1 //==- BlockFrequencyInfoImpl.h - Block Frequency Implementation -*- C++ -*-===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // Shared implementation of BlockFrequency for IR and Machine Instructions.
11 // See the documentation below for BlockFrequencyInfoImpl for details.
13 //===----------------------------------------------------------------------===//
15 #ifndef LLVM_ANALYSIS_BLOCKFREQUENCYINFOIMPL_H
16 #define LLVM_ANALYSIS_BLOCKFREQUENCYINFOIMPL_H
18 #include "llvm/ADT/DenseMap.h"
19 #include "llvm/ADT/PostOrderIterator.h"
20 #include "llvm/ADT/iterator_range.h"
21 #include "llvm/IR/BasicBlock.h"
22 #include "llvm/Support/BlockFrequency.h"
23 #include "llvm/Support/BranchProbability.h"
24 #include "llvm/Support/Debug.h"
25 #include "llvm/Support/ScaledNumber.h"
26 #include "llvm/Support/raw_ostream.h"
32 #define DEBUG_TYPE "block-freq"
34 //===----------------------------------------------------------------------===//
36 // BlockMass definition.
38 // TODO: Make this private to BlockFrequencyInfoImpl or delete.
40 //===----------------------------------------------------------------------===//
43 /// \brief Mass of a block.
45 /// This class implements a sort of fixed-point fraction always between 0.0 and
46 /// 1.0. getMass() == UINT64_MAX indicates a value of 1.0.
48 /// Masses can be added and subtracted. Simple saturation arithmetic is used,
49 /// so arithmetic operations never overflow or underflow.
51 /// Masses can be multiplied. Multiplication treats full mass as 1.0 and uses
52 /// an inexpensive floating-point algorithm that's off-by-one (almost, but not
53 /// quite, maximum precision).
55 /// Masses can be scaled by \a BranchProbability at maximum precision.
60 BlockMass() : Mass(0) {}
61 explicit BlockMass(uint64_t Mass) : Mass(Mass) {}
63 static BlockMass getEmpty() { return BlockMass(); }
64 static BlockMass getFull() { return BlockMass(UINT64_MAX); }
66 uint64_t getMass() const { return Mass; }
68 bool isFull() const { return Mass == UINT64_MAX; }
69 bool isEmpty() const { return !Mass; }
71 bool operator!() const { return isEmpty(); }
73 /// \brief Add another mass.
75 /// Adds another mass, saturating at \a isFull() rather than overflowing.
76 BlockMass &operator+=(const BlockMass &X) {
77 uint64_t Sum = Mass + X.Mass;
78 Mass = Sum < Mass ? UINT64_MAX : Sum;
82 /// \brief Subtract another mass.
84 /// Subtracts another mass, saturating at \a isEmpty() rather than
86 BlockMass &operator-=(const BlockMass &X) {
87 uint64_t Diff = Mass - X.Mass;
88 Mass = Diff > Mass ? 0 : Diff;
92 BlockMass &operator*=(const BranchProbability &P) {
97 bool operator==(const BlockMass &X) const { return Mass == X.Mass; }
98 bool operator!=(const BlockMass &X) const { return Mass != X.Mass; }
99 bool operator<=(const BlockMass &X) const { return Mass <= X.Mass; }
100 bool operator>=(const BlockMass &X) const { return Mass >= X.Mass; }
101 bool operator<(const BlockMass &X) const { return Mass < X.Mass; }
102 bool operator>(const BlockMass &X) const { return Mass > X.Mass; }
104 /// \brief Convert to scaled number.
106 /// Convert to \a ScaledNumber. \a isFull() gives 1.0, while \a isEmpty()
107 /// gives slightly above 0.0.
108 ScaledNumber<uint64_t> toScaled() const;
111 raw_ostream &print(raw_ostream &OS) const;
114 inline BlockMass operator+(const BlockMass &L, const BlockMass &R) {
115 return BlockMass(L) += R;
117 inline BlockMass operator-(const BlockMass &L, const BlockMass &R) {
118 return BlockMass(L) -= R;
120 inline BlockMass operator*(const BlockMass &L, const BranchProbability &R) {
121 return BlockMass(L) *= R;
123 inline BlockMass operator*(const BranchProbability &L, const BlockMass &R) {
124 return BlockMass(R) *= L;
127 inline raw_ostream &operator<<(raw_ostream &OS, const BlockMass &X) {
131 template <> struct isPodLike<BlockMass> {
132 static const bool value = true;
136 //===----------------------------------------------------------------------===//
138 // BlockFrequencyInfoImpl definition.
140 //===----------------------------------------------------------------------===//
144 class BranchProbabilityInfo;
148 class MachineBasicBlock;
149 class MachineBranchProbabilityInfo;
150 class MachineFunction;
152 class MachineLoopInfo;
154 namespace bfi_detail {
155 struct IrreducibleGraph;
157 // This is part of a workaround for a GCC 4.7 crash on lambdas.
158 template <class BT> struct BlockEdgesAdder;
161 /// \brief Base class for BlockFrequencyInfoImpl
163 /// BlockFrequencyInfoImplBase has supporting data structures and some
164 /// algorithms for BlockFrequencyInfoImplBase. Only algorithms that depend on
165 /// the block type (or that call such algorithms) are skipped here.
167 /// Nevertheless, the majority of the overall algorithm documention lives with
168 /// BlockFrequencyInfoImpl. See there for details.
169 class BlockFrequencyInfoImplBase {
171 typedef ScaledNumber<uint64_t> Scaled64;
173 /// \brief Representative of a block.
175 /// This is a simple wrapper around an index into the reverse-post-order
176 /// traversal of the blocks.
