Split the loop unroll mechanism logic out into a utility function.

Patch by Matthijs Kooijman!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@51083 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 14 insertions, 379 deletions

diff --git a/lib/Transforms/Scalar/LoopUnroll.cpp b/lib/Transforms/Scalar/LoopUnroll.cpp
index d329970204..ad8cfd4eeb 100644
--- a/lib/Transforms/Scalar/LoopUnroll.cpp
+++ b/lib/Transforms/Scalar/LoopUnroll.cpp
@@ -10,39 +10,19 @@
 // This pass implements a simple loop unroller.  It works best when loops have
 // been canonicalized by the -indvars pass, allowing it to determine the trip
 // counts of loops easily.
-//
-// This pass will multi-block loops only if they contain no non-unrolled 
-// subloops.  The process of unrolling can produce extraneous basic blocks 
-// linked with unconditional branches.  This will be corrected in the future.
-//
 //===----------------------------------------------------------------------===//
 
 #define DEBUG_TYPE "loop-unroll"
+#include "llvm/IntrinsicInst.h"
 #include "llvm/Transforms/Scalar.h"
-#include "llvm/Constants.h"
-#include "llvm/Function.h"
-#include "llvm/Instructions.h"
-#include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/LoopPass.h"
-#include "llvm/Transforms/Utils/Cloning.h"
-#include "llvm/Transforms/Utils/Local.h"
-#include "llvm/Support/CFG.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
-#include "llvm/Support/MathExtras.h"
-#include "llvm/ADT/Statistic.h"
-#include "llvm/ADT/STLExtras.h"
-#include "llvm/ADT/SmallPtrSet.h"
-#include "llvm/IntrinsicInst.h"
-#include <algorithm>
-#include <climits>
-#include <cstdio>
-using namespace llvm;
+#include "llvm/Transforms/Utils/UnrollLoop.h"
 
-STATISTIC(NumCompletelyUnrolled, "Number of loops completely unrolled");
-STATISTIC(NumUnrolled,    "Number of loops unrolled (completely or otherwise)");
+using namespace llvm;
 
 static cl::opt<unsigned>
 UnrollThreshold("unroll-threshold", cl::init(100), cl::Hidden,
@@ -54,7 +34,6 @@ UnrollCount("unroll-count", cl::init(0), cl::Hidden,
 
 namespace {
   class VISIBILITY_HIDDEN LoopUnroll : public LoopPass {
-    LoopInfo *LI;  // The current loop information
   public:
     static char ID; // Pass ID, replacement for typeid
     LoopUnroll() : LoopPass((intptr_t)&ID) {}
@@ -65,8 +44,6 @@ namespace {
     static const unsigned NoThreshold = UINT_MAX;
 
     bool runOnLoop(Loop *L, LPPassManager &LPM);
-    bool unrollLoop(Loop *L, unsigned Count, unsigned Threshold);
-    BasicBlock *FoldBlockIntoPredecessor(BasicBlock *BB);
 
     /// This transformation requires natural loop information & requires that
     /// loop preheaders be inserted into the CFG...
@@ -121,136 +98,18 @@ static unsigned ApproximateLoopSize(const Loop *L) {
   return Size;
 }
 
