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.
 //
 //===----------------------------------------------------------------------===//

 #include "SpillPlacement.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/BitVector.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/SparseSet.h"
 #include "llvm/CodeGen/EdgeBundles.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineLoopInfo.h"
 #include "llvm/CodeGen/Passes.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/BlockFrequency.h"
 #include <algorithm>
 #include <cassert>
 #include <cstdint>
 #include <utility>

 using namespace llvm;

 #define DEBUG_TYPE "spill-code-placement"

 char SpillPlacement::ID = 0;

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

 INITIALIZE_PASS_BEGIN(SpillPlacement, DEBUG_TYPE,
                       "Spill Code Placement Analysis", true, true)
 INITIALIZE_PASS_DEPENDENCY(EdgeBundles)
 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
 INITIALIZE_PASS_END(SpillPlacement, DEBUG_TYPE,
                     "Spill Code Placement Analysis", true, true)

 void SpillPlacement::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.setPreservesAll();
   AU.addRequired<MachineBlockFrequencyInfo>();
   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 {
   /// BiasN - Sum of blocks that prefer a spill.
   BlockFrequency BiasN;

   /// BiasP - Sum of blocks that prefer a register.
   BlockFrequency BiasP;

   /// Value - Output value of this node computed from the Bias and links.
   /// This is always on of the values {-1, 0, 1}. A positive number means the
   /// variable should go in a register through this bundle.
   int Value;

   using LinkVector = SmallVector<std::pair<BlockFrequency, unsigned>, 4>;

   /// Links - (Weight, BundleNo) for all transparent blocks connecting to other
   /// bundles. The weights are all positive block frequencies.
   LinkVector Links;

   /// SumLinkWeights - Cached sum of the weights of all links + ThresHold.
   BlockFrequency SumLinkWeights;

   /// 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 {
     // We must spill if Bias < -sum(weights) or the MustSpill flag was set.
     // BiasN is saturated when MustSpill is set, make sure this still returns
     // true when the RHS saturates. Note that SumLinkWeights includes Threshold.
     return BiasN >= BiasP + SumLinkWeights;
   }

   /// clear - Reset per-query data, but preserve frequencies that only depend on
   /// the CFG.
   void clear(const BlockFrequency &Threshold) {
     BiasN = BiasP = Value = 0;
     SumLinkWeights = Threshold;
     Links.clear();
   }

   /// addLink - Add a link to bundle b with weight w.
   void addLink(unsigned b, BlockFrequency w) {
     // Update cached sum.
     SumLinkWeights += w;

     // 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.
   void addBias(BlockFrequency freq, BorderConstraint direction) {
     switch (direction) {
     default:
       break;
     case PrefReg:
       BiasP += freq;
       break;
     case PrefSpill:
       BiasN += freq;
       break;
     case MustSpill:
       BiasN = BlockFrequency::getMaxFrequency();
       break;
     }
   }

   /// update - Recompute Value from Bias and Links. Return true when node
   /// preference changes.
   bool update(const Node nodes[], const BlockFrequency &Threshold) {
     // Compute the weighted sum of inputs.
     BlockFrequency SumN = BiasN;
     BlockFrequency SumP = BiasP;
     for (LinkVector::iterator I = Links.begin(), E = Links.end(); I != E; ++I) {
       if (nodes[I->second].Value == -1)
         SumN += I->first;
       else if (nodes[I->second].Value == 1)
         SumP += I->first;
     }

     // Each weighted sum is going to be less than the total frequency of the
     // bundle. Ideally, we should simply set Value = sign(SumP - SumN), 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.
     //
     bool Before = preferReg();
     if (SumN >= SumP + Threshold)
       Value = -1;
     else if (SumP >= SumN + Threshold)
       Value = 1;
     else
       Value = 0;
     return Before != preferReg();
   }

   void getDissentingNeighbors(SparseSet<unsigned> &List,
                               const Node nodes[]) const {
     for (const auto &Elt : Links) {
       unsigned n = Elt.second;
       // Neighbors that already have the same value are not going to
       // change because of this node changing.
       if (Value != nodes[n].Value)
         List.insert(n);
     }
   }
 };