178 /// Unlike a block pointer, its order has meaning (location in the
179 /// topological sort) and it's class is the same regardless of block type.
181 typedef uint32_t IndexType;
184 bool operator==(const BlockNode &X) const { return Index == X.Index; }
185 bool operator!=(const BlockNode &X) const { return Index != X.Index; }
186 bool operator<=(const BlockNode &X) const { return Index <= X.Index; }
187 bool operator>=(const BlockNode &X) const { return Index >= X.Index; }
188 bool operator<(const BlockNode &X) const { return Index < X.Index; }
189 bool operator>(const BlockNode &X) const { return Index > X.Index; }
191 BlockNode() : Index(UINT32_MAX) {}
192 BlockNode(IndexType Index) : Index(Index) {}
194 bool isValid() const { return Index <= getMaxIndex(); }
195 static size_t getMaxIndex() { return UINT32_MAX - 1; }
198 /// \brief Stats about a block itself.
199 struct FrequencyData {
204 /// \brief Data about a loop.
206 /// Contains the data necessary to represent represent a loop as a
207 /// pseudo-node once it's packaged.
209 typedef SmallVector<std::pair<BlockNode, BlockMass>, 4> ExitMap;
210 typedef SmallVector<BlockNode, 4> NodeList;
211 LoopData *Parent; ///< The parent loop.
212 bool IsPackaged; ///< Whether this has been packaged.
213 uint32_t NumHeaders; ///< Number of headers.
214 ExitMap Exits; ///< Successor edges (and weights).
215 NodeList Nodes; ///< Header and the members of the loop.
216 BlockMass BackedgeMass; ///< Mass returned to loop header.
220 LoopData(LoopData *Parent, const BlockNode &Header)
221 : Parent(Parent), IsPackaged(false), NumHeaders(1), Nodes(1, Header) {}
222 template <class It1, class It2>
223 LoopData(LoopData *Parent, It1 FirstHeader, It1 LastHeader, It2 FirstOther,
225 : Parent(Parent), IsPackaged(false), Nodes(FirstHeader, LastHeader) {
226 NumHeaders = Nodes.size();
227 Nodes.insert(Nodes.end(), FirstOther, LastOther);
229 bool isHeader(const BlockNode &Node) const {
231 return std::binary_search(Nodes.begin(), Nodes.begin() + NumHeaders,
233 return Node == Nodes[0];
235 BlockNode getHeader() const { return Nodes[0]; }
236 bool isIrreducible() const { return NumHeaders > 1; }
238 NodeList::const_iterator members_begin() const {
239 return Nodes.begin() + NumHeaders;
241 NodeList::const_iterator members_end() const { return Nodes.end(); }
242 iterator_range<NodeList::const_iterator> members() const {
243 return make_range(members_begin(), members_end());
247 /// \brief Index of loop information.
249 BlockNode Node; ///< This node.
250 LoopData *Loop; ///< The loop this block is inside.
251 BlockMass Mass; ///< Mass distribution from the entry block.
253 WorkingData(const BlockNode &Node) : Node(Node), Loop(nullptr) {}
255 bool isLoopHeader() const { return Loop && Loop->isHeader(Node); }
256 bool isDoubleLoopHeader() const {
257 return isLoopHeader() && Loop->Parent && Loop->Parent->isIrreducible() &&
258 Loop->Parent->isHeader(Node);
261 LoopData *getContainingLoop() const {
264 if (!isDoubleLoopHeader())
266 return Loop->Parent->Parent;
269 /// \brief Resolve a node to its representative.
271 /// Get the node currently representing Node, which could be a containing
274 /// This function should only be called when distributing mass. As long as
275 /// there are no irreducilbe edges to Node, then it will have complexity
276 /// O(1) in this context.
278 /// In general, the complexity is O(L), where L is the number of loop
279 /// headers Node has been packaged into. Since this method is called in
280 /// the context of distributing mass, L will be the number of loop headers
281 /// an early exit edge jumps out of.
282 BlockNode getResolvedNode() const {
283 auto L = getPackagedLoop();
284 return L ? L->getHeader() : Node;
286 LoopData *getPackagedLoop() const {
287 if (!Loop || !Loop->IsPackaged)
290 while (L->Parent && L->Parent->IsPackaged)
295 /// \brief Get the appropriate mass for a node.
297 /// Get appropriate mass for Node. If Node is a loop-header (whose loop
298 /// has been packaged), returns the mass of its pseudo-node. If it's a
299 /// node inside a packaged loop, it returns the loop's mass.
300 BlockMass &getMass() {
303 if (!isADoublePackage())
305 return Loop->Parent->Mass;
308 /// \brief Has ContainingLoop been packaged up?
309 bool isPackaged() const { return getResolvedNode() != Node; }
310 /// \brief Has Loop been packaged up?
311 bool isAPackage() const { return isLoopHeader() && Loop->IsPackaged; }
312 /// \brief Has Loop been packaged up twice?
313 bool isADoublePackage() const {
314 return isDoubleLoopHeader() && Loop->Parent->IsPackaged;
318 /// \brief Unscaled probability weight.
320 /// Probability weight for an edge in the graph (including the
321 /// successor/target node).
323 /// All edges in the original function are 32-bit. However, exit edges from
324 /// loop packages are taken from 64-bit exit masses, so we need 64-bits of
325 /// space in general.
327 /// In addition to the raw weight amount, Weight stores the type of the edge
328 /// in the current context (i.e., the context of the loop being processed).
329 /// Is this a local edge within the loop, an exit from the loop, or a
330 /// backedge to the loop header?
332 enum DistType { Local, Exit, Backedge };
334 BlockNode TargetNode;
336 Weight() : Type(Local), Amount(0) {}
339 /// \brief Distribution of unscaled probability weight.
