lld/COFF/CallGraphSort.cpp

   1 //===- CallGraphSort.cpp --------------------------------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 ///
   9 /// This is based on the ELF port, see ELF/CallGraphSort.cpp for the details
  10 /// about the algorithm.
  11 ///
  12 //===----------------------------------------------------------------------===//
  13
  14 #include "CallGraphSort.h"
  15 #include "COFFLinkerContext.h"
  16 #include "InputFiles.h"
  17 #include "SymbolTable.h"
  18 #include "Symbols.h"
  19 #include "lld/Common/ErrorHandler.h"
  20
  21 #include <numeric>
  22
  23 using namespace llvm;
  24 using namespace lld;
  25 using namespace lld::coff;
  26
  27 namespace {
  28 struct Edge {
  29   int from;
  30   uint64_t weight;
  31 };
  32
  33 struct Cluster {
  34   Cluster(int sec, size_t s) : next(sec), prev(sec), size(s) {}
  35
  36   double getDensity() const {
  37     if (size == 0)
  38       return 0;
  39     return double(weight) / double(size);
  40   }
  41
  42   int next;
  43   int prev;
  44   uint64_t size;
  45   uint64_t weight = 0;
  46   uint64_t initialWeight = 0;
  47   Edge bestPred = {-1, 0};
  48 };
  49
  50 class CallGraphSort {
  51 public:
  52   CallGraphSort(const COFFLinkerContext &ctx);
  53
  54   DenseMap<const SectionChunk *, int> run();
  55
  56 private:
  57   std::vector<Cluster> clusters;
  58   std::vector<const SectionChunk *> sections;
  59
  60   const COFFLinkerContext &ctx;
  61 };
  62
  63 // Maximum amount the combined cluster density can be worse than the original
  64 // cluster to consider merging.
  65 constexpr int MAX_DENSITY_DEGRADATION = 8;
  66
  67 // Maximum cluster size in bytes.
  68 constexpr uint64_t MAX_CLUSTER_SIZE = 1024 * 1024;
  69 } // end anonymous namespace
  70
  71 using SectionPair = std::pair<const SectionChunk *, const SectionChunk *>;
  72
  73 // Take the edge list in Config->CallGraphProfile, resolve symbol names to
  74 // Symbols, and generate a graph between InputSections with the provided
  75 // weights.
  76 CallGraphSort::CallGraphSort(const COFFLinkerContext &ctx) : ctx(ctx) {
  77   const MapVector<SectionPair, uint64_t> &profile = ctx.config.callGraphProfile;
  78   DenseMap<const SectionChunk *, int> secToCluster;
  79
  80   auto getOrCreateNode = [&](const SectionChunk *isec) -> int {
  81     auto res = secToCluster.try_emplace(isec, clusters.size());
  82     if (res.second) {
  83       sections.push_back(isec);
  84       clusters.emplace_back(clusters.size(), isec->getSize());
  85     }
  86     return res.first->second;
  87   };
  88
  89   // Create the graph.
  90   for (const std::pair<SectionPair, uint64_t> &c : profile) {
  91     const auto *fromSec = cast<SectionChunk>(c.first.first->repl);
  92     const auto *toSec = cast<SectionChunk>(c.first.second->repl);
  93     uint64_t weight = c.second;
  94
  95     // Ignore edges between input sections belonging to different output
  96     // sections.  This is done because otherwise we would end up with clusters
  97     // containing input sections that can't actually be placed adjacently in the
  98     // output.  This messes with the cluster size and density calculations.  We
  99     // would also end up moving input sections in other output sections without
 100     // moving them closer to what calls them.
 101     if (ctx.getOutputSection(fromSec) != ctx.getOutputSection(toSec))
 102       continue;
 103
 104     int from = getOrCreateNode(fromSec);
 105     int to = getOrCreateNode(toSec);
 106
 107     clusters[to].weight += weight;
 108
 109     if (from == to)
 110       continue;
 111
 112     // Remember the best edge.
 113     Cluster &toC = clusters[to];
 114     if (toC.bestPred.from == -1 || toC.bestPred.weight < weight) {
 115       toC.bestPred.from = from;
 116       toC.bestPred.weight = weight;
 117     }
 118   }
 119   for (Cluster &c : clusters)
 120     c.initialWeight = c.weight;
 121 }
 122
 123 // It's bad to merge clusters which would degrade the density too much.
 124 static bool isNewDensityBad(Cluster &a, Cluster &b) {
 125   double newDensity = double(a.weight + b.weight) / double(a.size + b.size);
 126   return newDensity < a.getDensity() / MAX_DENSITY_DEGRADATION;
 127 }
 128
 129 // Find the leader of V's belonged cluster (represented as an equivalence
 130 // class). We apply union-find path-halving technique (simple to implement) in
 131 // the meantime as it decreases depths and the time complexity.
