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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
/// * buildOutlinedFrame
/// * insertOutlinedCall
/// * 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/CodeGen/MachineOutliner.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/Twine.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineOptimizationRemarkEmitter.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/IR/DIBuilder.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Mangler.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <functional>
#include <map>
#include <sstream>
#include <tuple>
#include <vector>
#define DEBUG_TYPE "machine-outliner"
using namespace llvm;
using namespace ore;
using namespace outliner;
STATISTIC(NumOutlined, "Number of candidates outlined");
STATISTIC(FunctionsCreated, "Number of functions created");
// Set to true if the user wants the outliner to run on linkonceodr linkage
// functions. This is false by default because the linker can dedupe linkonceodr
// functions. Since the outliner is confined to a single module (modulo LTO),
// this is off by default. It should, however, be the default behaviour in
// LTO.
static cl::opt<bool> EnableLinkOnceODROutlining(
cl::desc("Enable the machine outliner on linkonceodr functions"),
namespace {
/// 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;
/// 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;
/// 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;
/// 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;
/// 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;
/// 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);
/// Maps \p MachineInstrs to unsigned integers and stores the mappings.
struct InstructionMapper {
/// 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;
/// 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;
/// 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;
/// 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;
/// 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 TII \p TargetInstrInfo for the function.
void convertToUnsignedVec(MachineBasicBlock &MBB,
const TargetInstrInfo &TII) {
unsigned Flags = TII.getMachineOutlinerMBBFlags(MBB);
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, Flags)) {
case InstrType::Illegal:
case InstrType::Legal:
case InstrType::LegalTerminator:
case InstrType::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!");
/// 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;
/// Set to true if the outliner should consider functions with
/// linkonceodr linkage.
bool OutlineFromLinkOnceODRs = false;
/// Set to true if the outliner should run on all functions in the module
/// considered safe for outlining.
/// Set to true by default for compatibility with llc's -run-pass option.
/// Set when the pass is constructed in TargetPassConfig.
bool RunOnAllFunctions = true;
// Collection of IR functions created by the outliner.
std::vector<Function *> CreatedIRFunctions;
StringRef getPassName() const override { return "Machine Outliner"; }
void getAnalysisUsage(AnalysisUsage &AU) const override {
MachineOutliner() : ModulePass(ID) {
/// Remark output explaining that not outlining a set of candidates would be
/// better than outlining that set.
void emitNotOutliningCheaperRemark(
unsigned StringLen, std::vector<Candidate> &CandidatesForRepeatedSeq,
OutlinedFunction &OF);
/// Remark output explaining that a function was outlined.
void emitOutlinedFunctionRemark(OutlinedFunction &OF);
/// 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 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,
InstructionMapper &Mapper,
std::vector<std::shared_ptr<Candidate>> &CandidateList,
std::vector<OutlinedFunction> &FunctionList);
/// 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.
/// \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);
/// Helper function for pruneOverlaps.
/// Removes \p C from the candidate list, and updates its \p OutlinedFunction.
void prune(Candidate &C, std::vector<OutlinedFunction> &FunctionList);
/// 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.
void pruneOverlaps(std::vector<std::shared_ptr<Candidate>> &CandidateList,
std::vector<OutlinedFunction> &FunctionList,
InstructionMapper &Mapper, unsigned MaxCandidateLen);
/// Construct a suffix tree on the instructions in \p M and outline repeated
/// strings from that tree.
bool runOnModule(Module &M) override;
/// Return a DISubprogram for OF if one exists, and null otherwise. Helper
/// function for remark emission.
