//===----------------------------------------------------------------------===//
//
// Shared implementation of BlockFrequency for IR and Machine Instructions.
+// See the documentation below for BlockFrequencyInfoImpl for details.
//
//===----------------------------------------------------------------------===//
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/PostOrderIterator.h"
+#include "llvm/ADT/SCCIterator.h"
#include "llvm/ADT/iterator_range.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/Support/BlockFrequency.h"
#include "llvm/Support/BranchProbability.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
+#include <deque>
+#include <list>
#include <string>
#include <vector>
-#include <list>
#define DEBUG_TYPE "block-freq"
return *this;
}
- /// \brief Scale by another mass.
- ///
- /// The current implementation is a little imprecise, but it's relatively
- /// fast, never overflows, and maintains the property that 1.0*1.0==1.0
- /// (where isFull represents the number 1.0). It's an approximation of
- /// 128-bit multiply that gets right-shifted by 64-bits.
- ///
- /// For a given digit size, multiplying two-digit numbers looks like:
- ///
- /// U1 . L1
- /// * U2 . L2
- /// ============
- /// 0 . . L1*L2
- /// + 0 . U1*L2 . 0 // (shift left once by a digit-size)
- /// + 0 . U2*L1 . 0 // (shift left once by a digit-size)
- /// + U1*L2 . 0 . 0 // (shift left twice by a digit-size)
- ///
- /// BlockMass has 64-bit numbers. Split each into two 32-bit digits, stored
- /// 64-bit. Add 1 to the lower digits, to model isFull as 1.0; this won't
- /// overflow, since we have 64-bit storage for each digit.
- ///
- /// To do this accurately, (a) multiply into two 64-bit digits, incrementing
- /// the upper digit on overflows of the lower digit (carry), (b) subtract 1
- /// from the lower digit, decrementing the upper digit on underflow (carry),
- /// and (c) truncate the lower digit. For the 1.0*1.0 case, the upper digit
- /// will be 0 at the end of step (a), and then will underflow back to isFull
- /// (1.0) in step (b).
- ///
- /// Instead, the implementation does something a little faster with a small
- /// loss of accuracy: ignore the lower 64-bit digit entirely. The loss of
- /// accuracy is small, since the sum of the unmodelled carries is 0 or 1
- /// (i.e., step (a) will overflow at most once, and step (b) will underflow
- /// only if step (a) overflows).
- ///
- /// This is the formula we're calculating:
- ///
- /// U1.L1 * U2.L2 == U1 * U2 + (U1 * (L2+1))>>32 + (U2 * (L1+1))>>32
- ///
- /// As a demonstration of 1.0*1.0, consider two 4-bit numbers that are both
- /// full (1111).
- ///
- /// U1.L1 * U2.L2 == U1 * U2 + (U1 * (L2+1))>>2 + (U2 * (L1+1))>>2
- /// 11.11 * 11.11 == 11 * 11 + (11 * (11+1))/4 + (11 * (11+1))/4
- /// == 1001 + (11 * 100)/4 + (11 * 100)/4
- /// == 1001 + 1100/4 + 1100/4
- /// == 1001 + 0011 + 0011
- /// == 1111
- BlockMass &operator*=(const BlockMass &X) {
- uint64_t U1 = Mass >> 32, L1 = Mass & UINT32_MAX, U2 = X.Mass >> 32,
- L2 = X.Mass & UINT32_MAX;
- Mass = U1 * U2 + (U1 * (L2 + 1) >> 32) + ((L1 + 1) * U2 >> 32);
+ BlockMass &operator*=(const BranchProbability &P) {
+ Mass = P.scale(Mass);
return *this;
}
- /// \brief Multiply by a branch probability.
- ///
- /// Multiply by P. Guarantees full precision.
- ///
- /// This could be naively implemented by multiplying by the numerator and
- /// dividing by the denominator, but in what order? Multiplying first can
- /// overflow, while dividing first will lose precision (potentially, changing
- /// a non-zero mass to zero).
- ///
- /// The implementation mixes the two methods. Since \a BranchProbability
- /// uses 32-bits and \a BlockMass 64-bits, shift the mass as far to the left
- /// as there is room, then divide by the denominator to get a quotient.
