llvm/lib/Transforms/Scalar/LoopUnswitch.cpp - toolchain/llvm-project - Gitiles

 //===-- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop ------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file was developed by the LLVM research group and is distributed under
 // the University of Illinois Open Source License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This pass transforms loops that contain branches on loop-invariant conditions
 // to have multiple loops.  For example, it turns the left into the right code:
 //
 //  for (...)                  if (lic)
 //    A                          for (...)
 //    if (lic)                     A; B; C
 //      B                      else
 //    C                          for (...)
 //                                 A; C
 //
 // This can increase the size of the code exponentially (doubling it every time
 // a loop is unswitched) so we only unswitch if the resultant code will be
 // smaller than a threshold.
 //
 // This pass expects LICM to be run before it to hoist invariant conditions out
 // of the loop, to make the unswitching opportunity obvious.
 //
 //===----------------------------------------------------------------------===//

 #define DEBUG_TYPE "loop-unswitch"
 #include "llvm/Transforms/Scalar.h"
 #include "llvm/Constants.h"
 #include "llvm/Function.h"
 #include "llvm/Instructions.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Transforms/Utils/Cloning.h"
 #include "llvm/Transforms/Utils/Local.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/CommandLine.h"
 #include <algorithm>
 #include <iostream>
 #include <set>
 using namespace llvm;

 namespace {
   Statistic<> NumUnswitched("loop-unswitch", "Number of loops unswitched");
   cl::opt<unsigned>
   Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
             cl::init(10), cl::Hidden);

   class LoopUnswitch : public FunctionPass {
     LoopInfo *LI;  // Loop information
   public:
     virtual bool runOnFunction(Function &F);
     bool visitLoop(Loop *L);

     /// This transformation requires natural loop information & requires that
     /// loop preheaders be inserted into the CFG...
     ///
     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
       AU.addRequiredID(LoopSimplifyID);
       AU.addPreservedID(LoopSimplifyID);
       AU.addRequired<LoopInfo>();
       AU.addPreserved<LoopInfo>();
     }

   private:
     unsigned getLoopUnswitchCost(Loop *L, Value *LIC);
     void VersionLoop(Value *LIC, Loop *L, Loop *&Out1, Loop *&Out2);
     BasicBlock *SplitBlock(BasicBlock *BB, bool SplitAtTop);
     void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, bool Val);
     void UnswitchTrivialCondition(Loop *L, Value *Cond, bool EntersLoopOnCond,
                                   BasicBlock *ExitBlock);
   };
   RegisterOpt<LoopUnswitch> X("loop-unswitch", "Unswitch loops");
 }

 FunctionPass *llvm::createLoopUnswitchPass() { return new LoopUnswitch(); }

 bool LoopUnswitch::runOnFunction(Function &F) {
   bool Changed = false;
   LI = &getAnalysis<LoopInfo>();

   // Transform all the top-level loops.  Copy the loop list so that the child
   // can update the loop tree if it needs to delete the loop.
   std::vector<Loop*> SubLoops(LI->begin(), LI->end());
   for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
     Changed |= visitLoop(SubLoops[i]);

   return Changed;
 }


 /// LoopValuesUsedOutsideLoop - Return true if there are any values defined in
 /// the loop that are used by instructions outside of it.
 static bool LoopValuesUsedOutsideLoop(Loop *L) {
   // We will be doing lots of "loop contains block" queries.  Loop::contains is
   // linear time, use a set to speed this up.
   std::set<BasicBlock*> LoopBlocks;

   for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
        BB != E; ++BB)
     LoopBlocks.insert(*BB);

   for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
        BB != E; ++BB) {
     for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ++I)
       for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
            ++UI) {
         BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
         if (!LoopBlocks.count(UserBB))
           return true;
       }
   }
   return false;
 }

 /// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
 /// trivial: that is, that the condition controls whether or not the loop does
 /// anything at all.  If this is a trivial condition, unswitching produces no
 /// code duplications (equivalently, it produces a simpler loop and a new empty
 /// loop, which gets deleted).
 ///
 /// If this is a trivial condition, return ConstantBool::True if the loop body
 /// runs when the condition is true, False if the loop body executes when the
 /// condition is false.  Otherwise, return null to indicate a complex condition.
 static bool IsTrivialUnswitchCondition(Loop *L, Value *Cond,
                                        bool *CondEntersLoop = 0,
                                        BasicBlock **LoopExit = 0) {
   BasicBlock *Header = L->getHeader();
   BranchInst *HeaderTerm = dyn_cast<BranchInst>(Header->getTerminator());

   // If the header block doesn't end with a conditional branch on Cond, we can't
   // handle it.
   if (!HeaderTerm || !HeaderTerm->isConditional() ||
       HeaderTerm->getCondition() != Cond)
     return false;

   // Check to see if the conditional branch goes to the latch block.  If not,
   // it's not trivial.  This also determines the value of Cond that will execute
   // the loop.
   BasicBlock *Latch = L->getLoopLatch();
   if (HeaderTerm->getSuccessor(1) == Latch) {
     if (CondEntersLoop) *CondEntersLoop = true;
   } else if (HeaderTerm->getSuccessor(0) == Latch)
     if (CondEntersLoop) *CondEntersLoop = false;
   else
     return false;  // Doesn't branch to latch block.

