lib/CodeGen/CodePlacementOpt.cpp - platform/external/llvm - Git at Google

 //===-- CodePlacementOpt.cpp - Code Placement pass. -----------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file implements the pass that optimizes code placement and aligns loop
 // headers to target-specific alignment boundaries.
 //
 //===----------------------------------------------------------------------===//

 #define DEBUG_TYPE "code-placement"
 #include "llvm/CodeGen/Passes.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
 #include "llvm/CodeGen/MachineLoopInfo.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Target/TargetInstrInfo.h"
 #include "llvm/Target/TargetLowering.h"
 #include "llvm/Target/TargetMachine.h"
 using namespace llvm;

 STATISTIC(NumLoopsAligned,  "Number of loops aligned");
 STATISTIC(NumIntraElim,     "Number of intra loop branches eliminated");
 STATISTIC(NumIntraMoved,    "Number of intra loop branches moved");

 namespace {
   class CodePlacementOpt : public MachineFunctionPass {
     const MachineLoopInfo *MLI;
     const TargetInstrInfo *TII;
     const TargetLowering  *TLI;

   public:
     static char ID;
     CodePlacementOpt() : MachineFunctionPass(ID) {}

     virtual bool runOnMachineFunction(MachineFunction &MF);

     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
       AU.addRequired<MachineLoopInfo>();
       AU.addPreservedID(MachineDominatorsID);
       MachineFunctionPass::getAnalysisUsage(AU);
     }

   private:
     bool HasFallthrough(MachineBasicBlock *MBB);
     bool HasAnalyzableTerminator(MachineBasicBlock *MBB);
     void Splice(MachineFunction &MF,
                 MachineFunction::iterator InsertPt,
                 MachineFunction::iterator Begin,
                 MachineFunction::iterator End);
     bool EliminateUnconditionalJumpsToTop(MachineFunction &MF,
                                           MachineLoop *L);
     bool MoveDiscontiguousLoopBlocks(MachineFunction &MF,
                                      MachineLoop *L);
     bool OptimizeIntraLoopEdgesInLoopNest(MachineFunction &MF, MachineLoop *L);
     bool OptimizeIntraLoopEdges(MachineFunction &MF);
     bool AlignLoops(MachineFunction &MF);
     bool AlignLoop(MachineFunction &MF, MachineLoop *L, unsigned Align);
   };

   char CodePlacementOpt::ID = 0;
 } // end anonymous namespace

 char &llvm::CodePlacementOptID = CodePlacementOpt::ID;
 INITIALIZE_PASS(CodePlacementOpt, "code-placement",
                 "Code Placement Optimizer", false, false)

 /// HasFallthrough - Test whether the given branch has a fallthrough, either as
 /// a plain fallthrough or as a fallthrough case of a conditional branch.
 ///
 bool CodePlacementOpt::HasFallthrough(MachineBasicBlock *MBB) {
   MachineBasicBlock *TBB = 0, *FBB = 0;
   SmallVector<MachineOperand, 4> Cond;
   if (TII->AnalyzeBranch(*MBB, TBB, FBB, Cond))
     return false;
   // This conditional branch has no fallthrough.
   if (FBB)
     return false;
   // An unconditional branch has no fallthrough.
   if (Cond.empty() && TBB)
     return false;
   // It has a fallthrough.
   return true;
 }

 /// HasAnalyzableTerminator - Test whether AnalyzeBranch will succeed on MBB.
 /// This is called before major changes are begun to test whether it will be
 /// possible to complete the changes.
 ///
 /// Target-specific code is hereby encouraged to make AnalyzeBranch succeed
 /// whenever possible.
 ///
 bool CodePlacementOpt::HasAnalyzableTerminator(MachineBasicBlock *MBB) {
   // Conservatively ignore EH landing pads.
   if (MBB->isLandingPad()) return false;

   // Aggressively handle return blocks and similar constructs.
   if (MBB->succ_empty()) return true;

