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//===---- MachineOutliner.cpp - Outline instructions -----------*- C++ -*-===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
/// \file
/// Replaces repeated sequences of instructions with function calls.
/// This works by placing every instruction from every basic block in a
/// suffix tree, and repeatedly querying that tree for repeated sequences of
/// instructions. If a sequence of instructions appears often, then it ought
/// to be beneficial to pull out into a function.
/// The MachineOutliner communicates with a given target using hooks defined in
/// TargetInstrInfo.h. The target supplies the outliner with information on how
/// a specific sequence of instructions should be outlined. This information
/// is used to deduce the number of instructions necessary to
/// * Create an outlined function
/// * Call that outlined function
/// Targets must implement
/// * getOutliningCandidateInfo
/// * insertOutlinerEpilogue
/// * insertOutlinedCall
/// * insertOutlinerPrologue
/// * isFunctionSafeToOutlineFrom
/// in order to make use of the MachineOutliner.
/// This was originally presented at the 2016 LLVM Developers' Meeting in the
/// talk "Reducing Code Size Using Outlining". For a high-level overview of
/// how this pass works, the talk is available on YouTube at
/// The slides for the talk are available at
/// The talk provides an overview of how the outliner finds candidates and
/// ultimately outlines them. It describes how the main data structure for this
/// pass, the suffix tree, is queried and purged for candidates. It also gives
/// a simplified suffix tree construction algorithm for suffix trees based off
/// of the algorithm actually used here, Ukkonen's algorithm.
/// For the original RFC for this pass, please see
/// For more information on the suffix tree data structure, please see
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/Twine.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineOptimizationRemarkEmitter.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetRegisterInfo.h"
#include "llvm/Target/TargetSubtargetInfo.h"
#include <functional>
#include <map>
#include <sstream>
#include <tuple>
#include <vector>
#define DEBUG_TYPE "machine-outliner"
using namespace llvm;
using namespace ore;
STATISTIC(NumOutlined, "Number of candidates outlined");
STATISTIC(FunctionsCreated, "Number of functions created");
namespace {
/// \brief An individual sequence of instructions to be replaced with a call to
/// an outlined function.
struct Candidate {
/// The start index of this \p Candidate in the instruction list.
unsigned StartIdx;
/// The number of instructions in this \p Candidate.
unsigned Len;
/// Set to false if the candidate overlapped with another candidate.
bool InCandidateList = true;
/// \brief The index of this \p Candidate's \p OutlinedFunction in the list of
/// \p OutlinedFunctions.
unsigned FunctionIdx;
/// Contains all target-specific information for this \p Candidate.
TargetInstrInfo::MachineOutlinerInfo MInfo;
/// Return the number of instructions in this Candidate.
unsigned getLength() const { return Len; }
/// Return the start index of this candidate.
unsigned getStartIdx() const { return StartIdx; }
// Return the end index of this candidate.
unsigned getEndIdx() const { return StartIdx + Len - 1; }
/// \brief The number of instructions that would be saved by outlining every
/// candidate of this type.
/// This is a fixed value which is not updated during the candidate pruning
/// process. It is only used for deciding which candidate to keep if two
/// candidates overlap. The true benefit is stored in the OutlinedFunction
/// for some given candidate.
unsigned Benefit = 0;
Candidate(unsigned StartIdx, unsigned Len, unsigned FunctionIdx)
: StartIdx(StartIdx), Len(Len), FunctionIdx(FunctionIdx) {}
Candidate() {}
/// \brief Used to ensure that \p Candidates are outlined in an order that
/// preserves the start and end indices of other \p Candidates.
bool operator<(const Candidate &RHS) const {
return getStartIdx() > RHS.getStartIdx();
/// \brief The information necessary to create an outlined function for some
/// class of candidate.
struct OutlinedFunction {
/// The number of candidates for this \p OutlinedFunction.
unsigned OccurrenceCount = 0;
std::vector<std::shared_ptr<Candidate>> Candidates;
/// The actual outlined function created.
/// This is initialized after we go through and create the actual function.
MachineFunction *MF = nullptr;
/// A number assigned to this function which appears at the end of its name.
unsigned Name;
/// \brief The sequence of integers corresponding to the instructions in this
/// function.
std::vector<unsigned> Sequence;
/// Contains all target-specific information for this \p OutlinedFunction.
TargetInstrInfo::MachineOutlinerInfo MInfo;
/// Return the number of candidates for this \p OutlinedFunction.
unsigned getOccurrenceCount() { return OccurrenceCount; }
/// Decrement the occurrence count of this OutlinedFunction and return the
/// new count.
unsigned decrement() {
assert(OccurrenceCount > 0 && "Can't decrement an empty function!");
return getOccurrenceCount();
/// \brief Return the number of instructions it would take to outline this
/// function.
unsigned getOutliningCost() {
return (OccurrenceCount * MInfo.CallOverhead) + Sequence.size() +
/// \brief Return the number of instructions that would be saved by outlining
/// this function.
unsigned getBenefit() {
unsigned NotOutlinedCost = OccurrenceCount * Sequence.size();
unsigned OutlinedCost = getOutliningCost();
return (NotOutlinedCost < OutlinedCost) ? 0
: NotOutlinedCost - OutlinedCost;
OutlinedFunction(unsigned Name, unsigned OccurrenceCount,
const std::vector<unsigned> &Sequence,
TargetInstrInfo::MachineOutlinerInfo &MInfo)
: OccurrenceCount(OccurrenceCount), Name(Name), Sequence(Sequence),
MInfo(MInfo) {}
/// Represents an undefined index in the suffix tree.
const unsigned EmptyIdx = -1;
/// A node in a suffix tree which represents a substring or suffix.
/// Each node has either no children or at least two children, with the root
/// being a exception in the empty tree.
