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, 371 insertions, 0 deletions

diff --git a/lib/Transforms/Utils/UnrollLoop.cpp b/lib/Transforms/Utils/UnrollLoop.cpp
new file mode 100644
index 0000000..a86306c
--- /dev/null
+++ b/lib/Transforms/Utils/UnrollLoop.cpp
@@ -0,0 +1,371 @@
+//===-- UnrollLoop.cpp - Loop unrolling utilities -------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements some loop unrolling utilities. It does not define any
+// actual pass or policy, but provides a single function to perform loop
+// unrolling.
+//
+// It works best when loops have been canonicalized by the -indvars pass,
+// allowing it to determine the trip counts of loops easily.
+//
+// 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/Transforms/Utils/UnrollLoop.h"
+#include "llvm/BasicBlock.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/ConstantFolding.h"
+#include "llvm/Analysis/LoopPass.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Transforms/Utils/Cloning.h"
+#include "llvm/Transforms/Utils/Local.h"
+
+using namespace llvm;
+
+/* TODO: Should these be here or in LoopUnroll? */
+STATISTIC(NumCompletelyUnrolled, "Number of loops completely unrolled");
+STATISTIC(NumUnrolled,    "Number of loops unrolled (completely or otherwise)");
+
+/// 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.
+/// The LoopInfo Analysis that is passed will be kept consistent.
+/// Returns the new combined block.
+static BasicBlock *FoldBlockIntoPredecessor(BasicBlock *BB, LoopInfo* LI) {
+  // 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;
+}
+
+/// Unroll the given loop by Count. The loop must be in LCSSA form. Returns true
+/// if unrolling was succesful, or false if the loop was unmodified. Unrolling
+/// can only fail when the loop's latch block is not terminated by a conditional
+/// branch instruction. However, if the trip count (and multiple) are not known,
+/// loop unrolling will mostly produce more code that is no faster.
+///
+/// The LoopInfo Analysis that is passed will be kept consistent.
+///
+/// If a LoopPassManager is passed in, and the loop is fully removed, it will be
+/// removed from the LoopPassManager as well. LPM can also be NULL.
+bool llvm::UnrollLoop(Loop *L, unsigned Count, LoopInfo* LI, LPPassManager* LPM) {
+  assert(L->isLCSSAForm());
+
+  BasicBlock *Header = L->getHeader();
+  BasicBlock *LatchBlock = L->getLoopLatch();
+  BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator());
+  
+  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;
+  }
+
+  // Find trip count
+  unsigned TripCount = L->getSmallConstantTripCount();
+  // Find trip multiple if count is not available
+  unsigned TripMultiple = 1;
+  if (TripCount == 0)
+    TripMultiple = L->getSmallConstantTripMultiple();
+
+  if (TripCount != 0)
+    DOUT << "  Trip Count = " << TripCount << "\n";
+  if (TripMultiple != 1)
+    DOUT << "  Trip Multiple = " << TripMultiple << "\n";
+
+  // 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);
+
+  // 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, LI)) {
+        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);
+      }
+    }
+
+  NumCompletelyUnrolled += CompletelyUnroll;
+  ++NumUnrolled;
+  // Remove the loop from the LoopPassManager if it's completely removed.
+  if (CompletelyUnroll && LPM != NULL)
+    LPM->deleteLoopFromQueue(L);
+
+  // If we didn't completely unroll the loop, it should still be in LCSSA form.
+  if (!CompletelyUnroll)
+    assert(L->isLCSSAForm());
+
+  return true;
+}