llvm/lib/CodeGen/SpillPlacement.cpp - toolchain/llvm-project - Gitiles

 //===-- SpillPlacement.cpp - Optimal Spill Code Placement -----------------===//
 //
 //                     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 spill code placement analysis.
 //
 // Each edge bundle corresponds to a node in a Hopfield network. Constraints on
 // basic blocks are weighted by the block frequency and added to become the node
 // bias.
 //
 // Transparent basic blocks have the variable live through, but don't care if it
 // is spilled or in a register. These blocks become connections in the Hopfield
 // network, again weighted by block frequency.
 //
 // The Hopfield network minimizes (possibly locally) its energy function:
 //
 //   E = -sum_n V_n * ( B_n + sum_{n, m linked by b} V_m * F_b )
 //
 // The energy function represents the expected spill code execution frequency,
 // or the cost of spilling. This is a Lyapunov function which never increases
 // when a node is updated. It is guaranteed to converge to a local minimum.
 //
 //===----------------------------------------------------------------------===//

 #define DEBUG_TYPE "spillplacement"
 #include "SpillPlacement.h"
 #include "llvm/CodeGen/EdgeBundles.h"
 #include "llvm/CodeGen/LiveIntervalAnalysis.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineLoopInfo.h"
 #include "llvm/CodeGen/Passes.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/Format.h"

 using namespace llvm;

 char SpillPlacement::ID = 0;
 INITIALIZE_PASS_BEGIN(SpillPlacement, "spill-code-placement",
                       "Spill Code Placement Analysis", true, true)
 INITIALIZE_PASS_DEPENDENCY(EdgeBundles)
 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
 INITIALIZE_PASS_END(SpillPlacement, "spill-code-placement",
                     "Spill Code Placement Analysis", true, true)

 char &llvm::SpillPlacementID = SpillPlacement::ID;

 void SpillPlacement::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.setPreservesAll();
   AU.addRequiredTransitive<EdgeBundles>();
   AU.addRequiredTransitive<MachineLoopInfo>();
   MachineFunctionPass::getAnalysisUsage(AU);
 }

 /// Node - Each edge bundle corresponds to a Hopfield node.
 ///
 /// The node contains precomputed frequency data that only depends on the CFG,
 /// but Bias and Links are computed each time placeSpills is called.
 ///
 /// The node Value is positive when the variable should be in a register. The
 /// value can change when linked nodes change, but convergence is very fast
 /// because all weights are positive.
 ///
 struct SpillPlacement::Node {
   /// Scale - Inverse block frequency feeding into[0] or out of[1] the bundle.
   /// Ideally, these two numbers should be identical, but inaccuracies in the
   /// block frequency estimates means that we need to normalize ingoing and
   /// outgoing frequencies separately so they are commensurate.
   float Scale[2];

   /// Bias - Normalized contributions from non-transparent blocks.
   /// A bundle connected to a MustSpill block has a huge negative bias,
   /// otherwise it is a number in the range [-2;2].
   float Bias;

   /// Value - Output value of this node computed from the Bias and links.
   /// This is always in the range [-1;1]. A positive number means the variable
   /// should go in a register through this bundle.
   float Value;

   typedef SmallVector<std::pair<float, unsigned>, 4> LinkVector;

   /// Links - (Weight, BundleNo) for all transparent blocks connecting to other
   /// bundles. The weights are all positive and add up to at most 2, weights
   /// from ingoing and outgoing nodes separately add up to a most 1. The weight
   /// sum can be less than 2 when the variable is not live into / out of some
   /// connected basic blocks.
   LinkVector Links;

   /// preferReg - Return true when this node prefers to be in a register.
   bool preferReg() const {
     // Undecided nodes (Value==0) go on the stack.
     return Value > 0;
   }

   /// mustSpill - Return True if this node is so biased that it must spill.
   bool mustSpill() const {
     // Actually, we must spill if Bias < sum(weights).
     // It may be worth it to compute the weight sum here?
     return Bias < -2.0f;
   }