/// Children are represented as a map between unsigned integers and nodes. If
/// a node N has a child M on unsigned integer k, then the mapping represented
/// by N is a proper prefix of the mapping represented by M. Note that this,
/// although similar to a trie is somewhat different: each node stores a full
/// substring of the full mapping rather than a single character state.
/// Each internal node contains a pointer to the internal node representing
/// the same string, but with the first character chopped off. This is stored
/// in \p Link. Each leaf node stores the start index of its respective
/// suffix in \p SuffixIdx.
struct SuffixTreeNode {
/// The children of this node.
/// A child existing on an unsigned integer implies that from the mapping
/// represented by the current node, there is a way to reach another
/// mapping by tacking that character on the end of the current string.
DenseMap<unsigned, SuffixTreeNode *> Children;
/// A flag set to false if the node has been pruned from the tree.
bool IsInTree = true;
/// The start index of this node's substring in the main string.
unsigned StartIdx = EmptyIdx;
/// The end index of this node's substring in the main string.
/// Every leaf node must have its \p EndIdx incremented at the end of every
/// step in the construction algorithm. To avoid having to update O(N)
/// nodes individually at the end of every step, the end index is stored
/// as a pointer.
unsigned *EndIdx = nullptr;
/// For leaves, the start index of the suffix represented by this node.
/// For all other nodes, this is ignored.
unsigned SuffixIdx = EmptyIdx;
/// \brief For internal nodes, a pointer to the internal node representing
/// the same sequence with the first character chopped off.
/// This acts as a shortcut in Ukkonen's algorithm. One of the things that
/// Ukkonen's algorithm does to achieve linear-time construction is
/// keep track of which node the next insert should be at. This makes each
/// insert O(1), and there are a total of O(N) inserts. The suffix link
/// helps with inserting children of internal nodes.
/// Say we add a child to an internal node with associated mapping S. The
/// next insertion must be at the node representing S - its first character.
/// This is given by the way that we iteratively build the tree in Ukkonen's
/// algorithm. The main idea is to look at the suffixes of each prefix in the
/// string, starting with the longest suffix of the prefix, and ending with
/// the shortest. Therefore, if we keep pointers between such nodes, we can
/// move to the next insertion point in O(1) time. If we don't, then we'd
/// have to query from the root, which takes O(N) time. This would make the
/// construction algorithm O(N^2) rather than O(N).
SuffixTreeNode *Link = nullptr;
/// The parent of this node. Every node except for the root has a parent.
SuffixTreeNode *Parent = nullptr;
/// The number of times this node's string appears in the tree.
/// This is equal to the number of leaf children of the string. It represents
/// the number of suffixes that the node's string is a prefix of.
unsigned OccurrenceCount = 0;
/// The length of the string formed by concatenating the edge labels from the
/// root to this node.
unsigned ConcatLen = 0;
/// Returns true if this node is a leaf.
bool isLeaf() const { return SuffixIdx != EmptyIdx; }
/// Returns true if this node is the root of its owning \p SuffixTree.
bool isRoot() const { return StartIdx == EmptyIdx; }
/// Return the number of elements in the substring associated with this node.
size_t size() const {
// Is it the root? If so, it's the empty string so return 0.
if (isRoot())
return 0;
assert(*EndIdx != EmptyIdx && "EndIdx is undefined!");
// Size = the number of elements in the string.
// For example, [0 1 2 3] has length 4, not 3. 3-0 = 3, so we have 3-0+1.
return *EndIdx - StartIdx + 1;
SuffixTreeNode(unsigned StartIdx, unsigned *EndIdx, SuffixTreeNode *Link,
SuffixTreeNode *Parent)
: StartIdx(StartIdx), EndIdx(EndIdx), Link(Link), Parent(Parent) {}
SuffixTreeNode() {}
/// A data structure for fast substring queries.
/// Suffix trees represent the suffixes of their input strings in their leaves.
/// A suffix tree is a type of compressed trie structure where each node
/// represents an entire substring rather than a single character. Each leaf
/// of the tree is a suffix.
/// A suffix tree can be seen as a type of state machine where each state is a
/// substring of the full string. The tree is structured so that, for a string
/// of length N, there are exactly N leaves in the tree. This structure allows
/// us to quickly find repeated substrings of the input string.
/// In this implementation, a "string" is a vector of unsigned integers.
/// These integers may result from hashing some data type. A suffix tree can
/// contain 1 or many strings, which can then be queried as one large string.
/// The suffix tree is implemented using Ukkonen's algorithm for linear-time
/// suffix tree construction. Ukkonen's algorithm is explained in more detail
/// in the paper by Esko Ukkonen "On-line construction of suffix trees. The
/// paper is available at
class SuffixTree {
/// Stores each leaf node in the tree.
/// This is used for finding outlining candidates.
std::vector<SuffixTreeNode *> LeafVector;
/// Each element is an integer representing an instruction in the module.
ArrayRef<unsigned> Str;
/// Maintains each node in the tree.
SpecificBumpPtrAllocator<SuffixTreeNode> NodeAllocator;
/// The root of the suffix tree.
/// The root represents the empty string. It is maintained by the
/// \p NodeAllocator like every other node in the tree.
SuffixTreeNode *Root = nullptr;
/// Maintains the end indices of the internal nodes in the tree.
/// Each internal node is guaranteed to never have its end index change
/// during the construction algorithm; however, leaves must be updated at
/// every step. Therefore, we need to store leaf end indices by reference
/// to avoid updating O(N) leaves at every step of construction. Thus,
/// every internal node must be allocated its own end index.