- /// Multiplying by the numerator and right shifting gives a first
- /// approximation.
- ///
- /// Calculate the error in this first approximation by calculating the
- /// opposite mass (multiply by the opposite numerator and shift) and
- /// subtracting both from teh original mass.
- ///
- /// Add to the first approximation the correct fraction of this error value.
- /// This time, multiply first and then divide, since there is no danger of
- /// overflow.
- ///
- /// \pre P represents a fraction between 0.0 and 1.0.
- BlockMass &operator*=(const BranchProbability &P);
-
bool operator==(const BlockMass &X) const { return Mass == X.Mass; }
bool operator!=(const BlockMass &X) const { return Mass != X.Mass; }
bool operator<=(const BlockMass &X) const { return Mass <= X.Mass; }
inline BlockMass operator-(const BlockMass &L, const BlockMass &R) {
return BlockMass(L) -= R;
}
-inline BlockMass operator*(const BlockMass &L, const BlockMass &R) {
- return BlockMass(L) *= R;
-}
inline BlockMass operator*(const BlockMass &L, const BranchProbability &R) {
return BlockMass(L) *= R;
}
class MachineLoop;
class MachineLoopInfo;
+namespace bfi_detail {
+struct IrreducibleGraph;
+
+// This is part of a workaround for a GCC 4.7 crash on lambdas.
+template <class BT> struct BlockEdgesAdder;
+}
+
/// \brief Base class for BlockFrequencyInfoImpl
///
/// BlockFrequencyInfoImplBase has supporting data structures and some
typedef SmallVector<BlockNode, 4> NodeList;
LoopData *Parent; ///< The parent loop.
bool IsPackaged; ///< Whether this has been packaged.
+ uint32_t NumHeaders; ///< Number of headers.
ExitMap Exits; ///< Successor edges (and weights).
NodeList Nodes; ///< Header and the members of the loop.
BlockMass BackedgeMass; ///< Mass returned to loop header.
Float Scale;
LoopData(LoopData *Parent, const BlockNode &Header)
- : Parent(Parent), IsPackaged(false), Nodes(1, Header) {}
- bool isHeader(const BlockNode &Node) const { return Node == Nodes[0]; }
+ : Parent(Parent), IsPackaged(false), NumHeaders(1), Nodes(1, Header) {}
+ template <class It1, class It2>
+ LoopData(LoopData *Parent, It1 FirstHeader, It1 LastHeader, It2 FirstOther,
+ It2 LastOther)
+ : Parent(Parent), IsPackaged(false), Nodes(FirstHeader, LastHeader) {
+ NumHeaders = Nodes.size();
+ Nodes.insert(Nodes.end(), FirstOther, LastOther);
+ }
+ bool isHeader(const BlockNode &Node) const {
+ if (isIrreducible())
+ return std::binary_search(Nodes.begin(), Nodes.begin() + NumHeaders,
+ Node);
+ return Node == Nodes[0];
+ }
BlockNode getHeader() const { return Nodes[0]; }
+ bool isIrreducible() const { return NumHeaders > 1; }
- NodeList::const_iterator members_begin() const { return Nodes.begin() + 1; }
+ NodeList::const_iterator members_begin() const {
+ return Nodes.begin() + NumHeaders;
+ }
NodeList::const_iterator members_end() const { return Nodes.end(); }
iterator_range<NodeList::const_iterator> members() const {
return make_range(members_begin(), members_end());
WorkingData(const BlockNode &Node) : Node(Node), Loop(nullptr) {}
bool isLoopHeader() const { return Loop && Loop->isHeader(Node); }
+ bool isDoubleLoopHeader() const {
+ return isLoopHeader() && Loop->Parent && Loop->Parent->isIrreducible() &&
+ Loop->Parent->isHeader(Node);
+ }
LoopData *getContainingLoop() const {
- return isLoopHeader() ? Loop->Parent : Loop;
+ if (!isLoopHeader())
+ return Loop;
+ if (!isDoubleLoopHeader())
+ return Loop->Parent;
+ return Loop->Parent->Parent;
}
/// \brief Resolve a node to its representative.
