blob: dc2f5ef15206e88dbab7ab6a9dc62a1ba6f0b901 [file] [log] [blame]
//===---- MachineOutliner.cpp - Outline instructions -----------*- C++ -*-===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
/// \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/SmallSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/Twine.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/CodeGen/LivePhysRegs.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineOptimizationRemarkEmitter.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/InitializePasses.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/SuffixTree.h"
#include "llvm/Support/raw_ostream.h"
#include <functional>
#include <tuple>
#include <vector>
#define DEBUG_TYPE "machine-outliner"
using namespace llvm;
using namespace ore;
using namespace outliner;
// Statistics for outlined functions.
STATISTIC(NumOutlined, "Number of candidates outlined");
STATISTIC(FunctionsCreated, "Number of functions created");
// Statistics for instruction mapping.
STATISTIC(NumLegalInUnsignedVec, "Outlinable instructions mapped");
"Unoutlinable instructions mapped + number of sentinel values");
STATISTIC(NumSentinels, "Sentinel values inserted during mapping");
"Invisible instructions skipped during mapping");
"Total number of instructions mapped and saved to mapping vector");
// 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(
"enable-linkonceodr-outlining", cl::Hidden,
cl::desc("Enable the machine outliner on linkonceodr functions"),
/// Number of times to re-run the outliner. This is not the total number of runs
/// as the outliner will run at least one time. The default value is set to 0,
/// meaning the outliner will run one time and rerun zero times after that.
static cl::opt<unsigned> OutlinerReruns(
"machine-outliner-reruns", cl::init(0), cl::Hidden,
"Number of times to rerun the outliner after the initial outline"));
static cl::opt<unsigned> OutlinerBenefitThreshold(
"outliner-benefit-threshold", cl::init(1), cl::Hidden,
"The minimum size in bytes before an outlining candidate is accepted"));
namespace {
/// 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>
/// Correspondence between \p MachineBasicBlocks and target-defined flags.
DenseMap<MachineBasicBlock *, unsigned> MBBFlagsMap;
/// The vector of unsigned integers that the module is mapped to.
SmallVector<unsigned> UnsignedVec;
/// Stores the location of the instruction associated with the integer
/// at index i in \p UnsignedVec for each index i.
SmallVector<MachineBasicBlock::iterator> InstrList;
// Set if we added an illegal number in the previous step.
// Since each illegal number is unique, we only need one of them between
// each range of legal numbers. This lets us make sure we don't add more
// than one illegal number per range.
bool AddedIllegalLastTime = false;
/// Maps \p *It to a legal integer.
/// Updates \p CanOutlineWithPrevInstr, \p HaveLegalRange, \p InstrListForMBB,
/// \p UnsignedVecForMBB, \p InstructionIntegerMap, and \p LegalInstrNumber.
/// \returns The integer that \p *It was mapped to.
unsigned mapToLegalUnsigned(
MachineBasicBlock::iterator &It, bool &CanOutlineWithPrevInstr,
bool &HaveLegalRange, unsigned &NumLegalInBlock,
SmallVector<unsigned> &UnsignedVecForMBB,
SmallVector<MachineBasicBlock::iterator> &InstrListForMBB) {
// We added something legal, so we should unset the AddedLegalLastTime
// flag.
AddedIllegalLastTime = false;
// If we have at least two adjacent legal instructions (which may have
// invisible instructions in between), remember that.
if (CanOutlineWithPrevInstr)
HaveLegalRange = true;
CanOutlineWithPrevInstr = true;
// Keep track of the number of legal instructions we insert.
// 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)
// 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.");
// Statistics.
return MINumber;
/// Maps \p *It to an illegal integer.
/// Updates \p InstrListForMBB, \p UnsignedVecForMBB, and \p
/// IllegalInstrNumber.
/// \returns The integer that \p *It was mapped to.
unsigned mapToIllegalUnsigned(
MachineBasicBlock::iterator &It, bool &CanOutlineWithPrevInstr,
SmallVector<unsigned> &UnsignedVecForMBB,
SmallVector<MachineBasicBlock::iterator> &InstrListForMBB) {
// Can't outline an illegal instruction. Set the flag.
CanOutlineWithPrevInstr = false;
// Only add one illegal number per range of legal numbers.
if (AddedIllegalLastTime)
return IllegalInstrNumber;
// Remember that we added an illegal number last time.