   /// Node - Create a blank Node.
   Node() {
     Scale[0] = Scale[1] = 0;
   }

   /// clear - Reset per-query data, but preserve frequencies that only depend on
   // the CFG.
   void clear() {
     Bias = Value = 0;
     Links.clear();
   }

   /// addLink - Add a link to bundle b with weight w.
   /// out=0 for an ingoing link, and 1 for an outgoing link.
   void addLink(unsigned b, float w, bool out) {
     // Normalize w relative to all connected blocks from that direction.
     w *= Scale[out];

     // There can be multiple links to the same bundle, add them up.
     for (LinkVector::iterator I = Links.begin(), E = Links.end(); I != E; ++I)
       if (I->second == b) {
         I->first += w;
         return;
       }
     // This must be the first link to b.
     Links.push_back(std::make_pair(w, b));
   }

   /// addBias - Bias this node from an ingoing[0] or outgoing[1] link.
   /// Return the change to the total number of positive biases.
   void addBias(float w, bool out) {
     // Normalize w relative to all connected blocks from that direction.
     w *= Scale[out];
     Bias += w;
   }

   /// update - Recompute Value from Bias and Links. Return true when node
   /// preference changes.
   bool update(const Node nodes[]) {
     // Compute the weighted sum of inputs.
     float Sum = Bias;
     for (LinkVector::iterator I = Links.begin(), E = Links.end(); I != E; ++I)
       Sum += I->first * nodes[I->second].Value;

     // The weighted sum is going to be in the range [-2;2]. Ideally, we should
     // simply set Value = sign(Sum), but we will add a dead zone around 0 for
     // two reasons:
     //  1. It avoids arbitrary bias when all links are 0 as is possible during
     //     initial iterations.
     //  2. It helps tame rounding errors when the links nominally sum to 0.
     const float Thres = 1e-4f;
     bool Before = preferReg();
     if (Sum < -Thres)
       Value = -1;
     else if (Sum > Thres)
       Value = 1;
     else
       Value = 0;
     return Before != preferReg();
   }
 };

 bool SpillPlacement::runOnMachineFunction(MachineFunction &mf) {
   MF = &mf;
   bundles = &getAnalysis<EdgeBundles>();
   loops = &getAnalysis<MachineLoopInfo>();

   assert(!nodes && "Leaking node array");
   nodes = new Node[bundles->getNumBundles()];

   // Compute total ingoing and outgoing block frequencies for all bundles.
   BlockFrequency.resize(mf.getNumBlockIDs());
   for (MachineFunction::iterator I = mf.begin(), E = mf.end(); I != E; ++I) {
     float Freq = LiveIntervals::getSpillWeight(true, false,
                                                loops->getLoopDepth(I));
     unsigned Num = I->getNumber();
     BlockFrequency[Num] = Freq;
     nodes[bundles->getBundle(Num, 1)].Scale[0] += Freq;
     nodes[bundles->getBundle(Num, 0)].Scale[1] += Freq;
   }

   // Scales are reciprocal frequencies.
   for (unsigned i = 0, e = bundles->getNumBundles(); i != e; ++i)
     for (unsigned d = 0; d != 2; ++d)
       if (nodes[i].Scale[d] > 0)
         nodes[i].Scale[d] = 1 / nodes[i].Scale[d];

   // We never change the function.
   return false;
 }

 void SpillPlacement::releaseMemory() {
   delete[] nodes;
   nodes = 0;
 }

 /// activate - mark node n as active if it wasn't already.
 void SpillPlacement::activate(unsigned n) {
   if (ActiveNodes->test(n))
     return;
   ActiveNodes->set(n);
   nodes[n].clear();

   // Very large bundles usually come from big switches, indirect branches,
   // landing pads, or loops with many 'continue' statements. It is difficult to
   // allocate registers when so many different blocks are involved.
   //
   // Give a small negative bias to large bundles such that 1/32 of the
   // connected blocks need to be interested before we consider expanding the
   // region through the bundle. This helps compile time by limiting the number
   // of blocks visited and the number of links in the Hopfield network.
   if (bundles->getBlocks(n).size() > 100)
     nodes[n].Bias = -0.0625f;
 }