BumpPtrAllocator InternalEndIdxAllocator;
/// The end index of each leaf in the tree.
unsigned LeafEndIdx = -1;
/// \brief Helper struct which keeps track of the next insertion point in
/// Ukkonen's algorithm.
struct ActiveState {
/// The next node to insert at.
SuffixTreeNode *Node;
/// The index of the first character in the substring currently being added.
unsigned Idx = EmptyIdx;
/// The length of the substring we have to add at the current step.
unsigned Len = 0;
/// \brief The point the next insertion will take place at in the
/// construction algorithm.
ActiveState Active;
/// Allocate a leaf node and add it to the tree.
/// \param Parent The parent of this node.
/// \param StartIdx The start index of this node's associated string.
/// \param Edge The label on the edge leaving \p Parent to this node.
/// \returns A pointer to the allocated leaf node.
SuffixTreeNode *insertLeaf(SuffixTreeNode &Parent, unsigned StartIdx,
unsigned Edge) {
assert(StartIdx <= LeafEndIdx && "String can't start after it ends!");
SuffixTreeNode *N = new (NodeAllocator.Allocate())
SuffixTreeNode(StartIdx, &LeafEndIdx, nullptr, &Parent);
Parent.Children[Edge] = N;
return N;
/// Allocate an internal node and add it to the tree.
/// \param Parent The parent of this node. Only null when allocating the root.
/// \param StartIdx The start index of this node's associated string.
/// \param EndIdx The end index of this node's associated string.
/// \param Edge The label on the edge leaving \p Parent to this node.
/// \returns A pointer to the allocated internal node.
SuffixTreeNode *insertInternalNode(SuffixTreeNode *Parent, unsigned StartIdx,
unsigned EndIdx, unsigned Edge) {
assert(StartIdx <= EndIdx && "String can't start after it ends!");
assert(!(!Parent && StartIdx != EmptyIdx) &&
"Non-root internal nodes must have parents!");
unsigned *E = new (InternalEndIdxAllocator) unsigned(EndIdx);
SuffixTreeNode *N = new (NodeAllocator.Allocate())
SuffixTreeNode(StartIdx, E, Root, Parent);
if (Parent)
Parent->Children[Edge] = N;
return N;
/// \brief Set the suffix indices of the leaves to the start indices of their
/// respective suffixes. Also stores each leaf in \p LeafVector at its
/// respective suffix index.
/// \param[in] CurrNode The node currently being visited.
/// \param CurrIdx The current index of the string being visited.
void setSuffixIndices(SuffixTreeNode &CurrNode, unsigned CurrIdx) {
bool IsLeaf = CurrNode.Children.size() == 0 && !CurrNode.isRoot();
// Store the length of the concatenation of all strings from the root to
// this node.
if (!CurrNode.isRoot()) {
if (CurrNode.ConcatLen == 0)
CurrNode.ConcatLen = CurrNode.size();
if (CurrNode.Parent)
CurrNode.ConcatLen += CurrNode.Parent->ConcatLen;
// Traverse the tree depth-first.
for (auto &ChildPair : CurrNode.Children) {
assert(ChildPair.second && "Node had a null child!");
setSuffixIndices(*ChildPair.second, CurrIdx + ChildPair.second->size());
// Is this node a leaf?
if (IsLeaf) {
// If yes, give it a suffix index and bump its parent's occurrence count.
CurrNode.SuffixIdx = Str.size() - CurrIdx;
assert(CurrNode.Parent && "CurrNode had no parent!");
// Store the leaf in the leaf vector for pruning later.
LeafVector[CurrNode.SuffixIdx] = &CurrNode;
/// \brief Construct the suffix tree for the prefix of the input ending at
/// \p EndIdx.
/// Used to construct the full suffix tree iteratively. At the end of each
/// step, the constructed suffix tree is either a valid suffix tree, or a
/// suffix tree with implicit suffixes. At the end of the final step, the
/// suffix tree is a valid tree.
/// \param EndIdx The end index of the current prefix in the main string.
/// \param SuffixesToAdd The number of suffixes that must be added
/// to complete the suffix tree at the current phase.
/// \returns The number of suffixes that have not been added at the end of
/// this step.
unsigned extend(unsigned EndIdx, unsigned SuffixesToAdd) {
SuffixTreeNode *NeedsLink = nullptr;
while (SuffixesToAdd > 0) {
// Are we waiting to add anything other than just the last character?
if (Active.Len == 0) {
// If not, then say the active index is the end index.
Active.Idx = EndIdx;
assert(Active.Idx <= EndIdx && "Start index can't be after end index!");
// The first character in the current substring we're looking at.
unsigned FirstChar = Str[Active.Idx];
// Have we inserted anything starting with FirstChar at the current node?
if (Active.Node->Children.count(FirstChar) == 0) {
// If not, then we can just insert a leaf and move too the next step.
insertLeaf(*Active.Node, EndIdx, FirstChar);
// The active node is an internal node, and we visited it, so it must
// need a link if it doesn't have one.
if (NeedsLink) {
NeedsLink->Link = Active.Node;
NeedsLink = nullptr;
} else {
// There's a match with FirstChar, so look for the point in the tree to
// insert a new node.
SuffixTreeNode *NextNode = Active.Node->Children[FirstChar];
unsigned SubstringLen = NextNode->size();
// Is the current suffix we're trying to insert longer than the size of
// the child we want to move to?
if (Active.Len >= SubstringLen) {
// If yes, then consume the characters we've seen and move to the next
// node.
Active.Idx += SubstringLen;
Active.Len -= SubstringLen;
Active.Node = NextNode;
// Otherwise, the suffix we're trying to insert must be contained in the
// next node we want to move to.
unsigned LastChar = Str[EndIdx];
// Is the string we're trying to insert a substring of the next node?
if (Str[NextNode->StartIdx + Active.Len] == LastChar) {
// If yes, then we're done for this step. Remember our insertion point
// and move to the next end index. At this point, we have an implicit
// suffix tree.
if (NeedsLink && !Active.Node->isRoot()) {
NeedsLink->Link = Active.Node;
NeedsLink = nullptr;
// The string we're trying to insert isn't a substring of the next node,
// but matches up to a point. Split the node.