AddedIllegalLastTime = true;
unsigned MINumber = IllegalInstrNumber;
// Statistics.
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) {
LLVM_DEBUG(dbgs() << "*** Converting MBB '" << MBB.getName()
<< "' to unsigned vector ***\n");
unsigned Flags = 0;
// Don't even map in this case.
if (!TII.isMBBSafeToOutlineFrom(MBB, Flags))
auto OutlinableRanges = TII.getOutlinableRanges(MBB, Flags);
LLVM_DEBUG(dbgs() << MBB.getName() << ": " << OutlinableRanges.size()
<< " outlinable range(s)\n");
if (OutlinableRanges.empty())
// Store info for the MBB for later outlining.
MBBFlagsMap[&MBB] = Flags;
MachineBasicBlock::iterator It = MBB.begin();
// The number of instructions in this block that will be considered for
// outlining.
unsigned NumLegalInBlock = 0;
// True if we have at least two legal instructions which aren't separated
// by an illegal instruction.
bool HaveLegalRange = false;
// True if we can perform outlining given the last mapped (non-invisible)
// instruction. This lets us know if we have a legal range.
bool CanOutlineWithPrevInstr = false;
// FIXME: Should this all just be handled in the target, rather than using
// repeated calls to getOutliningType?
SmallVector<unsigned> UnsignedVecForMBB;
SmallVector<MachineBasicBlock::iterator> InstrListForMBB;
LLVM_DEBUG(dbgs() << "*** Mapping outlinable ranges ***\n");
for (auto &OutlinableRange : OutlinableRanges) {
auto OutlinableRangeBegin = OutlinableRange.first;
auto OutlinableRangeEnd = OutlinableRange.second;
#ifndef NDEBUG
dbgs() << "Mapping "
<< std::distance(OutlinableRangeBegin, OutlinableRangeEnd)
<< " instruction range\n");
// Everything outside of an outlinable range is illegal.
unsigned NumSkippedInRange = 0;
for (; It != OutlinableRangeBegin; ++It) {
#ifndef NDEBUG
mapToIllegalUnsigned(It, CanOutlineWithPrevInstr, UnsignedVecForMBB,
#ifndef NDEBUG
LLVM_DEBUG(dbgs() << "Skipped " << NumSkippedInRange
<< " instructions outside outlinable range\n");
assert(It != MBB.end() && "Should still have instructions?");
// `It` is now positioned at the beginning of a range of instructions
// which may be outlinable. Check if each instruction is known to be safe.
for (; It != OutlinableRangeEnd; ++It) {
// Keep track of where this instruction is in the module.
switch (TII.getOutliningType(It, Flags)) {
case InstrType::Illegal:
mapToIllegalUnsigned(It, CanOutlineWithPrevInstr, UnsignedVecForMBB,
case InstrType::Legal:
mapToLegalUnsigned(It, CanOutlineWithPrevInstr, HaveLegalRange,
NumLegalInBlock, UnsignedVecForMBB,
case InstrType::LegalTerminator:
mapToLegalUnsigned(It, CanOutlineWithPrevInstr, HaveLegalRange,
NumLegalInBlock, UnsignedVecForMBB,
// The instruction also acts as a terminator, so we have to record
// that in the string.
mapToIllegalUnsigned(It, CanOutlineWithPrevInstr, UnsignedVecForMBB,
case InstrType::Invisible:
// Normally this is set by mapTo(Blah)Unsigned, but we just want to
// skip this instruction. So, unset the flag here.
AddedIllegalLastTime = false;
LLVM_DEBUG(dbgs() << "HaveLegalRange = " << HaveLegalRange << "\n");
// Are there enough legal instructions in the block for outlining to be
// possible?
if (HaveLegalRange) {
// 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.
mapToIllegalUnsigned(It, CanOutlineWithPrevInstr, UnsignedVecForMBB,
append_range(InstrList, InstrListForMBB);
append_range(UnsignedVec, UnsignedVecForMBB);
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;
/// The current repeat number of machine outlining.
unsigned OutlineRepeatedNum = 0;
/// 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;
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 by
/// constructing a suffix tree.
/// 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 Mapper Contains outlining mapping information.
/// \param[out] FunctionList Filled with a list of \p OutlinedFunctions
/// each type of candidate.
void findCandidates(InstructionMapper &Mapper,
std::vector<OutlinedFunction> &FunctionList);
/// Replace the sequences of instructions represented by \p OutlinedFunctions
/// with calls to functions.