   // The latch block must end with a conditional branch where one edge goes to
   // the header (this much we know) and one edge goes OUT of the loop.
   BranchInst *LatchBranch = dyn_cast<BranchInst>(Latch->getTerminator());
   if (!LatchBranch || !LatchBranch->isConditional()) return false;

   if (LatchBranch->getSuccessor(0) == Header) {
     if (L->contains(LatchBranch->getSuccessor(1))) return false;
     if (LoopExit) *LoopExit = LatchBranch->getSuccessor(1);
   } else {
     assert(LatchBranch->getSuccessor(1) == Header);
     if (L->contains(LatchBranch->getSuccessor(0))) return false;
     if (LoopExit) *LoopExit = LatchBranch->getSuccessor(0);
   }

   // We already know that nothing uses any scalar values defined inside of this
   // loop.  As such, we just have to check to see if this loop will execute any
   // side-effecting instructions (e.g. stores, calls, volatile loads) in the
   // part of the loop that the code *would* execute.
   for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
     if (I->mayWriteToMemory())
       return false;
   for (BasicBlock::iterator I = Latch->begin(), E = Latch->end(); I != E; ++I)
     if (I->mayWriteToMemory())
       return false;
   return true;
 }

 /// getLoopUnswitchCost - Return the cost (code size growth) that will happen if
 /// we choose to unswitch the specified loop on the specified value.
 ///
 unsigned LoopUnswitch::getLoopUnswitchCost(Loop *L, Value *LIC) {
   // If the condition is trivial, always unswitch.  There is no code growth for
   // this case.
   if (IsTrivialUnswitchCondition(L, LIC))
     return 0;

   unsigned Cost = 0;
   // FIXME: this is brain dead.  It should take into consideration code
   // shrinkage.
   for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
        I != E; ++I) {
     BasicBlock *BB = *I;
     // Do not include empty blocks in the cost calculation.  This happen due to
     // loop canonicalization and will be removed.
     if (BB->begin() == BasicBlock::iterator(BB->getTerminator()))
       continue;

     // Count basic blocks.
     ++Cost;
   }

   return Cost;
 }

 bool LoopUnswitch::visitLoop(Loop *L) {
   bool Changed = false;

   // Recurse through all subloops before we process this loop.  Copy the loop
   // list so that the child can update the loop tree if it needs to delete the
   // loop.
   std::vector<Loop*> SubLoops(L->begin(), L->end());
   for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
     Changed |= visitLoop(SubLoops[i]);

   // Loop over all of the basic blocks in the loop.  If we find an interior
   // block that is branching on a loop-invariant condition, we can unswitch this
   // loop.
   for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
        I != E; ++I) {
     TerminatorInst *TI = (*I)->getTerminator();
     if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
       if (!isa<Constant>(SI) && L->isLoopInvariant(SI->getCondition()))
         DEBUG(std::cerr << "TODO: Implement unswitching 'switch' loop %"
               << L->getHeader()->getName() << ", cost = "
               << L->getBlocks().size() << "\n" << **I);
       continue;
     }

     BranchInst *BI = dyn_cast<BranchInst>(TI);
     if (!BI) continue;

     // If this isn't branching on an invariant condition, we can't unswitch it.
     if (!BI->isConditional() || isa<Constant>(BI->getCondition()) ||
         !L->isLoopInvariant(BI->getCondition()))
       continue;

     // Check to see if it would be profitable to unswitch this loop.
     if (getLoopUnswitchCost(L, BI->getCondition()) > Threshold) {
       // FIXME: this should estimate growth by the amount of code shared by the
       // resultant unswitched loops.  This should have no code growth:
       //    for () { if (iv) {...} }
       // as one copy of the loop will be empty.
       //
       DEBUG(std::cerr << "NOT unswitching loop %"
             << L->getHeader()->getName() << ", cost too high: "
             << L->getBlocks().size() << "\n");
       continue;
     }

     // If this loop has live-out values, we can't unswitch it. We need something
     // like loop-closed SSA form in order to know how to insert PHI nodes for
     // these values.
     if (LoopValuesUsedOutsideLoop(L)) {
       DEBUG(std::cerr << "NOT unswitching loop %"
                       << L->getHeader()->getName()
                       << ", a loop value is used outside loop!\n");
       continue;
     }