 /// addConstraints - Compute node biases and weights from a set of constraints.
 /// Set a bit in NodeMask for each active node.
 void SpillPlacement::addConstraints(ArrayRef<BlockConstraint> LiveBlocks) {
   for (ArrayRef<BlockConstraint>::iterator I = LiveBlocks.begin(),
        E = LiveBlocks.end(); I != E; ++I) {
     float Freq = getBlockFrequency(I->Number);
     const float Bias[] = {
       0,           // DontCare,
       1,           // PrefReg,
       -1,          // PrefSpill
       0,           // PrefBoth
       -HUGE_VALF   // MustSpill
     };

     // Live-in to block?
     if (I->Entry != DontCare) {
       unsigned ib = bundles->getBundle(I->Number, 0);
       activate(ib);
       nodes[ib].addBias(Freq * Bias[I->Entry], 1);
     }

     // Live-out from block?
     if (I->Exit != DontCare) {
       unsigned ob = bundles->getBundle(I->Number, 1);
       activate(ob);
       nodes[ob].addBias(Freq * Bias[I->Exit], 0);
     }
   }
 }

 /// addPrefSpill - Same as addConstraints(PrefSpill)
 void SpillPlacement::addPrefSpill(ArrayRef<unsigned> Blocks, bool Strong) {
   for (ArrayRef<unsigned>::iterator I = Blocks.begin(), E = Blocks.end();
        I != E; ++I) {
     float Freq = getBlockFrequency(*I);
     if (Strong)
       Freq += Freq;
     unsigned ib = bundles->getBundle(*I, 0);
     unsigned ob = bundles->getBundle(*I, 1);
     activate(ib);
     activate(ob);
     nodes[ib].addBias(-Freq, 1);
     nodes[ob].addBias(-Freq, 0);
   }
 }

 void SpillPlacement::addLinks(ArrayRef<unsigned> Links) {
   for (ArrayRef<unsigned>::iterator I = Links.begin(), E = Links.end(); I != E;
        ++I) {
     unsigned Number = *I;
     unsigned ib = bundles->getBundle(Number, 0);
     unsigned ob = bundles->getBundle(Number, 1);

     // Ignore self-loops.
     if (ib == ob)
       continue;
     activate(ib);
     activate(ob);
     if (nodes[ib].Links.empty() && !nodes[ib].mustSpill())
       Linked.push_back(ib);
     if (nodes[ob].Links.empty() && !nodes[ob].mustSpill())
       Linked.push_back(ob);
     float Freq = getBlockFrequency(Number);
     nodes[ib].addLink(ob, Freq, 1);
     nodes[ob].addLink(ib, Freq, 0);
   }
 }

 bool SpillPlacement::scanActiveBundles() {
   Linked.clear();
   RecentPositive.clear();
   for (int n = ActiveNodes->find_first(); n>=0; n = ActiveNodes->find_next(n)) {
     nodes[n].update(nodes);
     // A node that must spill, or a node without any links is not going to
     // change its value ever again, so exclude it from iterations.
     if (nodes[n].mustSpill())
       continue;
     if (!nodes[n].Links.empty())
       Linked.push_back(n);
     if (nodes[n].preferReg())
       RecentPositive.push_back(n);
   }
   return !RecentPositive.empty();
 }

 /// iterate - Repeatedly update the Hopfield nodes until stability or the
 /// maximum number of iterations is reached.
 /// @param Linked - Numbers of linked nodes that need updating.
 void SpillPlacement::iterate() {
   // First update the recently positive nodes. They have likely received new
   // negative bias that will turn them off.
   while (!RecentPositive.empty())
     nodes[RecentPositive.pop_back_val()].update(nodes);

   if (Linked.empty())
     return;

