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//===- MachineBlockPlacement.cpp - Basic Block Code Layout optimization ---===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This file implements basic block placement transformations using the CFG
// structure and branch probability estimates.
// The pass strives to preserve the structure of the CFG (that is, retain
// a topological ordering of basic blocks) in the absence of a *strong* signal
// to the contrary from probabilities. However, within the CFG structure, it
// attempts to choose an ordering which favors placing more likely sequences of
// blocks adjacent to each other.
// The algorithm works from the inner-most loop within a function outward, and
// at each stage walks through the basic blocks, trying to coalesce them into
// sequential chains where allowed by the CFG (or demanded by heavy
// probabilities). Finally, it walks the blocks in topological order, and the
// first time it reaches a chain of basic blocks, it schedules them in the
// function in-order.
#include "BranchFolding.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/BlockFrequencyInfoImpl.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachinePostDominators.h"
#include "llvm/CodeGen/TailDuplicator.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetPassConfig.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/Function.h"
#include "llvm/Pass.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/BlockFrequency.h"
#include "llvm/Support/BranchProbability.h"
#include "llvm/Support/CodeGen.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetLowering.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetSubtargetInfo.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <iterator>
#include <memory>
#include <string>
#include <tuple>
#include <utility>
#include <vector>
using namespace llvm;
#define DEBUG_TYPE "block-placement"
STATISTIC(NumCondBranches, "Number of conditional branches");
STATISTIC(NumUncondBranches, "Number of unconditional branches");
"Potential frequency of taking conditional branches");
"Potential frequency of taking unconditional branches");
static cl::opt<unsigned> AlignAllBlock("align-all-blocks",
cl::desc("Force the alignment of all "
"blocks in the function."),
cl::init(0), cl::Hidden);
static cl::opt<unsigned> AlignAllNonFallThruBlocks(
cl::desc("Force the alignment of all "
"blocks that have no fall-through predecessors (i.e. don't add "
"nops that are executed)."),
cl::init(0), cl::Hidden);
// FIXME: Find a good default for this flag and remove the flag.
static cl::opt<unsigned> ExitBlockBias(
cl::desc("Block frequency percentage a loop exit block needs "
"over the original exit to be considered the new exit."),
cl::init(0), cl::Hidden);
// Definition:
// - Outlining: placement of a basic block outside the chain or hot path.
static cl::opt<unsigned> LoopToColdBlockRatio(
cl::desc("Outline loop blocks from loop chain if (frequency of loop) / "
"(frequency of block) is greater than this ratio"),
cl::init(5), cl::Hidden);
static cl::opt<bool> ForceLoopColdBlock(
cl::desc("Force outlining cold blocks from loops."),
cl::init(false), cl::Hidden);
static cl::opt<bool>
cl::desc("Model the cost of loop rotation more "
"precisely by using profile data."),
cl::init(false), cl::Hidden);
static cl::opt<bool>
cl::desc("Force the use of precise cost "
"loop rotation strategy."),
cl::init(false), cl::Hidden);
static cl::opt<unsigned> MisfetchCost(
cl::desc("Cost that models the probabilistic risk of an instruction "
"misfetch due to a jump comparing to falling through, whose cost "
"is zero."),
cl::init(1), cl::Hidden);
static cl::opt<unsigned> JumpInstCost("jump-inst-cost",
cl::desc("Cost of jump instructions."),
cl::init(1), cl::Hidden);
static cl::opt<bool>
cl::desc("Perform tail duplication during placement. "
"Creates more fallthrough opportunites in "
"outline branches."),
cl::init(true), cl::Hidden);
static cl::opt<bool>
cl::desc("Perform branch folding during placement. "
"Reduces code size."),
cl::init(true), cl::Hidden);
// Heuristic for tail duplication.
static cl::opt<unsigned> TailDupPlacementThreshold(
cl::desc("Instruction cutoff for tail duplication during layout. "
"Tail merging during layout is forced to have a threshold "
"that won't conflict."), cl::init(2),
// Heuristic for aggressive tail duplication.
static cl::opt<unsigned> TailDupPlacementAggressiveThreshold(
cl::desc("Instruction cutoff for aggressive tail duplication during "
"layout. Used at -O3. Tail merging during layout is forced to "
"have a threshold that won't conflict."), cl::init(4),
// Heuristic for tail duplication.
static cl::opt<unsigned> TailDupPlacementPenalty(
cl::desc("Cost penalty for blocks that can avoid breaking CFG by copying. "
"Copying can increase fallthrough, but it also increases icache "
"pressure. This parameter controls the penalty to account for that. "
"Percent as integer."),
// Heuristic for triangle chains.
static cl::opt<unsigned> TriangleChainCount(
cl::desc("Number of triangle-shaped-CFG's that need to be in a row for the "
"triangle tail duplication heuristic to kick in. 0 to disable."),
extern cl::opt<unsigned> StaticLikelyProb;
extern cl::opt<unsigned> ProfileLikelyProb;
// Internal option used to control BFI display only after MBP pass.