// For example, say we ended our search at a node n and we're trying to
// insert ABD. Then we'll create a new node s for AB, reduce n to just
// representing C, and insert a new leaf node l to represent d. This
// allows us to ensure that if n was a leaf, it remains a leaf.
// | ABC ---split---> | AB
// n s
// C / \ D
// n l
// The node s from the diagram
SuffixTreeNode *SplitNode =
insertInternalNode(Active.Node, NextNode->StartIdx,
NextNode->StartIdx + Active.Len - 1, FirstChar);
// Insert the new node representing the new substring into the tree as
// a child of the split node. This is the node l from the diagram.
insertLeaf(*SplitNode, EndIdx, LastChar);
// Make the old node a child of the split node and update its start
// index. This is the node n from the diagram.
NextNode->StartIdx += Active.Len;
NextNode->Parent = SplitNode;
SplitNode->Children[Str[NextNode->StartIdx]] = NextNode;
// SplitNode is an internal node, update the suffix link.
if (NeedsLink)
NeedsLink->Link = SplitNode;
NeedsLink = SplitNode;
// We've added something new to the tree, so there's one less suffix to
// add.
if (Active.Node->isRoot()) {
if (Active.Len > 0) {
Active.Idx = EndIdx - SuffixesToAdd + 1;
} else {
// Start the next phase at the next smallest suffix.
Active.Node = Active.Node->Link;
return SuffixesToAdd;
/// Construct a suffix tree from a sequence of unsigned integers.
/// \param Str The string to construct the suffix tree for.
SuffixTree(const std::vector<unsigned> &Str) : Str(Str) {
Root = insertInternalNode(nullptr, EmptyIdx, EmptyIdx, 0);
Root->IsInTree = true;
Active.Node = Root;
LeafVector = std::vector<SuffixTreeNode *>(Str.size());
// Keep track of the number of suffixes we have to add of the current
// prefix.
unsigned SuffixesToAdd = 0;
Active.Node = Root;
// Construct the suffix tree iteratively on each prefix of the string.
// PfxEndIdx is the end index of the current prefix.
// End is one past the last element in the string.
for (unsigned PfxEndIdx = 0, End = Str.size(); PfxEndIdx < End;
PfxEndIdx++) {
LeafEndIdx = PfxEndIdx; // Extend each of the leaves.
SuffixesToAdd = extend(PfxEndIdx, SuffixesToAdd);
// Set the suffix indices of each leaf.
assert(Root && "Root node can't be nullptr!");
setSuffixIndices(*Root, 0);
/// \brief Maps \p MachineInstrs to unsigned integers and stores the mappings.
struct InstructionMapper {
/// \brief The next available integer to assign to a \p MachineInstr that
/// cannot be outlined.
/// Set to -3 for compatability with \p DenseMapInfo<unsigned>.
unsigned IllegalInstrNumber = -3;
/// \brief The next available integer to assign to a \p MachineInstr that can
/// be outlined.
unsigned LegalInstrNumber = 0;
/// Correspondence from \p MachineInstrs to unsigned integers.
DenseMap<MachineInstr *, unsigned, MachineInstrExpressionTrait>
/// Corresponcence from unsigned integers to \p MachineInstrs.
/// Inverse of \p InstructionIntegerMap.
DenseMap<unsigned, MachineInstr *> IntegerInstructionMap;
/// The vector of unsigned integers that the module is mapped to.
std::vector<unsigned> UnsignedVec;
/// \brief Stores the location of the instruction associated with the integer
/// at index i in \p UnsignedVec for each index i.
std::vector<MachineBasicBlock::iterator> InstrList;
/// \brief Maps \p *It to a legal integer.
/// Updates \p InstrList, \p UnsignedVec, \p InstructionIntegerMap,
/// \p IntegerInstructionMap, and \p LegalInstrNumber.
/// \returns The integer that \p *It was mapped to.
unsigned mapToLegalUnsigned(MachineBasicBlock::iterator &It) {
// Get the integer for this instruction or give it the current
// LegalInstrNumber.
MachineInstr &MI = *It;
bool WasInserted;
DenseMap<MachineInstr *, unsigned, MachineInstrExpressionTrait>::iterator
std::tie(ResultIt, WasInserted) =
InstructionIntegerMap.insert(std::make_pair(&MI, LegalInstrNumber));
unsigned MINumber = ResultIt->second;
// There was an insertion.
if (WasInserted) {
IntegerInstructionMap.insert(std::make_pair(MINumber, &MI));
// Make sure we don't overflow or use any integers reserved by the DenseMap.
if (LegalInstrNumber >= IllegalInstrNumber)
report_fatal_error("Instruction mapping overflow!");
assert(LegalInstrNumber != DenseMapInfo<unsigned>::getEmptyKey() &&
"Tried to assign DenseMap tombstone or empty key to instruction.");
assert(LegalInstrNumber != DenseMapInfo<unsigned>::getTombstoneKey() &&
"Tried to assign DenseMap tombstone or empty key to instruction.");
return MINumber;
/// Maps \p *It to an illegal integer.
/// Updates \p InstrList, \p UnsignedVec, and \p IllegalInstrNumber.