   // Run up to 10 iterations. The edge bundle numbering is closely related to
   // basic block numbering, so there is a strong tendency towards chains of
   // linked nodes with sequential numbers. By scanning the linked nodes
   // backwards and forwards, we make it very likely that a single node can
   // affect the entire network in a single iteration. That means very fast
   // convergence, usually in a single iteration.
   for (unsigned iteration = 0; iteration != 10; ++iteration) {
     // Scan backwards, skipping the last node which was just updated.
     bool Changed = false;
     for (SmallVectorImpl<unsigned>::const_reverse_iterator I =
            llvm::next(Linked.rbegin()), E = Linked.rend(); I != E; ++I) {
       unsigned n = *I;
       if (nodes[n].update(nodes)) {
         Changed = true;
         if (nodes[n].preferReg())
           RecentPositive.push_back(n);
       }
     }
     if (!Changed || !RecentPositive.empty())
       return;

     // Scan forwards, skipping the first node which was just updated.
     Changed = false;
     for (SmallVectorImpl<unsigned>::const_iterator I =
            llvm::next(Linked.begin()), E = Linked.end(); I != E; ++I) {
       unsigned n = *I;
       if (nodes[n].update(nodes)) {
         Changed = true;
         if (nodes[n].preferReg())
           RecentPositive.push_back(n);
       }
     }
     if (!Changed || !RecentPositive.empty())
       return;
   }
 }

 void SpillPlacement::prepare(BitVector &RegBundles) {
   Linked.clear();
   RecentPositive.clear();
   // Reuse RegBundles as our ActiveNodes vector.
   ActiveNodes = &RegBundles;
   ActiveNodes->clear();
   ActiveNodes->resize(bundles->getNumBundles());
 }

 bool
 SpillPlacement::finish() {
   assert(ActiveNodes && "Call prepare() first");

   // Write preferences back to ActiveNodes.
   bool Perfect = true;
   for (int n = ActiveNodes->find_first(); n>=0; n = ActiveNodes->find_next(n))
     if (!nodes[n].preferReg()) {
       ActiveNodes->reset(n);
       Perfect = false;
     }
   ActiveNodes = 0;
   return Perfect;
 }
	//===-- SpillPlacement.cpp - Optimal Spill Code Placement -----------------===//
	//
	// 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 spill code placement analysis.
	//
	// Each edge bundle corresponds to a node in a Hopfield network. Constraints on
	// basic blocks are weighted by the block frequency and added to become the node
	// bias.
	//
	// Transparent basic blocks have the variable live through, but don't care if it
	// is spilled or in a register. These blocks become connections in the Hopfield
	// network, again weighted by block frequency.
	//
	// The Hopfield network minimizes (possibly locally) its energy function:
	//
	// E = -sum_n V_n * ( B_n + sum_{n, m linked by b} V_m * F_b )
	//
	// The energy function represents the expected spill code execution frequency,
	// or the cost of spilling. This is a Lyapunov function which never increases
	// when a node is updated. It is guaranteed to converge to a local minimum.
	//
	//===----------------------------------------------------------------------===//

	#define DEBUG_TYPE "spillplacement"
	#include "SpillPlacement.h"
	#include "llvm/CodeGen/EdgeBundles.h"
	#include "llvm/CodeGen/LiveIntervalAnalysis.h"
	#include "llvm/CodeGen/MachineBasicBlock.h"
	#include "llvm/CodeGen/MachineFunction.h"
	#include "llvm/CodeGen/MachineLoopInfo.h"
	#include "llvm/CodeGen/Passes.h"
	#include "llvm/Support/Debug.h"
	#include "llvm/Support/Format.h"

	using namespace llvm;

	char SpillPlacement::ID = 0;
	INITIALIZE_PASS_BEGIN(SpillPlacement, "spill-code-placement",
	"Spill Code Placement Analysis", true, true)
	INITIALIZE_PASS_DEPENDENCY(EdgeBundles)
	INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
	INITIALIZE_PASS_END(SpillPlacement, "spill-code-placement",
	"Spill Code Placement Analysis", true, true)

	char &llvm::SpillPlacementID = SpillPlacement::ID;

	void SpillPlacement::getAnalysisUsage(AnalysisUsage &AU) const {
	AU.setPreservesAll();
	AU.addRequiredTransitive<EdgeBundles>();
	AU.addRequiredTransitive<MachineLoopInfo>();
	MachineFunctionPass::getAnalysisUsage(AU);
	}