   // Ask the target's AnalyzeBranch if it can handle this block.
   MachineBasicBlock *TBB = 0, *FBB = 0;
   SmallVector<MachineOperand, 4> Cond;
   // Make sure the terminator is understood.
   if (TII->AnalyzeBranch(*MBB, TBB, FBB, Cond))
     return false;
    // Ignore blocks which look like they might have EH-related control flow.
    // AnalyzeBranch thinks it knows how to analyze such things, but it doesn't
    // recognize the possibility of a control transfer through an unwind.
    // Such blocks contain EH_LABEL instructions, however they may be in the
    // middle of the block. Instead of searching for them, just check to see
    // if the CFG disagrees with AnalyzeBranch.
   if (1u + !Cond.empty() != MBB->succ_size())
     return false;
   // Make sure we have the option of reversing the condition.
   if (!Cond.empty() && TII->ReverseBranchCondition(Cond))
     return false;
   return true;
 }

 /// Splice - Move the sequence of instructions [Begin,End) to just before
 /// InsertPt. Update branch instructions as needed to account for broken
 /// fallthrough edges and to take advantage of newly exposed fallthrough
 /// opportunities.
 ///
 void CodePlacementOpt::Splice(MachineFunction &MF,
                               MachineFunction::iterator InsertPt,
                               MachineFunction::iterator Begin,
                               MachineFunction::iterator End) {
   assert(Begin != MF.begin() && End != MF.begin() && InsertPt != MF.begin() &&
          "Splice can't change the entry block!");
   MachineFunction::iterator OldBeginPrior = prior(Begin);
   MachineFunction::iterator OldEndPrior = prior(End);

   MF.splice(InsertPt, Begin, End);

   prior(Begin)->updateTerminator();
   OldBeginPrior->updateTerminator();
   OldEndPrior->updateTerminator();
 }

 /// EliminateUnconditionalJumpsToTop - Move blocks which unconditionally jump
 /// to the loop top to the top of the loop so that they have a fall through.
 /// This can introduce a branch on entry to the loop, but it can eliminate a
 /// branch within the loop. See the @simple case in
 /// test/CodeGen/X86/loop_blocks.ll for an example of this.
 bool CodePlacementOpt::EliminateUnconditionalJumpsToTop(MachineFunction &MF,
                                                         MachineLoop *L) {
   bool Changed = false;
   MachineBasicBlock *TopMBB = L->getTopBlock();

   bool BotHasFallthrough = HasFallthrough(L->getBottomBlock());

   if (TopMBB == MF.begin() ||
       HasAnalyzableTerminator(prior(MachineFunction::iterator(TopMBB)))) {
   new_top:
     for (MachineBasicBlock::pred_iterator PI = TopMBB->pred_begin(),
          PE = TopMBB->pred_end(); PI != PE; ++PI) {
       MachineBasicBlock *Pred = *PI;
       if (Pred == TopMBB) continue;
       if (HasFallthrough(Pred)) continue;
       if (!L->contains(Pred)) continue;

       // Verify that we can analyze all the loop entry edges before beginning
       // any changes which will require us to be able to analyze them.
       if (Pred == MF.begin())
         continue;
       if (!HasAnalyzableTerminator(Pred))
         continue;
       if (!HasAnalyzableTerminator(prior(MachineFunction::iterator(Pred))))
         continue;

       // Move the block.
       DEBUG(dbgs() << "CGP: Moving blocks starting at BB#" << Pred->getNumber()
                    << " to top of loop.\n");
       Changed = true;

       // Move it and all the blocks that can reach it via fallthrough edges
       // exclusively, to keep existing fallthrough edges intact.
       MachineFunction::iterator Begin = Pred;
       MachineFunction::iterator End = llvm::next(Begin);
       while (Begin != MF.begin()) {
         MachineFunction::iterator Prior = prior(Begin);
         if (Prior == MF.begin())
           break;
         // Stop when a non-fallthrough edge is found.
         if (!HasFallthrough(Prior))
           break;
         // Stop if a block which could fall-through out of the loop is found.
         if (Prior->isSuccessor(End))
           break;
         // If we've reached the top, stop scanning.
         if (Prior == MachineFunction::iterator(TopMBB)) {
           // We know top currently has a fall through (because we just checked
           // it) which would be lost if we do the transformation, so it isn't
           // worthwhile to do the transformation unless it would expose a new
           // fallthrough edge.
           if (!Prior->isSuccessor(End))
             goto next_pred;
           // Otherwise we can stop scanning and proceed to move the blocks.
           break;
         }
         // If we hit a switch or something complicated, don't move anything
         // for this predecessor.
         if (!HasAnalyzableTerminator(prior(MachineFunction::iterator(Prior))))
           break;
         // Ok, the block prior to Begin will be moved along with the rest.
         // Extend the range to include it.
         Begin = Prior;
         ++NumIntraMoved;
       }