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

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

   // Compute total ingoing and outgoing block frequencies for all bundles.
   BlockFrequencies.resize(mf.getNumBlockIDs());
   MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
   setThreshold(MBFI->getEntryFreq());
   for (auto &I : mf) {
     unsigned Num = I.getNumber();
     BlockFrequencies[Num] = MBFI->getBlockFreq(&I);
   }

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

 void SpillPlacement::releaseMemory() {
   delete[] nodes;
   nodes = nullptr;
   TodoList.clear();
 }

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

   // 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 a substantial
   // fraction 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].BiasP = 0;
     nodes[n].BiasN = (MBFI->getEntryFreq() / 16);
   }
 }

 /// Set the threshold for a given entry frequency.
 ///
 /// Set the threshold relative to \c Entry.  Since the threshold is used as a
 /// bound on the open interval (-Threshold;Threshold), 1 is the minimum
 /// threshold.
 void SpillPlacement::setThreshold(const BlockFrequency &Entry) {
   // Apparently 2 is a good threshold when Entry==2^14, but we need to scale
   // it.  Divide by 2^13, rounding as appropriate.
   uint64_t Freq = Entry.getFrequency();
   uint64_t Scaled = (Freq >> 13) + bool(Freq & (1 << 12));
   Threshold = std::max(UINT64_C(1), Scaled);
 }

 /// 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) {
     BlockFrequency Freq = BlockFrequencies[I->Number];

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

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

 /// 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) {
     BlockFrequency Freq = BlockFrequencies[*I];
     if (Strong)
       Freq += Freq;
     unsigned ib = bundles->getBundle(*I, false);
     unsigned ob = bundles->getBundle(*I, true);
     activate(ib);
     activate(ob);
     nodes[ib].addBias(Freq, PrefSpill);
     nodes[ob].addBias(Freq, PrefSpill);
   }
 }

 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, false);
     unsigned ob = bundles->getBundle(Number, true);

     // Ignore self-loops.
     if (ib == ob)
       continue;
     activate(ib);
     activate(ob);
     BlockFrequency Freq = BlockFrequencies[Number];
     nodes[ib].addLink(ob, Freq);
     nodes[ob].addLink(ib, Freq);
   }
 }

 bool SpillPlacement::scanActiveBundles() {
   RecentPositive.clear();
   for (unsigned n : ActiveNodes->set_bits()) {
     update(n);
     // 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].preferReg())
       RecentPositive.push_back(n);
   }
   return !RecentPositive.empty();
 }

 bool SpillPlacement::update(unsigned n) {
   if (!nodes[n].update(nodes, Threshold))
     return false;
   nodes[n].getDissentingNeighbors(TodoList, nodes);
   return true;
 }

 /// iterate - Repeatedly update the Hopfield nodes until stability or the
 /// maximum number of iterations is reached.
 void SpillPlacement::iterate() {
   // We do not need to push those node in the todolist.
   // They are already been proceeded as part of the previous iteration.
   RecentPositive.clear();

   // Since the last iteration, the todolist have been augmented by calls
   // to addConstraints, addLinks, and co.
   // Update the network energy starting at this new frontier.
   // The call to ::update will add the nodes that changed into the todolist.
   unsigned Limit = bundles->getNumBundles() * 10;
   while(Limit-- > 0 && !TodoList.empty()) {
     unsigned n = TodoList.pop_back_val();
     if (!update(n))
       continue;
     if (nodes[n].preferReg())
       RecentPositive.push_back(n);
   }
 }

 void SpillPlacement::prepare(BitVector &RegBundles) {
   RecentPositive.clear();
   TodoList.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 (unsigned n : ActiveNodes->set_bits())
     if (!nodes[n].preferReg()) {
       ActiveNodes->reset(n);
       Perfect = false;
     }
   ActiveNodes = nullptr;
   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.
	//
	//===----------------------------------------------------------------------===//