341 /// Distribution of unscaled probability weight to a set of successors.
343 /// This class collates the successor edge weights for later processing.
345 /// \a DidOverflow indicates whether \a Total did overflow while adding to
346 /// the distribution. It should never overflow twice.
347 struct Distribution {
348 typedef SmallVector<Weight, 4> WeightList;
349 WeightList Weights; ///< Individual successor weights.
350 uint64_t Total; ///< Sum of all weights.
351 bool DidOverflow; ///< Whether \a Total did overflow.
353 Distribution() : Total(0), DidOverflow(false) {}
354 void addLocal(const BlockNode &Node, uint64_t Amount) {
355 add(Node, Amount, Weight::Local);
357 void addExit(const BlockNode &Node, uint64_t Amount) {
358 add(Node, Amount, Weight::Exit);
360 void addBackedge(const BlockNode &Node, uint64_t Amount) {
361 add(Node, Amount, Weight::Backedge);
364 /// \brief Normalize the distribution.
366 /// Combines multiple edges to the same \a Weight::TargetNode and scales
367 /// down so that \a Total fits into 32-bits.
369 /// This is linear in the size of \a Weights. For the vast majority of
370 /// cases, adjacent edge weights are combined by sorting WeightList and
371 /// combining adjacent weights. However, for very large edge lists an
372 /// auxiliary hash table is used.
376 void add(const BlockNode &Node, uint64_t Amount, Weight::DistType Type);
379 /// \brief Data about each block. This is used downstream.
380 std::vector<FrequencyData> Freqs;
382 /// \brief Loop data: see initializeLoops().
383 std::vector<WorkingData> Working;
385 /// \brief Indexed information about loops.
386 std::list<LoopData> Loops;
388 /// \brief Add all edges out of a packaged loop to the distribution.
390 /// Adds all edges from LocalLoopHead to Dist. Calls addToDist() to add each
393 /// \return \c true unless there's an irreducible backedge.
394 bool addLoopSuccessorsToDist(const LoopData *OuterLoop, LoopData &Loop,
397 /// \brief Add an edge to the distribution.
399 /// Adds an edge to Succ to Dist. If \c LoopHead.isValid(), then whether the
400 /// edge is local/exit/backedge is in the context of LoopHead. Otherwise,
401 /// every edge should be a local edge (since all the loops are packaged up).
403 /// \return \c true unless aborted due to an irreducible backedge.
404 bool addToDist(Distribution &Dist, const LoopData *OuterLoop,
405 const BlockNode &Pred, const BlockNode &Succ, uint64_t Weight);
407 LoopData &getLoopPackage(const BlockNode &Head) {
408 assert(Head.Index < Working.size());
409 assert(Working[Head.Index].isLoopHeader());
410 return *Working[Head.Index].Loop;
413 /// \brief Analyze irreducible SCCs.
415 /// Separate irreducible SCCs from \c G, which is an explict graph of \c
416 /// OuterLoop (or the top-level function, if \c OuterLoop is \c nullptr).
417 /// Insert them into \a Loops before \c Insert.
419 /// \return the \c LoopData nodes representing the irreducible SCCs.
420 iterator_range<std::list<LoopData>::iterator>
421 analyzeIrreducible(const bfi_detail::IrreducibleGraph &G, LoopData *OuterLoop,
422 std::list<LoopData>::iterator Insert);
424 /// \brief Update a loop after packaging irreducible SCCs inside of it.
426 /// Update \c OuterLoop. Before finding irreducible control flow, it was
427 /// partway through \a computeMassInLoop(), so \a LoopData::Exits and \a
428 /// LoopData::BackedgeMass need to be reset. Also, nodes that were packaged
429 /// up need to be removed from \a OuterLoop::Nodes.
430 void updateLoopWithIrreducible(LoopData &OuterLoop);
432 /// \brief Distribute mass according to a distribution.
434 /// Distributes the mass in Source according to Dist. If LoopHead.isValid(),
435 /// backedges and exits are stored in its entry in Loops.
437 /// Mass is distributed in parallel from two copies of the source mass.
438 void distributeMass(const BlockNode &Source, LoopData *OuterLoop,
441 /// \brief Compute the loop scale for a loop.
442 void computeLoopScale(LoopData &Loop);
444 /// \brief Package up a loop.
445 void packageLoop(LoopData &Loop);
447 /// \brief Unwrap loops.
450 /// \brief Finalize frequency metrics.
452 /// Calculates final frequencies and cleans up no-longer-needed data
454 void finalizeMetrics();
456 /// \brief Clear all memory.
459 virtual std::string getBlockName(const BlockNode &Node) const;
460 std::string getLoopName(const LoopData &Loop) const;
462 virtual raw_ostream &print(raw_ostream &OS) const { return OS; }
463 void dump() const { print(dbgs()); }
465 Scaled64 getFloatingBlockFreq(const BlockNode &Node) const;
467 BlockFrequency getBlockFreq(const BlockNode &Node) const;
469 raw_ostream &printBlockFreq(raw_ostream &OS, const BlockNode &Node) const;
470 raw_ostream &printBlockFreq(raw_ostream &OS,
471 const BlockFrequency &Freq) const;
473 uint64_t getEntryFreq() const {
474 assert(!Freqs.empty());
475 return Freqs[0].Integer;
477 /// \brief Virtual destructor.