       // Move the blocks.
       Splice(MF, TopMBB, Begin, End);

       // Update TopMBB.
       TopMBB = L->getTopBlock();

       // We have a new loop top. Iterate on it. We shouldn't have to do this
       // too many times if BranchFolding has done a reasonable job.
       goto new_top;
     next_pred:;
     }
   }

   // If the loop previously didn't exit with a fall-through and it now does,
   // we eliminated a branch.
   if (Changed &&
       !BotHasFallthrough &&
       HasFallthrough(L->getBottomBlock())) {
     ++NumIntraElim;
   }

   return Changed;
 }

 /// MoveDiscontiguousLoopBlocks - Move any loop blocks that are not in the
 /// portion of the loop contiguous with the header. This usually makes the loop
 /// contiguous, provided that AnalyzeBranch can handle all the relevant
 /// branching. See the @cfg_islands case in test/CodeGen/X86/loop_blocks.ll
 /// for an example of this.
 bool CodePlacementOpt::MoveDiscontiguousLoopBlocks(MachineFunction &MF,
                                                    MachineLoop *L) {
   bool Changed = false;
   MachineBasicBlock *TopMBB = L->getTopBlock();
   MachineBasicBlock *BotMBB = L->getBottomBlock();

   // Determine a position to move orphaned loop blocks to. If TopMBB is not
   // entered via fallthrough and BotMBB is exited via fallthrough, prepend them
   // to the top of the loop to avoid losing that fallthrough. Otherwise append
   // them to the bottom, even if it previously had a fallthrough, on the theory
   // that it's worth an extra branch to keep the loop contiguous.
   MachineFunction::iterator InsertPt =
     llvm::next(MachineFunction::iterator(BotMBB));
   bool InsertAtTop = false;
   if (TopMBB != MF.begin() &&
       !HasFallthrough(prior(MachineFunction::iterator(TopMBB))) &&
       HasFallthrough(BotMBB)) {
     InsertPt = TopMBB;
     InsertAtTop = true;
   }

   // Keep a record of which blocks are in the portion of the loop contiguous
   // with the loop header.
   SmallPtrSet<MachineBasicBlock *, 8> ContiguousBlocks;
   for (MachineFunction::iterator I = TopMBB,
        E = llvm::next(MachineFunction::iterator(BotMBB)); I != E; ++I)
     ContiguousBlocks.insert(I);

   // Find non-contigous blocks and fix them.
   if (InsertPt != MF.begin() && HasAnalyzableTerminator(prior(InsertPt)))
     for (MachineLoop::block_iterator BI = L->block_begin(), BE = L->block_end();
          BI != BE; ++BI) {
       MachineBasicBlock *BB = *BI;

       // Verify that we can analyze all the loop entry edges before beginning
       // any changes which will require us to be able to analyze them.
       if (!HasAnalyzableTerminator(BB))
         continue;
       if (!HasAnalyzableTerminator(prior(MachineFunction::iterator(BB))))
         continue;

       // If the layout predecessor is part of the loop, this block will be
       // processed along with it. This keeps them in their relative order.
       if (BB != MF.begin() &&
           L->contains(prior(MachineFunction::iterator(BB))))
         continue;

       // Check to see if this block is already contiguous with the main
       // portion of the loop.
       if (!ContiguousBlocks.insert(BB))
         continue;

       // Move the block.
       DEBUG(dbgs() << "CGP: Moving blocks starting at BB#" << BB->getNumber()
                    << " to be contiguous with loop.\n");
       Changed = true;