     //std::cerr << "BEFORE:\n"; LI->dump();
     Loop *NewLoop1 = 0, *NewLoop2 = 0;

     // If this is a trivial condition to unswitch (which results in no code
     // duplication), do it now.
     bool EntersLoopOnCond;
     BasicBlock *ExitBlock;
     if (IsTrivialUnswitchCondition(L, BI->getCondition(), &EntersLoopOnCond,
                                    &ExitBlock)) {
       UnswitchTrivialCondition(L, BI->getCondition(),
                                EntersLoopOnCond, ExitBlock);
       NewLoop1 = L;
     } else {
       VersionLoop(BI->getCondition(), L, NewLoop1, NewLoop2);
     }

     //std::cerr << "AFTER:\n"; LI->dump();

     // Try to unswitch each of our new loops now!
     if (NewLoop1) visitLoop(NewLoop1);
     if (NewLoop2) visitLoop(NewLoop2);
     return true;
   }

   return Changed;
 }

 /// SplitBlock - Split the specified basic block into two pieces.  If SplitAtTop
 /// is false, this splits the block so the second half only has an unconditional
 /// branch.  If SplitAtTop is true, it makes it so the first half of the block
 /// only has an unconditional branch in it.
 ///
 /// This method updates the LoopInfo for this function to correctly reflect the
 /// CFG changes made.
 ///
 /// This routine returns the new basic block that was inserted, which is always
 /// the later part of the block.
 BasicBlock *LoopUnswitch::SplitBlock(BasicBlock *BB, bool SplitAtTop) {
   BasicBlock::iterator SplitPoint;
   if (!SplitAtTop)
     SplitPoint = BB->getTerminator();
   else {
     SplitPoint = BB->begin();
     while (isa<PHINode>(SplitPoint)) ++SplitPoint;
   }

   BasicBlock *New = BB->splitBasicBlock(SplitPoint, BB->getName()+".tail");
   // New now lives in whichever loop that BB used to.
   if (Loop *L = LI->getLoopFor(BB))
     L->addBasicBlockToLoop(New, *LI);
   return New;
 }


 // RemapInstruction - Convert the instruction operands from referencing the
 // current values into those specified by ValueMap.
 //
 static inline void RemapInstruction(Instruction *I,
                                     std::map<const Value *, Value*> &ValueMap) {
   for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
     Value *Op = I->getOperand(op);
     std::map<const Value *, Value*>::iterator It = ValueMap.find(Op);
     if (It != ValueMap.end()) Op = It->second;
     I->setOperand(op, Op);
   }
 }

 /// CloneLoop - Recursively clone the specified loop and all of its children,
 /// mapping the blocks with the specified map.
 static Loop *CloneLoop(Loop *L, Loop *PL, std::map<const Value*, Value*> &VM,
                        LoopInfo *LI) {
   Loop *New = new Loop();

   if (PL)
     PL->addChildLoop(New);
   else
     LI->addTopLevelLoop(New);

   // Add all of the blocks in L to the new loop.
   for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
        I != E; ++I)
     if (LI->getLoopFor(*I) == L)
       New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI);

   // Add all of the subloops to the new loop.
   for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
     CloneLoop(*I, New, VM, LI);

   return New;
 }

 /// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable
 /// condition in it (a cond branch from its header block to its latch block,
 /// where the path through the loop that doesn't execute its body has no
 /// side-effects), unswitch it.  This doesn't involve any code duplication, just
 /// moving the conditional branch outside of the loop and updating loop info.
 void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
                                             bool EnterOnCond,
                                             BasicBlock *ExitBlock) {
   DEBUG(std::cerr << "loop-unswitch: Trivial-Unswitch loop %"
         << L->getHeader()->getName() << " [" << L->getBlocks().size()
         << " blocks] in Function " << L->getHeader()->getParent()->getName()
         << " on cond:" << *Cond << "\n");

   // First step, split the preahder, so that we know that there is a safe place
   // to insert the conditional branch.  We will change 'OrigPH' to have a
   // conditional branch on Cond.
   BasicBlock *OrigPH = L->getLoopPreheader();
   BasicBlock *NewPH = SplitBlock(OrigPH, false);

   // Now that we have a place to insert the conditional branch, create a place
   // to branch to: this is the exit block out of the loop that we should
   // short-circuit to.

