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llvm-project/llvm/lib/CodeGen/MachineOutliner.cpp

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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.
///
/// 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
///
/// https://www.youtube.com/watch?v=yorld-WSOeU
///
/// The slides for the talk are available at
///
/// http://www.llvm.org/devmtg/2016-11/Slides/Paquette-Outliner.pdf
///
/// 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
///
/// http://lists.llvm.org/pipermail/llvm-dev/2016-August/104170.html
///
/// For more information on the suffix tree data structure, please see
/// https://www.cs.helsinki.fi/u/ukkonen/SuffixT1withFigs.pdf
///
//===----------------------------------------------------------------------===//
#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/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;
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 {
/// Set to false if the candidate overlapped with another candidate.
bool InCandidateList = true;
/// The start index of this \p Candidate.
size_t StartIdx;
/// The number of instructions in this \p Candidate.
size_t Len;
/// The index of this \p Candidate's \p OutlinedFunction in the list of
/// \p OutlinedFunctions.
size_t FunctionIdx;
/// \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(size_t StartIdx, size_t Len, size_t 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 StartIdx > RHS.StartIdx; }
};
/// \brief The information necessary to create an outlined function for some
/// class of candidate.
struct OutlinedFunction {
/// 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.
size_t Name;
/// The number of candidates for this OutlinedFunction.
size_t OccurrenceCount = 0;
/// \brief The sequence of integers corresponding to the instructions in this
/// function.
std::vector<unsigned> Sequence;
/// The number of instructions this function would save.
unsigned Benefit = 0;
/// \brief Set to true if candidates for this outlined function should be
/// replaced with tail calls to this OutlinedFunction.
bool IsTailCall = false;
OutlinedFunction(size_t Name, size_t OccurrenceCount,
const std::vector<unsigned> &Sequence,
unsigned Benefit, bool IsTailCall)
: Name(Name), OccurrenceCount(OccurrenceCount), Sequence(Sequence),
Benefit(Benefit), IsTailCall(IsTailCall)
{}
};
/// Represents an undefined index in the suffix tree.
const size_t 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.
size_t 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.
size_t *EndIdx = nullptr;
/// For leaves, the start index of the suffix represented by this node.
///
/// For all other nodes, this is ignored.
size_t SuffixIdx = EmptyIdx;
/// \brief For internal nodes, a pointer to the internal node representing
/// the same sequence with the first character chopped off.
///
/// This has two major purposes in the suffix tree. The first is as a
/// shortcut in Ukkonen's construction 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).
///
/// The suffix link is also used during the tree pruning process to let us
/// quickly throw out a bunch of potential overlaps. Say we have a sequence
/// S we want to outline. Then each of its suffixes contribute to at least
/// one overlapping case. Therefore, we can follow the suffix links
/// starting at the node associated with S to the root and "delete" those
/// nodes, save for the root. For each candidate, this removes
/// O(|candidate|) overlaps from the search space. We don't actually
/// completely invalidate these nodes though; doing that is far too
/// aggressive. Consider the following pathological string:
///
/// 1 2 3 1 2 3 2 3 2 3 2 3 2 3 2 3 2 3
///
/// If we, for the sake of example, outlined 1 2 3, then we would throw
/// out all instances of 2 3. This isn't desirable. To get around this,
/// when we visit a link node, we decrement its occurrence count by the
/// number of sequences we outlined in the current step. In the pathological
/// example, the 2 3 node would have an occurrence count of 8, while the
/// 1 2 3 node would have an occurrence count of 2. Thus, the 2 3 node
/// would survive to the next round allowing us to outline the extra
/// instances of 2 3.
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.
size_t OccurrenceCount = 0;
/// The length of the string formed by concatenating the edge labels from the
/// root to this node.
size_t 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(size_t StartIdx, size_t *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
///
/// https://www.cs.helsinki.fi/u/ukkonen/SuffixT1withFigs.pdf
class SuffixTree {
private:
/// 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;
/// Stores each leaf node in the tree.
///
/// This is used for finding outlining candidates.
std::vector<SuffixTreeNode *> LeafVector;
/// 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.
size_t 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.
size_t Idx = EmptyIdx;
/// The length of the substring we have to add at the current step.
size_t 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, size_t 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, size_t StartIdx,
size_t EndIdx, unsigned Edge) {
assert(StartIdx <= EndIdx && "String can't start after it ends!");
assert(!(!Parent && StartIdx != EmptyIdx) &&
"Non-root internal nodes must have parents!");
size_t *E = new (InternalEndIdxAllocator) size_t(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, size_t 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!");
CurrNode.Parent->OccurrenceCount++;
// 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(size_t EndIdx, size_t 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];
size_t 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;
continue;
}
// 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;
}
Active.Len++;
break;
}
// 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.