/// \param M The module we are outlining from.
/// \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, std::vector<OutlinedFunction> &FunctionList,
InstructionMapper &Mapper, unsigned &OutlinedFunctionNum);
/// Creates a function for \p OF and inserts it into the module.
MachineFunction *createOutlinedFunction(Module &M, OutlinedFunction &OF,
InstructionMapper &Mapper,
unsigned Name);
/// Calls 'doOutline()' 1 + OutlinerReruns times.
bool runOnModule(Module &M) override;
/// Construct a suffix tree on the instructions in \p M and outline repeated
/// strings from that tree.
bool doOutline(Module &M, unsigned &OutlinedFunctionNum);
/// Return a DISubprogram for OF if one exists, and null otherwise. Helper
/// function for remark emission.
DISubprogram *getSubprogramOrNull(const OutlinedFunction &OF) {
for (const Candidate &C : OF.Candidates)
if (MachineFunction *MF = C.getMF())
if (DISubprogram *SP = MF->getFunction().getSubprogram())
return SP;
return nullptr;
/// Populate and \p InstructionMapper with instruction-to-integer mappings.
/// These are used to construct a suffix tree.
void populateMapper(InstructionMapper &Mapper, Module &M,
MachineModuleInfo &MMI);
/// Initialize information necessary to output a size remark.
/// FIXME: This should be handled by the pass manager, not the outliner.
/// FIXME: This is nearly identical to the initSizeRemarkInfo in the legacy
/// pass manager.
void initSizeRemarkInfo(const Module &M, const MachineModuleInfo &MMI,
StringMap<unsigned> &FunctionToInstrCount);
/// Emit the remark.
// FIXME: This should be handled by the pass manager, not the outliner.
emitInstrCountChangedRemark(const Module &M, const MachineModuleInfo &MMI,
const StringMap<unsigned> &FunctionToInstrCount);
} // 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) {
// FIXME: Right now, we arbitrarily choose some Candidate from the
// OutlinedFunction. This isn't necessarily fixed, nor does it have to be.
// We should probably sort these by function name or something to make sure
// the remarks are stable.
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.getNumInstrs()) << " 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++) {
R << NV((Twine("StartLoc") + Twine(i)).str(),
if (i != e - 1)
R << ", ";
R << ")";
void MachineOutliner::findCandidates(
InstructionMapper &Mapper, std::vector<OutlinedFunction> &FunctionList) {
SuffixTree ST(Mapper.UnsignedVec);
// First, find all of the repeated substrings in the tree of minimum length
// 2.
std::vector<Candidate> CandidatesForRepeatedSeq;
LLVM_DEBUG(dbgs() << "*** Discarding overlapping candidates *** \n");
dbgs() << "Searching for overlaps in all repeated sequences...\n");
for (const SuffixTree::RepeatedSubstring &RS : ST) {
unsigned StringLen = RS.Length;
LLVM_DEBUG(dbgs() << " Sequence length: " << StringLen << "\n");
// Debug code to keep track of how many candidates we removed.
#ifndef NDEBUG
unsigned NumDiscarded = 0;
unsigned NumKept = 0;
for (const unsigned &StartIdx : RS.StartIndices) {
// 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
unsigned EndIdx = StartIdx + StringLen - 1;
auto FirstOverlap = find_if(
CandidatesForRepeatedSeq, [StartIdx, EndIdx](const Candidate &C) {
return EndIdx >= C.getStartIdx() && StartIdx <= C.getEndIdx();
if (FirstOverlap != CandidatesForRepeatedSeq.end()) {
#ifndef NDEBUG
LLVM_DEBUG(dbgs() << " .. DISCARD candidate @ [" << StartIdx
<< ", " << EndIdx << "]; overlaps with candidate @ ["
<< FirstOverlap->getStartIdx() << ", "
<< FirstOverlap->getEndIdx() << "]\n");
// It doesn't overlap with anything, so we can outline it.
// Each sequence is over [StartIt, EndIt].
// Save the candidate and its location.