479 /// Need a virtual destructor to mask the compiler warning about
481 virtual ~BlockFrequencyInfoImplBase() {}
484 namespace bfi_detail {
485 template <class BlockT> struct TypeMap {};
486 template <> struct TypeMap<BasicBlock> {
487 typedef BasicBlock BlockT;
488 typedef Function FunctionT;
489 typedef BranchProbabilityInfo BranchProbabilityInfoT;
491 typedef LoopInfo LoopInfoT;
493 template <> struct TypeMap<MachineBasicBlock> {
494 typedef MachineBasicBlock BlockT;
495 typedef MachineFunction FunctionT;
496 typedef MachineBranchProbabilityInfo BranchProbabilityInfoT;
497 typedef MachineLoop LoopT;
498 typedef MachineLoopInfo LoopInfoT;
501 /// \brief Get the name of a MachineBasicBlock.
503 /// Get the name of a MachineBasicBlock. It's templated so that including from
504 /// CodeGen is unnecessary (that would be a layering issue).
506 /// This is used mainly for debug output. The name is similar to
507 /// MachineBasicBlock::getFullName(), but skips the name of the function.
508 template <class BlockT> std::string getBlockName(const BlockT *BB) {
509 assert(BB && "Unexpected nullptr");
510 auto MachineName = "BB" + Twine(BB->getNumber());
511 if (BB->getBasicBlock())
512 return (MachineName + "[" + BB->getName() + "]").str();
513 return MachineName.str();
515 /// \brief Get the name of a BasicBlock.
516 template <> inline std::string getBlockName(const BasicBlock *BB) {
517 assert(BB && "Unexpected nullptr");
518 return BB->getName().str();
521 /// \brief Graph of irreducible control flow.
523 /// This graph is used for determining the SCCs in a loop (or top-level
524 /// function) that has irreducible control flow.
526 /// During the block frequency algorithm, the local graphs are defined in a
527 /// light-weight way, deferring to the \a BasicBlock or \a MachineBasicBlock
528 /// graphs for most edges, but getting others from \a LoopData::ExitMap. The
529 /// latter only has successor information.
531 /// \a IrreducibleGraph makes this graph explicit. It's in a form that can use
532 /// \a GraphTraits (so that \a analyzeIrreducible() can use \a scc_iterator),
533 /// and it explicitly lists predecessors and successors. The initialization
534 /// that relies on \c MachineBasicBlock is defined in the header.
535 struct IrreducibleGraph {
536 typedef BlockFrequencyInfoImplBase BFIBase;
540 typedef BFIBase::BlockNode BlockNode;
544 std::deque<const IrrNode *> Edges;
545 IrrNode(const BlockNode &Node) : Node(Node), NumIn(0) {}
547 typedef std::deque<const IrrNode *>::const_iterator iterator;
548 iterator pred_begin() const { return Edges.begin(); }
549 iterator succ_begin() const { return Edges.begin() + NumIn; }
550 iterator pred_end() const { return succ_begin(); }
551 iterator succ_end() const { return Edges.end(); }
554 const IrrNode *StartIrr;
555 std::vector<IrrNode> Nodes;
556 SmallDenseMap<uint32_t, IrrNode *, 4> Lookup;
558 /// \brief Construct an explicit graph containing irreducible control flow.
560 /// Construct an explicit graph of the control flow in \c OuterLoop (or the
561 /// top-level function, if \c OuterLoop is \c nullptr). Uses \c
562 /// addBlockEdges to add block successors that have not been packaged into
565 /// \a BlockFrequencyInfoImpl::computeIrreducibleMass() is the only expected
567 template <class BlockEdgesAdder>
568 IrreducibleGraph(BFIBase &BFI, const BFIBase::LoopData *OuterLoop,
569 BlockEdgesAdder addBlockEdges)
570 : BFI(BFI), StartIrr(nullptr) {
571 initialize(OuterLoop, addBlockEdges);
574 template <class BlockEdgesAdder>
575 void initialize(const BFIBase::LoopData *OuterLoop,
576 BlockEdgesAdder addBlockEdges);
577 void addNodesInLoop(const BFIBase::LoopData &OuterLoop);
578 void addNodesInFunction();
579 void addNode(const BlockNode &Node) {
580 Nodes.emplace_back(Node);
581 BFI.Working[Node.Index].getMass() = BlockMass::getEmpty();
584 template <class BlockEdgesAdder>
585 void addEdges(const BlockNode &Node, const BFIBase::LoopData *OuterLoop,
586 BlockEdgesAdder addBlockEdges);
587 void addEdge(IrrNode &Irr, const BlockNode &Succ,
588 const BFIBase::LoopData *OuterLoop);
590 template <class BlockEdgesAdder>
591 void IrreducibleGraph::initialize(const BFIBase::LoopData *OuterLoop,
592 BlockEdgesAdder addBlockEdges) {
594 addNodesInLoop(*OuterLoop);
595 for (auto N : OuterLoop->Nodes)
596 addEdges(N, OuterLoop, addBlockEdges);
598 addNodesInFunction();
599 for (uint32_t Index = 0; Index < BFI.Working.size(); ++Index)
600 addEdges(Index, OuterLoop, addBlockEdges);
602 StartIrr = Lookup[Start.Index];
604 template <class BlockEdgesAdder>
605 void IrreducibleGraph::addEdges(const BlockNode &Node,
606 const BFIBase::LoopData *OuterLoop,
607 BlockEdgesAdder addBlockEdges) {
608 auto L = Lookup.find(Node.Index);
609 if (L == Lookup.end())
611 IrrNode &Irr = *L->second;
612 const auto &Working = BFI.Working[Node.Index];
614 if (Working.isAPackage())
615 for (const auto &I : Working.Loop->Exits)
616 addEdge(Irr, I.first, OuterLoop);
618 addBlockEdges(*this, Irr, OuterLoop);
622 /// \brief Shared implementation for block frequency analysis.