SuffixesToAdd--;
if (Active.Node->isRoot()) {
if (Active.Len > 0) {
Active.Len--;
Active.Idx = EndIdx - SuffixesToAdd + 1;
}
} else {
// Start the next phase at the next smallest suffix.
Active.Node = Active.Node->Link;
}
}
return SuffixesToAdd;
}
public:
/// Find all repeated substrings that satisfy \p BenefitFn.
///
/// 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[out] CandidateList Filled with candidates representing each
/// beneficial substring.
/// \param[out] FunctionList Filled with a list of \p OutlinedFunctions each
/// type of candidate.
/// \param BenefitFn The function to satisfy.
///
/// \returns The length of the longest candidate found.
size_t findCandidates(std::vector<Candidate> &CandidateList,
std::vector<OutlinedFunction> &FunctionList,
const std::function<unsigned(SuffixTreeNode &, size_t, unsigned)>
&BenefitFn) {
CandidateList.clear();
FunctionList.clear();
size_t FnIdx = 0;
size_t MaxLen = 0;
for (SuffixTreeNode* Leaf : LeafVector) {
assert(Leaf && "Leaves in LeafVector cannot be null!");
if (!Leaf->IsInTree)
continue;
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)
continue;
size_t StringLen = Leaf->ConcatLen - Leaf->size();
// How many instructions would outlining this string save?
unsigned Benefit = BenefitFn(Parent,
StringLen, Str[Leaf->SuffixIdx + StringLen - 1]);
// If it's not beneficial, skip it.
if (Benefit < 1)
continue;
if (StringLen > MaxLen)
MaxLen = StringLen;
unsigned OccurrenceCount = 0;
for (auto &ChildPair : Parent.Children) {
SuffixTreeNode *M = ChildPair.second;
// Is it a leaf? If so, we have an occurrence of this candidate.
if (M && M->IsInTree && M->isLeaf()) {
OccurrenceCount++;
CandidateList.emplace_back(M->SuffixIdx, StringLen, FnIdx);
CandidateList.back().Benefit = Benefit;
M->IsInTree = false;
}
}
// Save the function for the new candidate sequence.
std::vector<unsigned> CandidateSequence;
for (unsigned i = Leaf->SuffixIdx; i < Leaf->SuffixIdx + StringLen; i++)
CandidateSequence.push_back(Str[i]);
FunctionList.emplace_back(FnIdx, OccurrenceCount, CandidateSequence,
Benefit, false);
// Move to the next function.
FnIdx++;
Parent.IsInTree = false;
}
return MaxLen;
}
/// 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.
size_t 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 (size_t PfxEndIdx = 0, End = Str.size(); PfxEndIdx < End; PfxEndIdx++) {
SuffixesToAdd++;
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>
InstructionIntegerMap;
/// 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.
InstrList.push_back(It);
MachineInstr &MI = *It;
bool WasInserted;
DenseMap<MachineInstr *, unsigned, MachineInstrExpressionTrait>::iterator
ResultIt;
std::tie(ResultIt, WasInserted) =
InstructionIntegerMap.insert(std::make_pair(&MI, LegalInstrNumber));
unsigned MINumber = ResultIt->second;
// There was an insertion.
if (WasInserted) {
LegalInstrNumber++;
IntegerInstructionMap.insert(std::make_pair(MINumber, &MI));
}
UnsignedVec.push_back(MINumber);
// 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;
InstrList.push_back(It);
UnsignedVec.push_back(IllegalInstrNumber);
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:
mapToIllegalUnsigned(It);
break;
case TargetInstrInfo::MachineOutlinerInstrType::Legal:
mapToLegalUnsigned(It);
break;
case TargetInstrInfo::MachineOutlinerInstrType::Invisible:
break;
}
}
// 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.