#ifndef NDEBUG
MachineBasicBlock::iterator StartIt = Mapper.InstrList[StartIdx];
MachineBasicBlock::iterator EndIt = Mapper.InstrList[EndIdx];
MachineBasicBlock *MBB = StartIt->getParent();
CandidatesForRepeatedSeq.emplace_back(StartIdx, StringLen, StartIt, EndIt,
MBB, FunctionList.size(),
#ifndef NDEBUG
LLVM_DEBUG(dbgs() << " Candidates discarded: " << NumDiscarded
<< "\n");
LLVM_DEBUG(dbgs() << " Candidates kept: " << NumKept << "\n\n");
// 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.size() < 2)
// Arbitrarily choose a TII from the first candidate.
// FIXME: Should getOutliningCandidateInfo move to TargetMachine?
const TargetInstrInfo *TII =
std::optional<OutlinedFunction> OF =
// If we deleted too many candidates, then there's nothing worth outlining.
// FIXME: This should take target-specified instruction sizes into account.
if (!OF || OF->Candidates.size() < 2)
// Is it better to outline this candidate than not?
if (OF->getBenefit() < OutlinerBenefitThreshold) {
emitNotOutliningCheaperRemark(StringLen, CandidatesForRepeatedSeq, *OF);
MachineFunction *MachineOutliner::createOutlinedFunction(
Module &M, OutlinedFunction &OF, InstructionMapper &Mapper, unsigned Name) {
// Create the function name. This should be unique.
// FIXME: We should have a better naming scheme. This should be stable,
// regardless of changes to the outliner's cost model/traversal order.
std::string FunctionName = "OUTLINED_FUNCTION_";
if (OutlineRepeatedNum > 0)
FunctionName += std::to_string(OutlineRepeatedNum + 1) + "_";
FunctionName += std::to_string(Name);
LLVM_DEBUG(dbgs() << "NEW FUNCTION: " << FunctionName << "\n");
// Create the function using an IR-level function.
LLVMContext &C = M.getContext();
Function *F = Function::Create(FunctionType::get(Type::getVoidTy(C), false),
Function::ExternalLinkage, FunctionName, M);
// NOTE: If this is linkonceodr, then we can take advantage of linker deduping
// which gives us better results when we outline from linkonceodr functions.
// Set optsize/minsize, so we don't insert padding between outlined
// functions.
Candidate &FirstCand = OF.Candidates.front();
const TargetInstrInfo &TII =
TII.mergeOutliningCandidateAttributes(*F, OF.Candidates);
// Set uwtable, so we generate eh_frame.
UWTableKind UW = std::accumulate(
OF.Candidates.cbegin(), OF.Candidates.cend(), UWTableKind::None,
[](UWTableKind K, const outliner::Candidate &C) {
return std::max(K, C.getMF()->getFunction().getUWTableKind());
if (UW != UWTableKind::None)
BasicBlock *EntryBB = BasicBlock::Create(C, "entry", F);
IRBuilder<> Builder(EntryBB);
MachineModuleInfo &MMI = getAnalysis<MachineModuleInfoWrapperPass>().getMMI();
MachineFunction &MF = MMI.getOrCreateMachineFunction(*F);
MachineBasicBlock &MBB = *MF.CreateMachineBasicBlock();
// Insert the new function into the module.
MF.insert(MF.begin(), &MBB);
MachineFunction *OriginalMF = FirstCand.front().getMF();
const std::vector<MCCFIInstruction> &Instrs =
for (auto &MI : FirstCand) {
if (MI.isDebugInstr())
// Don't keep debug information for outlined instructions.
auto DL = DebugLoc();
if (MI.isCFIInstruction()) {
unsigned CFIIndex = MI.getOperand(0).getCFIIndex();
MCCFIInstruction CFI = Instrs[CFIIndex];
BuildMI(MBB, MBB.end(), DL, TII.get(TargetOpcode::CFI_INSTRUCTION))
} else {
MachineInstr *NewMI = MF.CloneMachineInstr(&MI);
MBB.insert(MBB.end(), NewMI);
// Set normal properties for a late MachineFunction.
// Compute live-in set for outlined fn
const MachineRegisterInfo &MRI = MF.getRegInfo();
const TargetRegisterInfo &TRI = *MRI.getTargetRegisterInfo();
LivePhysRegs LiveIns(TRI);
for (auto &Cand : OF.Candidates) {
// Figure out live-ins at the first instruction.