-// RemapInstruction - Convert the instruction operands from referencing the
-// current values into those specified by ValueMap.
-//
-static inline void RemapInstruction(Instruction *I,
-                                    DenseMap<const Value *, Value*> &ValueMap) {
-  for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
-    Value *Op = I->getOperand(op);
-    DenseMap<const Value *, Value*>::iterator It = ValueMap.find(Op);
-    if (It != ValueMap.end()) Op = It->second;
-    I->setOperand(op, Op);
-  }
-}
-
-// FoldBlockIntoPredecessor - Folds a basic block into its predecessor if it
-// only has one predecessor, and that predecessor only has one successor.
-// Returns the new combined block.
-BasicBlock *LoopUnroll::FoldBlockIntoPredecessor(BasicBlock *BB) {
-  // Merge basic blocks into their predecessor if there is only one distinct
-  // pred, and if there is only one distinct successor of the predecessor, and
-  // if there are no PHI nodes.
-  //
-  BasicBlock *OnlyPred = BB->getSinglePredecessor();
-  if (!OnlyPred) return 0;
-
-  if (OnlyPred->getTerminator()->getNumSuccessors() != 1)
-    return 0;
-
-  DOUT << "Merging: " << *BB << "into: " << *OnlyPred;
-
-  // Resolve any PHI nodes at the start of the block.  They are all
-  // guaranteed to have exactly one entry if they exist, unless there are
-  // multiple duplicate (but guaranteed to be equal) entries for the
-  // incoming edges.  This occurs when there are multiple edges from
-  // OnlyPred to OnlySucc.
-  //
-  while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
-    PN->replaceAllUsesWith(PN->getIncomingValue(0));
-    BB->getInstList().pop_front();  // Delete the phi node...
-  }
-
-  // Delete the unconditional branch from the predecessor...
-  OnlyPred->getInstList().pop_back();
-
-  // Move all definitions in the successor to the predecessor...
-  OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
-
-  // Make all PHI nodes that referred to BB now refer to Pred as their
-  // source...
-  BB->replaceAllUsesWith(OnlyPred);
-
-  std::string OldName = BB->getName();
-
-  // Erase basic block from the function...
-  LI->removeBlock(BB);
-  BB->eraseFromParent();
-
-  // Inherit predecessor's name if it exists...
-  if (!OldName.empty() && !OnlyPred->hasName())
-    OnlyPred->setName(OldName);
-
-  return OnlyPred;
-}
-
 bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
-  LI = &getAnalysis<LoopInfo>();
-
-  // Unroll the loop.
-  if (!unrollLoop(L, UnrollCount, UnrollThreshold))
-    return false;
-
-  // Update the loop information for this loop.
-  // If we completely unrolled the loop, remove it from the parent.
-  if (L->getNumBackEdges() == 0)
-    LPM.deleteLoopFromQueue(L);
-
-  return true;
-}
-
-/// Unroll the given loop by UnrollCount, or by a heuristically-determined
-/// value if Count is zero. If Threshold is not NoThreshold, it is a value
-/// to limit code size expansion. If the loop size would expand beyond the
-/// threshold value, unrolling is suppressed. The return value is true if
-/// any transformations are performed.
-///
-bool LoopUnroll::unrollLoop(Loop *L, unsigned Count, unsigned Threshold) {
   assert(L->isLCSSAForm());
+  LoopInfo *LI = &getAnalysis<LoopInfo>();
 
   BasicBlock *Header = L->getHeader();
-  BasicBlock *LatchBlock = L->getLoopLatch();
-  BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator());
-
   DOUT << "Loop Unroll: F[" << Header->getParent()->getName()
        << "] Loop %" << Header->getName() << "\n";
 