	/// Node - Each edge bundle corresponds to a Hopfield node.
	///
	/// The node contains precomputed frequency data that only depends on the CFG,
	/// but Bias and Links are computed each time placeSpills is called.
	///
	/// The node Value is positive when the variable should be in a register. The
	/// value can change when linked nodes change, but convergence is very fast
	/// because all weights are positive.
	///
	struct SpillPlacement::Node {
	/// Scale - Inverse block frequency feeding into[0] or out of[1] the bundle.
	/// Ideally, these two numbers should be identical, but inaccuracies in the
	/// block frequency estimates means that we need to normalize ingoing and
	/// outgoing frequencies separately so they are commensurate.
	float Scale[2];

	/// Bias - Normalized contributions from non-transparent blocks.
	/// A bundle connected to a MustSpill block has a huge negative bias,
	/// otherwise it is a number in the range [-2;2].
	float Bias;

	/// Value - Output value of this node computed from the Bias and links.
	/// This is always in the range [-1;1]. A positive number means the variable
	/// should go in a register through this bundle.
	float Value;

	typedef SmallVector<std::pair<float, unsigned>, 4> LinkVector;

	/// Links - (Weight, BundleNo) for all transparent blocks connecting to other
	/// bundles. The weights are all positive and add up to at most 2, weights
	/// from ingoing and outgoing nodes separately add up to a most 1. The weight
	/// sum can be less than 2 when the variable is not live into / out of some
	/// connected basic blocks.
	LinkVector Links;

	/// preferReg - Return true when this node prefers to be in a register.
	bool preferReg() const {
	// Undecided nodes (Value==0) go on the stack.
	return Value > 0;
	}

	/// mustSpill - Return True if this node is so biased that it must spill.
	bool mustSpill() const {
	// Actually, we must spill if Bias < sum(weights).
	// It may be worth it to compute the weight sum here?
	return Bias < -2.0f;
	}

	/// Node - Create a blank Node.
	Node() {
	Scale[0] = Scale[1] = 0;
	}

	/// clear - Reset per-query data, but preserve frequencies that only depend on
	// the CFG.
	void clear() {
	Bias = Value = 0;
	Links.clear();
	}

	/// addLink - Add a link to bundle b with weight w.
	/// out=0 for an ingoing link, and 1 for an outgoing link.
	void addLink(unsigned b, float w, bool out) {
	// Normalize w relative to all connected blocks from that direction.
	w *= Scale[out];

	// There can be multiple links to the same bundle, add them up.
	for (LinkVector::iterator I = Links.begin(), E = Links.end(); I != E; ++I)
	if (I->second == b) {
	I->first += w;
	return;
	}
	// This must be the first link to b.
	Links.push_back(std::make_pair(w, b));
	}

	/// addBias - Bias this node from an ingoing[0] or outgoing[1] link.
	/// Return the change to the total number of positive biases.
	void addBias(float w, bool out) {
	// Normalize w relative to all connected blocks from that direction.
	w *= Scale[out];
	Bias += w;
	}

	/// update - Recompute Value from Bias and Links. Return true when node
	/// preference changes.
	bool update(const Node nodes[]) {
	// Compute the weighted sum of inputs.
	float Sum = Bias;
	for (LinkVector::iterator I = Links.begin(), E = Links.end(); I != E; ++I)
	Sum += I->first * nodes[I->second].Value;

	// The weighted sum is going to be in the range [-2;2]. Ideally, we should
	// simply set Value = sign(Sum), but we will add a dead zone around 0 for
	// two reasons:
	// 1. It avoids arbitrary bias when all links are 0 as is possible during
	// initial iterations.
	// 2. It helps tame rounding errors when the links nominally sum to 0.
	const float Thres = 1e-4f;
	bool Before = preferReg();
	if (Sum < -Thres)
	Value = -1;
	else if (Sum > Thres)
	Value = 1;
	else
	Value = 0;
	return Before != preferReg();
	}
	};

	bool SpillPlacement::runOnMachineFunction(MachineFunction &mf) {
	MF = &mf;
	bundles = &getAnalysis<EdgeBundles>();
	loops = &getAnalysis<MachineLoopInfo>();

	assert(!nodes && "Leaking node array");
	nodes = new Node[bundles->getNumBundles()];