       // Process this block and all loop blocks contiguous with it, to keep
       // them in their relative order.
       MachineFunction::iterator Begin = BB;
       MachineFunction::iterator End = llvm::next(MachineFunction::iterator(BB));
       for (; End != MF.end(); ++End) {
         if (!L->contains(End)) break;
         if (!HasAnalyzableTerminator(End)) break;
         ContiguousBlocks.insert(End);
         ++NumIntraMoved;
       }

       // If we're inserting at the bottom of the loop, and the code we're
       // moving originally had fall-through successors, bring the sucessors
       // up with the loop blocks to preserve the fall-through edges.
       if (!InsertAtTop)
         for (; End != MF.end(); ++End) {
           if (L->contains(End)) break;
           if (!HasAnalyzableTerminator(End)) break;
           if (!HasFallthrough(prior(End))) break;
         }

       // Move the blocks. This may invalidate TopMBB and/or BotMBB, but
       // we don't need them anymore at this point.
       Splice(MF, InsertPt, Begin, End);
     }

   return Changed;
 }

 /// OptimizeIntraLoopEdgesInLoopNest - Reposition loop blocks to minimize
 /// intra-loop branching and to form contiguous loops.
 ///
 /// This code takes the approach of making minor changes to the existing
 /// layout to fix specific loop-oriented problems. Also, it depends on
 /// AnalyzeBranch, which can't understand complex control instructions.
 ///
 bool CodePlacementOpt::OptimizeIntraLoopEdgesInLoopNest(MachineFunction &MF,
                                                         MachineLoop *L) {
   bool Changed = false;

   // Do optimization for nested loops.
   for (MachineLoop::iterator I = L->begin(), E = L->end(); I != E; ++I)
     Changed |= OptimizeIntraLoopEdgesInLoopNest(MF, *I);

   // Do optimization for this loop.
   Changed |= EliminateUnconditionalJumpsToTop(MF, L);
   Changed |= MoveDiscontiguousLoopBlocks(MF, L);

   return Changed;
 }

 /// OptimizeIntraLoopEdges - Reposition loop blocks to minimize
 /// intra-loop branching and to form contiguous loops.
 ///
 bool CodePlacementOpt::OptimizeIntraLoopEdges(MachineFunction &MF) {
   bool Changed = false;

   if (!TLI->shouldOptimizeCodePlacement())
     return Changed;

   // Do optimization for each loop in the function.
   for (MachineLoopInfo::iterator I = MLI->begin(), E = MLI->end();
        I != E; ++I)
     if (!(*I)->getParentLoop())
       Changed |= OptimizeIntraLoopEdgesInLoopNest(MF, *I);

   return Changed;
 }

 /// AlignLoops - Align loop headers to target preferred alignments.
 ///
 bool CodePlacementOpt::AlignLoops(MachineFunction &MF) {
   const Function *F = MF.getFunction();
   if (F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
                                       Attribute::OptimizeForSize))
     return false;

   unsigned Align = TLI->getPrefLoopAlignment();
   if (!Align)
     return false;  // Don't care about loop alignment.

   bool Changed = false;

   for (MachineLoopInfo::iterator I = MLI->begin(), E = MLI->end();
        I != E; ++I)
     Changed |= AlignLoop(MF, *I, Align);

   return Changed;
 }

 /// AlignLoop - Align loop headers to target preferred alignments.
 ///
 bool CodePlacementOpt::AlignLoop(MachineFunction &MF, MachineLoop *L,
                                  unsigned Align) {
   bool Changed = false;

   // Do alignment for nested loops.
   for (MachineLoop::iterator I = L->begin(), E = L->end(); I != E; ++I)
     Changed |= AlignLoop(MF, *I, Align);

   L->getTopBlock()->setAlignment(Align);
   Changed = true;
   ++NumLoopsAligned;

   return Changed;
 }

 bool CodePlacementOpt::runOnMachineFunction(MachineFunction &MF) {
   MLI = &getAnalysis<MachineLoopInfo>();
   if (MLI->empty())
     return false;  // No loops.