// Defined in CodeGen/MachineBlockFrequencyInfo.cpp:
// -view-block-layout-with-bfi=
extern cl::opt<GVDAGType> ViewBlockLayoutWithBFI;
// Command line option to specify the name of the function for CFG dump
// Defined in Analysis/BlockFrequencyInfo.cpp: -view-bfi-func-name=
extern cl::opt<std::string> ViewBlockFreqFuncName;
namespace {
class BlockChain;
/// \brief Type for our function-wide basic block -> block chain mapping.
using BlockToChainMapType = DenseMap<const MachineBasicBlock *, BlockChain *>;
/// \brief A chain of blocks which will be laid out contiguously.
/// This is the datastructure representing a chain of consecutive blocks that
/// are profitable to layout together in order to maximize fallthrough
/// probabilities and code locality. We also can use a block chain to represent
/// a sequence of basic blocks which have some external (correctness)
/// requirement for sequential layout.
/// Chains can be built around a single basic block and can be merged to grow
/// them. They participate in a block-to-chain mapping, which is updated
/// automatically as chains are merged together.
class BlockChain {
/// \brief The sequence of blocks belonging to this chain.
/// This is the sequence of blocks for a particular chain. These will be laid
/// out in-order within the function.
SmallVector<MachineBasicBlock *, 4> Blocks;
/// \brief A handle to the function-wide basic block to block chain mapping.
/// This is retained in each block chain to simplify the computation of child
/// block chains for SCC-formation and iteration. We store the edges to child
/// basic blocks, and map them back to their associated chains using this
/// structure.
BlockToChainMapType &BlockToChain;
/// \brief Construct a new BlockChain.
/// This builds a new block chain representing a single basic block in the
/// function. It also registers itself as the chain that block participates
/// in with the BlockToChain mapping.
BlockChain(BlockToChainMapType &BlockToChain, MachineBasicBlock *BB)
: Blocks(1, BB), BlockToChain(BlockToChain) {
assert(BB && "Cannot create a chain with a null basic block");
BlockToChain[BB] = this;
/// \brief Iterator over blocks within the chain.
using iterator = SmallVectorImpl<MachineBasicBlock *>::iterator;
using const_iterator = SmallVectorImpl<MachineBasicBlock *>::const_iterator;
/// \brief Beginning of blocks within the chain.
iterator begin() { return Blocks.begin(); }
const_iterator begin() const { return Blocks.begin(); }
/// \brief End of blocks within the chain.
iterator end() { return Blocks.end(); }
const_iterator end() const { return Blocks.end(); }
bool remove(MachineBasicBlock* BB) {
for(iterator i = begin(); i != end(); ++i) {
if (*i == BB) {
return true;
return false;
/// \brief Merge a block chain into this one.
/// This routine merges a block chain into this one. It takes care of forming
/// a contiguous sequence of basic blocks, updating the edge list, and
/// updating the block -> chain mapping. It does not free or tear down the
/// old chain, but the old chain's block list is no longer valid.
void merge(MachineBasicBlock *BB, BlockChain *Chain) {
assert(BB && "Can't merge a null block.");
assert(!Blocks.empty() && "Can't merge into an empty chain.");
// Fast path in case we don't have a chain already.
if (!Chain) {
assert(!BlockToChain[BB] &&
"Passed chain is null, but BB has entry in BlockToChain.");
BlockToChain[BB] = this;
assert(BB == *Chain->begin() && "Passed BB is not head of Chain.");
assert(Chain->begin() != Chain->end());
// Update the incoming blocks to point to this chain, and add them to the
// chain structure.
for (MachineBasicBlock *ChainBB : *Chain) {
assert(BlockToChain[ChainBB] == Chain && "Incoming blocks not in chain.");
BlockToChain[ChainBB] = this;
#ifndef NDEBUG
/// \brief Dump the blocks in this chain.
LLVM_DUMP_METHOD void dump() {
for (MachineBasicBlock *MBB : *this)
#endif // NDEBUG
/// \brief Count of predecessors of any block within the chain which have not
/// yet been scheduled. In general, we will delay scheduling this chain
/// until those predecessors are scheduled (or we find a sufficiently good
/// reason to override this heuristic.) Note that when forming loop chains,
/// blocks outside the loop are ignored and treated as if they were already
/// scheduled.