	// Compute total ingoing and outgoing block frequencies for all bundles.
	BlockFrequency.resize(mf.getNumBlockIDs());
	for (MachineFunction::iterator I = mf.begin(), E = mf.end(); I != E; ++I) {
	float Freq = LiveIntervals::getSpillWeight(true, false,
	loops->getLoopDepth(I));
	unsigned Num = I->getNumber();
	BlockFrequency[Num] = Freq;
	nodes[bundles->getBundle(Num, 1)].Scale[0] += Freq;
	nodes[bundles->getBundle(Num, 0)].Scale[1] += Freq;
	}

	// Scales are reciprocal frequencies.
	for (unsigned i = 0, e = bundles->getNumBundles(); i != e; ++i)
	for (unsigned d = 0; d != 2; ++d)
	if (nodes[i].Scale[d] > 0)
	nodes[i].Scale[d] = 1 / nodes[i].Scale[d];

	// We never change the function.
	return false;
	}

	void SpillPlacement::releaseMemory() {
	delete[] nodes;
	nodes = 0;
	}

	/// activate - mark node n as active if it wasn't already.
	void SpillPlacement::activate(unsigned n) {
	if (ActiveNodes->test(n))
	return;
	ActiveNodes->set(n);
	nodes[n].clear();

	// Very large bundles usually come from big switches, indirect branches,
	// landing pads, or loops with many 'continue' statements. It is difficult to
	// allocate registers when so many different blocks are involved.
	//
	// Give a small negative bias to large bundles such that 1/32 of the
	// connected blocks need to be interested before we consider expanding the
	// region through the bundle. This helps compile time by limiting the number
	// of blocks visited and the number of links in the Hopfield network.
	if (bundles->getBlocks(n).size() > 100)
	nodes[n].Bias = -0.0625f;
	}


	/// addConstraints - Compute node biases and weights from a set of constraints.
	/// Set a bit in NodeMask for each active node.
	void SpillPlacement::addConstraints(ArrayRef<BlockConstraint> LiveBlocks) {
	for (ArrayRef<BlockConstraint>::iterator I = LiveBlocks.begin(),
	E = LiveBlocks.end(); I != E; ++I) {
	float Freq = getBlockFrequency(I->Number);
	const float Bias[] = {
	0, // DontCare,
	1, // PrefReg,
	-1, // PrefSpill
	0, // PrefBoth
	-HUGE_VALF // MustSpill
	};

	// Live-in to block?
	if (I->Entry != DontCare) {
	unsigned ib = bundles->getBundle(I->Number, 0);
	activate(ib);
	nodes[ib].addBias(Freq * Bias[I->Entry], 1);
	}

	// Live-out from block?
	if (I->Exit != DontCare) {
	unsigned ob = bundles->getBundle(I->Number, 1);
	activate(ob);
	nodes[ob].addBias(Freq * Bias[I->Exit], 0);
	}
	}
	}

	/// addPrefSpill - Same as addConstraints(PrefSpill)
	void SpillPlacement::addPrefSpill(ArrayRef<unsigned> Blocks, bool Strong) {
	for (ArrayRef<unsigned>::iterator I = Blocks.begin(), E = Blocks.end();
	I != E; ++I) {
	float Freq = getBlockFrequency(*I);
	if (Strong)
	Freq += Freq;
	unsigned ib = bundles->getBundle(*I, 0);
	unsigned ob = bundles->getBundle(*I, 1);
	activate(ib);
	activate(ob);
	nodes[ib].addBias(-Freq, 1);
	nodes[ob].addBias(-Freq, 0);
	}
	}

	void SpillPlacement::addLinks(ArrayRef<unsigned> Links) {
	for (ArrayRef<unsigned>::iterator I = Links.begin(), E = Links.end(); I != E;
	++I) {
	unsigned Number = *I;
	unsigned ib = bundles->getBundle(Number, 0);
	unsigned ob = bundles->getBundle(Number, 1);