   TLI = MF.getTarget().getTargetLowering();
   TII = MF.getTarget().getInstrInfo();

   bool Changed = OptimizeIntraLoopEdges(MF);

   Changed |= AlignLoops(MF);

   return Changed;
 }
	//===-- CodePlacementOpt.cpp - Code Placement pass. -----------------------===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file is distributed under the University of Illinois Open Source
	// License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//
	//
	// This file implements the pass that optimizes code placement and aligns loop
	// headers to target-specific alignment boundaries.
	//
	//===----------------------------------------------------------------------===//

	#define DEBUG_TYPE "code-placement"
	#include "llvm/CodeGen/Passes.h"
	#include "llvm/ADT/Statistic.h"
	#include "llvm/CodeGen/MachineFunctionPass.h"
	#include "llvm/CodeGen/MachineLoopInfo.h"
	#include "llvm/Support/Compiler.h"
	#include "llvm/Support/Debug.h"
	#include "llvm/Target/TargetInstrInfo.h"
	#include "llvm/Target/TargetLowering.h"
	#include "llvm/Target/TargetMachine.h"
	using namespace llvm;

	STATISTIC(NumLoopsAligned, "Number of loops aligned");
	STATISTIC(NumIntraElim, "Number of intra loop branches eliminated");
	STATISTIC(NumIntraMoved, "Number of intra loop branches moved");

	namespace {
	class CodePlacementOpt : public MachineFunctionPass {
	const MachineLoopInfo *MLI;
	const TargetInstrInfo *TII;
	const TargetLowering *TLI;

	public:
	static char ID;
	CodePlacementOpt() : MachineFunctionPass(ID) {}

	virtual bool runOnMachineFunction(MachineFunction &MF);

	virtual void getAnalysisUsage(AnalysisUsage &AU) const {
	AU.addRequired<MachineLoopInfo>();
	AU.addPreservedID(MachineDominatorsID);
	MachineFunctionPass::getAnalysisUsage(AU);
	}

	private:
	bool HasFallthrough(MachineBasicBlock *MBB);
	bool HasAnalyzableTerminator(MachineBasicBlock *MBB);
	void Splice(MachineFunction &MF,
	MachineFunction::iterator InsertPt,
	MachineFunction::iterator Begin,
	MachineFunction::iterator End);
	bool EliminateUnconditionalJumpsToTop(MachineFunction &MF,
	MachineLoop *L);
	bool MoveDiscontiguousLoopBlocks(MachineFunction &MF,
	MachineLoop *L);
	bool OptimizeIntraLoopEdgesInLoopNest(MachineFunction &MF, MachineLoop *L);
	bool OptimizeIntraLoopEdges(MachineFunction &MF);
	bool AlignLoops(MachineFunction &MF);
	bool AlignLoop(MachineFunction &MF, MachineLoop *L, unsigned Align);
	};

	char CodePlacementOpt::ID = 0;
	} // end anonymous namespace

	char &llvm::CodePlacementOptID = CodePlacementOpt::ID;
	INITIALIZE_PASS(CodePlacementOpt, "code-placement",
	"Code Placement Optimizer", false, false)

	/// HasFallthrough - Test whether the given branch has a fallthrough, either as
	/// a plain fallthrough or as a fallthrough case of a conditional branch.
	///
	bool CodePlacementOpt::HasFallthrough(MachineBasicBlock *MBB) {
	MachineBasicBlock TBB = 0, FBB = 0;
	SmallVector<MachineOperand, 4> Cond;
	if (TII->AnalyzeBranch(*MBB, TBB, FBB, Cond))
	return false;
	// This conditional branch has no fallthrough.
	if (FBB)
	return false;
	// An unconditional branch has no fallthrough.
	if (Cond.empty() && TBB)
	return false;
	// It has a fallthrough.
	return true;
	}

	/// HasAnalyzableTerminator - Test whether AnalyzeBranch will succeed on MBB.
	/// This is called before major changes are begun to test whether it will be
	/// possible to complete the changes.
	///
	/// Target-specific code is hereby encouraged to make AnalyzeBranch succeed
	/// whenever possible.
	///
	bool CodePlacementOpt::HasAnalyzableTerminator(MachineBasicBlock *MBB) {
	// Conservatively ignore EH landing pads.
	if (MBB->isLandingPad()) return false;

	// Aggressively handle return blocks and similar constructs.
	if (MBB->succ_empty()) return true;