	#include "SpillPlacement.h"
	#include "llvm/ADT/ArrayRef.h"
	#include "llvm/ADT/BitVector.h"
	#include "llvm/ADT/SmallVector.h"
	#include "llvm/ADT/SparseSet.h"
	#include "llvm/CodeGen/EdgeBundles.h"
	#include "llvm/CodeGen/MachineBasicBlock.h"
	#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
	#include "llvm/CodeGen/MachineFunction.h"
	#include "llvm/CodeGen/MachineLoopInfo.h"
	#include "llvm/CodeGen/Passes.h"
	#include "llvm/Pass.h"
	#include "llvm/Support/BlockFrequency.h"
	#include <algorithm>
	#include <cassert>
	#include <cstdint>
	#include <utility>

	using namespace llvm;

	#define DEBUG_TYPE "spill-code-placement"

	char SpillPlacement::ID = 0;

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

	INITIALIZE_PASS_BEGIN(SpillPlacement, DEBUG_TYPE,
	"Spill Code Placement Analysis", true, true)
	INITIALIZE_PASS_DEPENDENCY(EdgeBundles)
	INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
	INITIALIZE_PASS_END(SpillPlacement, DEBUG_TYPE,
	"Spill Code Placement Analysis", true, true)

	void SpillPlacement::getAnalysisUsage(AnalysisUsage &AU) const {
	AU.setPreservesAll();
	AU.addRequired<MachineBlockFrequencyInfo>();
	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 {
	/// BiasN - Sum of blocks that prefer a spill.
	BlockFrequency BiasN;

	/// BiasP - Sum of blocks that prefer a register.
	BlockFrequency BiasP;

	/// Value - Output value of this node computed from the Bias and links.
	/// This is always on of the values {-1, 0, 1}. A positive number means the
	/// variable should go in a register through this bundle.
	int Value;

	using LinkVector = SmallVector<std::pair<BlockFrequency, unsigned>, 4>;

	/// Links - (Weight, BundleNo) for all transparent blocks connecting to other
	/// bundles. The weights are all positive block frequencies.
	LinkVector Links;

	/// SumLinkWeights - Cached sum of the weights of all links + ThresHold.
	BlockFrequency SumLinkWeights;

	/// 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 {
	// We must spill if Bias < -sum(weights) or the MustSpill flag was set.
	// BiasN is saturated when MustSpill is set, make sure this still returns
	// true when the RHS saturates. Note that SumLinkWeights includes Threshold.
	return BiasN >= BiasP + SumLinkWeights;
	}

	/// clear - Reset per-query data, but preserve frequencies that only depend on
	/// the CFG.
	void clear(const BlockFrequency &Threshold) {
	BiasN = BiasP = Value = 0;
	SumLinkWeights = Threshold;
	Links.clear();
	}

	/// addLink - Add a link to bundle b with weight w.
	void addLink(unsigned b, BlockFrequency w) {
	// Update cached sum.
	SumLinkWeights += w;

	// 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.
	void addBias(BlockFrequency freq, BorderConstraint direction) {
	switch (direction) {
	default:
	break;
	case PrefReg:
	BiasP += freq;
	break;
	case PrefSpill:
	BiasN += freq;
	break;
	case MustSpill:
	BiasN = BlockFrequency::getMaxFrequency();
	break;
	}
	}

	/// update - Recompute Value from Bias and Links. Return true when node
	/// preference changes.
	bool update(const Node nodes[], const BlockFrequency &Threshold) {
	// Compute the weighted sum of inputs.
	BlockFrequency SumN = BiasN;
	BlockFrequency SumP = BiasP;
	for (LinkVector::iterator I = Links.begin(), E = Links.end(); I != E; ++I) {
	if (nodes[I->second].Value == -1)
	SumN += I->first;
	else if (nodes[I->second].Value == 1)
	SumP += I->first;
	}

	// Each weighted sum is going to be less than the total frequency of the
	// bundle. Ideally, we should simply set Value = sign(SumP - SumN), 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.
	//
	bool Before = preferReg();
	if (SumN >= SumP + Threshold)
	Value = -1;
	else if (SumP >= SumN + Threshold)
	Value = 1;
	else
	Value = 0;
	return Before != preferReg();
	}

	void getDissentingNeighbors(SparseSet<unsigned> &List,
	const Node nodes[]) const {
	for (const auto &Elt : Links) {
	unsigned n = Elt.second;
	// Neighbors that already have the same value are not going to
	// change because of this node changing.
	if (Value != nodes[n].Value)
	List.insert(n);
	}
	}
	};