	// Ignore self-loops.
	if (ib == ob)
	continue;
	activate(ib);
	activate(ob);
	if (nodes[ib].Links.empty() && !nodes[ib].mustSpill())
	Linked.push_back(ib);
	if (nodes[ob].Links.empty() && !nodes[ob].mustSpill())
	Linked.push_back(ob);
	float Freq = getBlockFrequency(Number);
	nodes[ib].addLink(ob, Freq, 1);
	nodes[ob].addLink(ib, Freq, 0);
	}
	}

	bool SpillPlacement::scanActiveBundles() {
	Linked.clear();
	RecentPositive.clear();
	for (int n = ActiveNodes->find_first(); n>=0; n = ActiveNodes->find_next(n)) {
	nodes[n].update(nodes);
	// A node that must spill, or a node without any links is not going to
	// change its value ever again, so exclude it from iterations.
	if (nodes[n].mustSpill())
	continue;
	if (!nodes[n].Links.empty())
	Linked.push_back(n);
	if (nodes[n].preferReg())
	RecentPositive.push_back(n);
	}
	return !RecentPositive.empty();
	}

	/// iterate - Repeatedly update the Hopfield nodes until stability or the
	/// maximum number of iterations is reached.
	/// @param Linked - Numbers of linked nodes that need updating.
	void SpillPlacement::iterate() {
	// First update the recently positive nodes. They have likely received new
	// negative bias that will turn them off.
	while (!RecentPositive.empty())
	nodes[RecentPositive.pop_back_val()].update(nodes);

	if (Linked.empty())
	return;

	// Run up to 10 iterations. The edge bundle numbering is closely related to
	// basic block numbering, so there is a strong tendency towards chains of
	// linked nodes with sequential numbers. By scanning the linked nodes
	// backwards and forwards, we make it very likely that a single node can
	// affect the entire network in a single iteration. That means very fast
	// convergence, usually in a single iteration.
	for (unsigned iteration = 0; iteration != 10; ++iteration) {
	// Scan backwards, skipping the last node which was just updated.
	bool Changed = false;
	for (SmallVectorImpl<unsigned>::const_reverse_iterator I =
	llvm::next(Linked.rbegin()), E = Linked.rend(); I != E; ++I) {
	unsigned n = *I;
	if (nodes[n].update(nodes)) {
	Changed = true;
	if (nodes[n].preferReg())
	RecentPositive.push_back(n);
	}
	}
	if (!Changed \|\| !RecentPositive.empty())
	return;

	// Scan forwards, skipping the first node which was just updated.
	Changed = false;
	for (SmallVectorImpl<unsigned>::const_iterator I =
	llvm::next(Linked.begin()), E = Linked.end(); I != E; ++I) {
	unsigned n = *I;
	if (nodes[n].update(nodes)) {
	Changed = true;
	if (nodes[n].preferReg())
	RecentPositive.push_back(n);
	}
	}
	if (!Changed \|\| !RecentPositive.empty())
	return;
	}
	}

	void SpillPlacement::prepare(BitVector &RegBundles) {
	Linked.clear();
	RecentPositive.clear();
	// Reuse RegBundles as our ActiveNodes vector.
	ActiveNodes = &RegBundles;
	ActiveNodes->clear();
	ActiveNodes->resize(bundles->getNumBundles());
	}

	bool
	SpillPlacement::finish() {
	assert(ActiveNodes && "Call prepare() first");

	// Write preferences back to ActiveNodes.
	bool Perfect = true;
	for (int n = ActiveNodes->find_first(); n>=0; n = ActiveNodes->find_next(n))
	if (!nodes[n].preferReg()) {
	ActiveNodes->reset(n);
	Perfect = false;
	}
	ActiveNodes = 0;
	return Perfect;
	}