   // Split this block now, so that the loop maintains its exit block.
   assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
   BasicBlock *NewExit = SplitBlock(ExitBlock, true);

   // Okay, now we have a position to branch from and a position to branch to,
   // insert the new conditional branch.
   new BranchInst(EnterOnCond ? NewPH : NewExit, EnterOnCond ? NewExit : NewPH,
                  Cond, OrigPH->getTerminator());
   OrigPH->getTerminator()->eraseFromParent();

   // Now that we know that the loop is never entered when this condition is a
   // particular value, rewrite the loop with this info.  We know that this will
   // at least eliminate the old branch.
   RewriteLoopBodyWithConditionConstant(L, Cond, EnterOnCond);

   ++NumUnswitched;
 }


 /// VersionLoop - We determined that the loop is profitable to unswitch and
 /// contains a branch on a loop invariant condition.  Split it into loop
 /// versions and test the condition outside of either loop.  Return the loops
 /// created as Out1/Out2.
 void LoopUnswitch::VersionLoop(Value *LIC, Loop *L, Loop *&Out1, Loop *&Out2) {
   Function *F = L->getHeader()->getParent();

   DEBUG(std::cerr << "loop-unswitch: Unswitching loop %"
         << L->getHeader()->getName() << " [" << L->getBlocks().size()
         << " blocks] in Function " << F->getName()
         << " on cond:" << *LIC << "\n");

   std::vector<BasicBlock*> LoopBlocks;

   // First step, split the preheader and exit blocks, and add these blocks to
   // the LoopBlocks list.
   BasicBlock *OrigPreheader = L->getLoopPreheader();
   LoopBlocks.push_back(SplitBlock(OrigPreheader, false));

   // We want the loop to come after the preheader, but before the exit blocks.
   LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());

   std::vector<BasicBlock*> ExitBlocks;
   L->getExitBlocks(ExitBlocks);
   std::sort(ExitBlocks.begin(), ExitBlocks.end());
   ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()),
                    ExitBlocks.end());
   for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
     SplitBlock(ExitBlocks[i], true);
     LoopBlocks.push_back(ExitBlocks[i]);
   }

   // Next step, clone all of the basic blocks that make up the loop (including
   // the loop preheader and exit blocks), keeping track of the mapping between
   // the instructions and blocks.
   std::vector<BasicBlock*> NewBlocks;
   NewBlocks.reserve(LoopBlocks.size());
   std::map<const Value*, Value*> ValueMap;
   for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
     NewBlocks.push_back(CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F));
     ValueMap[LoopBlocks[i]] = NewBlocks.back();  // Keep the BB mapping.
   }

   // Splice the newly inserted blocks into the function right before the
   // original preheader.
   F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(),
                                 NewBlocks[0], F->end());

   // Now we create the new Loop object for the versioned loop.
   Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI);
   if (Loop *Parent = L->getParentLoop()) {
     // Make sure to add the cloned preheader and exit blocks to the parent loop
     // as well.
     Parent->addBasicBlockToLoop(NewBlocks[0], *LI);
     for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
       Parent->addBasicBlockToLoop(cast<BasicBlock>(ValueMap[ExitBlocks[i]]),
                                   *LI);
   }

   // Rewrite the code to refer to itself.
   for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
     for (BasicBlock::iterator I = NewBlocks[i]->begin(),
            E = NewBlocks[i]->end(); I != E; ++I)
       RemapInstruction(I, ValueMap);

   // Rewrite the original preheader to select between versions of the loop.
   assert(isa<BranchInst>(OrigPreheader->getTerminator()) &&
          cast<BranchInst>(OrigPreheader->getTerminator())->isUnconditional() &&
          OrigPreheader->getTerminator()->getSuccessor(0) == LoopBlocks[0] &&
          "Preheader splitting did not work correctly!");
   // Remove the unconditional branch to LoopBlocks[0].
   OrigPreheader->getInstList().pop_back();

   // Insert a conditional branch on LIC to the two preheaders.  The original
   // code is the true version and the new code is the false version.
   new BranchInst(LoopBlocks[0], NewBlocks[0], LIC, OrigPreheader);

   // Now we rewrite the original code to know that the condition is true and the
   // new code to know that the condition is false.
   RewriteLoopBodyWithConditionConstant(L, LIC, true);
   RewriteLoopBodyWithConditionConstant(NewLoop, LIC, false);
   ++NumUnswitched;
   Out1 = L;
   Out2 = NewLoop;
 }

 // RewriteLoopBodyWithConditionConstant - We know that the boolean value LIC has
 // the value specified by Val in the specified loop.  Rewrite any uses of LIC or
 // of properties correlated to it.
 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
                                                         bool Val) {
   assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
   // FIXME: Support correlated properties, like:
   //  for (...)
   //    if (li1 < li2)
   //      ...
   //    if (li1 > li2)
   //      ...
   ConstantBool *BoolVal = ConstantBool::get(Val);

   std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
   for (unsigned i = 0, e = Users.size(); i != e; ++i)
     if (Instruction *U = cast<Instruction>(Users[i]))
       if (L->contains(U->getParent()))
         U->replaceUsesOfWith(LIC, BoolVal);
 }
	//===-- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop ------===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file was developed by the LLVM research group and is distributed under
	// the University of Illinois Open Source License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//
	//
	// This pass transforms loops that contain branches on loop-invariant conditions
	// to have multiple loops. For example, it turns the left into the right code:
	//
	// for (...) if (lic)
	// A for (...)
	// if (lic) A; B; C
	// B else
	// C for (...)
	// A; C
	//
	// This can increase the size of the code exponentially (doubling it every time
	// a loop is unswitched) so we only unswitch if the resultant code will be
	// smaller than a threshold.
	//
	// This pass expects LICM to be run before it to hoist invariant conditions out
	// of the loop, to make the unswitching opportunity obvious.
	//
	//===----------------------------------------------------------------------===//