DISubprogram *getSubprogramOrNull(const OutlinedFunction &OF) {
DISubprogram *SP;
for (const std::shared_ptr<Candidate> &C : OF.Candidates)
if (C && C->getMF() && (SP = C->getMF()->getFunction().getSubprogram()))
return SP;
return nullptr;
} // Anonymous namespace.
char MachineOutliner::ID = 0;
namespace llvm {
ModulePass *createMachineOutlinerPass(bool RunOnAllFunctions) {
MachineOutliner *OL = new MachineOutliner();
OL->RunOnAllFunctions = RunOnAllFunctions;
return OL;
} // namespace llvm
INITIALIZE_PASS(MachineOutliner, DEBUG_TYPE, "Machine Function Outliner", false,
void MachineOutliner::emitNotOutliningCheaperRemark(
unsigned StringLen, std::vector<Candidate> &CandidatesForRepeatedSeq,
OutlinedFunction &OF) {
Candidate &C = CandidatesForRepeatedSeq.front();
MachineOptimizationRemarkEmitter MORE(*(C.getMF()), nullptr);
MORE.emit([&]() {
MachineOptimizationRemarkMissed R(DEBUG_TYPE, "NotOutliningCheaper",
C.front()->getDebugLoc(), C.getMBB());
R << "Did not outline " << NV("Length", StringLen) << " instructions"
<< " from " << NV("NumOccurrences", CandidatesForRepeatedSeq.size())
<< " locations."
<< " Bytes from outlining all occurrences ("
<< NV("OutliningCost", OF.getOutliningCost()) << ")"
<< " >= Unoutlined instruction bytes ("
<< NV("NotOutliningCost", OF.getNotOutlinedCost()) << ")"
<< " (Also found at: ";
// Tell the user the other places the candidate was found.
for (unsigned i = 1, e = CandidatesForRepeatedSeq.size(); i < e; i++) {
R << NV((Twine("OtherStartLoc") + Twine(i)).str(),
if (i != e - 1)
R << ", ";
R << ")";
return R;
void MachineOutliner::emitOutlinedFunctionRemark(OutlinedFunction &OF) {
MachineBasicBlock *MBB = &*OF.MF->begin();
MachineOptimizationRemarkEmitter MORE(*OF.MF, nullptr);
MachineOptimizationRemark R(DEBUG_TYPE, "OutlinedFunction",
MBB->findDebugLoc(MBB->begin()), MBB);
R << "Saved " << NV("OutliningBenefit", OF.getBenefit()) << " bytes 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 << ")";
unsigned MachineOutliner::findCandidates(
SuffixTree &ST, 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;
// 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()) {
// Never visit this leaf again.
M->IsInTree = false;
unsigned StartIdx = M->SuffixIdx;
unsigned EndIdx = StartIdx + StringLen - 1;
// Trick: Discard some candidates that would be incompatible with the
// ones we've already found for this sequence. This will save us some
// work in candidate selection.
// If two candidates overlap, then we can't outline them both. This
// happens when we have candidates that look like, say
// AA (where each "A" is an instruction).
// We might have some portion of the module that looks like this:
// AAAAAA (6 A's)
// In this case, there are 5 different copies of "AA" in this range, but
// at most 3 can be outlined. If only outlining 3 of these is going to
// be unbeneficial, then we ought to not bother.
// Note that two things DON'T overlap when they look like this:
// start1...end1 .... start2...end2
// That is, one must either
// * End before the other starts
// * Start after the other ends
if (std::all_of(CandidatesForRepeatedSeq.begin(),
[&StartIdx, &EndIdx](const Candidate &C) {
return (EndIdx < C.getStartIdx() ||
StartIdx > C.getEndIdx());
})) {
// It doesn't overlap with anything, so we can outline it.
// Each sequence is over [StartIt, EndIt].
// Save the candidate and its location.
MachineBasicBlock::iterator StartIt = Mapper.InstrList[StartIdx];
MachineBasicBlock::iterator EndIt = Mapper.InstrList[EndIdx];
CandidatesForRepeatedSeq.emplace_back(StartIdx, StringLen, StartIt,
EndIt, StartIt->getParent(),
// We've found something we might want to outline.