	// Ask the target's AnalyzeBranch if it can handle this block.
	MachineBasicBlock TBB = 0, FBB = 0;
	SmallVector<MachineOperand, 4> Cond;
	// Make sure the terminator is understood.
	if (TII->AnalyzeBranch(*MBB, TBB, FBB, Cond))
	return false;
	// Ignore blocks which look like they might have EH-related control flow.
	// AnalyzeBranch thinks it knows how to analyze such things, but it doesn't
	// recognize the possibility of a control transfer through an unwind.
	// Such blocks contain EH_LABEL instructions, however they may be in the
	// middle of the block. Instead of searching for them, just check to see
	// if the CFG disagrees with AnalyzeBranch.
	if (1u + !Cond.empty() != MBB->succ_size())
	return false;
	// Make sure we have the option of reversing the condition.
	if (!Cond.empty() && TII->ReverseBranchCondition(Cond))
	return false;
	return true;
	}

	/// Splice - Move the sequence of instructions [Begin,End) to just before
	/// InsertPt. Update branch instructions as needed to account for broken
	/// fallthrough edges and to take advantage of newly exposed fallthrough
	/// opportunities.
	///
	void CodePlacementOpt::Splice(MachineFunction &MF,
	MachineFunction::iterator InsertPt,
	MachineFunction::iterator Begin,
	MachineFunction::iterator End) {
	assert(Begin != MF.begin() && End != MF.begin() && InsertPt != MF.begin() &&
	"Splice can't change the entry block!");
	MachineFunction::iterator OldBeginPrior = prior(Begin);
	MachineFunction::iterator OldEndPrior = prior(End);

	MF.splice(InsertPt, Begin, End);

	prior(Begin)->updateTerminator();
	OldBeginPrior->updateTerminator();
	OldEndPrior->updateTerminator();
	}

	/// EliminateUnconditionalJumpsToTop - Move blocks which unconditionally jump
	/// to the loop top to the top of the loop so that they have a fall through.
	/// This can introduce a branch on entry to the loop, but it can eliminate a
	/// branch within the loop. See the @simple case in
	/// test/CodeGen/X86/loop_blocks.ll for an example of this.
	bool CodePlacementOpt::EliminateUnconditionalJumpsToTop(MachineFunction &MF,
	MachineLoop *L) {
	bool Changed = false;
	MachineBasicBlock *TopMBB = L->getTopBlock();

	bool BotHasFallthrough = HasFallthrough(L->getBottomBlock());

	if (TopMBB == MF.begin() \|\|
	HasAnalyzableTerminator(prior(MachineFunction::iterator(TopMBB)))) {
	new_top:
	for (MachineBasicBlock::pred_iterator PI = TopMBB->pred_begin(),
	PE = TopMBB->pred_end(); PI != PE; ++PI) {
	MachineBasicBlock Pred = PI;
	if (Pred == TopMBB) continue;
	if (HasFallthrough(Pred)) continue;
	if (!L->contains(Pred)) continue;

	// Verify that we can analyze all the loop entry edges before beginning
	// any changes which will require us to be able to analyze them.
	if (Pred == MF.begin())
	continue;
	if (!HasAnalyzableTerminator(Pred))
	continue;
	if (!HasAnalyzableTerminator(prior(MachineFunction::iterator(Pred))))
	continue;

	// Move the block.
	DEBUG(dbgs() << "CGP: Moving blocks starting at BB#" << Pred->getNumber()
	<< " to top of loop.\n");
	Changed = true;

	// Move it and all the blocks that can reach it via fallthrough edges
	// exclusively, to keep existing fallthrough edges intact.
	MachineFunction::iterator Begin = Pred;
	MachineFunction::iterator End = llvm::next(Begin);
	while (Begin != MF.begin()) {
	MachineFunction::iterator Prior = prior(Begin);
	if (Prior == MF.begin())
	break;
	// Stop when a non-fallthrough edge is found.
	if (!HasFallthrough(Prior))
	break;
	// Stop if a block which could fall-through out of the loop is found.
	if (Prior->isSuccessor(End))
	break;
	// If we've reached the top, stop scanning.
	if (Prior == MachineFunction::iterator(TopMBB)) {
	// We know top currently has a fall through (because we just checked
	// it) which would be lost if we do the transformation, so it isn't
	// worthwhile to do the transformation unless it would expose a new
	// fallthrough edge.
	if (!Prior->isSuccessor(End))
	goto next_pred;
	// Otherwise we can stop scanning and proceed to move the blocks.
	break;
	}
	// If we hit a switch or something complicated, don't move anything
	// for this predecessor.
	if (!HasAnalyzableTerminator(prior(MachineFunction::iterator(Prior))))
	break;
	// Ok, the block prior to Begin will be moved along with the rest.
	// Extend the range to include it.
	Begin = Prior;
	++NumIntraMoved;
	}

