llvm-project/lld/ELF/CallGraphSort.cpp

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//===- CallGraphSort.cpp --------------------------------------------------===//
//
// The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
///
/// Implementation of Call-Chain Clustering from: Optimizing Function Placement
/// for Large-Scale Data-Center Applications
/// https://research.fb.com/wp-content/uploads/2017/01/cgo2017-hfsort-final1.pdf
///
/// The goal of this algorithm is to improve runtime performance of the final
/// executable by arranging code sections such that page table and i-cache
/// misses are minimized.
///
/// Definitions:
/// * Cluster
/// * An ordered list of input sections which are layed out as a unit. At the
/// beginning of the algorithm each input section has its own cluster and
/// the weight of the cluster is the sum of the weight of all incomming
/// edges.
/// * Call-Chain Clustering (C³) Heuristic
/// * Defines when and how clusters are combined. Pick the highest weighted
/// input section then add it to its most likely predecessor if it wouldn't
/// penalize it too much.
/// * Density
/// * The weight of the cluster divided by the size of the cluster. This is a
/// proxy for the ammount of execution time spent per byte of the cluster.
///
/// It does so given a call graph profile by the following:
/// * Build a weighted call graph from the call graph profile
/// * Sort input sections by weight
/// * For each input section starting with the highest weight
/// * Find its most likely predecessor cluster
/// * Check if the combined cluster would be too large, or would have too low
/// a density.
/// * If not, then combine the clusters.
/// * Sort non-empty clusters by density
///
//===----------------------------------------------------------------------===//
#include "CallGraphSort.h"
#include "OutputSections.h"
#include "SymbolTable.h"
#include "Symbols.h"
using namespace llvm;
using namespace lld;
using namespace lld::elf;
namespace {
struct Edge {
int From;
uint64_t Weight;
};
struct Cluster {
Cluster(int Sec, size_t S) {
Sections.push_back(Sec);
Size = S;
}
double getDensity() const {
if (Size == 0)
return 0;
return double(Weight) / double(Size);
}
std::vector<int> Sections;
size_t Size = 0;
uint64_t Weight = 0;
uint64_t InitialWeight = 0;
std::vector<Edge> Preds;
};
class CallGraphSort {
public:
CallGraphSort();
DenseMap<const InputSectionBase *, int> run();
private:
std::vector<Cluster> Clusters;
std::vector<const InputSectionBase *> Sections;
void groupClusters();
};
// Maximum ammount the combined cluster density can be worse than the original
// cluster to consider merging.
constexpr int MAX_DENSITY_DEGRADATION = 8;
// Maximum cluster size in bytes.
constexpr uint64_t MAX_CLUSTER_SIZE = 1024 * 1024;
} // end anonymous namespace
// Take the edge list in Config->CallGraphProfile, resolve symbol names to
// Symbols, and generate a graph between InputSections with the provided
// weights.
CallGraphSort::CallGraphSort() {
llvm::MapVector<std::pair<const InputSectionBase *, const InputSectionBase *>,
uint64_t> &Profile = Config->CallGraphProfile;
DenseMap<const InputSectionBase *, int> SecToCluster;
auto GetOrCreateNode = [&](const InputSectionBase *IS) -> int {
auto Res = SecToCluster.insert(std::make_pair(IS, Clusters.size()));
if (Res.second) {
Sections.push_back(IS);
Clusters.emplace_back(Clusters.size(), IS->getSize());
}
return Res.first->second;
};
// Create the graph.
for (const auto &C : Profile) {
const auto *FromSB = cast<InputSectionBase>(C.first.first->Repl);
const auto *ToSB = cast<InputSectionBase>(C.first.second->Repl);
uint64_t Weight = C.second;
// Ignore edges between input sections belonging to different output
// sections. This is done because otherwise we would end up with clusters
// containing input sections that can't actually be placed adjacently in the
// output. This messes with the cluster size and density calculations. We
// would also end up moving input sections in other output sections without
// moving them closer to what calls them.
if (FromSB->getOutputSection() != ToSB->getOutputSection())
continue;
int From = GetOrCreateNode(FromSB);
int To = GetOrCreateNode(ToSB);
Clusters[To].Weight += Weight;
if (From == To)
continue;
// Add an edge
Clusters[To].Preds.push_back({From, Weight});
}
for (Cluster &C : Clusters)
C.InitialWeight = C.Weight;
}
// It's bad to merge clusters which would degrade the density too much.
static bool isNewDensityBad(Cluster &A, Cluster &B) {
double NewDensity = double(A.Weight + B.Weight) / double(A.Size + B.Size);
if (NewDensity < A.getDensity() / MAX_DENSITY_DEGRADATION)
return true;
return false;
}
static void mergeClusters(Cluster &Into, Cluster &From) {
Into.Sections.insert(Into.Sections.end(), From.Sections.begin(),
From.Sections.end());
Into.Size += From.Size;
Into.Weight += From.Weight;
From.Sections.clear();
From.Size = 0;
From.Weight = 0;
}
// Group InputSections into clusters using the Call-Chain Clustering heuristic
// then sort the clusters by density.
void CallGraphSort::groupClusters() {
std::vector<int> SortedSecs(Clusters.size());
std::vector<Cluster *> SecToCluster(Clusters.size());
for (int SI = 0, SE = Clusters.size(); SI != SE; ++SI) {
SortedSecs[SI] = SI;
SecToCluster[SI] = &Clusters[SI];
}
std::stable_sort(SortedSecs.begin(), SortedSecs.end(), [&](int A, int B) {
return Clusters[B].getDensity() < Clusters[A].getDensity();
});
for (int SI : SortedSecs) {
// Clusters[SI] is the same as SecToClusters[SI] here because it has not
// been merged into another cluster yet.
Cluster &C = Clusters[SI];
int BestPred = -1;
uint64_t BestWeight = 0;
for (Edge &E : C.Preds) {
if (BestPred == -1 || E.Weight > BestWeight) {
BestPred = E.From;
BestWeight = E.Weight;
}
}
// don't consider merging if the edge is unlikely.
if (BestWeight * 10 <= C.InitialWeight)
continue;
Cluster *PredC = SecToCluster[BestPred];
if (PredC == &C)
continue;
if (C.Size + PredC->Size > MAX_CLUSTER_SIZE)
continue;
if (isNewDensityBad(*PredC, C))
continue;
// NOTE: Consider using a disjoint-set to track section -> cluster mapping
// if this is ever slow.
for (int SI : C.Sections)
SecToCluster[SI] = PredC;
mergeClusters(*PredC, C);
}
// Remove empty or dead nodes. Invalidates all cluster indices.
llvm::erase_if(Clusters, [](const Cluster &C) {
return C.Size == 0 || C.Sections.empty();
});
// Sort by density.
std::stable_sort(Clusters.begin(), Clusters.end(),
[](const Cluster &A, const Cluster &B) {
return A.getDensity() > B.getDensity();
});
}
DenseMap<const InputSectionBase *, int> CallGraphSort::run() {
groupClusters();
// Generate order.
llvm::DenseMap<const InputSectionBase *, int> OrderMap;
ssize_t CurOrder = 1;
for (const Cluster &C : Clusters)
for (int SecIndex : C.Sections)
OrderMap[Sections[SecIndex]] = CurOrder++;
return OrderMap;
}
// Sort sections by the profile data provided by -callgraph-profile-file
//
// This first builds a call graph based on the profile data then merges sections
// according to the C³ huristic. All clusters are then sorted by a density
// metric to further improve locality.
DenseMap<const InputSectionBase *, int> elf::computeCallGraphProfileOrder() {
return CallGraphSort().run();
}