/// Get appropriate mass for Node. If Node is a loop-header (whose loop
/// has been packaged), returns the mass of its pseudo-node. If it's a
/// node inside a packaged loop, it returns the loop's mass.
- BlockMass &getMass() { return isAPackage() ? Loop->Mass : Mass; }
+ BlockMass &getMass() {
+ if (!isAPackage())
+ return Mass;
+ if (!isADoublePackage())
+ return Loop->Mass;
+ return Loop->Parent->Mass;
+ }
/// \brief Has ContainingLoop been packaged up?
bool isPackaged() const { return getResolvedNode() != Node; }
/// \brief Has Loop been packaged up?
bool isAPackage() const { return isLoopHeader() && Loop->IsPackaged; }
+ /// \brief Has Loop been packaged up twice?
+ bool isADoublePackage() const {
+ return isDoubleLoopHeader() && Loop->Parent->IsPackaged;
+ }
};
/// \brief Unscaled probability weight.
///
/// Adds all edges from LocalLoopHead to Dist. Calls addToDist() to add each
/// successor edge.
- void addLoopSuccessorsToDist(const LoopData *OuterLoop, LoopData &Loop,
+ ///
+ /// \return \c true unless there's an irreducible backedge.
+ bool addLoopSuccessorsToDist(const LoopData *OuterLoop, LoopData &Loop,
Distribution &Dist);
/// \brief Add an edge to the distribution.
/// Adds an edge to Succ to Dist. If \c LoopHead.isValid(), then whether the
/// edge is local/exit/backedge is in the context of LoopHead. Otherwise,
/// every edge should be a local edge (since all the loops are packaged up).
- void addToDist(Distribution &Dist, const LoopData *OuterLoop,
+ ///
+ /// \return \c true unless aborted due to an irreducible backedge.
+ bool addToDist(Distribution &Dist, const LoopData *OuterLoop,
const BlockNode &Pred, const BlockNode &Succ, uint64_t Weight);
LoopData &getLoopPackage(const BlockNode &Head) {
return *Working[Head.Index].Loop;
}
+ /// \brief Analyze irreducible SCCs.
+ ///
+ /// Separate irreducible SCCs from \c G, which is an explict graph of \c
+ /// OuterLoop (or the top-level function, if \c OuterLoop is \c nullptr).
+ /// Insert them into \a Loops before \c Insert.
+ ///
+ /// \return the \c LoopData nodes representing the irreducible SCCs.
+ iterator_range<std::list<LoopData>::iterator>
+ analyzeIrreducible(const bfi_detail::IrreducibleGraph &G, LoopData *OuterLoop,
+ std::list<LoopData>::iterator Insert);
+
+ /// \brief Update a loop after packaging irreducible SCCs inside of it.
+ ///
+ /// Update \c OuterLoop. Before finding irreducible control flow, it was
+ /// partway through \a computeMassInLoop(), so \a LoopData::Exits and \a
+ /// LoopData::BackedgeMass need to be reset. Also, nodes that were packaged
+ /// up need to be removed from \a OuterLoop::Nodes.
+ void updateLoopWithIrreducible(LoopData &OuterLoop);
+
/// \brief Distribute mass according to a distribution.
///
/// Distributes the mass in Source according to Dist. If LoopHead.isValid(),
void clear();
virtual std::string getBlockName(const BlockNode &Node) const;
+ std::string getLoopName(const LoopData &Loop) const;
virtual raw_ostream &print(raw_ostream &OS) const { return OS; }
void dump() const { print(dbgs()); }
assert(BB && "Unexpected nullptr");
return BB->getName().str();
}
+
+/// \brief Graph of irreducible control flow.
+///
+/// This graph is used for determining the SCCs in a loop (or top-level
+/// function) that has irreducible control flow.
+///
+/// During the block frequency algorithm, the local graphs are defined in a
+/// light-weight way, deferring to the \a BasicBlock or \a MachineBasicBlock
+/// graphs for most edges, but getting others from \a LoopData::ExitMap. The
+/// latter only has successor information.
+///
+/// \a IrreducibleGraph makes this graph explicit. It's in a form that can use
+/// \a GraphTraits (so that \a analyzeIrreducible() can use \a scc_iterator),
+/// and it explicitly lists predecessors and successors. The initialization
+/// that relies on \c MachineBasicBlock is defined in the header.