MachineBasicBlock &OutlineBB = *Cand.front().getParent();
LivePhysRegs CandLiveIns(TRI);
for (const MachineInstr &MI :
reverse(make_range(Cand.begin(), OutlineBB.end())))
// The live-in set for the outlined function is the union of the live-ins
// from all the outlining points.
for (MCPhysReg Reg : CandLiveIns)
addLiveIns(MBB, LiveIns);
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;
// Get the mangled name of the function for the linkage name.
std::string Dummy;
raw_string_ostream MangledNameStream(Dummy);
Mg.getNameWithPrefix(MangledNameStream, F, false);
DISubprogram *OutlinedSP = DB.createFunction(
Unit /* Context */, F->getName(), StringRef(MangledNameStream.str()),
Unit /* File */,
0 /* Line 0 is reserved for compiler-generated code. */,
DB.getOrCreateTypeArray(std::nullopt)), /* void type */
0, /* Line 0 is reserved for compiler-generated code. */
DINode::DIFlags::FlagArtificial /* Compiler-generated code. */,
/* Outlined code is optimized code by definition. */
DISubprogram::SPFlagDefinition | DISubprogram::SPFlagOptimized);
// Don't add any new variables to the subprogram.
// Attach subprogram to the function.
// We're done with the DIBuilder.
return &MF;
bool MachineOutliner::outline(Module &M,
std::vector<OutlinedFunction> &FunctionList,
InstructionMapper &Mapper,
unsigned &OutlinedFunctionNum) {
LLVM_DEBUG(dbgs() << "*** Outlining ***\n");
LLVM_DEBUG(dbgs() << "NUMBER OF POTENTIAL FUNCTIONS: " << FunctionList.size()
<< "\n");
bool OutlinedSomething = false;
// Sort by benefit. The most beneficial functions should be outlined first.
[](const OutlinedFunction &LHS, const OutlinedFunction &RHS) {
return LHS.getBenefit() > RHS.getBenefit();
// Walk over each function, outlining them as we go along. Functions are
// outlined greedily, based off the sort above.
auto *UnsignedVecBegin = Mapper.UnsignedVec.begin();
for (OutlinedFunction &OF : FunctionList) {
#ifndef NDEBUG
auto NumCandidatesBefore = OF.Candidates.size();
// If we outlined something that overlapped with a candidate in a previous
// step, then we can't outline from it.
erase_if(OF.Candidates, [&UnsignedVecBegin](Candidate &C) {
return std::any_of(UnsignedVecBegin + C.getStartIdx(),
UnsignedVecBegin + C.getEndIdx() + 1, [](unsigned I) {
return I == static_cast<unsigned>(-1);
#ifndef NDEBUG
auto NumCandidatesAfter = OF.Candidates.size();
LLVM_DEBUG(dbgs() << "PRUNED: " << NumCandidatesBefore - NumCandidatesAfter
<< "/" << NumCandidatesBefore << " candidates\n");
// If we made it unbeneficial to outline this function, skip it.
if (OF.getBenefit() < OutlinerBenefitThreshold) {
LLVM_DEBUG(dbgs() << "SKIP: Expected benefit (" << OF.getBenefit()
<< " B) < threshold (" << OutlinerBenefitThreshold
<< " B)\n");
LLVM_DEBUG(dbgs() << "OUTLINE: Expected benefit (" << OF.getBenefit()
<< " B) > threshold (" << OutlinerBenefitThreshold
<< " B)\n");
// It's beneficial. Create the function and outline its sequence's
// occurrences.
OF.MF = createOutlinedFunction(M, OF, Mapper, OutlinedFunctionNum);
OutlinedFunctionNum++; // Created a function, move to the next name.
MachineFunction *MF = OF.MF;
const TargetSubtargetInfo &STI = MF->getSubtarget();
const TargetInstrInfo &TII = *STI.getInstrInfo();
// Replace occurrences of the sequence with calls to the new function.
for (Candidate &C : OF.Candidates) {
MachineBasicBlock &MBB = *C.getMBB();
MachineBasicBlock::iterator StartIt = C.begin();
MachineBasicBlock::iterator EndIt = std::prev(C.end());
// Insert the call.
auto CallInst = TII.insertOutlinedCall(M, MBB, StartIt, *MF, C);
// Insert the call.
#ifndef NDEBUG
auto MBBBeingOutlinedFromName =
MBB.getName().empty() ? "<unknown>" : MBB.getName().str();
auto MFBeingOutlinedFromName = MBB.getParent()->getName().empty()
? "<unknown>"
: MBB.getParent()->getName().str();
LLVM_DEBUG(dbgs() << " CALL: " << MF->getName() << " in "
<< MFBeingOutlinedFromName << ":"
<< MBBBeingOutlinedFromName << "\n");
LLVM_DEBUG(dbgs() << " .. " << *CallInst);
// 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)) {
// The following code is to add implicit def operands to the call
// instruction. It also updates call site information for moved
// code.