/// \returns The integer that \p *It was mapped to.
unsigned mapToIllegalUnsigned(MachineBasicBlock::iterator &It) {
unsigned MINumber = IllegalInstrNumber;
assert(LegalInstrNumber < IllegalInstrNumber &&
"Instruction mapping overflow!");
assert(IllegalInstrNumber != DenseMapInfo<unsigned>::getEmptyKey() &&
"IllegalInstrNumber cannot be DenseMap tombstone or empty key!");
assert(IllegalInstrNumber != DenseMapInfo<unsigned>::getTombstoneKey() &&
"IllegalInstrNumber cannot be DenseMap tombstone or empty key!");
return MINumber;
/// \brief Transforms a \p MachineBasicBlock into a \p vector of \p unsigneds
/// and appends it to \p UnsignedVec and \p InstrList.
/// Two instructions are assigned the same integer if they are identical.
/// If an instruction is deemed unsafe to outline, then it will be assigned an
/// unique integer. The resulting mapping is placed into a suffix tree and
/// queried for candidates.
/// \param MBB The \p MachineBasicBlock to be translated into integers.
/// \param TRI \p TargetRegisterInfo for the module.
/// \param TII \p TargetInstrInfo for the module.
void convertToUnsignedVec(MachineBasicBlock &MBB,
const TargetRegisterInfo &TRI,
const TargetInstrInfo &TII) {
for (MachineBasicBlock::iterator It = MBB.begin(), Et = MBB.end(); It != Et;
It++) {
// Keep track of where this instruction is in the module.
switch (TII.getOutliningType(*It)) {
case TargetInstrInfo::MachineOutlinerInstrType::Illegal:
case TargetInstrInfo::MachineOutlinerInstrType::Legal:
case TargetInstrInfo::MachineOutlinerInstrType::Invisible:
// After we're done every insertion, uniquely terminate this part of the
// "string". This makes sure we won't match across basic block or function
// boundaries since the "end" is encoded uniquely and thus appears in no
// repeated substring.
InstructionMapper() {
// Make sure that the implementation of DenseMapInfo<unsigned> hasn't
// changed.
assert(DenseMapInfo<unsigned>::getEmptyKey() == (unsigned)-1 &&
"DenseMapInfo<unsigned>'s empty key isn't -1!");
assert(DenseMapInfo<unsigned>::getTombstoneKey() == (unsigned)-2 &&
"DenseMapInfo<unsigned>'s tombstone key isn't -2!");
/// \brief An interprocedural pass which finds repeated sequences of
/// instructions and replaces them with calls to functions.
/// Each instruction is mapped to an unsigned integer and placed in a string.
/// The resulting mapping is then placed in a \p SuffixTree. The \p SuffixTree
/// is then repeatedly queried for repeated sequences of instructions. Each
/// non-overlapping repeated sequence is then placed in its own
/// \p MachineFunction and each instance is then replaced with a call to that
/// function.
struct MachineOutliner : public ModulePass {
static char ID;
/// \brief Set to true if the outliner should consider functions with
/// linkonceodr linkage.
bool OutlineFromLinkOnceODRs = false;
StringRef getPassName() const override { return "Machine Outliner"; }
void getAnalysisUsage(AnalysisUsage &AU) const override {
MachineOutliner(bool OutlineFromLinkOnceODRs = false)
: ModulePass(ID), OutlineFromLinkOnceODRs(OutlineFromLinkOnceODRs) {
/// Find all repeated substrings that satisfy the outlining cost model.
/// If a substring appears at least twice, then it must be represented by
/// an internal node which appears in at least two suffixes. Each suffix is
/// represented by a leaf node. To do this, we visit each internal node in
/// the tree, using the leaf children of each internal node. If an internal
/// node represents a beneficial substring, then we use each of its leaf
/// children to find the locations of its substring.
/// \param ST A suffix tree to query.
/// \param TII TargetInstrInfo for the target.
/// \param Mapper Contains outlining mapping information.
/// \param[out] CandidateList Filled with candidates representing each
/// beneficial substring.
/// \param[out] FunctionList Filled with a list of \p OutlinedFunctions each
/// type of candidate.
/// \returns The length of the longest candidate found.
findCandidates(SuffixTree &ST, const TargetInstrInfo &TII,
InstructionMapper &Mapper,
std::vector<std::shared_ptr<Candidate>> &CandidateList,
std::vector<OutlinedFunction> &FunctionList);
/// \brief Replace the sequences of instructions represented by the
/// \p Candidates in \p CandidateList with calls to \p MachineFunctions
/// described in \p FunctionList.
/// \param M The module we are outlining from.
/// \param CandidateList A list of candidates to be outlined.
/// \param FunctionList A list of functions to be inserted into the module.
/// \param Mapper Contains the instruction mappings for the module.
bool outline(Module &M,
const ArrayRef<std::shared_ptr<Candidate>> &CandidateList,
std::vector<OutlinedFunction> &FunctionList,
InstructionMapper &Mapper);
/// Creates a function for \p OF and inserts it into the module.
MachineFunction *createOutlinedFunction(Module &M, const OutlinedFunction &OF,
InstructionMapper &Mapper);
/// Find potential outlining candidates and store them in \p CandidateList.
/// For each type of potential candidate, also build an \p OutlinedFunction
/// struct containing the information to build the function for that
/// candidate.
/// \param[out] CandidateList Filled with outlining candidates for the module.
/// \param[out] FunctionList Filled with functions corresponding to each type
/// of \p Candidate.
/// \param ST The suffix tree for the module.
/// \param TII TargetInstrInfo for the module.
/// \returns The length of the longest candidate found. 0 if there are none.
buildCandidateList(std::vector<std::shared_ptr<Candidate>> &CandidateList,
std::vector<OutlinedFunction> &FunctionList,
SuffixTree &ST, InstructionMapper &Mapper,
const TargetInstrInfo &TII);
/// Helper function for pruneOverlaps.