+struct IrreducibleGraph {
+ typedef BlockFrequencyInfoImplBase BFIBase;
+
+ BFIBase &BFI;
+
+ typedef BFIBase::BlockNode BlockNode;
+ struct IrrNode {
+ BlockNode Node;
+ unsigned NumIn;
+ std::deque<const IrrNode *> Edges;
+ IrrNode(const BlockNode &Node) : Node(Node), NumIn(0) {}
+
+ typedef std::deque<const IrrNode *>::const_iterator iterator;
+ iterator pred_begin() const { return Edges.begin(); }
+ iterator succ_begin() const { return Edges.begin() + NumIn; }
+ iterator pred_end() const { return succ_begin(); }
+ iterator succ_end() const { return Edges.end(); }
+ };
+ BlockNode Start;
+ const IrrNode *StartIrr;
+ std::vector<IrrNode> Nodes;
+ SmallDenseMap<uint32_t, IrrNode *, 4> Lookup;
+
+ /// \brief Construct an explicit graph containing irreducible control flow.
+ ///
+ /// Construct an explicit graph of the control flow in \c OuterLoop (or the
+ /// top-level function, if \c OuterLoop is \c nullptr). Uses \c
+ /// addBlockEdges to add block successors that have not been packaged into
+ /// loops.
+ ///
+ /// \a BlockFrequencyInfoImpl::computeIrreducibleMass() is the only expected
+ /// user of this.
+ template <class BlockEdgesAdder>
+ IrreducibleGraph(BFIBase &BFI, const BFIBase::LoopData *OuterLoop,
+ BlockEdgesAdder addBlockEdges)
+ : BFI(BFI), StartIrr(nullptr) {
+ initialize(OuterLoop, addBlockEdges);
+ }
+
+ template <class BlockEdgesAdder>
+ void initialize(const BFIBase::LoopData *OuterLoop,
+ BlockEdgesAdder addBlockEdges);
+ void addNodesInLoop(const BFIBase::LoopData &OuterLoop);
+ void addNodesInFunction();
+ void addNode(const BlockNode &Node) {
+ Nodes.emplace_back(Node);
+ BFI.Working[Node.Index].getMass() = BlockMass::getEmpty();
+ }
+ void indexNodes();
+ template <class BlockEdgesAdder>
+ void addEdges(const BlockNode &Node, const BFIBase::LoopData *OuterLoop,
+ BlockEdgesAdder addBlockEdges);
+ void addEdge(IrrNode &Irr, const BlockNode &Succ,
+ const BFIBase::LoopData *OuterLoop);
+};
+template <class BlockEdgesAdder>
+void IrreducibleGraph::initialize(const BFIBase::LoopData *OuterLoop,
+ BlockEdgesAdder addBlockEdges) {
+ if (OuterLoop) {
+ addNodesInLoop(*OuterLoop);
+ for (auto N : OuterLoop->Nodes)
+ addEdges(N, OuterLoop, addBlockEdges);
+ } else {
+ addNodesInFunction();
+ for (uint32_t Index = 0; Index < BFI.Working.size(); ++Index)
+ addEdges(Index, OuterLoop, addBlockEdges);
+ }
+ StartIrr = Lookup[Start.Index];
+}
+template <class BlockEdgesAdder>
+void IrreducibleGraph::addEdges(const BlockNode &Node,
+ const BFIBase::LoopData *OuterLoop,
+ BlockEdgesAdder addBlockEdges) {
+ auto L = Lookup.find(Node.Index);
+ if (L == Lookup.end())
+ return;
+ IrrNode &Irr = *L->second;
+ const auto &Working = BFI.Working[Node.Index];
+
+ if (Working.isAPackage())
+ for (const auto &I : Working.Loop->Exits)
+ addEdge(Irr, I.first, OuterLoop);
+ else
+ addBlockEdges(*this, Irr, OuterLoop);
+}
}
/// \brief Shared implementation for block frequency analysis.
/// MachineBlockFrequencyInfo, and calculates the relative frequencies of
/// blocks.