SmallSet<Register, 2> UseRegs, DefRegs;
// 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. Also remove call site information for outlined block
// of code. The exposed uses need to be copied in the outlined range.
for (MachineBasicBlock::reverse_iterator
Iter = EndIt.getReverse(),
Last = std::next(CallInst.getReverse());
Iter != Last; Iter++) {
MachineInstr *MI = &*Iter;
SmallSet<Register, 2> InstrUseRegs;
for (MachineOperand &MOP : MI->operands()) {
// Skip over anything that isn't a register.
if (!MOP.isReg())
if (MOP.isDef()) {
// Introduce DefRegs set to skip the redundant register.
if (UseRegs.count(MOP.getReg()) &&
// Since the regiester is modeled as defined,
// it is not necessary to be put in use register set.
} else if (!MOP.isUndef()) {
// Any register which is not undefined should
// be put in the use register set.
if (MI->isCandidateForCallSiteEntry())
for (const Register &I : DefRegs)
// If it's a def, add it to the call instruction.
MachineOperand::CreateReg(I, true, /* isDef = true */
true /* isImp = true */));
for (const Register &I : UseRegs)
// If it's a exposed use, add it to the call instruction.
MachineOperand::CreateReg(I, false, /* isDef = false */
true /* isImp = true */));
// 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));
// Keep track of what we removed by marking them all as -1.
for (unsigned &I : make_range(UnsignedVecBegin + C.getStartIdx(),
UnsignedVecBegin + C.getEndIdx() + 1))
I = static_cast<unsigned>(-1);
OutlinedSomething = true;
// Statistics.
LLVM_DEBUG(dbgs() << "OutlinedSomething = " << OutlinedSomething << "\n";);
return OutlinedSomething;
void MachineOutliner::populateMapper(InstructionMapper &Mapper, Module &M,
MachineModuleInfo &MMI) {
// Build instruction mappings for each function in the module. Start by
// iterating over each Function in M.
LLVM_DEBUG(dbgs() << "*** Populating mapper ***\n");
for (Function &F : M) {
LLVM_DEBUG(dbgs() << "MAPPING FUNCTION: " << F.getName() << "\n");
if (F.hasFnAttribute("nooutline")) {
LLVM_DEBUG(dbgs() << "SKIP: Function has nooutline attribute\n");
// 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) {
LLVM_DEBUG(dbgs() << "SKIP: Function does not have a MachineFunction\n");
const TargetInstrInfo *TII = MF->getSubtarget().getInstrInfo();
if (!RunOnAllFunctions && !TII->shouldOutlineFromFunctionByDefault(*MF)) {
LLVM_DEBUG(dbgs() << "SKIP: Target does not want to outline from "
"function by default\n");
// 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)) {
LLVM_DEBUG(dbgs() << "SKIP: " << MF->getName()
<< ": unsafe to outline from\n");
// 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.
const unsigned MinMBBSize = 2;
for (MachineBasicBlock &MBB : *MF) {
LLVM_DEBUG(dbgs() << " MAPPING MBB: '" << MBB.getName() << "'\n");
// If there isn't anything in MBB, then there's no point in outlining from
// it.
// If there are fewer than 2 instructions in the MBB, then it can't ever
// contain something worth outlining.
// FIXME: This should be based off of the maximum size in B of an outlined
// call versus the size in B of the MBB.
if (MBB.size() < MinMBBSize) {
LLVM_DEBUG(dbgs() << " SKIP: MBB size less than minimum size of "
<< MinMBBSize << "\n");
// 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()) {
LLVM_DEBUG(dbgs() << " SKIP: MBB's address is taken\n");
// MBB is suitable for outlining. Map it to a list of unsigneds.
Mapper.convertToUnsignedVec(MBB, *TII);
// Statistics.