/// Removes \p C from the candidate list, and updates its \p OutlinedFunction.
void prune(Candidate &C, std::vector<OutlinedFunction> &FunctionList);
/// \brief Remove any overlapping candidates that weren't handled by the
/// suffix tree's pruning method.
/// Pruning from the suffix tree doesn't necessarily remove all overlaps.
/// If a short candidate is chosen for outlining, then a longer candidate
/// which has that short candidate as a suffix is chosen, the tree's pruning
/// method will not find it. Thus, we need to prune before outlining as well.
/// \param[in,out] CandidateList A list of outlining candidates.
/// \param[in,out] FunctionList A list of functions to be outlined.
/// \param Mapper Contains instruction mapping info for outlining.
/// \param MaxCandidateLen The length of the longest candidate.
/// \param TII TargetInstrInfo for the module.
void pruneOverlaps(std::vector<std::shared_ptr<Candidate>> &CandidateList,
std::vector<OutlinedFunction> &FunctionList,
InstructionMapper &Mapper, unsigned MaxCandidateLen,
const TargetInstrInfo &TII);
/// Construct a suffix tree on the instructions in \p M and outline repeated
/// strings from that tree.
bool runOnModule(Module &M) override;
} // Anonymous namespace.
char MachineOutliner::ID = 0;
namespace llvm {
ModulePass *createMachineOutlinerPass(bool OutlineFromLinkOnceODRs) {
return new MachineOutliner(OutlineFromLinkOnceODRs);
} // namespace llvm
INITIALIZE_PASS(MachineOutliner, DEBUG_TYPE, "Machine Function Outliner", false,
unsigned MachineOutliner::findCandidates(
SuffixTree &ST, const TargetInstrInfo &TII, InstructionMapper &Mapper,
std::vector<std::shared_ptr<Candidate>> &CandidateList,
std::vector<OutlinedFunction> &FunctionList) {
unsigned MaxLen = 0;
// FIXME: Visit internal nodes instead of leaves.
for (SuffixTreeNode *Leaf : ST.LeafVector) {
assert(Leaf && "Leaves in LeafVector cannot be null!");
if (!Leaf->IsInTree)
assert(Leaf->Parent && "All leaves must have parents!");
SuffixTreeNode &Parent = *(Leaf->Parent);
// If it doesn't appear enough, or we already outlined from it, skip it.
if (Parent.OccurrenceCount < 2 || Parent.isRoot() || !Parent.IsInTree)
// Figure out if this candidate is beneficial.
unsigned StringLen = Leaf->ConcatLen - (unsigned)Leaf->size();
// Too short to be beneficial; skip it.
// FIXME: This isn't necessarily true for, say, X86. If we factor in
// instruction lengths we need more information than this.
if (StringLen < 2)
// If this is a beneficial class of candidate, then every one is stored in
// this vector.
std::vector<Candidate> CandidatesForRepeatedSeq;
// Describes the start and end point of each candidate. This allows the
// target to infer some information about each occurrence of each repeated
// sequence.
// FIXME: CandidatesForRepeatedSeq and this should be combined.
std::pair<MachineBasicBlock::iterator, MachineBasicBlock::iterator>>
// Figure out the call overhead for each instance of the sequence.
for (auto &ChildPair : Parent.Children) {
SuffixTreeNode *M = ChildPair.second;
if (M && M->IsInTree && M->isLeaf()) {
// Each sequence is over [StartIt, EndIt].
MachineBasicBlock::iterator StartIt = Mapper.InstrList[M->SuffixIdx];
MachineBasicBlock::iterator EndIt =
Mapper.InstrList[M->SuffixIdx + StringLen - 1];
CandidatesForRepeatedSeq.emplace_back(M->SuffixIdx, StringLen,
RepeatedSequenceLocs.emplace_back(std::make_pair(StartIt, EndIt));
// Never visit this leaf again.
M->IsInTree = false;
// We've found something we might want to outline.
// Create an OutlinedFunction to store it and check if it'd be beneficial
// to outline.
TargetInstrInfo::MachineOutlinerInfo MInfo =
std::vector<unsigned> Seq;
for (unsigned i = Leaf->SuffixIdx; i < Leaf->SuffixIdx + StringLen; i++)
OutlinedFunction OF(FunctionList.size(), Parent.OccurrenceCount, Seq,
unsigned Benefit = OF.getBenefit();
// Is it better to outline this candidate than not?
if (Benefit < 1) {
// Outlining this candidate would take more instructions than not
// outlining.
// Emit a remark explaining why we didn't outline this candidate.
std::pair<MachineBasicBlock::iterator, MachineBasicBlock::iterator> C =
MachineOptimizationRemarkEmitter MORE(
*(C.first->getParent()->getParent()), nullptr);
MORE.emit([&]() {
MachineOptimizationRemarkMissed R(DEBUG_TYPE, "NotOutliningCheaper",
R << "Did not outline " << NV("Length", StringLen) << " instructions"
<< " from " << NV("NumOccurrences", RepeatedSequenceLocs.size())
<< " locations."
<< " Instructions from outlining all occurrences ("
<< NV("OutliningCost", OF.getOutliningCost()) << ")"
<< " >= Unoutlined instruction count ("
<< NV("NotOutliningCost", StringLen * OF.getOccurrenceCount()) << ")"
<< " (Also found at: ";
// Tell the user the other places the candidate was found.
for (unsigned i = 1, e = RepeatedSequenceLocs.size(); i < e; i++) {
R << NV((Twine("OtherStartLoc") + Twine(i)).str(),
if (i != e - 1)
R << ", ";
R << ")";
return R;
// Move to the next candidate.
if (StringLen > MaxLen)
MaxLen = StringLen;
// At this point, the candidate class is seen as beneficial. Set their
// benefit values and save them in the candidate list.
std::vector<std::shared_ptr<Candidate>> CandidatesForFn;
for (Candidate &C : CandidatesForRepeatedSeq) {
C.Benefit = Benefit;
C.MInfo = MInfo;
std::shared_ptr<Candidate> Cptr = std::make_shared<Candidate>(C);
FunctionList.back().Candidates = CandidatesForFn;
// Move to the next function.