///
+/// LoopInfo defines a loop as a "non-trivial" SCC dominated by a single block,
+/// which is called the header. A given loop, L, can have sub-loops, which are
+/// loops within the subgraph of L that exclude its header. (A "trivial" SCC
+/// consists of a single block that does not have a self-edge.)
+///
+/// In addition to loops, this algorithm has limited support for irreducible
+/// SCCs, which are SCCs with multiple entry blocks. Irreducible SCCs are
+/// discovered on they fly, and modelled as loops with multiple headers.
+///
+/// The headers of irreducible sub-SCCs consist of its entry blocks and all
+/// nodes that are targets of a backedge within it (excluding backedges within
+/// true sub-loops). Block frequency calculations act as if a block is
+/// inserted that intercepts all the edges to the headers. All backedges and
+/// entries point to this block. Its successors are the headers, which split
+/// the frequency evenly.
+///
/// This algorithm leverages BlockMass and UnsignedFloat to maintain precision,
/// separates mass distribution from loop scaling, and dithers to eliminate
/// probability mass loss.
/// All other stages make use of this ordering. Save a lookup from BlockT
/// to BlockNode (the index into RPOT) in Nodes.
///
-/// 1. Loop indexing (\a initializeLoops()).
+/// 1. Loop initialization (\a initializeLoops()).
///
/// Translate LoopInfo/MachineLoopInfo into a form suitable for the rest of
/// the algorithm. In particular, store the immediate members of each loop
/// For each loop (bottom-up), distribute mass through the DAG resulting
/// from ignoring backedges and treating sub-loops as a single pseudo-node.
/// Track the backedge mass distributed to the loop header, and use it to
-/// calculate the loop scale (number of loop iterations).
-///
-/// Visiting loops bottom-up is a post-order traversal of loop headers.
-/// For each loop, immediate members that represent sub-loops will already
-/// have been visited and packaged into a pseudo-node.
+/// calculate the loop scale (number of loop iterations). Immediate
+/// members that represent sub-loops will already have been visited and
+/// packaged into a pseudo-node.
///
/// Distributing mass in a loop is a reverse-post-order traversal through
/// the loop. Start by assigning full mass to the Loop header. For each
/// The weight, the successor, and its category are stored in \a
/// Distribution. There can be multiple edges to each successor.
///
+/// - If there's a backedge to a non-header, there's an irreducible SCC.
+/// The usual flow is temporarily aborted. \a
+/// computeIrreducibleMass() finds the irreducible SCCs within the
+/// loop, packages them up, and restarts the flow.
+///
/// - Normalize the distribution: scale weights down so that their sum
/// is 32-bits, and coalesce multiple edges to the same node.
///
/// loops in the function. This uses the same algorithm as distributing
/// mass in a loop, except that there are no exit or backedge edges.
///
-/// 4. Loop unpackaging and cleanup (\a finalizeMetrics()).
+/// 4. Unpackage loops (\a unwrapLoops()).
+///
+/// Initialize each block's frequency to a floating point representation of
+/// its mass.
///
-/// Initialize the frequency to a floating point representation of its
-/// mass.
+/// Visit loops top-down, scaling the frequencies of its immediate members
+/// by the loop's pseudo-node's frequency.
///
-/// Visit loops top-down (reverse post-order), scaling the loop header's
-/// frequency by its psuedo-node's mass and loop scale. Keep track of the
-/// minimum and maximum final frequencies.
+/// 5. Convert frequencies to a 64-bit range (\a finalizeMetrics()).
///
/// Using the min and max frequencies as a guide, translate floating point
/// frequencies to an appropriate range in uint64_t.
///
/// It has some known flaws.
///
-/// - Irreducible control flow isn't modelled correctly. In particular,
-/// LoopInfo and MachineLoopInfo ignore irreducible backedges. The main
-/// result is that irreducible SCCs will under-scaled. No mass is lost,
-/// but the computed branch weights for the loop pseudo-node will be
-/// incorrect.
+/// - Loop scale is limited to 4096 per loop (2^12) to avoid exhausting
+/// BlockFrequency's 64-bit integer precision.
+///
+/// - The model of irreducible control flow is a rough approximation.