-  if (!BI || BI->isUnconditional()) {
-    // The loop-rotate pass can be helpful to avoid this in many cases.
-    DOUT << "  Can't unroll; loop not terminated by a conditional branch.\n";
-    return false;
-  }
-
-  // Determine the trip count and/or trip multiple. A TripCount value of zero
-  // is used to mean an unknown trip count. The TripMultiple value is the
-  // greatest known integer multiple of the trip count.
-  unsigned TripCount = 0;
-  unsigned TripMultiple = 1;
-  if (Value *TripCountValue = L->getTripCount()) {
-    if (ConstantInt *TripCountC = dyn_cast<ConstantInt>(TripCountValue)) {
-      // Guard against huge trip counts. This also guards against assertions in
-      // APInt from the use of getZExtValue, below.
-      if (TripCountC->getValue().getActiveBits() <= 32) {
-        TripCount = (unsigned)TripCountC->getZExtValue();
-      }
-    } else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(TripCountValue)) {
-      switch (BO->getOpcode()) {
-      case BinaryOperator::Mul:
-        if (ConstantInt *MultipleC = dyn_cast<ConstantInt>(BO->getOperand(1))) {
-          if (MultipleC->getValue().getActiveBits() <= 32) {
-            TripMultiple = (unsigned)MultipleC->getZExtValue();
-          }
-        }
-        break;
-      default: break;
-      }
-    }
-  }
-  if (TripCount != 0)
-    DOUT << "  Trip Count = " << TripCount << "\n";
-  if (TripMultiple != 1)
-    DOUT << "  Trip Multiple = " << TripMultiple << "\n";
-
+  // Find trip count
+  unsigned TripCount = L->getSmallConstantTripCount();
+  unsigned Count = UnrollCount;
+ 
   // Automatically select an unroll count.
   if (Count == 0) {
     // Conservative heuristic: if we know the trip count, see if we can
@@ -263,245 +122,21 @@ bool LoopUnroll::unrollLoop(Loop *L, unsigned Count, unsigned Threshold) {
     }
   }
 
-  // Effectively "DCE" unrolled iterations that are beyond the tripcount
-  // and will never be executed.
-  if (TripCount != 0 && Count > TripCount)
-    Count = TripCount;
-
-  assert(Count > 0);
-  assert(TripMultiple > 0);
-  assert(TripCount == 0 || TripCount % TripMultiple == 0);
-
   // Enforce the threshold.
-  if (Threshold != NoThreshold) {
+  if (UnrollThreshold != NoThreshold) {
     unsigned LoopSize = ApproximateLoopSize(L);
     DOUT << "  Loop Size = " << LoopSize << "\n";
     uint64_t Size = (uint64_t)LoopSize*Count;
-    if (TripCount != 1 && Size > Threshold) {
+    if (TripCount != 1 && Size > UnrollThreshold) {
       DOUT << "  TOO LARGE TO UNROLL: "
-           << Size << ">" << Threshold << "\n";
+           << Size << ">" << UnrollThreshold << "\n";
       return false;
     }
   }
 