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

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

	// Compute total ingoing and outgoing block frequencies for all bundles.
	BlockFrequencies.resize(mf.getNumBlockIDs());
	MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
	setThreshold(MBFI->getEntryFreq());
	for (auto &I : mf) {
	unsigned Num = I.getNumber();
	BlockFrequencies[Num] = MBFI->getBlockFreq(&I);
	}

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

	void SpillPlacement::releaseMemory() {
	delete[] nodes;
	nodes = nullptr;
	TodoList.clear();
	}

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

	// 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 a substantial
	// fraction 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].BiasP = 0;
	nodes[n].BiasN = (MBFI->getEntryFreq() / 16);
	}
	}

	/// Set the threshold for a given entry frequency.
	///
	/// Set the threshold relative to \c Entry. Since the threshold is used as a
	/// bound on the open interval (-Threshold;Threshold), 1 is the minimum
	/// threshold.
	void SpillPlacement::setThreshold(const BlockFrequency &Entry) {
	// Apparently 2 is a good threshold when Entry==2^14, but we need to scale
	// it. Divide by 2^13, rounding as appropriate.
	uint64_t Freq = Entry.getFrequency();
	uint64_t Scaled = (Freq >> 13) + bool(Freq & (1 << 12));
	Threshold = std::max(UINT64_C(1), Scaled);
	}

	/// 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) {
	BlockFrequency Freq = BlockFrequencies[I->Number];

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

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

	/// 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) {
	BlockFrequency Freq = BlockFrequencies[*I];
	if (Strong)
	Freq += Freq;
	unsigned ib = bundles->getBundle(*I, false);
	unsigned ob = bundles->getBundle(*I, true);
	activate(ib);
	activate(ob);
	nodes[ib].addBias(Freq, PrefSpill);
	nodes[ob].addBias(Freq, PrefSpill);
	}
	}

	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, false);
	unsigned ob = bundles->getBundle(Number, true);

	// Ignore self-loops.
	if (ib == ob)
	continue;
	activate(ib);
	activate(ob);
	BlockFrequency Freq = BlockFrequencies[Number];
	nodes[ib].addLink(ob, Freq);
	nodes[ob].addLink(ib, Freq);
	}
	}

	bool SpillPlacement::scanActiveBundles() {
	RecentPositive.clear();
	for (unsigned n : ActiveNodes->set_bits()) {
	update(n);
	// 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].preferReg())
	RecentPositive.push_back(n);
	}
	return !RecentPositive.empty();
	}

	bool SpillPlacement::update(unsigned n) {
	if (!nodes[n].update(nodes, Threshold))
	return false;
	nodes[n].getDissentingNeighbors(TodoList, nodes);
	return true;
	}

	/// iterate - Repeatedly update the Hopfield nodes until stability or the
	/// maximum number of iterations is reached.
	void SpillPlacement::iterate() {
	// We do not need to push those node in the todolist.
	// They are already been proceeded as part of the previous iteration.
	RecentPositive.clear();

	// Since the last iteration, the todolist have been augmented by calls
	// to addConstraints, addLinks, and co.
	// Update the network energy starting at this new frontier.
	// The call to ::update will add the nodes that changed into the todolist.
	unsigned Limit = bundles->getNumBundles() * 10;
	while(Limit-- > 0 && !TodoList.empty()) {
	unsigned n = TodoList.pop_back_val();
	if (!update(n))
	continue;
	if (nodes[n].preferReg())
	RecentPositive.push_back(n);
	}
	}

	void SpillPlacement::prepare(BitVector &RegBundles) {
	RecentPositive.clear();
	TodoList.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 (unsigned n : ActiveNodes->set_bits())
	if (!nodes[n].preferReg()) {
	ActiveNodes->reset(n);
	Perfect = false;
	}
	ActiveNodes = nullptr;
	return Perfect;
	}