///
/// Modelling irreducible control flow exactly involves setting up and
/// solving a group of infinite geometric series. Such precision is
/// unlikely to be worthwhile, since most of our algorithms give up on
/// irreducible control flow anyway.
///
-/// Nevertheless, we might find that we need to get closer. If
-/// LoopInfo/MachineLoopInfo flags loops with irreducible control flow
-/// (and/or the function as a whole), we can find the SCCs, compute an
-/// approximate exit frequency for the SCC as a whole, and scale up
-/// accordingly.
+/// Nevertheless, we might find that we need to get closer. Here's a sort
+/// of TODO list for the model with diminishing returns, to be completed as
+/// necessary.
///
-/// - Loop scale is limited to 4096 per loop (2^12) to avoid exhausting
-/// BlockFrequency's 64-bit integer precision.
+/// - The headers for the \a LoopData representing an irreducible SCC
+/// include non-entry blocks. When these extra blocks exist, they
+/// indicate a self-contained irreducible sub-SCC. We could treat them
+/// as sub-loops, rather than arbitrarily shoving the problematic
+/// blocks into the headers of the main irreducible SCC.
+///
+/// - Backedge frequencies are assumed to be evenly split between the
+/// headers of a given irreducible SCC. Instead, we could track the
+/// backedge mass separately for each header, and adjust their relative
+/// frequencies.
+///
+/// - Entry frequencies are assumed to be evenly split between the
+/// headers of a given irreducible SCC, which is the only option if we
+/// need to compute mass in the SCC before its parent loop. Instead,
+/// we could partially compute mass in the parent loop, and stop when
+/// we get to the SCC. Here, we have the correct ratio of entry
+/// masses, which we can use to adjust their relative frequencies.
+/// Compute mass in the SCC, and then continue propagation in the
+/// parent.
+///
+/// - We can propagate mass iteratively through the SCC, for some fixed
+/// number of iterations. Each iteration starts by assigning the entry
+/// blocks their backedge mass from the prior iteration. The final
+/// mass for each block (and each exit, and the total backedge mass
+/// used for computing loop scale) is the sum of all iterations.
+/// (Running this until fixed point would "solve" the geometric
+/// series by simulation.)
template <class BT> class BlockFrequencyInfoImpl : BlockFrequencyInfoImplBase {
typedef typename bfi_detail::TypeMap<BT>::BlockT BlockT;
typedef typename bfi_detail::TypeMap<BT>::FunctionT FunctionT;
typedef typename bfi_detail::TypeMap<BT>::LoopT LoopT;
typedef typename bfi_detail::TypeMap<BT>::LoopInfoT LoopInfoT;
+ // This is part of a workaround for a GCC 4.7 crash on lambdas.
+ friend struct bfi_detail::BlockEdgesAdder<BT>;
+
typedef GraphTraits<const BlockT *> Successor;
typedef GraphTraits<Inverse<const BlockT *>> Predecessor;
///
/// In the context of distributing mass through \c OuterLoop, divide the mass
/// currently assigned to \c Node between its successors.
- void propagateMassToSuccessors(LoopData *OuterLoop, const BlockNode &Node);
+ ///
+ /// \return \c true unless there's an irreducible backedge.
+ bool propagateMassToSuccessors(LoopData *OuterLoop, const BlockNode &Node);
/// \brief Compute mass in a particular loop.
///
/// that have not been packaged into sub-loops.
///
/// \pre \a computeMassInLoop() has been called for each subloop of \c Loop.
- void computeMassInLoop(LoopData &Loop);
+ /// \return \c true unless there's an irreducible backedge.
+ bool computeMassInLoop(LoopData &Loop);
+
+ /// \brief Try to compute mass in the top-level function.
+ ///
+ /// Assign mass to the entry block, and then for each block in reverse
+ /// post-order, distribute mass to its successors. Skips nodes that have
+ /// been packaged into loops.
+ ///
+ /// \pre \a computeMassInLoops() has been called.
+ /// \return \c true unless there's an irreducible backedge.
+ bool tryToComputeMassInFunction();
+
+ /// \brief Compute mass in (and package up) irreducible SCCs.
+ ///
+ /// Find the irreducible SCCs in \c OuterLoop, add them to \a Loops (in front
+ /// of \c Insert), and call \a computeMassInLoop() on each of them.