InstrList.push_back(MBB.end());
UnsignedVec.push_back(IllegalInstrNumber);
IllegalInstrNumber--;
}
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;
StringRef getPassName() const override { return "Machine Outliner"; }
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<MachineModuleInfo>();
AU.addPreserved<MachineModuleInfo>();
AU.setPreservesAll();
ModulePass::getAnalysisUsage(AU);
}
MachineOutliner() : ModulePass(ID) {
initializeMachineOutlinerPass(*PassRegistry::getPassRegistry());
}
/// \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<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.
unsigned buildCandidateList(std::vector<Candidate> &CandidateList,
std::vector<OutlinedFunction> &FunctionList,
SuffixTree &ST,
InstructionMapper &Mapper,
const TargetInstrInfo &TII);
/// \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 MaxCandidateLen The length of the longest candidate.
/// \param TII TargetInstrInfo for the module.
void pruneOverlaps(std::vector<Candidate> &CandidateList,
std::vector<OutlinedFunction> &FunctionList,
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() { return new MachineOutliner(); }
}
INITIALIZE_PASS(MachineOutliner, "machine-outliner",
"Machine Function Outliner", false, false)
void MachineOutliner::pruneOverlaps(std::vector<Candidate> &CandidateList,
std::vector<OutlinedFunction> &FunctionList,
unsigned MaxCandidateLen,
const TargetInstrInfo &TII) {
// 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;
OutlinedFunction &F1 = FunctionList[C1.FunctionIdx];
// If we removed this candidate, skip it.
if (!C1.InCandidateList)
continue;
// Is it still worth it to outline C1?
if (F1.Benefit < 1 || F1.OccurrenceCount < 2) {
assert(F1.OccurrenceCount > 0 &&
"Can't remove OutlinedFunction with no occurrences!");
F1.OccurrenceCount--;
C1.InCandidateList = false;
continue;
}
// 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.StartIdx > MaxCandidateLen)
FarthestPossibleIdx = C1.StartIdx - 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;
OutlinedFunction &F2 = FunctionList[C2.FunctionIdx];
// Is this candidate too far away to overlap?
if (C2.StartIdx < FarthestPossibleIdx)
break;
// Did we already remove this candidate in a previous step?
if (!C2.InCandidateList)
continue;
// Is the function beneficial to outline?
if (F2.OccurrenceCount < 2 || F2.Benefit < 1) {
// If not, remove this candidate and move to the next one.
assert(F2.OccurrenceCount > 0 &&
"Can't remove OutlinedFunction with no occurrences!");
F2.OccurrenceCount--;
C2.InCandidateList = false;
continue;
}
size_t C2End = C2.StartIdx + C2.Len - 1;
// 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 (C2End < C1.StartIdx)
continue;
// 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.
if (C1.Benefit >= C2.Benefit) {
// C1 is better, so remove C2 and update C2's OutlinedFunction to
// reflect the removal.
assert(F2.OccurrenceCount > 0 &&
"Can't remove OutlinedFunction with no occurrences!");
F2.OccurrenceCount--;
F2.Benefit = TII.getOutliningBenefit(F2.Sequence.size(),
F2.OccurrenceCount,
F2.IsTailCall
);
C2.InCandidateList = false;
DEBUG (
dbgs() << "- Removed C2. \n";
dbgs() << "--- Num fns left for C2: " << F2.OccurrenceCount << "\n";
dbgs() << "--- C2's benefit: " << F2.Benefit << "\n";
);
} else {
// C2 is better, so remove C1 and update C1's OutlinedFunction to
// reflect the removal.
assert(F1.OccurrenceCount > 0 &&
"Can't remove OutlinedFunction with no occurrences!");
F1.OccurrenceCount--;
F1.Benefit = TII.getOutliningBenefit(F1.Sequence.size(),
F1.OccurrenceCount,
F1.IsTailCall
);
C1.InCandidateList = false;
DEBUG (
dbgs() << "- Removed C1. \n";
dbgs() << "--- Num fns left for C1: " << F1.OccurrenceCount << "\n";
dbgs() << "--- C1's benefit: " << F1.Benefit << "\n";
);
// C1 is out, so we don't have to compare it against anyone else.
break;
}
}
}
}
unsigned
MachineOutliner::buildCandidateList(std::vector<Candidate> &CandidateList,
std::vector<OutlinedFunction> &FunctionList,
SuffixTree &ST,
InstructionMapper &Mapper,
const TargetInstrInfo &TII) {
std::vector<unsigned> CandidateSequence; // Current outlining candidate.
size_t MaxCandidateLen = 0; // Length of the longest candidate.
// Function for maximizing query in the suffix tree.
