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diff --git a/llvm/lib/CodeGen/SpillPlacement.cpp b/llvm/lib/CodeGen/SpillPlacement.cpp
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+//===- SpillPlacement.cpp - Optimal Spill Code Placement ------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// 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;
+}