	#define DEBUG_TYPE "loop-unswitch"
	#include "llvm/Transforms/Scalar.h"
	#include "llvm/Constants.h"
	#include "llvm/Function.h"
	#include "llvm/Instructions.h"
	#include "llvm/Analysis/LoopInfo.h"
	#include "llvm/Transforms/Utils/Cloning.h"
	#include "llvm/Transforms/Utils/Local.h"
	#include "llvm/ADT/Statistic.h"
	#include "llvm/Support/Debug.h"
	#include "llvm/Support/CommandLine.h"
	#include <algorithm>
	#include <iostream>
	#include <set>
	using namespace llvm;

	namespace {
	Statistic<> NumUnswitched("loop-unswitch", "Number of loops unswitched");
	cl::opt<unsigned>
	Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
	cl::init(10), cl::Hidden);

	class LoopUnswitch : public FunctionPass {
	LoopInfo *LI; // Loop information
	public:
	virtual bool runOnFunction(Function &F);
	bool visitLoop(Loop *L);

	/// This transformation requires natural loop information & requires that
	/// loop preheaders be inserted into the CFG...
	///
	virtual void getAnalysisUsage(AnalysisUsage &AU) const {
	AU.addRequiredID(LoopSimplifyID);
	AU.addPreservedID(LoopSimplifyID);
	AU.addRequired<LoopInfo>();
	AU.addPreserved<LoopInfo>();
	}

	private:
	unsigned getLoopUnswitchCost(Loop L, Value LIC);
	void VersionLoop(Value LIC, Loop L, Loop &Out1, Loop &Out2);
	BasicBlock SplitBlock(BasicBlock BB, bool SplitAtTop);
	void RewriteLoopBodyWithConditionConstant(Loop L, Value LIC, bool Val);
	void UnswitchTrivialCondition(Loop L, Value Cond, bool EntersLoopOnCond,
	BasicBlock *ExitBlock);
	};
	RegisterOpt<LoopUnswitch> X("loop-unswitch", "Unswitch loops");
	}

	FunctionPass *llvm::createLoopUnswitchPass() { return new LoopUnswitch(); }

	bool LoopUnswitch::runOnFunction(Function &F) {
	bool Changed = false;
	LI = &getAnalysis<LoopInfo>();

	// Transform all the top-level loops. Copy the loop list so that the child
	// can update the loop tree if it needs to delete the loop.
	std::vector<Loop*> SubLoops(LI->begin(), LI->end());
	for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
	Changed \|= visitLoop(SubLoops[i]);

	return Changed;
	}


	/// LoopValuesUsedOutsideLoop - Return true if there are any values defined in
	/// the loop that are used by instructions outside of it.
	static bool LoopValuesUsedOutsideLoop(Loop *L) {
	// We will be doing lots of "loop contains block" queries. Loop::contains is
	// linear time, use a set to speed this up.
	std::set<BasicBlock*> LoopBlocks;

	for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
	BB != E; ++BB)
	LoopBlocks.insert(*BB);

	for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
	BB != E; ++BB) {
	for (BasicBlock::iterator I = (BB)->begin(), E = (BB)->end(); I != E; ++I)
	for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
	++UI) {
	BasicBlock UserBB = cast<Instruction>(UI)->getParent();
	if (!LoopBlocks.count(UserBB))
	return true;
	}
	}
	return false;
	}

	/// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
	/// trivial: that is, that the condition controls whether or not the loop does
	/// anything at all. If this is a trivial condition, unswitching produces no
	/// code duplications (equivalently, it produces a simpler loop and a new empty
	/// loop, which gets deleted).
	///
	/// If this is a trivial condition, return ConstantBool::True if the loop body
	/// runs when the condition is true, False if the loop body executes when the
	/// condition is false. Otherwise, return null to indicate a complex condition.
	static bool IsTrivialUnswitchCondition(Loop L, Value Cond,
	bool *CondEntersLoop = 0,
	BasicBlock **LoopExit = 0) {
	BasicBlock *Header = L->getHeader();
	BranchInst *HeaderTerm = dyn_cast<BranchInst>(Header->getTerminator());

	// If the header block doesn't end with a conditional branch on Cond, we can't
	// handle it.
	if (!HeaderTerm \|\| !HeaderTerm->isConditional() \|\|
	HeaderTerm->getCondition() != Cond)
	return false;

	// Check to see if the conditional branch goes to the latch block. If not,
	// it's not trivial. This also determines the value of Cond that will execute
	// the loop.
	BasicBlock *Latch = L->getLoopLatch();
	if (HeaderTerm->getSuccessor(1) == Latch) {
	if (CondEntersLoop) *CondEntersLoop = true;
	} else if (HeaderTerm->getSuccessor(0) == Latch)
	if (CondEntersLoop) *CondEntersLoop = false;
	else
	return false; // Doesn't branch to latch block.