+ ///
+ /// If \c OuterLoop is \c nullptr, it refers to the top-level function.
+ ///
+ /// \pre \a computeMassInLoop() has been called for each subloop of \c
+ /// OuterLoop.
+ /// \pre \c Insert points at the the last loop successfully processed by \a
+ /// computeMassInLoop().
+ /// \pre \c OuterLoop has irreducible SCCs.
+ void computeIrreducibleMass(LoopData *OuterLoop,
+ std::list<LoopData>::iterator Insert);
/// \brief Compute mass in all loops.
///
/// For each loop bottom-up, call \a computeMassInLoop().
+ ///
+ /// \a computeMassInLoop() aborts (and returns \c false) on loops that
+ /// contain a irreducible sub-SCCs. Use \a computeIrreducibleMass() and then
+ /// re-enter \a computeMassInLoop().
+ ///
+ /// \post \a computeMassInLoop() has returned \c true for every loop.
void computeMassInLoops();
/// \brief Compute mass in the top-level function.
///
- /// Assign mass to the entry block, and then for each block in reverse
- /// post-order, distribute mass to its successors. Skips nodes that have
- /// been packaged into loops.
+ /// Uses \a tryToComputeMassInFunction() and \a computeIrreducibleMass() to
+ /// compute mass in the top-level function.
///
- /// \pre \a computeMassInLoops() has been called.
+ /// \post \a tryToComputeMassInFunction() has returned \c true.
void computeMassInFunction();
std::string getBlockName(const BlockNode &Node) const override {
template <class BT> void BlockFrequencyInfoImpl<BT>::computeMassInLoops() {
// Visit loops with the deepest first, and the top-level loops last.
- for (auto L = Loops.rbegin(), E = Loops.rend(); L != E; ++L)
- computeMassInLoop(*L);
+ for (auto L = Loops.rbegin(), E = Loops.rend(); L != E; ++L) {
+ if (computeMassInLoop(*L))
+ continue;
+ auto Next = std::next(L);
+ computeIrreducibleMass(&*L, L.base());
+ L = std::prev(Next);
+ if (computeMassInLoop(*L))
+ continue;
+ llvm_unreachable("unhandled irreducible control flow");
+ }
}
template <class BT>
-void BlockFrequencyInfoImpl<BT>::computeMassInLoop(LoopData &Loop) {
+bool BlockFrequencyInfoImpl<BT>::computeMassInLoop(LoopData &Loop) {
// Compute mass in loop.
- DEBUG(dbgs() << "compute-mass-in-loop: " << getBlockName(Loop.getHeader())
- << "\n");
-
- Working[Loop.getHeader().Index].getMass() = BlockMass::getFull();
- propagateMassToSuccessors(&Loop, Loop.getHeader());
-
- for (const BlockNode &M : Loop.members())
- propagateMassToSuccessors(&Loop, M);
+ DEBUG(dbgs() << "compute-mass-in-loop: " << getLoopName(Loop) << "\n");
+
+ if (Loop.isIrreducible()) {
+ BlockMass Remaining = BlockMass::getFull();
+ for (uint32_t H = 0; H < Loop.NumHeaders; ++H) {
+ auto &Mass = Working[Loop.Nodes[H].Index].getMass();
+ Mass = Remaining * BranchProbability(1, Loop.NumHeaders - H);
+ Remaining -= Mass;
+ }
+ for (const BlockNode &M : Loop.Nodes)
+ if (!propagateMassToSuccessors(&Loop, M))
+ llvm_unreachable("unhandled irreducible control flow");
+ } else {
+ Working[Loop.getHeader().Index].getMass() = BlockMass::getFull();
+ if (!propagateMassToSuccessors(&Loop, Loop.getHeader()))
+ llvm_unreachable("irreducible control flow to loop header!?");
+ for (const BlockNode &M : Loop.members())
+ if (!propagateMassToSuccessors(&Loop, M))
+ // Irreducible backedge.
+ return false;
+ }
computeLoopScale(Loop);
packageLoop(Loop);
+ return true;
}
-template <class BT> void BlockFrequencyInfoImpl<BT>::computeMassInFunction() {
+template <class BT>
+bool BlockFrequencyInfoImpl<BT>::tryToComputeMassInFunction() {
// Compute mass in function.