-  // Are we eliminating the loop control altogether?
-  bool CompletelyUnroll = Count == TripCount;
-
-  // If we know the trip count, we know the multiple...
-  unsigned BreakoutTrip = 0;
-  if (TripCount != 0) {
-    BreakoutTrip = TripCount % Count;
-    TripMultiple = 0;
-  } else {
-    // Figure out what multiple to use.
-    BreakoutTrip = TripMultiple =
-      (unsigned)GreatestCommonDivisor64(Count, TripMultiple);
-  }
-
-  if (CompletelyUnroll) {
-    DOUT << "COMPLETELY UNROLLING loop %" << Header->getName()
-         << " with trip count " << TripCount << "!\n";
-  } else {
-    DOUT << "UNROLLING loop %" << Header->getName()
-         << " by " << Count;
-    if (TripMultiple == 0 || BreakoutTrip != TripMultiple) {
-      DOUT << " with a breakout at trip " << BreakoutTrip;
-    } else if (TripMultiple != 1) {
-      DOUT << " with " << TripMultiple << " trips per branch";
-    }
-    DOUT << "!\n";
-  }
-
-  std::vector<BasicBlock*> LoopBlocks = L->getBlocks();
-
-  bool ContinueOnTrue = L->contains(BI->getSuccessor(0));
-  BasicBlock *LoopExit = BI->getSuccessor(ContinueOnTrue);
-
-  // For the first iteration of the loop, we should use the precloned values for
-  // PHI nodes.  Insert associations now.
-  typedef DenseMap<const Value*, Value*> ValueMapTy;
-  ValueMapTy LastValueMap;
-  std::vector<PHINode*> OrigPHINode;
-  for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
-    PHINode *PN = cast<PHINode>(I);
-    OrigPHINode.push_back(PN);
-    if (Instruction *I = 
-                dyn_cast<Instruction>(PN->getIncomingValueForBlock(LatchBlock)))
-      if (L->contains(I->getParent()))
-        LastValueMap[I] = I;
-  }
-
-  std::vector<BasicBlock*> Headers;
-  std::vector<BasicBlock*> Latches;
-  Headers.push_back(Header);
-  Latches.push_back(LatchBlock);
-
-  for (unsigned It = 1; It != Count; ++It) {
-    char SuffixBuffer[100];
-    sprintf(SuffixBuffer, ".%d", It);
-    
-    std::vector<BasicBlock*> NewBlocks;
-    
-    for (std::vector<BasicBlock*>::iterator BB = LoopBlocks.begin(),
-         E = LoopBlocks.end(); BB != E; ++BB) {
-      ValueMapTy ValueMap;
-      BasicBlock *New = CloneBasicBlock(*BB, ValueMap, SuffixBuffer);
-      Header->getParent()->getBasicBlockList().push_back(New);
-
-      // Loop over all of the PHI nodes in the block, changing them to use the
-      // incoming values from the previous block.
-      if (*BB == Header)
-        for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) {
-          PHINode *NewPHI = cast<PHINode>(ValueMap[OrigPHINode[i]]);
-          Value *InVal = NewPHI->getIncomingValueForBlock(LatchBlock);
-          if (Instruction *InValI = dyn_cast<Instruction>(InVal))
-            if (It > 1 && L->contains(InValI->getParent()))
-              InVal = LastValueMap[InValI];
-          ValueMap[OrigPHINode[i]] = InVal;
-          New->getInstList().erase(NewPHI);
-        }
-
-      // Update our running map of newest clones
-      LastValueMap[*BB] = New;
-      for (ValueMapTy::iterator VI = ValueMap.begin(), VE = ValueMap.end();
-           VI != VE; ++VI)
-        LastValueMap[VI->first] = VI->second;
-
-      L->addBasicBlockToLoop(New, LI->getBase());
-
-      // Add phi entries for newly created values to all exit blocks except
-      // the successor of the latch block.  The successor of the exit block will
-      // be updated specially after unrolling all the way.
-      if (*BB != LatchBlock)
-        for (Value::use_iterator UI = (*BB)->use_begin(), UE = (*BB)->use_end();
-             UI != UE;) {
-          Instruction *UseInst = cast<Instruction>(*UI);
-          ++UI;
-          if (isa<PHINode>(UseInst) && !L->contains(UseInst->getParent())) {
-            PHINode *phi = cast<PHINode>(UseInst);
-            Value *Incoming = phi->getIncomingValueForBlock(*BB);
-            phi->addIncoming(Incoming, New);
-          }
-        }
-
-      // Keep track of new headers and latches as we create them, so that
-      // we can insert the proper branches later.
-      if (*BB == Header)
-        Headers.push_back(New);
-      if (*BB == LatchBlock) {
-        Latches.push_back(New);
-
-        // Also, clear out the new latch's back edge so that it doesn't look
-        // like a new loop, so that it's amenable to being merged with adjacent
-        // blocks later on.