	// The latch block must end with a conditional branch where one edge goes to
	// the header (this much we know) and one edge goes OUT of the loop.
	BranchInst *LatchBranch = dyn_cast<BranchInst>(Latch->getTerminator());
	if (!LatchBranch \|\| !LatchBranch->isConditional()) return false;

	if (LatchBranch->getSuccessor(0) == Header) {
	if (L->contains(LatchBranch->getSuccessor(1))) return false;
	if (LoopExit) *LoopExit = LatchBranch->getSuccessor(1);
	} else {
	assert(LatchBranch->getSuccessor(1) == Header);
	if (L->contains(LatchBranch->getSuccessor(0))) return false;
	if (LoopExit) *LoopExit = LatchBranch->getSuccessor(0);
	}

	// We already know that nothing uses any scalar values defined inside of this
	// loop. As such, we just have to check to see if this loop will execute any
	// side-effecting instructions (e.g. stores, calls, volatile loads) in the
	// part of the loop that the code would execute.
	for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
	if (I->mayWriteToMemory())
	return false;
	for (BasicBlock::iterator I = Latch->begin(), E = Latch->end(); I != E; ++I)
	if (I->mayWriteToMemory())
	return false;
	return true;
	}

	/// getLoopUnswitchCost - Return the cost (code size growth) that will happen if
	/// we choose to unswitch the specified loop on the specified value.
	///
	unsigned LoopUnswitch::getLoopUnswitchCost(Loop L, Value LIC) {
	// If the condition is trivial, always unswitch. There is no code growth for
	// this case.
	if (IsTrivialUnswitchCondition(L, LIC))
	return 0;

	unsigned Cost = 0;
	// FIXME: this is brain dead. It should take into consideration code
	// shrinkage.
	for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
	I != E; ++I) {
	BasicBlock BB = I;
	// Do not include empty blocks in the cost calculation. This happen due to
	// loop canonicalization and will be removed.
	if (BB->begin() == BasicBlock::iterator(BB->getTerminator()))
	continue;

	// Count basic blocks.
	++Cost;
	}

	return Cost;
	}

	bool LoopUnswitch::visitLoop(Loop *L) {
	bool Changed = false;

	// Recurse through all subloops before we process this loop. Copy the loop
	// list so that the child can update the loop tree if it needs to delete the
	// loop.
	std::vector<Loop*> SubLoops(L->begin(), L->end());
	for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
	Changed \|= visitLoop(SubLoops[i]);

	// Loop over all of the basic blocks in the loop. If we find an interior
	// block that is branching on a loop-invariant condition, we can unswitch this
	// loop.
	for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
	I != E; ++I) {
	TerminatorInst TI = (I)->getTerminator();
	if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
	if (!isa<Constant>(SI) && L->isLoopInvariant(SI->getCondition()))
	DEBUG(std::cerr << "TODO: Implement unswitching 'switch' loop %"
	<< L->getHeader()->getName() << ", cost = "
	<< L->getBlocks().size() << "\n" << **I);
	continue;
	}

	BranchInst *BI = dyn_cast<BranchInst>(TI);
	if (!BI) continue;

	// If this isn't branching on an invariant condition, we can't unswitch it.
	if (!BI->isConditional() \|\| isa<Constant>(BI->getCondition()) \|\|
	!L->isLoopInvariant(BI->getCondition()))
	continue;

	// Check to see if it would be profitable to unswitch this loop.
	if (getLoopUnswitchCost(L, BI->getCondition()) > Threshold) {
	// FIXME: this should estimate growth by the amount of code shared by the
	// resultant unswitched loops. This should have no code growth:
	// for () { if (iv) {...} }
	// as one copy of the loop will be empty.
	//
	DEBUG(std::cerr << "NOT unswitching loop %"
	<< L->getHeader()->getName() << ", cost too high: "
	<< L->getBlocks().size() << "\n");
	continue;
	}

	// If this loop has live-out values, we can't unswitch it. We need something
	// like loop-closed SSA form in order to know how to insert PHI nodes for
	// these values.
	if (LoopValuesUsedOutsideLoop(L)) {
	DEBUG(std::cerr << "NOT unswitching loop %"
	<< L->getHeader()->getName()
	<< ", a loop value is used outside loop!\n");
	continue;
	}

