has only one predecessor, OrigPreHeader.
+      PHINode *NPH = new PHINode(In->getType(), In->getName());
+      NPH->addIncoming(PN->getIncomingValueForBlock(OrigPreHeader), 
+		       OrigPreHeader);
+      NewPreHeader->getInstList().push_back(NPH);
+      
+      // Create new PHI node with two incoming values for NewHeader.
+      // One incoming value is from OrigLatch (through OrigHeader) and 
+      // second incoming value is from NewPreHeader.
+      PHINode *NH = new PHINode(In->getType(), In->getName());
+      NH->addIncoming(PN->getIncomingValueForBlock(OrigLatch), OrigHeader);
+      NH->addIncoming(NPH, NewPreHeader);
+      NewHeader->getInstList().push_front(NH);
+
+      RD.push_back(InsnReplacementData(In, NPH, NH));
+    } else {
+      // This is not a PHI instruction. Insert its clone into NewPreHeader.
+      // If this instruction is using a value from same basic block then
+      // update it to use value from cloned instruction.
+      Instruction *C = In->clone();
+      C->setName(In->getName());
+      NewPreHeader->getInstList().push_back(C);
+
+      // If this instruction is used outside this basic block then
+      // create new PHINode for this instruction.
+      Instruction *NewHeaderReplacement = NULL;
+      if (usedOutsideOriginalHeader(In)) {
+        PHINode *PN = new PHINode(In->getType(), In->getName());
+        PN->addIncoming(In, OrigHeader);
+        PN->addIncoming(C, NewPreHeader);
+        NewHeader->getInstList().push_front(PN);
+        NewHeaderReplacement = PN;
+      } 
+      RD.push_back(InsnReplacementData(In, C, NewHeaderReplacement));
+    }
+  }
+
+  // Update new pre-header.
+  // Rename values that are defined in original header to reflects values
+  // defined in new pre-header.
+  for (SmallVector<InsnReplacementData, MAX_HEADER_SIZE>::iterator 
+         I = RD.begin(), E = RD.end(); I != E; ++I) {
+    
+    InsnReplacementData IRD = (*I);
+    Instruction *In = IRD.Original;
+    Instruction *C = IRD.PreHeader;
+    
+    if (C->getParent() != NewPreHeader)
+      continue;
+
+    // PHINodes uses value from pre-header predecessors.
+    if (isa<PHINode>(In))
+      continue;
+
+    for (unsigned opi = 0; opi < In->getNumOperands(); ++opi) {
+      if (Instruction *OpPhi = dyn_cast<PHINode>(In->getOperand(opi))) {
+        if (InsnReplacementData *D = findReplacementData(OpPhi))
+          C->setOperand(opi, D->PreHeader);
+      }
+      else if (Instruction *OpInsn = 
+               dyn_cast<Instruction>(In->getOperand(opi))) {
+        if (InsnReplacementData *D = findReplacementData(OpInsn))
+          C->setOperand(opi, D->PreHeader);
+      }
+    }
+  }
+
+  // Rename uses of original header instructions to reflect their new
+  // definitions (either from new pre-header node or from newly created
+  // new header PHINodes.
+  //
+  // Original header instructions are used in
+  // 1) Original header:
+  //
+  //    If instruction is used in non-phi instructions then it is using
+  //    defintion from original heder iteself. Do not replace this use
+  //    with definition from new header or new pre-header.
+  //
+  //    If instruction is used in phi node then it is an incoming 
+  //    value. Rename its use to reflect new definition from new-preheader
+  //    or new header.
+  //
+  // 2) Inside loop but not in original header
+  //
+  //    Replace this use to reflect definition from new header.
+  for (SmallVector<InsnReplacementData, MAX_HEADER_SIZE>::iterator 
+         I = RD.begin(), E = RD.end(); I != E; ++I) {
+
+    InsnReplacementData IRD = (*I);
+    if (!IRD.Header)
+      continue;
+
+    Instruction *OldPhi = IRD.Original;
+    Instruction *NewPhi = IRD.Header;
+
+    // Before replacing uses, collect them first, so that iterator is
+    // not invalidated.
+    SmallVector<Instruction *, 16> AllUses;
+    for (Value::use_iterator UI = OldPhi->use_begin(), UE = OldPhi->use_end();
+         UI != UE; ++UI ) {
+      Instruction *U = cast<Instruction>(UI);
+      AllUses.push_back(U);
+    }
+
+    for (SmallVector<Instruction *, 16>::iterator UI = AllUses.begin(), 
+           UE = AllUses.end(); UI != UE; ++UI) {
+      Instruction *U = *UI;
+      BasicBlock *Parent = U->getParent();
+
+      // Used inside original header
+      if (Parent == OrigHeader) {
+        // Do not rename uses inside original header non-phi instructions.