Parent.IsInTree = false;
return MaxLen;
// Remove C from the candidate space, and update its OutlinedFunction.
void MachineOutliner::prune(Candidate &C,
std::vector<OutlinedFunction> &FunctionList) {
// Get the OutlinedFunction associated with this Candidate.
OutlinedFunction &F = FunctionList[C.FunctionIdx];
// Update C's associated function's occurrence count.
// Remove C from the CandidateList.
C.InCandidateList = false;
DEBUG(dbgs() << "- Removed a Candidate \n";
dbgs() << "--- Num fns left for candidate: " << F.getOccurrenceCount()
<< "\n";
dbgs() << "--- Candidate's functions's benefit: " << F.getBenefit()
<< "\n";);
void MachineOutliner::pruneOverlaps(
std::vector<std::shared_ptr<Candidate>> &CandidateList,
std::vector<OutlinedFunction> &FunctionList, InstructionMapper &Mapper,
unsigned MaxCandidateLen, const TargetInstrInfo &TII) {
// Return true if this candidate became unbeneficial for outlining in a
// previous step.
auto ShouldSkipCandidate = [&FunctionList, this](Candidate &C) {
// Check if the candidate was removed in a previous step.
if (!C.InCandidateList)
return true;
// C must be alive. Check if we should remove it.
if (FunctionList[C.FunctionIdx].getBenefit() < 1) {
prune(C, FunctionList);
return true;
// C is in the list, and F is still beneficial.
return false;
// TODO: Experiment with interval trees or other interval-checking structures
// to lower the time complexity of this function.
// TODO: Can we do better than the simple greedy choice?
// Check for overlaps in the range.
// This is O(MaxCandidateLen * CandidateList.size()).
for (auto It = CandidateList.begin(), Et = CandidateList.end(); It != Et;
It++) {
Candidate &C1 = **It;
// If C1 was already pruned, or its function is no longer beneficial for
// outlining, move to the next candidate.
if (ShouldSkipCandidate(C1))
// The minimum start index of any candidate that could overlap with this
// one.
unsigned FarthestPossibleIdx = 0;
// Either the index is 0, or it's at most MaxCandidateLen indices away.
if (C1.getStartIdx() > MaxCandidateLen)
FarthestPossibleIdx = C1.getStartIdx() - MaxCandidateLen;
// Compare against the candidates in the list that start at at most
// FarthestPossibleIdx indices away from C1. There are at most
// MaxCandidateLen of these.
for (auto Sit = It + 1; Sit != Et; Sit++) {
Candidate &C2 = **Sit;
// Is this candidate too far away to overlap?
if (C2.getStartIdx() < FarthestPossibleIdx)
// If C2 was already pruned, or its function is no longer beneficial for
// outlining, move to the next candidate.
if (ShouldSkipCandidate(C2))
// Do C1 and C2 overlap?
// Not overlapping:
// High indices... [C1End ... C1Start][C2End ... C2Start] ...Low indices
// We sorted our candidate list so C2Start <= C1Start. We know that
// C2End > C2Start since each candidate has length >= 2. Therefore, all we
// have to check is C2End < C2Start to see if we overlap.
if (C2.getEndIdx() < C1.getStartIdx())
// C1 and C2 overlap.
// We need to choose the better of the two.
// Approximate this by picking the one which would have saved us the
// most instructions before any pruning.
// Is C2 a better candidate?
if (C2.Benefit > C1.Benefit) {
// Yes, so prune C1. Since C1 is dead, we don't have to compare it
// against anything anymore, so break.
prune(C1, FunctionList);
// Prune C2 and move on to the next candidate.
prune(C2, FunctionList);
unsigned MachineOutliner::buildCandidateList(
std::vector<std::shared_ptr<Candidate>> &CandidateList,
std::vector<OutlinedFunction> &FunctionList, SuffixTree &ST,
InstructionMapper &Mapper, const TargetInstrInfo &TII) {
std::vector<unsigned> CandidateSequence; // Current outlining candidate.
unsigned MaxCandidateLen = 0; // Length of the longest candidate.
MaxCandidateLen =
findCandidates(ST, TII, Mapper, CandidateList, FunctionList);
// Sort the candidates in decending order. This will simplify the outlining
// process when we have to remove the candidates from the mapping by
// allowing us to cut them out without keeping track of an offset.
CandidateList.begin(), CandidateList.end(),
[](const std::shared_ptr<Candidate> &LHS,
const std::shared_ptr<Candidate> &RHS) { return *LHS < *RHS; });
return MaxCandidateLen;
MachineFunction *
MachineOutliner::createOutlinedFunction(Module &M, const OutlinedFunction &OF,
InstructionMapper &Mapper) {
// Create the function name. This should be unique. For now, just hash the
// module name and include it in the function name plus the number of this
// function.
std::ostringstream NameStream;
NameStream << "OUTLINED_FUNCTION_" << OF.Name;
// Create the function using an IR-level function.
LLVMContext &C = M.getContext();
Function *F = dyn_cast<Function>(
M.getOrInsertFunction(NameStream.str(), Type::getVoidTy(C)));
assert(F && "Function was null!");
// NOTE: If this is linkonceodr, then we can take advantage of linker deduping
// which gives us better results when we outline from linkonceodr functions.