624 /// This is a shared implementation of BlockFrequencyInfo and
625 /// MachineBlockFrequencyInfo, and calculates the relative frequencies of
628 /// LoopInfo defines a loop as a "non-trivial" SCC dominated by a single block,
629 /// which is called the header. A given loop, L, can have sub-loops, which are
630 /// loops within the subgraph of L that exclude its header. (A "trivial" SCC
631 /// consists of a single block that does not have a self-edge.)
633 /// In addition to loops, this algorithm has limited support for irreducible
634 /// SCCs, which are SCCs with multiple entry blocks. Irreducible SCCs are
635 /// discovered on they fly, and modelled as loops with multiple headers.
637 /// The headers of irreducible sub-SCCs consist of its entry blocks and all
638 /// nodes that are targets of a backedge within it (excluding backedges within
639 /// true sub-loops). Block frequency calculations act as if a block is
640 /// inserted that intercepts all the edges to the headers. All backedges and
641 /// entries point to this block. Its successors are the headers, which split
642 /// the frequency evenly.
644 /// This algorithm leverages BlockMass and ScaledNumber to maintain precision,
645 /// separates mass distribution from loop scaling, and dithers to eliminate
646 /// probability mass loss.
648 /// The implementation is split between BlockFrequencyInfoImpl, which knows the
649 /// type of graph being modelled (BasicBlock vs. MachineBasicBlock), and
650 /// BlockFrequencyInfoImplBase, which doesn't. The base class uses \a
651 /// BlockNode, a wrapper around a uint32_t. BlockNode is numbered from 0 in
652 /// reverse-post order. This gives two advantages: it's easy to compare the
653 /// relative ordering of two nodes, and maps keyed on BlockT can be represented
656 /// This algorithm is O(V+E), unless there is irreducible control flow, in
657 /// which case it's O(V*E) in the worst case.
659 /// These are the main stages:
661 /// 0. Reverse post-order traversal (\a initializeRPOT()).
663 /// Run a single post-order traversal and save it (in reverse) in RPOT.
664 /// All other stages make use of this ordering. Save a lookup from BlockT
665 /// to BlockNode (the index into RPOT) in Nodes.
667 /// 1. Loop initialization (\a initializeLoops()).
669 /// Translate LoopInfo/MachineLoopInfo into a form suitable for the rest of
670 /// the algorithm. In particular, store the immediate members of each loop
671 /// in reverse post-order.
673 /// 2. Calculate mass and scale in loops (\a computeMassInLoops()).
675 /// For each loop (bottom-up), distribute mass through the DAG resulting
676 /// from ignoring backedges and treating sub-loops as a single pseudo-node.
677 /// Track the backedge mass distributed to the loop header, and use it to
678 /// calculate the loop scale (number of loop iterations). Immediate
679 /// members that represent sub-loops will already have been visited and
680 /// packaged into a pseudo-node.
682 /// Distributing mass in a loop is a reverse-post-order traversal through
683 /// the loop. Start by assigning full mass to the Loop header. For each
684 /// node in the loop:
686 /// - Fetch and categorize the weight distribution for its successors.
687 /// If this is a packaged-subloop, the weight distribution is stored
688 /// in \a LoopData::Exits. Otherwise, fetch it from
689 /// BranchProbabilityInfo.
691 /// - Each successor is categorized as \a Weight::Local, a local edge
692 /// within the current loop, \a Weight::Backedge, a backedge to the
693 /// loop header, or \a Weight::Exit, any successor outside the loop.
694 /// The weight, the successor, and its category are stored in \a
695 /// Distribution. There can be multiple edges to each successor.
697 /// - If there's a backedge to a non-header, there's an irreducible SCC.
698 /// The usual flow is temporarily aborted. \a
699 /// computeIrreducibleMass() finds the irreducible SCCs within the
700 /// loop, packages them up, and restarts the flow.
702 /// - Normalize the distribution: scale weights down so that their sum
703 /// is 32-bits, and coalesce multiple edges to the same node.
705 /// - Distribute the mass accordingly, dithering to minimize mass loss,
706 /// as described in \a distributeMass().
708 /// Finally, calculate the loop scale from the accumulated backedge mass.
710 /// 3. Distribute mass in the function (\a computeMassInFunction()).
712 /// Finally, distribute mass through the DAG resulting from packaging all
713 /// loops in the function. This uses the same algorithm as distributing
714 /// mass in a loop, except that there are no exit or backedge edges.
716 /// 4. Unpackage loops (\a unwrapLoops()).
718 /// Initialize each block's frequency to a floating point representation of
721 /// Visit loops top-down, scaling the frequencies of its immediate members
722 /// by the loop's pseudo-node's frequency.
724 /// 5. Convert frequencies to a 64-bit range (\a finalizeMetrics()).
726 /// Using the min and max frequencies as a guide, translate floating point
727 /// frequencies to an appropriate range in uint64_t.
729 /// It has some known flaws.
731 /// - Loop scale is limited to 4096 per loop (2^12) to avoid exhausting
732 /// BlockFrequency's 64-bit integer precision.
734 /// - The model of irreducible control flow is a rough approximation.
736 /// Modelling irreducible control flow exactly involves setting up and
737 /// solving a group of infinite geometric series. Such precision is
738 /// unlikely to be worthwhile, since most of our algorithms give up on
739 /// irreducible control flow anyway.
741 /// Nevertheless, we might find that we need to get closer. Here's a sort
742 /// of TODO list for the model with diminishing returns, to be completed as
745 /// - The headers for the \a LoopData representing an irreducible SCC
746 /// include non-entry blocks. When these extra blocks exist, they
747 /// indicate a self-contained irreducible sub-SCC. We could treat them
748 /// as sub-loops, rather than arbitrarily shoving the problematic
749 /// blocks into the headers of the main irreducible SCC.