UnsignedVecSize = Mapper.UnsignedVec.size();
void MachineOutliner::initSizeRemarkInfo(
const Module &M, const MachineModuleInfo &MMI,
StringMap<unsigned> &FunctionToInstrCount) {
// Collect instruction counts for every function. We'll use this to emit
// per-function size remarks later.
for (const Function &F : M) {
MachineFunction *MF = MMI.getMachineFunction(F);
// We only care about MI counts here. If there's no MachineFunction at this
// point, then there won't be after the outliner runs, so let's move on.
if (!MF)
FunctionToInstrCount[F.getName().str()] = MF->getInstructionCount();
void MachineOutliner::emitInstrCountChangedRemark(
const Module &M, const MachineModuleInfo &MMI,
const StringMap<unsigned> &FunctionToInstrCount) {
// Iterate over each function in the module and emit remarks.
// Note that we won't miss anything by doing this, because the outliner never
// deletes functions.
for (const Function &F : M) {
MachineFunction *MF = MMI.getMachineFunction(F);
// The outliner never deletes functions. If we don't have a MF here, then we
// didn't have one prior to outlining either.
if (!MF)
std::string Fname = std::string(F.getName());
unsigned FnCountAfter = MF->getInstructionCount();
unsigned FnCountBefore = 0;
// Check if the function was recorded before.
auto It = FunctionToInstrCount.find(Fname);
// Did we have a previously-recorded size? If yes, then set FnCountBefore
// to that.
if (It != FunctionToInstrCount.end())
FnCountBefore = It->second;
// Compute the delta and emit a remark if there was a change.
int64_t FnDelta = static_cast<int64_t>(FnCountAfter) -
if (FnDelta == 0)
MachineOptimizationRemarkEmitter MORE(*MF, nullptr);
MORE.emit([&]() {
MachineOptimizationRemarkAnalysis R("size-info", "FunctionMISizeChange",
DiagnosticLocation(), &MF->front());
R << DiagnosticInfoOptimizationBase::Argument("Pass", "Machine Outliner")
<< ": Function: "
<< DiagnosticInfoOptimizationBase::Argument("Function", F.getName())
<< ": MI instruction count changed from "
<< DiagnosticInfoOptimizationBase::Argument("MIInstrsBefore",
<< " to "
<< DiagnosticInfoOptimizationBase::Argument("MIInstrsAfter",
<< "; Delta: "
<< DiagnosticInfoOptimizationBase::Argument("Delta", FnDelta);
return R;
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;
// Number to append to the current outlined function.
unsigned OutlinedFunctionNum = 0;
OutlineRepeatedNum = 0;
if (!doOutline(M, OutlinedFunctionNum))
return false;
for (unsigned I = 0; I < OutlinerReruns; ++I) {
OutlinedFunctionNum = 0;
if (!doOutline(M, OutlinedFunctionNum)) {
dbgs() << "Did not outline on iteration " << I + 2 << " out of "
<< OutlinerReruns + 1 << "\n";
return true;
bool MachineOutliner::doOutline(Module &M, unsigned &OutlinedFunctionNum) {
MachineModuleInfo &MMI = getAnalysis<MachineModuleInfoWrapperPass>().getMMI();
// 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;
// Prepare instruction mappings for the suffix tree.
populateMapper(Mapper, M, MMI);
std::vector<OutlinedFunction> FunctionList;
// Find all of the outlining candidates.
findCandidates(Mapper, FunctionList);
// If we've requested size remarks, then collect the MI counts of every
// function before outlining, and the MI counts after outlining.
// FIXME: This shouldn't be in the outliner at all; it should ultimately be
// the pass manager's responsibility.
// This could pretty easily be placed in outline instead, but because we
// really ultimately *don't* want this here, it's done like this for now
// instead.
// Check if we want size remarks.
bool ShouldEmitSizeRemarks = M.shouldEmitInstrCountChangedRemark();
StringMap<unsigned> FunctionToInstrCount;
if (ShouldEmitSizeRemarks)
initSizeRemarkInfo(M, MMI, FunctionToInstrCount);
// Outline each of the candidates and return true if something was outlined.
bool OutlinedSomething =
outline(M, FunctionList, Mapper, OutlinedFunctionNum);
// If we outlined something, we definitely changed the MI count of the
// module. If we've asked for size remarks, then output them.
// FIXME: This should be in the pass manager.
if (ShouldEmitSizeRemarks && OutlinedSomething)
emitInstrCountChangedRemark(M, MMI, FunctionToInstrCount);
if (!OutlinedSomething)
dbgs() << "Stopped outlining at iteration " << OutlineRepeatedNum
<< " because no changes were found.\n";
return OutlinedSomething;