// Create an OutlinedFunction to store it and check if it'd be beneficial
// to outline.
if (CandidatesForRepeatedSeq.empty())
// Arbitrarily choose a TII from the first candidate.
// FIXME: Should getOutliningCandidateInfo move to TargetMachine?
const TargetInstrInfo *TII =
OutlinedFunction OF =
// If we deleted every candidate, then there's nothing to outline.
if (OF.Candidates.empty())
std::vector<unsigned> Seq;
for (unsigned i = Leaf->SuffixIdx; i < Leaf->SuffixIdx + StringLen; i++)
OF.Sequence = Seq;
OF.Name = FunctionList.size();
// Is it better to outline this candidate than not?
if (OF.getBenefit() < 1) {
emitNotOutliningCheaperRemark(StringLen, CandidatesForRepeatedSeq, OF);
if (StringLen > MaxLen)
MaxLen = StringLen;
// The function is beneficial. Save its candidates to the candidate list
// for pruning.
for (std::shared_ptr<Candidate> &C : OF.Candidates)
// 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;
LLVM_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) {
// 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 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) {
std::vector<unsigned> CandidateSequence; // Current outlining candidate.
unsigned MaxCandidateLen = 0; // Length of the longest candidate.
MaxCandidateLen =
findCandidates(ST, 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.
// FIXME: Set nounwind, so we don't generate eh_frame? Haven't verified it's
// necessary.
// Set optsize/minsize, so we don't insert padding between outlined
// functions.
// Save F so that we can add debug info later if we need to.
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);
// 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.
MBB.insert(MBB.end(), NewMI);
TII.buildOutlinedFrame(MBB, MF, OF);
// If there's a DISubprogram associated with this outlined function, then
// emit debug info for the outlined function.
if (DISubprogram *SP = getSubprogramOrNull(OF)) {
// We have a DISubprogram. Get its DICompileUnit.
DICompileUnit *CU = SP->getUnit();
DIBuilder DB(M, true, CU);
DIFile *Unit = SP->getFile();
Mangler Mg;
// Walk over each IR function we created in the outliner and create
// DISubprograms for each function.
for (Function *F : CreatedIRFunctions) {
// Get the mangled name of the function for the linkage name.
std::string Dummy;
llvm::raw_string_ostream MangledNameStream(Dummy);
Mg.getNameWithPrefix(MangledNameStream, F, false);
DISubprogram *SP = DB.createFunction(
Unit /* Context */, F->getName(), StringRef(MangledNameStream.str()),
Unit /* File */,
0 /* Line 0 is reserved for compiler-generated code. */,
DB.getOrCreateTypeArray(None)), /* void type */
false, true, 0, /* Line 0 is reserved for compiler-generated code. */
DINode::DIFlags::FlagArtificial /* Compiler-generated code. */,
true /* Outlined code is optimized code by definition. */);
// Don't add any new variables to the subprogram.
// Attach subprogram to the function.
// We're done with the DIBuilder.
// Outlined functions shouldn't preserve liveness.
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)
// Does this candidate have a function yet?
if (!OF.MF) {
OF.MF = createOutlinedFunction(M, OF, Mapper);
MachineFunction *MF = OF.MF;
MachineBasicBlock &MBB = *C.getMBB();
MachineBasicBlock::iterator StartIt = C.front();
MachineBasicBlock::iterator EndIt = C.back();
assert(StartIt != C.getMBB()->end() && "StartIt out of bounds!");
assert(EndIt != C.getMBB()->end() && "EndIt out of bounds!");
const TargetSubtargetInfo &STI = MF->getSubtarget();
const TargetInstrInfo &TII = *STI.getInstrInfo();
// Insert a call to the new function and erase the old sequence.
auto CallInst = TII.insertOutlinedCall(M, MBB, StartIt, *OF.MF, C);
// If the caller tracks liveness, then we need to make sure that anything
// we outline doesn't break liveness assumptions.