	//std::cerr << "BEFORE:\n"; LI->dump();
	Loop NewLoop1 = 0, NewLoop2 = 0;

	// If this is a trivial condition to unswitch (which results in no code
	// duplication), do it now.
	bool EntersLoopOnCond;
	BasicBlock *ExitBlock;
	if (IsTrivialUnswitchCondition(L, BI->getCondition(), &EntersLoopOnCond,
	&ExitBlock)) {
	UnswitchTrivialCondition(L, BI->getCondition(),
	EntersLoopOnCond, ExitBlock);
	NewLoop1 = L;
	} else {
	VersionLoop(BI->getCondition(), L, NewLoop1, NewLoop2);
	}

	//std::cerr << "AFTER:\n"; LI->dump();

	// Try to unswitch each of our new loops now!
	if (NewLoop1) visitLoop(NewLoop1);
	if (NewLoop2) visitLoop(NewLoop2);
	return true;
	}

	return Changed;
	}

	/// SplitBlock - Split the specified basic block into two pieces. If SplitAtTop
	/// is false, this splits the block so the second half only has an unconditional
	/// branch. If SplitAtTop is true, it makes it so the first half of the block
	/// only has an unconditional branch in it.
	///
	/// This method updates the LoopInfo for this function to correctly reflect the
	/// CFG changes made.
	///
	/// This routine returns the new basic block that was inserted, which is always
	/// the later part of the block.
	BasicBlock LoopUnswitch::SplitBlock(BasicBlock BB, bool SplitAtTop) {
	BasicBlock::iterator SplitPoint;
	if (!SplitAtTop)
	SplitPoint = BB->getTerminator();
	else {
	SplitPoint = BB->begin();
	while (isa<PHINode>(SplitPoint)) ++SplitPoint;
	}

	BasicBlock *New = BB->splitBasicBlock(SplitPoint, BB->getName()+".tail");
	// New now lives in whichever loop that BB used to.
	if (Loop *L = LI->getLoopFor(BB))
	L->addBasicBlockToLoop(New, *LI);
	return New;
	}


	// RemapInstruction - Convert the instruction operands from referencing the
	// current values into those specified by ValueMap.
	//
	static inline void RemapInstruction(Instruction *I,
	std::map<const Value , Value> &ValueMap) {
	for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
	Value *Op = I->getOperand(op);
	std::map<const Value , Value>::iterator It = ValueMap.find(Op);
	if (It != ValueMap.end()) Op = It->second;
	I->setOperand(op, Op);
	}
	}

	/// CloneLoop - Recursively clone the specified loop and all of its children,
	/// mapping the blocks with the specified map.
	static Loop CloneLoop(Loop L, Loop PL, std::map<const Value, Value*> &VM,
	LoopInfo *LI) {
	Loop *New = new Loop();

	if (PL)
	PL->addChildLoop(New);
	else
	LI->addTopLevelLoop(New);

	// Add all of the blocks in L to the new loop.
	for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
	I != E; ++I)
	if (LI->getLoopFor(*I) == L)
	New->addBasicBlockToLoop(cast<BasicBlock>(VM[I]), LI);

	// Add all of the subloops to the new loop.
	for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
	CloneLoop(*I, New, VM, LI);

	return New;
	}

	/// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable
	/// condition in it (a cond branch from its header block to its latch block,
	/// where the path through the loop that doesn't execute its body has no
	/// side-effects), unswitch it. This doesn't involve any code duplication, just
	/// moving the conditional branch outside of the loop and updating loop info.
	void LoopUnswitch::UnswitchTrivialCondition(Loop L, Value Cond,
	bool EnterOnCond,
	BasicBlock *ExitBlock) {
	DEBUG(std::cerr << "loop-unswitch: Trivial-Unswitch loop %"
	<< L->getHeader()->getName() << " [" << L->getBlocks().size()
	<< " blocks] in Function " << L->getHeader()->getParent()->getName()
	<< " on cond:" << *Cond << "\n");

	// First step, split the preahder, so that we know that there is a safe place
	// to insert the conditional branch. We will change 'OrigPH' to have a
	// conditional branch on Cond.
	BasicBlock *OrigPH = L->getLoopPreheader();
	BasicBlock *NewPH = SplitBlock(OrigPH, false);

	// Now that we have a place to insert the conditional branch, create a place
	// to branch to: this is the exit block out of the loop that we should
	// short-circuit to.