 132 static int getLeader(std::vector<int> &leaders, int v) {
 133   while (leaders[v] != v) {
 134     leaders[v] = leaders[leaders[v]];
 135     v = leaders[v];
 136   }
 137   return v;
 138 }
 139
 140 static void mergeClusters(std::vector<Cluster> &cs, Cluster &into, int intoIdx,
 141                           Cluster &from, int fromIdx) {
 142   int tail1 = into.prev, tail2 = from.prev;
 143   into.prev = tail2;
 144   cs[tail2].next = intoIdx;
 145   from.prev = tail1;
 146   cs[tail1].next = fromIdx;
 147   into.size += from.size;
 148   into.weight += from.weight;
 149   from.size = 0;
 150   from.weight = 0;
 151 }
 152
 153 // Group InputSections into clusters using the Call-Chain Clustering heuristic
 154 // then sort the clusters by density.
 155 DenseMap<const SectionChunk *, int> CallGraphSort::run() {
 156   std::vector<int> sorted(clusters.size());
 157   std::vector<int> leaders(clusters.size());
 158
 159   std::iota(leaders.begin(), leaders.end(), 0);
 160   std::iota(sorted.begin(), sorted.end(), 0);
 161   llvm::stable_sort(sorted, [&](int a, int b) {
 162     return clusters[a].getDensity() > clusters[b].getDensity();
 163   });
 164
 165   for (int l : sorted) {
 166     // The cluster index is the same as the index of its leader here because
 167     // clusters[L] has not been merged into another cluster yet.
 168     Cluster &c = clusters[l];
 169
 170     // Don't consider merging if the edge is unlikely.
 171     if (c.bestPred.from == -1 || c.bestPred.weight * 10 <= c.initialWeight)
 172       continue;
 173
 174     int predL = getLeader(leaders, c.bestPred.from);
 175     if (l == predL)
 176       continue;
 177
 178     Cluster *predC = &clusters[predL];
 179     if (c.size + predC->size > MAX_CLUSTER_SIZE)
 180       continue;
 181
 182     if (isNewDensityBad(*predC, c))
 183       continue;
 184
 185     leaders[l] = predL;
 186     mergeClusters(clusters, *predC, predL, c, l);
 187   }
 188
 189   // Sort remaining non-empty clusters by density.
 190   sorted.clear();
 191   for (int i = 0, e = (int)clusters.size(); i != e; ++i)
 192     if (clusters[i].size > 0)
 193       sorted.push_back(i);
 194   llvm::stable_sort(sorted, [&](int a, int b) {
 195     return clusters[a].getDensity() > clusters[b].getDensity();
 196   });
 197
 198   DenseMap<const SectionChunk *, int> orderMap;
 199   // Sections will be sorted by increasing order. Absent sections will have
 200   // priority 0 and be placed at the end of sections.
 201   int curOrder = INT_MIN;
 202   for (int leader : sorted) {
 203     for (int i = leader;;) {
 204       orderMap[sections[i]] = curOrder++;
 205       i = clusters[i].next;
 206       if (i == leader)
 207         break;
 208     }
 209   }
 210   if (!ctx.config.printSymbolOrder.empty()) {
 211     std::error_code ec;
 212     raw_fd_ostream os(ctx.config.printSymbolOrder, ec, sys::fs::OF_None);
 213     if (ec) {
 214       error("cannot open " + ctx.config.printSymbolOrder + ": " + ec.message());
 215       return orderMap;
 216     }
 217     // Print the symbols ordered by C3, in the order of increasing curOrder
 218     // Instead of sorting all the orderMap, just repeat the loops above.
 219     for (int leader : sorted)
 220       for (int i = leader;;) {
 221         const SectionChunk *sc = sections[i];
 222
 223         // Search all the symbols in the file of the section
 224         // and find out a DefinedCOFF symbol with name that is within the
 225         // section.
 226         for (Symbol *sym : sc->file->getSymbols())
 227           if (auto *d = dyn_cast_or_null<DefinedCOFF>(sym))
 228             // Filter out non-COMDAT symbols and section symbols.
 229             if (d->isCOMDAT && !d->getCOFFSymbol().isSection() &&
 230                 sc == d->getChunk())
 231               os << sym->getName() << "\n";
 232         i = clusters[i].next;
 233         if (i == leader)
 234           break;
 235       }
 236   }
 237
 238   return orderMap;
 239 }
 240
 241 // Sort sections by the profile data provided by  /call-graph-ordering-file
 242 //
 243 // This first builds a call graph based on the profile data then merges sections
 244 // according to the C³ heuristic. All clusters are then sorted by a density
 245 // metric to further improve locality.
 246 DenseMap<const SectionChunk *, int>
 247 coff::computeCallGraphProfileOrder(const COFFLinkerContext &ctx) {
 248   return CallGraphSort(ctx).run();
 249 }