// This allows us to define more fine-grained types of things to outline in
// the target without putting target-specific info in the suffix tree.
auto BenefitFn = [&TII, &Mapper](const SuffixTreeNode &Curr,
size_t StringLen, unsigned EndVal) {
// The root represents the empty string.
if (Curr.isRoot())
return 0u;
// Is this long enough to outline?
// TODO: Let the target decide how "long" a string is in terms of the sizes
// of the instructions in the string. For example, if a call instruction
// is smaller than a one instruction string, we should outline that string.
if (StringLen < 2)
return 0u;
size_t Occurrences = Curr.OccurrenceCount;
// Anything we want to outline has to appear at least twice.
if (Occurrences < 2)
return 0u;
// Check if the last instruction in the sequence is a return.
MachineInstr *LastInstr =
Mapper.IntegerInstructionMap[EndVal];
assert(LastInstr && "Last instruction in sequence was unmapped!");
// The only way a terminator could be mapped as legal is if it was safe to
// tail call.
bool IsTailCall = LastInstr->isTerminator();
return TII.getOutliningBenefit(StringLen, Occurrences, IsTailCall);
};
MaxCandidateLen = ST.findCandidates(CandidateList, FunctionList, BenefitFn);
for (auto &OF : FunctionList)
OF.IsTailCall = Mapper.
IntegerInstructionMap[OF.Sequence.back()]->isTerminator();
// 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.
std::stable_sort(CandidateList.begin(), CandidateList.end());
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.
F->setLinkage(GlobalValue::PrivateLinkage);
F->setUnnamedAddr(GlobalValue::UnnamedAddr::Global);
BasicBlock *EntryBB = BasicBlock::Create(C, "entry", F);
IRBuilder<> Builder(EntryBB);
Builder.CreateRetVoid();
MachineModuleInfo &MMI = getAnalysis<MachineModuleInfo>();
MachineFunction &MF = MMI.getMachineFunction(*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.IsTailCall);
// Copy over the instructions for the function using the integer mappings in
// its sequence.
for (unsigned Str : OF.Sequence) {
MachineInstr *NewMI =
MF.CloneMachineInstr(Mapper.IntegerInstructionMap.find(Str)->second);
NewMI->dropMemRefs();
// Don't keep debug information for outlined instructions.
// FIXME: This means outlined functions are currently undebuggable.
NewMI->setDebugLoc(DebugLoc());
MBB.insert(MBB.end(), NewMI);
}
TII.insertOutlinerEpilogue(MBB, MF, OF.IsTailCall);
return &MF;
}
bool MachineOutliner::outline(Module &M,
const ArrayRef<Candidate> &CandidateList,
std::vector<OutlinedFunction> &FunctionList,
InstructionMapper &Mapper) {
bool OutlinedSomething = false;
// Replace the candidates with calls to their respective outlined functions.
for (const Candidate &C : CandidateList) {
// Was the candidate removed during pruneOverlaps?
if (!C.InCandidateList)
continue;
// If not, then look at its OutlinedFunction.
OutlinedFunction &OF = FunctionList[C.FunctionIdx];
// Was its OutlinedFunction made unbeneficial during pruneOverlaps?
if (OF.OccurrenceCount < 2 || OF.Benefit < 1)
continue;
// If not, then outline it.
assert(C.StartIdx < Mapper.InstrList.size() && "Candidate out of bounds!");
MachineBasicBlock *MBB = (*Mapper.InstrList[C.StartIdx]).getParent();
MachineBasicBlock::iterator StartIt = Mapper.InstrList[C.StartIdx];
unsigned EndIdx = C.StartIdx + C.Len - 1;
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);
FunctionsCreated++;
}
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, OF.IsTailCall);
StartIt = Mapper.InstrList[C.StartIdx];
MBB->erase(StartIt, EndIt);
OutlinedSomething = true;
// Statistics.
NumOutlined++;
}
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 = MMI.getMachineFunction(*M.begin())
.getSubtarget();
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.getMachineFunction(F);
// Is the function empty? Safe to outline from?
if (F.empty() || !TII->isFunctionSafeToOutlineFrom(MF))
continue;
// If it is, look at each MachineBasicBlock in the function.
for (MachineBasicBlock &MBB : MF) {
// Is there anything in MBB?
if (MBB.empty())
continue;
// 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<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, MaxCandidateLen, *TII);
// Outline each of the candidates and return true if something was outlined.
return outline(M, CandidateList, FunctionList, Mapper);
}
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