	// Split this block now, so that the loop maintains its exit block.
	assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
	BasicBlock *NewExit = SplitBlock(ExitBlock, true);

	// Okay, now we have a position to branch from and a position to branch to,
	// insert the new conditional branch.
	new BranchInst(EnterOnCond ? NewPH : NewExit, EnterOnCond ? NewExit : NewPH,
	Cond, OrigPH->getTerminator());
	OrigPH->getTerminator()->eraseFromParent();

	// Now that we know that the loop is never entered when this condition is a
	// particular value, rewrite the loop with this info. We know that this will
	// at least eliminate the old branch.
	RewriteLoopBodyWithConditionConstant(L, Cond, EnterOnCond);

	++NumUnswitched;
	}


	/// VersionLoop - We determined that the loop is profitable to unswitch and
	/// contains a branch on a loop invariant condition. Split it into loop
	/// versions and test the condition outside of either loop. Return the loops
	/// created as Out1/Out2.
	void LoopUnswitch::VersionLoop(Value LIC, Loop L, Loop &Out1, Loop &Out2) {
	Function *F = L->getHeader()->getParent();

	DEBUG(std::cerr << "loop-unswitch: Unswitching loop %"
	<< L->getHeader()->getName() << " [" << L->getBlocks().size()
	<< " blocks] in Function " << F->getName()
	<< " on cond:" << *LIC << "\n");

	std::vector<BasicBlock*> LoopBlocks;

	// First step, split the preheader and exit blocks, and add these blocks to
	// the LoopBlocks list.
	BasicBlock *OrigPreheader = L->getLoopPreheader();
	LoopBlocks.push_back(SplitBlock(OrigPreheader, false));

	// We want the loop to come after the preheader, but before the exit blocks.
	LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());

	std::vector<BasicBlock*> ExitBlocks;
	L->getExitBlocks(ExitBlocks);
	std::sort(ExitBlocks.begin(), ExitBlocks.end());
	ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()),
	ExitBlocks.end());
	for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
	SplitBlock(ExitBlocks[i], true);
	LoopBlocks.push_back(ExitBlocks[i]);
	}

	// Next step, clone all of the basic blocks that make up the loop (including
	// the loop preheader and exit blocks), keeping track of the mapping between
	// the instructions and blocks.
	std::vector<BasicBlock*> NewBlocks;
	NewBlocks.reserve(LoopBlocks.size());
	std::map<const Value, Value> ValueMap;
	for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
	NewBlocks.push_back(CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F));
	ValueMap[LoopBlocks[i]] = NewBlocks.back(); // Keep the BB mapping.
	}

	// Splice the newly inserted blocks into the function right before the
	// original preheader.
	F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(),
	NewBlocks[0], F->end());

	// Now we create the new Loop object for the versioned loop.
	Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI);
	if (Loop *Parent = L->getParentLoop()) {
	// Make sure to add the cloned preheader and exit blocks to the parent loop
	// as well.
	Parent->addBasicBlockToLoop(NewBlocks[0], *LI);
	for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
	Parent->addBasicBlockToLoop(cast<BasicBlock>(ValueMap[ExitBlocks[i]]),
	*LI);
	}

	// Rewrite the code to refer to itself.
	for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
	for (BasicBlock::iterator I = NewBlocks[i]->begin(),
	E = NewBlocks[i]->end(); I != E; ++I)
	RemapInstruction(I, ValueMap);

	// Rewrite the original preheader to select between versions of the loop.
	assert(isa<BranchInst>(OrigPreheader->getTerminator()) &&
	cast<BranchInst>(OrigPreheader->getTerminator())->isUnconditional() &&
	OrigPreheader->getTerminator()->getSuccessor(0) == LoopBlocks[0] &&
	"Preheader splitting did not work correctly!");
	// Remove the unconditional branch to LoopBlocks[0].
	OrigPreheader->getInstList().pop_back();

	// Insert a conditional branch on LIC to the two preheaders. The original
	// code is the true version and the new code is the false version.
	new BranchInst(LoopBlocks[0], NewBlocks[0], LIC, OrigPreheader);

	// Now we rewrite the original code to know that the condition is true and the
	// new code to know that the condition is false.
	RewriteLoopBodyWithConditionConstant(L, LIC, true);
	RewriteLoopBodyWithConditionConstant(NewLoop, LIC, false);
	++NumUnswitched;
	Out1 = L;
	Out2 = NewLoop;
	}

	// RewriteLoopBodyWithConditionConstant - We know that the boolean value LIC has
	// the value specified by Val in the specified loop. Rewrite any uses of LIC or
	// of properties correlated to it.
	void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop L, Value LIC,
	bool Val) {
	assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
	// FIXME: Support correlated properties, like:
	// for (...)
	// if (li1 < li2)
	// ...
	// if (li1 > li2)
	// ...
	ConstantBool *BoolVal = ConstantBool::get(Val);

	std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
	for (unsigned i = 0, e = Users.size(); i != e; ++i)
	if (Instruction *U = cast<Instruction>(Users[i]))
	if (L->contains(U->getParent()))
	U->replaceUsesOfWith(LIC, BoolVal);
	}