+        if (!isa<PHINode>(U))
+          continue;
+        PHINode *PU = dyn_cast<PHINode>(U);
+        // Do not rename uses inside original header phi nodes, if the
+        // incoming value is for new header.
+        if (PU->getBasicBlockIndex(NewHeader) != -1
+            && PU->getIncomingValueForBlock(NewHeader) == U)
+          continue;
+
+       U->replaceUsesOfWith(OldPhi, NewPhi);
+       continue;
+      }
+
+      // Used inside loop, but not in original header.
+      if (L->contains(U->getParent())) {
+        if (U != NewPhi )
+          U->replaceUsesOfWith(OldPhi, NewPhi);
+        continue;
+      }
+
+      // Used inside Exit Block. Since we are in LCSSA form, U must be PHINode.
+      assert ( U->getParent() == Exit && "Need to propagate new PHI into Exit blocks");
+      assert (isa<PHINode>(U) && "Use in Exit Block that is not PHINode");        
+
+      PHINode *UPhi = cast<PHINode>(U);
+
+      // UPhi already has one incoming argument from original header. 
+      // Add second incoming argument from new Pre header.
+      
+      UPhi->addIncoming(IRD.PreHeader, NewPreHeader);
+    }
+  }
+  
+  /// Make sure all Exit block PHINodes have required incoming values.
+  updateExitBlock();
+
+  // Update CFG
+
+  // Removing incoming branch from loop preheader to original header.
+  // Now original header is inside the loop.
+  OrigHeader->removePredecessor(OrigPreHeader);
+
+  // Establish NewPreHeader as loop preheader. Add unconditional branch
+  // from original loop pre-header to new loop pre-header. Add NewPreHEader
+  // in loop nest.
+  BranchInst *PH_BI = cast<BranchInst>(OrigPreHeader->getTerminator());
+  PH_BI->setSuccessor(0, NewPreHeader);
+  LoopInfo &LI = LPM.getAnalysis<LoopInfo>();
+  if (Loop *PL = LI.getLoopFor(OrigPreHeader))
+    PL->addBasicBlockToLoop(NewPreHeader, LI);
+
+  // Make NewHeader as the new header for the loop.
+  L->moveToHeader(NewHeader);
+
+  NumRotated++;
+  return true;
+}
+
+
+/// Make sure all Exit block PHINodes have required incoming values.
+/// If incoming value is constant or defined outside the loop then
+/// PHINode may not have an entry for new pre-header. 
+void LoopRotate::updateExitBlock() {
+
+  for (BasicBlock::iterator I = Exit->begin(), E = Exit->end();
+       I != E; ++I) {
+
+    if (!isa<PHINode>(I))
+      break;
+
+    PHINode *PN = dyn_cast<PHINode>(I);
+
+    if (PN->getBasicBlockIndex(NewPreHeader) == -1) {
+      Value *V = PN->getIncomingValueForBlock(OrigHeader);
+      if (isa<Constant>(V))
+        PN->addIncoming(V, NewPreHeader);
+      else {
+        InsnReplacementData *IRD = findReplacementData(cast<Instruction>(V));
+        assert (IRD && IRD->PreHeader && "Missing New Preheader Instruction");
+        PN->addIncoming(IRD->PreHeader, NewPreHeader);
+      }
+    }
+  }
+}
+
+
+/// Initialize local data
+void LoopRotate::initialize() {
+  L = NULL;
+  OrigHeader = NULL;
+  OrigPreHeader = NULL;
+  NewHeader = NULL;
+  NewPreHeader = NULL;
+  Exit = NULL;
+
+  RD.clear();
+}
+
+/// Return true if this instruction is used outside original header.
+bool LoopRotate::usedOutsideOriginalHeader(Instruction *In) {
+
+  for (Value::use_iterator UI = In->use_begin(), UE = In->use_end();
+       UI != UE; ++UI) {
+    Instruction *U = cast<Instruction>(UI);
+    if (U->getParent() != OrigHeader) {
+      if (L->contains(U->getParent()))
+        return true;
+    }
+  }
+
+  return false;
+}
+
+/// Find Replacement information for instruction. Return NULL if it is
+/// not available.
+InsnReplacementData *LoopRotate::findReplacementData(Instruction *In) {
+
+  // Since RD is small, linear walk is OK.
+  for (SmallVector<InsnReplacementData, MAX_HEADER_SIZE>::iterator 
+         I = RD.begin(), E = RD.end(); I != E; ++I) 
+    if ((*I).Original == In)
+      return &(*I);
+
+  return NULL;
+}