// The outlined functions themselves currently don't track liveness, but
// we should make sure that the ranges we yank things out of aren't
// wrong.
if (MBB.getParent()->getProperties().hasProperty(
MachineFunctionProperties::Property::TracksLiveness)) {
// Helper lambda for adding implicit def operands to the call instruction.
auto CopyDefs = [&CallInst](MachineInstr &MI) {
for (MachineOperand &MOP : MI.operands()) {
// Skip over anything that isn't a register.
if (!MOP.isReg())
// If it's a def, add it to the call instruction.
if (MOP.isDef())
MachineOperand::CreateReg(MOP.getReg(), true, /* isDef = true */
true /* isImp = true */));
// Copy over the defs in the outlined range.
// First inst in outlined range <-- Anything that's defined in this
// ... .. range has to be added as an implicit
// Last inst in outlined range <-- def to the call instruction.
std::for_each(CallInst, std::next(EndIt), CopyDefs);
// Erase from the point after where the call was inserted up to, and
// including, the final instruction in the sequence.
// Erase needs one past the end, so we need std::next there too.
MBB.erase(std::next(StartIt), std::next(EndIt));
OutlinedSomething = true;
// Statistics.
LLVM_DEBUG(dbgs() << "OutlinedSomething = " << OutlinedSomething << "\n";);
return OutlinedSomething;
bool MachineOutliner::runOnModule(Module &M) {
// Check if there's anything in the module. If it's empty, then there's
// nothing to outline.
if (M.empty())
return false;
MachineModuleInfo &MMI = getAnalysis<MachineModuleInfo>();
// If the user passed -enable-machine-outliner=always or
// -enable-machine-outliner, the pass will run on all functions in the module.
// Otherwise, if the target supports default outlining, it will run on all
// functions deemed by the target to be worth outlining from by default. Tell
// the user how the outliner is running.
dbgs() << "Machine Outliner: Running on ";
if (RunOnAllFunctions)
dbgs() << "all functions";
dbgs() << "target-default functions";
dbgs() << "\n"
// If the user specifies that they want to outline from linkonceodrs, set
// it here.
OutlineFromLinkOnceODRs = EnableLinkOnceODROutlining;
InstructionMapper Mapper;
// Build instruction mappings for each function in the module. Start by
// iterating over each Function in M.
for (Function &F : M) {
// If there's nothing in F, then there's no reason to try and outline from
// it.
if (F.empty())
// There's something in F. Check if it has a MachineFunction associated with
// it.
MachineFunction *MF = MMI.getMachineFunction(F);
// If it doesn't, then there's nothing to outline from. Move to the next
// Function.
if (!MF)
const TargetInstrInfo *TII = MF->getSubtarget().getInstrInfo();
if (!RunOnAllFunctions && !TII->shouldOutlineFromFunctionByDefault(*MF))
// We have a MachineFunction. Ask the target if it's suitable for outlining.
// If it isn't, then move on to the next Function in the module.
if (!TII->isFunctionSafeToOutlineFrom(*MF, OutlineFromLinkOnceODRs))
// We have a function suitable for outlining. Iterate over every
// MachineBasicBlock in MF and try to map its instructions to a list of
// unsigned integers.
for (MachineBasicBlock &MBB : *MF) {
// If there isn't anything in MBB, then there's no point in outlining from
// it.
if (MBB.empty())
// Check if MBB could be the target of an indirect branch. If it is, then
// we don't want to outline from it.
if (MBB.hasAddressTaken())
// MBB is suitable for outlining. Map it to a list of unsigneds.
Mapper.convertToUnsignedVec(MBB, *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);
// Remove candidates that overlap with other candidates.
pruneOverlaps(CandidateList, FunctionList, Mapper, MaxCandidateLen);
// Outline each of the candidates and return true if something was outlined.
bool OutlinedSomething = outline(M, CandidateList, FunctionList, Mapper);
return OutlinedSomething;