DEBUG(dbgs() << "compute-mass-in-function\n");
assert(!Working.empty() && "no blocks in function");
if (Working[Node.Index].isPackaged())
continue;
- propagateMassToSuccessors(nullptr, Node);
+ if (!propagateMassToSuccessors(nullptr, Node))
+ return false;
}
+ return true;
}
+template <class BT> void BlockFrequencyInfoImpl<BT>::computeMassInFunction() {
+ if (tryToComputeMassInFunction())
+ return;
+ computeIrreducibleMass(nullptr, Loops.begin());
+ if (tryToComputeMassInFunction())
+ return;
+ llvm_unreachable("unhandled irreducible control flow");
+}
+
+/// \note This should be a lambda, but that crashes GCC 4.7.
+namespace bfi_detail {
+template <class BT> struct BlockEdgesAdder {
+ typedef BT BlockT;
+ typedef BlockFrequencyInfoImplBase::LoopData LoopData;
+ typedef GraphTraits<const BlockT *> Successor;
+
+ const BlockFrequencyInfoImpl<BT> &BFI;
+ explicit BlockEdgesAdder(const BlockFrequencyInfoImpl<BT> &BFI)
+ : BFI(BFI) {}
+ void operator()(IrreducibleGraph &G, IrreducibleGraph::IrrNode &Irr,
+ const LoopData *OuterLoop) {
+ const BlockT *BB = BFI.RPOT[Irr.Node.Index];
+ for (auto I = Successor::child_begin(BB), E = Successor::child_end(BB);
+ I != E; ++I)
+ G.addEdge(Irr, BFI.getNode(*I), OuterLoop);
+ }
+};
+}
template <class BT>
-void
+void BlockFrequencyInfoImpl<BT>::computeIrreducibleMass(
+ LoopData *OuterLoop, std::list<LoopData>::iterator Insert) {
+ DEBUG(dbgs() << "analyze-irreducible-in-";
+ if (OuterLoop) dbgs() << "loop: " << getLoopName(*OuterLoop) << "\n";
+ else dbgs() << "function\n");
+
+ using namespace bfi_detail;
+ // Ideally, addBlockEdges() would be declared here as a lambda, but that
+ // crashes GCC 4.7.
+ BlockEdgesAdder<BT> addBlockEdges(*this);
+ IrreducibleGraph G(*this, OuterLoop, addBlockEdges);
+
+ for (auto &L : analyzeIrreducible(G, OuterLoop, Insert))
+ computeMassInLoop(L);
+
+ if (!OuterLoop)
+ return;
+ updateLoopWithIrreducible(*OuterLoop);
+}
+
+template <class BT>
+bool
BlockFrequencyInfoImpl<BT>::propagateMassToSuccessors(LoopData *OuterLoop,
const BlockNode &Node) {
DEBUG(dbgs() << " - node: " << getBlockName(Node) << "\n");
Distribution Dist;
if (auto *Loop = Working[Node.Index].getPackagedLoop()) {
assert(Loop != OuterLoop && "Cannot propagate mass in a packaged loop");
- addLoopSuccessorsToDist(OuterLoop, *Loop, Dist);
+ if (!addLoopSuccessorsToDist(OuterLoop, *Loop, Dist))
+ // Irreducible backedge.
+ return false;
} else {
const BlockT *BB = getBlock(Node);
for (auto SI = Successor::child_begin(BB), SE = Successor::child_end(BB);
SI != SE; ++SI)
// Do not dereference SI, or getEdgeWeight() is linear in the number of
// successors.
- addToDist(Dist, OuterLoop, Node, getNode(*SI),
- BPI->getEdgeWeight(BB, SI));
+ if (!addToDist(Dist, OuterLoop, Node, getNode(*SI),
+ BPI->getEdgeWeight(BB, SI)))
+ // Irreducible backedge.
+ return false;
}
// Distribute mass to successors, saving exit and backedge data in the
// loop header.
distributeMass(Node, OuterLoop, Dist);
+ return true;
}
template <class BT>
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