	// Move the blocks.
	Splice(MF, TopMBB, Begin, End);

	// Update TopMBB.
	TopMBB = L->getTopBlock();

	// We have a new loop top. Iterate on it. We shouldn't have to do this
	// too many times if BranchFolding has done a reasonable job.
	goto new_top;
	next_pred:;
	}
	}

	// If the loop previously didn't exit with a fall-through and it now does,
	// we eliminated a branch.
	if (Changed &&
	!BotHasFallthrough &&
	HasFallthrough(L->getBottomBlock())) {
	++NumIntraElim;
	}

	return Changed;
	}

	/// MoveDiscontiguousLoopBlocks - Move any loop blocks that are not in the
	/// portion of the loop contiguous with the header. This usually makes the loop
	/// contiguous, provided that AnalyzeBranch can handle all the relevant
	/// branching. See the @cfg_islands case in test/CodeGen/X86/loop_blocks.ll
	/// for an example of this.
	bool CodePlacementOpt::MoveDiscontiguousLoopBlocks(MachineFunction &MF,
	MachineLoop *L) {
	bool Changed = false;
	MachineBasicBlock *TopMBB = L->getTopBlock();
	MachineBasicBlock *BotMBB = L->getBottomBlock();

	// Determine a position to move orphaned loop blocks to. If TopMBB is not
	// entered via fallthrough and BotMBB is exited via fallthrough, prepend them
	// to the top of the loop to avoid losing that fallthrough. Otherwise append
	// them to the bottom, even if it previously had a fallthrough, on the theory
	// that it's worth an extra branch to keep the loop contiguous.
	MachineFunction::iterator InsertPt =
	llvm::next(MachineFunction::iterator(BotMBB));
	bool InsertAtTop = false;
	if (TopMBB != MF.begin() &&
	!HasFallthrough(prior(MachineFunction::iterator(TopMBB))) &&
	HasFallthrough(BotMBB)) {
	InsertPt = TopMBB;
	InsertAtTop = true;
	}

	// Keep a record of which blocks are in the portion of the loop contiguous
	// with the loop header.
	SmallPtrSet<MachineBasicBlock *, 8> ContiguousBlocks;
	for (MachineFunction::iterator I = TopMBB,
	E = llvm::next(MachineFunction::iterator(BotMBB)); I != E; ++I)
	ContiguousBlocks.insert(I);

	// Find non-contigous blocks and fix them.
	if (InsertPt != MF.begin() && HasAnalyzableTerminator(prior(InsertPt)))
	for (MachineLoop::block_iterator BI = L->block_begin(), BE = L->block_end();
	BI != BE; ++BI) {
	MachineBasicBlock BB = BI;

	// Verify that we can analyze all the loop entry edges before beginning
	// any changes which will require us to be able to analyze them.
	if (!HasAnalyzableTerminator(BB))
	continue;
	if (!HasAnalyzableTerminator(prior(MachineFunction::iterator(BB))))
	continue;

	// If the layout predecessor is part of the loop, this block will be
	// processed along with it. This keeps them in their relative order.
	if (BB != MF.begin() &&
	L->contains(prior(MachineFunction::iterator(BB))))
	continue;

	// Check to see if this block is already contiguous with the main
	// portion of the loop.
	if (!ContiguousBlocks.insert(BB))
	continue;

	// Move the block.
	DEBUG(dbgs() << "CGP: Moving blocks starting at BB#" << BB->getNumber()
	<< " to be contiguous with loop.\n");
	Changed = true;