-        TerminatorInst *Term = New->getTerminator();
-        assert(L->contains(Term->getSuccessor(!ContinueOnTrue)));
-        assert(Term->getSuccessor(ContinueOnTrue) == LoopExit);
-        Term->setSuccessor(!ContinueOnTrue, NULL);
-      }
-
-      NewBlocks.push_back(New);
-    }
-    
-    // Remap all instructions in the most recent iteration
-    for (unsigned i = 0; i < NewBlocks.size(); ++i)
-      for (BasicBlock::iterator I = NewBlocks[i]->begin(),
-           E = NewBlocks[i]->end(); I != E; ++I)
-        RemapInstruction(I, LastValueMap);
-  }
-  
-  // The latch block exits the loop.  If there are any PHI nodes in the
-  // successor blocks, update them to use the appropriate values computed as the
-  // last iteration of the loop.
-  if (Count != 1) {
-    SmallPtrSet<PHINode*, 8> Users;
-    for (Value::use_iterator UI = LatchBlock->use_begin(),
-         UE = LatchBlock->use_end(); UI != UE; ++UI)
-      if (PHINode *phi = dyn_cast<PHINode>(*UI))
-        Users.insert(phi);
-    
-    BasicBlock *LastIterationBB = cast<BasicBlock>(LastValueMap[LatchBlock]);
-    for (SmallPtrSet<PHINode*,8>::iterator SI = Users.begin(), SE = Users.end();
-         SI != SE; ++SI) {
-      PHINode *PN = *SI;
-      Value *InVal = PN->removeIncomingValue(LatchBlock, false);
-      // If this value was defined in the loop, take the value defined by the
-      // last iteration of the loop.
-      if (Instruction *InValI = dyn_cast<Instruction>(InVal)) {
-        if (L->contains(InValI->getParent()))
-          InVal = LastValueMap[InVal];
-      }
-      PN->addIncoming(InVal, LastIterationBB);
-    }
-  }
-
-  // Now, if we're doing complete unrolling, loop over the PHI nodes in the
-  // original block, setting them to their incoming values.
-  if (CompletelyUnroll) {
-    BasicBlock *Preheader = L->getLoopPreheader();
-    for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) {
-      PHINode *PN = OrigPHINode[i];
-      PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader));
-      Header->getInstList().erase(PN);
-    }
-  }
-
-  // Now that all the basic blocks for the unrolled iterations are in place,
-  // set up the branches to connect them.
-  for (unsigned i = 0, e = Latches.size(); i != e; ++i) {
-    // The original branch was replicated in each unrolled iteration.
-    BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator());
-
-    // The branch destination.
-    unsigned j = (i + 1) % e;
-    BasicBlock *Dest = Headers[j];
-    bool NeedConditional = true;
-
-    // For a complete unroll, make the last iteration end with a branch
-    // to the exit block.
-    if (CompletelyUnroll && j == 0) {
-      Dest = LoopExit;
-      NeedConditional = false;
-    }
-
-    // If we know the trip count or a multiple of it, we can safely use an
-    // unconditional branch for some iterations.
-    if (j != BreakoutTrip && (TripMultiple == 0 || j % TripMultiple != 0)) {
-      NeedConditional = false;
-    }
-
-    if (NeedConditional) {
-      // Update the conditional branch's successor for the following
-      // iteration.
-      Term->setSuccessor(!ContinueOnTrue, Dest);
-    } else {
-      Term->setUnconditionalDest(Dest);
-      // Merge adjacent basic blocks, if possible.
-      if (BasicBlock *Fold = FoldBlockIntoPredecessor(Dest)) {
-        std::replace(Latches.begin(), Latches.end(), Dest, Fold);
-        std::replace(Headers.begin(), Headers.end(), Dest, Fold);
-      }
-    }
-  }
-  
-  // At this point, the code is well formed.  We now do a quick sweep over the
-  // inserted code, doing constant propagation and dead code elimination as we
-  // go.
-  const std::vector<BasicBlock*> &NewLoopBlocks = L->getBlocks();
-  for (std::vector<BasicBlock*>::const_iterator BB = NewLoopBlocks.begin(),
-       BBE = NewLoopBlocks.end(); BB != BBE; ++BB)
-    for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ) {
-      Instruction *Inst = I++;
-
-      if (isInstructionTriviallyDead(Inst))
-        (*BB)->getInstList().erase(Inst);
-      else if (Constant *C = ConstantFoldInstruction(Inst)) {
-        Inst->replaceAllUsesWith(C);
-        (*BB)->getInstList().erase(Inst);
-      }
-    }
+  // Unroll the loop.
+  if (!UnrollLoop(L, Count, LI, &LPM))
+    return false;
 
-  NumCompletelyUnrolled += CompletelyUnroll;
-  ++NumUnrolled;
   return true;
 }