751 /// - Backedge frequencies are assumed to be evenly split between the
752 /// headers of a given irreducible SCC. Instead, we could track the
753 /// backedge mass separately for each header, and adjust their relative
756 /// - Entry frequencies are assumed to be evenly split between the
757 /// headers of a given irreducible SCC, which is the only option if we
758 /// need to compute mass in the SCC before its parent loop. Instead,
759 /// we could partially compute mass in the parent loop, and stop when
760 /// we get to the SCC. Here, we have the correct ratio of entry
761 /// masses, which we can use to adjust their relative frequencies.
762 /// Compute mass in the SCC, and then continue propagation in the
765 /// - We can propagate mass iteratively through the SCC, for some fixed
766 /// number of iterations. Each iteration starts by assigning the entry
767 /// blocks their backedge mass from the prior iteration. The final
768 /// mass for each block (and each exit, and the total backedge mass
769 /// used for computing loop scale) is the sum of all iterations.
770 /// (Running this until fixed point would "solve" the geometric
771 /// series by simulation.)
772 template <class BT> class BlockFrequencyInfoImpl : BlockFrequencyInfoImplBase {
773 typedef typename bfi_detail::TypeMap<BT>::BlockT BlockT;
774 typedef typename bfi_detail::TypeMap<BT>::FunctionT FunctionT;
775 typedef typename bfi_detail::TypeMap<BT>::BranchProbabilityInfoT
776 BranchProbabilityInfoT;
777 typedef typename bfi_detail::TypeMap<BT>::LoopT LoopT;
778 typedef typename bfi_detail::TypeMap<BT>::LoopInfoT LoopInfoT;
780 // This is part of a workaround for a GCC 4.7 crash on lambdas.
781 friend struct bfi_detail::BlockEdgesAdder<BT>;
783 typedef GraphTraits<const BlockT *> Successor;
784 typedef GraphTraits<Inverse<const BlockT *>> Predecessor;
786 const BranchProbabilityInfoT *BPI;
790 // All blocks in reverse postorder.
791 std::vector<const BlockT *> RPOT;
792 DenseMap<const BlockT *, BlockNode> Nodes;
794 typedef typename std::vector<const BlockT *>::const_iterator rpot_iterator;
796 rpot_iterator rpot_begin() const { return RPOT.begin(); }
797 rpot_iterator rpot_end() const { return RPOT.end(); }
799 size_t getIndex(const rpot_iterator &I) const { return I - rpot_begin(); }
801 BlockNode getNode(const rpot_iterator &I) const {
802 return BlockNode(getIndex(I));
804 BlockNode getNode(const BlockT *BB) const { return Nodes.lookup(BB); }
806 const BlockT *getBlock(const BlockNode &Node) const {
807 assert(Node.Index < RPOT.size());
808 return RPOT[Node.Index];
811 /// \brief Run (and save) a post-order traversal.
813 /// Saves a reverse post-order traversal of all the nodes in \a F.
814 void initializeRPOT();
816 /// \brief Initialize loop data.
818 /// Build up \a Loops using \a LoopInfo. \a LoopInfo gives us a mapping from
819 /// each block to the deepest loop it's in, but we need the inverse. For each
820 /// loop, we store in reverse post-order its "immediate" members, defined as
821 /// the header, the headers of immediate sub-loops, and all other blocks in
822 /// the loop that are not in sub-loops.
823 void initializeLoops();
825 /// \brief Propagate to a block's successors.
827 /// In the context of distributing mass through \c OuterLoop, divide the mass
828 /// currently assigned to \c Node between its successors.
830 /// \return \c true unless there's an irreducible backedge.
831 bool propagateMassToSuccessors(LoopData *OuterLoop, const BlockNode &Node);
833 /// \brief Compute mass in a particular loop.
835 /// Assign mass to \c Loop's header, and then for each block in \c Loop in
836 /// reverse post-order, distribute mass to its successors. Only visits nodes
837 /// that have not been packaged into sub-loops.
839 /// \pre \a computeMassInLoop() has been called for each subloop of \c Loop.
840 /// \return \c true unless there's an irreducible backedge.
841 bool computeMassInLoop(LoopData &Loop);
843 /// \brief Try to compute mass in the top-level function.
845 /// Assign mass to the entry block, and then for each block in reverse
846 /// post-order, distribute mass to its successors. Skips nodes that have
847 /// been packaged into loops.
849 /// \pre \a computeMassInLoops() has been called.
850 /// \return \c true unless there's an irreducible backedge.
851 bool tryToComputeMassInFunction();
853 /// \brief Compute mass in (and package up) irreducible SCCs.
855 /// Find the irreducible SCCs in \c OuterLoop, add them to \a Loops (in front
856 /// of \c Insert), and call \a computeMassInLoop() on each of them.
858 /// If \c OuterLoop is \c nullptr, it refers to the top-level function.
860 /// \pre \a computeMassInLoop() has been called for each subloop of \c
862 /// \pre \c Insert points at the the last loop successfully processed by \a
863 /// computeMassInLoop().
864 /// \pre \c OuterLoop has irreducible SCCs.
865 void computeIrreducibleMass(LoopData *OuterLoop,
866 std::list<LoopData>::iterator Insert);
868 /// \brief Compute mass in all loops.
870 /// For each loop bottom-up, call \a computeMassInLoop().
872 /// \a computeMassInLoop() aborts (and returns \c false) on loops that
873 /// contain a irreducible sub-SCCs. Use \a computeIrreducibleMass() and then
874 /// re-enter \a computeMassInLoop().
876 /// \post \a computeMassInLoop() has returned \c true for every loop.
877 void computeMassInLoops();
879 /// \brief Compute mass in the top-level function.
881 /// Uses \a tryToComputeMassInFunction() and \a computeIrreducibleMass() to
882 /// compute mass in the top-level function.