BasicBlock *EntryBB = BasicBlock::Create(C, "entry", F);
IRBuilder<> Builder(EntryBB);
MachineModuleInfo &MMI = getAnalysis<MachineModuleInfo>();
MachineFunction &MF = MMI.getOrCreateMachineFunction(*F);
MachineBasicBlock &MBB = *MF.CreateMachineBasicBlock();
const TargetSubtargetInfo &STI = MF.getSubtarget();
const TargetInstrInfo &TII = *STI.getInstrInfo();
// Insert the new function into the module.
MF.insert(MF.begin(), &MBB);
TII.insertOutlinerPrologue(MBB, MF, OF.MInfo);
// Copy over the instructions for the function using the integer mappings in
// its sequence.
for (unsigned Str : OF.Sequence) {
MachineInstr *NewMI =
// Don't keep debug information for outlined instructions.
// FIXME: This means outlined functions are currently undebuggable.
MBB.insert(MBB.end(), NewMI);
TII.insertOutlinerEpilogue(MBB, MF, OF.MInfo);
return &MF;
bool MachineOutliner::outline(
Module &M, const ArrayRef<std::shared_ptr<Candidate>> &CandidateList,
std::vector<OutlinedFunction> &FunctionList, InstructionMapper &Mapper) {
bool OutlinedSomething = false;
// Replace the candidates with calls to their respective outlined functions.
for (const std::shared_ptr<Candidate> &Cptr : CandidateList) {
Candidate &C = *Cptr;
// Was the candidate removed during pruneOverlaps?
if (!C.InCandidateList)
// If not, then look at its OutlinedFunction.
OutlinedFunction &OF = FunctionList[C.FunctionIdx];
// Was its OutlinedFunction made unbeneficial during pruneOverlaps?
if (OF.getBenefit() < 1)
// If not, then outline it.
assert(C.getStartIdx() < Mapper.InstrList.size() &&
"Candidate out of bounds!");
MachineBasicBlock *MBB = (*Mapper.InstrList[C.getStartIdx()]).getParent();
MachineBasicBlock::iterator StartIt = Mapper.InstrList[C.getStartIdx()];
unsigned EndIdx = C.getEndIdx();
assert(EndIdx < Mapper.InstrList.size() && "Candidate out of bounds!");
MachineBasicBlock::iterator EndIt = Mapper.InstrList[EndIdx];
assert(EndIt != MBB->end() && "EndIt out of bounds!");
EndIt++; // Erase needs one past the end index.
// Does this candidate have a function yet?
if (!OF.MF) {
OF.MF = createOutlinedFunction(M, OF, Mapper);
MachineBasicBlock *MBB = &*OF.MF->begin();
// Output a remark telling the user that an outlined function was created,
// and explaining where it came from.
MachineOptimizationRemarkEmitter MORE(*OF.MF, nullptr);
MachineOptimizationRemark R(DEBUG_TYPE, "OutlinedFunction",
MBB->findDebugLoc(MBB->begin()), MBB);
R << "Saved " << NV("OutliningBenefit", OF.getBenefit())
<< " instructions by "
<< "outlining " << NV("Length", OF.Sequence.size()) << " instructions "
<< "from " << NV("NumOccurrences", OF.getOccurrenceCount())
<< " locations. "
<< "(Found at: ";
// Tell the user the other places the candidate was found.
for (size_t i = 0, e = OF.Candidates.size(); i < e; i++) {
// Skip over things that were pruned.
if (!OF.Candidates[i]->InCandidateList)
R << NV(
(Twine("StartLoc") + Twine(i)).str(),
if (i != e - 1)
R << ", ";
R << ")";
MachineFunction *MF = OF.MF;
const TargetSubtargetInfo &STI = MF->getSubtarget();
const TargetInstrInfo &TII = *STI.getInstrInfo();
// Insert a call to the new function and erase the old sequence.
TII.insertOutlinedCall(M, *MBB, StartIt, *MF, C.MInfo);
StartIt = Mapper.InstrList[C.getStartIdx()];
MBB->erase(StartIt, EndIt);
OutlinedSomething = true;
// Statistics.
DEBUG(dbgs() << "OutlinedSomething = " << OutlinedSomething << "\n";);
return OutlinedSomething;
bool MachineOutliner::runOnModule(Module &M) {
// Is there anything in the module at all?
if (M.empty())
return false;
MachineModuleInfo &MMI = getAnalysis<MachineModuleInfo>();
const TargetSubtargetInfo &STI =
const TargetRegisterInfo *TRI = STI.getRegisterInfo();
const TargetInstrInfo *TII = STI.getInstrInfo();
InstructionMapper Mapper;
// Build instruction mappings for each function in the module.
for (Function &F : M) {
MachineFunction &MF = MMI.getOrCreateMachineFunction(F);
// Is the function empty? Safe to outline from?
if (F.empty() ||
!TII->isFunctionSafeToOutlineFrom(MF, OutlineFromLinkOnceODRs))
// If it is, look at each MachineBasicBlock in the function.
for (MachineBasicBlock &MBB : MF) {
// Is there anything in MBB?
if (MBB.empty())
// If yes, map it.
Mapper.convertToUnsignedVec(MBB, *TRI, *TII);
// Construct a suffix tree, use it to find candidates, and then outline them.
SuffixTree ST(Mapper.UnsignedVec);
std::vector<std::shared_ptr<Candidate>> CandidateList;
std::vector<OutlinedFunction> FunctionList;
// Find all of the outlining candidates.
unsigned MaxCandidateLen =
buildCandidateList(CandidateList, FunctionList, ST, Mapper, *TII);
// Remove candidates that overlap with other candidates.
pruneOverlaps(CandidateList, FunctionList, Mapper, MaxCandidateLen, *TII);
// Outline each of the candidates and return true if something was outlined.
return outline(M, CandidateList, FunctionList, Mapper);