	// Process this block and all loop blocks contiguous with it, to keep
	// them in their relative order.
	MachineFunction::iterator Begin = BB;
	MachineFunction::iterator End = llvm::next(MachineFunction::iterator(BB));
	for (; End != MF.end(); ++End) {
	if (!L->contains(End)) break;
	if (!HasAnalyzableTerminator(End)) break;
	ContiguousBlocks.insert(End);
	++NumIntraMoved;
	}

	// If we're inserting at the bottom of the loop, and the code we're
	// moving originally had fall-through successors, bring the sucessors
	// up with the loop blocks to preserve the fall-through edges.
	if (!InsertAtTop)
	for (; End != MF.end(); ++End) {
	if (L->contains(End)) break;
	if (!HasAnalyzableTerminator(End)) break;
	if (!HasFallthrough(prior(End))) break;
	}

	// Move the blocks. This may invalidate TopMBB and/or BotMBB, but
	// we don't need them anymore at this point.
	Splice(MF, InsertPt, Begin, End);
	}

	return Changed;
	}

	/// OptimizeIntraLoopEdgesInLoopNest - Reposition loop blocks to minimize
	/// intra-loop branching and to form contiguous loops.
	///
	/// This code takes the approach of making minor changes to the existing
	/// layout to fix specific loop-oriented problems. Also, it depends on
	/// AnalyzeBranch, which can't understand complex control instructions.
	///
	bool CodePlacementOpt::OptimizeIntraLoopEdgesInLoopNest(MachineFunction &MF,
	MachineLoop *L) {
	bool Changed = false;

	// Do optimization for nested loops.
	for (MachineLoop::iterator I = L->begin(), E = L->end(); I != E; ++I)
	Changed \|= OptimizeIntraLoopEdgesInLoopNest(MF, *I);

	// Do optimization for this loop.
	Changed \|= EliminateUnconditionalJumpsToTop(MF, L);
	Changed \|= MoveDiscontiguousLoopBlocks(MF, L);

	return Changed;
	}

	/// OptimizeIntraLoopEdges - Reposition loop blocks to minimize
	/// intra-loop branching and to form contiguous loops.
	///
	bool CodePlacementOpt::OptimizeIntraLoopEdges(MachineFunction &MF) {
	bool Changed = false;

	if (!TLI->shouldOptimizeCodePlacement())
	return Changed;

	// Do optimization for each loop in the function.
	for (MachineLoopInfo::iterator I = MLI->begin(), E = MLI->end();
	I != E; ++I)
	if (!(*I)->getParentLoop())
	Changed \|= OptimizeIntraLoopEdgesInLoopNest(MF, *I);

	return Changed;
	}

	/// AlignLoops - Align loop headers to target preferred alignments.
	///
	bool CodePlacementOpt::AlignLoops(MachineFunction &MF) {
	const Function *F = MF.getFunction();
	if (F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
	Attribute::OptimizeForSize))
	return false;

	unsigned Align = TLI->getPrefLoopAlignment();
	if (!Align)
	return false; // Don't care about loop alignment.

	bool Changed = false;

	for (MachineLoopInfo::iterator I = MLI->begin(), E = MLI->end();
	I != E; ++I)
	Changed \|= AlignLoop(MF, *I, Align);

	return Changed;
	}

	/// AlignLoop - Align loop headers to target preferred alignments.
	///
	bool CodePlacementOpt::AlignLoop(MachineFunction &MF, MachineLoop *L,
	unsigned Align) {
	bool Changed = false;

	// Do alignment for nested loops.
	for (MachineLoop::iterator I = L->begin(), E = L->end(); I != E; ++I)
	Changed \|= AlignLoop(MF, *I, Align);

	L->getTopBlock()->setAlignment(Align);
	Changed = true;
	++NumLoopsAligned;

	return Changed;
	}

	bool CodePlacementOpt::runOnMachineFunction(MachineFunction &MF) {
	MLI = &getAnalysis<MachineLoopInfo>();
	if (MLI->empty())
	return false; // No loops.

	TLI = MF.getTarget().getTargetLowering();
	TII = MF.getTarget().getInstrInfo();

	bool Changed = OptimizeIntraLoopEdges(MF);

	Changed \|= AlignLoops(MF);

	return Changed;
	}