884 /// \post \a tryToComputeMassInFunction() has returned \c true.
885 void computeMassInFunction();
887 std::string getBlockName(const BlockNode &Node) const override {
888 return bfi_detail::getBlockName(getBlock(Node));
892 const FunctionT *getFunction() const { return F; }
894 void doFunction(const FunctionT *F, const BranchProbabilityInfoT *BPI,
895 const LoopInfoT *LI);
896 BlockFrequencyInfoImpl() : BPI(nullptr), LI(nullptr), F(nullptr) {}
898 using BlockFrequencyInfoImplBase::getEntryFreq;
899 BlockFrequency getBlockFreq(const BlockT *BB) const {
900 return BlockFrequencyInfoImplBase::getBlockFreq(getNode(BB));
902 Scaled64 getFloatingBlockFreq(const BlockT *BB) const {
903 return BlockFrequencyInfoImplBase::getFloatingBlockFreq(getNode(BB));
906 /// \brief Print the frequencies for the current function.
908 /// Prints the frequencies for the blocks in the current function.
910 /// Blocks are printed in the natural iteration order of the function, rather
911 /// than reverse post-order. This provides two advantages: writing -analyze
912 /// tests is easier (since blocks come out in source order), and even
913 /// unreachable blocks are printed.
915 /// \a BlockFrequencyInfoImplBase::print() only knows reverse post-order, so
916 /// we need to override it here.
917 raw_ostream &print(raw_ostream &OS) const override;
918 using BlockFrequencyInfoImplBase::dump;
920 using BlockFrequencyInfoImplBase::printBlockFreq;
921 raw_ostream &printBlockFreq(raw_ostream &OS, const BlockT *BB) const {
922 return BlockFrequencyInfoImplBase::printBlockFreq(OS, getNode(BB));
927 void BlockFrequencyInfoImpl<BT>::doFunction(const FunctionT *F,
928 const BranchProbabilityInfoT *BPI,
929 const LoopInfoT *LI) {
930 // Save the parameters.
935 // Clean up left-over data structures.
936 BlockFrequencyInfoImplBase::clear();
941 DEBUG(dbgs() << "\nblock-frequency: " << F->getName() << "\n================="
942 << std::string(F->getName().size(), '=') << "\n");
946 // Visit loops in post-order to find thelocal mass distribution, and then do
947 // the full function.
948 computeMassInLoops();
949 computeMassInFunction();
954 template <class BT> void BlockFrequencyInfoImpl<BT>::initializeRPOT() {
955 const BlockT *Entry = F->begin();
956 RPOT.reserve(F->size());
957 std::copy(po_begin(Entry), po_end(Entry), std::back_inserter(RPOT));
958 std::reverse(RPOT.begin(), RPOT.end());
960 assert(RPOT.size() - 1 <= BlockNode::getMaxIndex() &&
961 "More nodes in function than Block Frequency Info supports");
963 DEBUG(dbgs() << "reverse-post-order-traversal\n");
964 for (rpot_iterator I = rpot_begin(), E = rpot_end(); I != E; ++I) {
965 BlockNode Node = getNode(I);
966 DEBUG(dbgs() << " - " << getIndex(I) << ": " << getBlockName(Node) << "\n");
970 Working.reserve(RPOT.size());
971 for (size_t Index = 0; Index < RPOT.size(); ++Index)
972 Working.emplace_back(Index);
973 Freqs.resize(RPOT.size());
976 template <class BT> void BlockFrequencyInfoImpl<BT>::initializeLoops() {
977 DEBUG(dbgs() << "loop-detection\n");
981 // Visit loops top down and assign them an index.
982 std::deque<std::pair<const LoopT *, LoopData *>> Q;
983 for (const LoopT *L : *LI)
984 Q.emplace_back(L, nullptr);
986 const LoopT *Loop = Q.front().first;
987 LoopData *Parent = Q.front().second;
990 BlockNode Header = getNode(Loop->getHeader());
991 assert(Header.isValid());
993 Loops.emplace_back(Parent, Header);
994 Working[Header.Index].Loop = &Loops.back();
995 DEBUG(dbgs() << " - loop = " << getBlockName(Header) << "\n");
997 for (const LoopT *L : *Loop)
998 Q.emplace_back(L, &Loops.back());
1001 // Visit nodes in reverse post-order and add them to their deepest containing
1003 for (size_t Index = 0; Index < RPOT.size(); ++Index) {
1004 // Loop headers have already been mostly mapped.
1005 if (Working[Index].isLoopHeader()) {
1006 LoopData *ContainingLoop = Working[Index].getContainingLoop();
1008 ContainingLoop->Nodes.push_back(Index);
1012 const LoopT *Loop = LI->getLoopFor(RPOT[Index]);
1016 // Add this node to its containing loop's member list.
1017 BlockNode Header = getNode(Loop->getHeader());
1018 assert(Header.isValid());
1019 const auto &HeaderData = Working[Header.Index];
1020 assert(HeaderData.isLoopHeader());
1022 Working[Index].Loop = HeaderData.Loop;
1023 HeaderData.Loop->Nodes.push_back(Index);
1024 DEBUG(dbgs() << " - loop = " << getBlockName(Header)
1025 << ": member = " << getBlockName(Index) << "\n");
1029 template <class BT> void BlockFrequencyInfoImpl<BT>::computeMassInLoops() {
1030 // Visit loops with the deepest first, and the top-level loops last.
1031 for (auto L = Loops.rbegin(), E = Loops.rend(); L != E; ++L) {
1032 if (computeMassInLoop(*L))
1034 auto Next = std::next(L);
1035 computeIrreducibleMass(&*L, L.base());
1036 L = std::prev(Next);
1037 if (computeMassInLoop(*L))
1039 llvm_unreachable("unhandled irreducible control flow");
1044 bool BlockFrequencyInfoImpl<BT>::computeMassInLoop(LoopData &Loop) {
1045 // Compute mass in loop.
1046 DEBUG(dbgs() << "compute-mass-in-loop: " << getLoopName(Loop) << "\n");
1048 if (Loop.isIrreducible()) {
1049 BlockMass Remaining = BlockMass::getFull();
1050 for (uint32_t H = 0; H < Loop.NumHeaders; ++H) {
1051 auto &Mass = Working[Loop.Nodes[H].Index].getMass();
1052 Mass = Remaining * BranchProbability(1, Loop.NumHeaders - H);
1055 for (const BlockNode &M : Loop.Nodes)
1056 if (!propagateMassToSuccessors(&Loop, M))
1057 llvm_unreachable("unhandled irreducible control flow");
1059 Working[Loop.getHeader().Index].getMass() = BlockMass::getFull();
1060 if (!propagateMassToSuccessors(&Loop, Loop.getHeader()))
1061 llvm_unreachable("irreducible control flow to loop header!?");
1062 for (const BlockNode &M : Loop.members())
1063 if (!propagateMassToSuccessors(&Loop, M))
1064 // Irreducible backedge.
1068 computeLoopScale(Loop);
1074 bool BlockFrequencyInfoImpl<BT>::tryToComputeMassInFunction() {
1075 // Compute mass in function.
1076 DEBUG(dbgs() << "compute-mass-in-function\n");
1077 assert(!Working.empty() && "no blocks in function");
1078 assert(!Working[0].isLoopHeader() && "entry block is a loop header");
1080 Working[0].getMass() = BlockMass::getFull();
1081 for (rpot_iterator I = rpot_begin(), IE = rpot_end(); I != IE; ++I) {
1082 // Check for nodes that have been packaged.
1083 BlockNode Node = getNode(I);
1084 if (Working[Node.Index].isPackaged())
1087 if (!propagateMassToSuccessors(nullptr, Node))
1093 template <class BT> void BlockFrequencyInfoImpl<BT>::computeMassInFunction() {
1094 if (tryToComputeMassInFunction())
1096 computeIrreducibleMass(nullptr, Loops.begin());
1097 if (tryToComputeMassInFunction())
1099 llvm_unreachable("unhandled irreducible control flow");
1102 /// \note This should be a lambda, but that crashes GCC 4.7.
1103 namespace bfi_detail {
1104 template <class BT> struct BlockEdgesAdder {
1106 typedef BlockFrequencyInfoImplBase::LoopData LoopData;
1107 typedef GraphTraits<const BlockT *> Successor;
1109 const BlockFrequencyInfoImpl<BT> &BFI;
1110 explicit BlockEdgesAdder(const BlockFrequencyInfoImpl<BT> &BFI)
1112 void operator()(IrreducibleGraph &G, IrreducibleGraph::IrrNode &Irr,
1113 const LoopData *OuterLoop) {
1114 const BlockT *BB = BFI.RPOT[Irr.Node.Index];
1115 for (auto I = Successor::child_begin(BB), E = Successor::child_end(BB);
1117 G.addEdge(Irr, BFI.getNode(*I), OuterLoop);
1122 void BlockFrequencyInfoImpl<BT>::computeIrreducibleMass(
1123 LoopData *OuterLoop, std::list<LoopData>::iterator Insert) {
1124 DEBUG(dbgs() << "analyze-irreducible-in-";
1125 if (OuterLoop) dbgs() << "loop: " << getLoopName(*OuterLoop) << "\n";
1126 else dbgs() << "function\n");
1128 using namespace bfi_detail;
1129 // Ideally, addBlockEdges() would be declared here as a lambda, but that
1131 BlockEdgesAdder<BT> addBlockEdges(*this);
1132 IrreducibleGraph G(*this, OuterLoop, addBlockEdges);
1134 for (auto &L : analyzeIrreducible(G, OuterLoop, Insert))
1135 computeMassInLoop(L);
1139 updateLoopWithIrreducible(*OuterLoop);
1144 BlockFrequencyInfoImpl<BT>::propagateMassToSuccessors(LoopData *OuterLoop,
1145 const BlockNode &Node) {
1146 DEBUG(dbgs() << " - node: " << getBlockName(Node) << "\n");
1147 // Calculate probability for successors.
1149 if (auto *Loop = Working[Node.Index].getPackagedLoop()) {
1150 assert(Loop != OuterLoop && "Cannot propagate mass in a packaged loop");
1151 if (!addLoopSuccessorsToDist(OuterLoop, *Loop, Dist))
1152 // Irreducible backedge.
1155 const BlockT *BB = getBlock(Node);
1156 for (auto SI = Successor::child_begin(BB), SE = Successor::child_end(BB);
1158 // Do not dereference SI, or getEdgeWeight() is linear in the number of
1160 if (!addToDist(Dist, OuterLoop, Node, getNode(*SI),
1161 BPI->getEdgeWeight(BB, SI)))
1162 // Irreducible backedge.
1166 // Distribute mass to successors, saving exit and backedge data in the
1168 distributeMass(Node, OuterLoop, Dist);
1173 raw_ostream &BlockFrequencyInfoImpl<BT>::print(raw_ostream &OS) const {
1176 OS << "block-frequency-info: " << F->getName() << "\n";
1177 for (const BlockT &BB : *F)
1178 OS << " - " << bfi_detail::getBlockName(&BB)
1179 << ": float = " << getFloatingBlockFreq(&BB)
1180 << ", int = " << getBlockFreq(&BB).getFrequency() << "\n";
1182 // Add an extra newline for readability.