/// Note: This field is reinitialized multiple times - once for each loop,
/// and then once for the function as a whole.
unsigned UnscheduledPredecessors = 0;
class MachineBlockPlacement : public MachineFunctionPass {
/// \brief A type for a block filter set.
using BlockFilterSet = SmallSetVector<const MachineBasicBlock *, 16>;
/// Pair struct containing basic block and taildup profitiability
struct BlockAndTailDupResult {
MachineBasicBlock *BB;
bool ShouldTailDup;
/// Triple struct containing edge weight and the edge.
struct WeightedEdge {
BlockFrequency Weight;
MachineBasicBlock *Src;
MachineBasicBlock *Dest;
/// \brief work lists of blocks that are ready to be laid out
SmallVector<MachineBasicBlock *, 16> BlockWorkList;
SmallVector<MachineBasicBlock *, 16> EHPadWorkList;
/// Edges that have already been computed as optimal.
DenseMap<const MachineBasicBlock *, BlockAndTailDupResult> ComputedEdges;
/// \brief Machine Function
MachineFunction *F;
/// \brief A handle to the branch probability pass.
const MachineBranchProbabilityInfo *MBPI;
/// \brief A handle to the function-wide block frequency pass.
std::unique_ptr<BranchFolder::MBFIWrapper> MBFI;
/// \brief A handle to the loop info.
MachineLoopInfo *MLI;
/// \brief Preferred loop exit.
/// Member variable for convenience. It may be removed by duplication deep
/// in the call stack.
MachineBasicBlock *PreferredLoopExit;
/// \brief A handle to the target's instruction info.
const TargetInstrInfo *TII;
/// \brief A handle to the target's lowering info.
const TargetLoweringBase *TLI;
/// \brief A handle to the post dominator tree.
MachinePostDominatorTree *MPDT;
/// \brief Duplicator used to duplicate tails during placement.
/// Placement decisions can open up new tail duplication opportunities, but
/// since tail duplication affects placement decisions of later blocks, it
/// must be done inline.
TailDuplicator TailDup;
/// \brief Allocator and owner of BlockChain structures.
/// We build BlockChains lazily while processing the loop structure of
/// a function. To reduce malloc traffic, we allocate them using this
/// slab-like allocator, and destroy them after the pass completes. An
/// important guarantee is that this allocator produces stable pointers to
/// the chains.
SpecificBumpPtrAllocator<BlockChain> ChainAllocator;
/// \brief Function wide BasicBlock to BlockChain mapping.
/// This mapping allows efficiently moving from any given basic block to the
/// BlockChain it participates in, if any. We use it to, among other things,
/// allow implicitly defining edges between chains as the existing edges
/// between basic blocks.
DenseMap<const MachineBasicBlock *, BlockChain *> BlockToChain;
#ifndef NDEBUG
/// The set of basic blocks that have terminators that cannot be fully
/// analyzed. These basic blocks cannot be re-ordered safely by
/// MachineBlockPlacement, and we must preserve physical layout of these
/// blocks and their successors through the pass.
SmallPtrSet<MachineBasicBlock *, 4> BlocksWithUnanalyzableExits;
/// Decrease the UnscheduledPredecessors count for all blocks in chain, and
/// if the count goes to 0, add them to the appropriate work list.
void markChainSuccessors(
const BlockChain &Chain, const MachineBasicBlock *LoopHeaderBB,
const BlockFilterSet *BlockFilter = nullptr);
/// Decrease the UnscheduledPredecessors count for a single block, and
/// if the count goes to 0, add them to the appropriate work list.
void markBlockSuccessors(
const BlockChain &Chain, const MachineBasicBlock *BB,
const MachineBasicBlock *LoopHeaderBB,
const BlockFilterSet *BlockFilter = nullptr);
const MachineBasicBlock *BB, const BlockChain &Chain,
const BlockFilterSet *BlockFilter,
SmallVector<MachineBasicBlock *, 4> &Successors);
bool shouldPredBlockBeOutlined(
const MachineBasicBlock *BB, const MachineBasicBlock *Succ,
const BlockChain &Chain, const BlockFilterSet *BlockFilter,
BranchProbability SuccProb, BranchProbability HotProb);
bool repeatedlyTailDuplicateBlock(
MachineBasicBlock *BB, MachineBasicBlock *&LPred,
const MachineBasicBlock *LoopHeaderBB,
BlockChain &Chain, BlockFilterSet *BlockFilter,
MachineFunction::iterator &PrevUnplacedBlockIt);
bool maybeTailDuplicateBlock(
MachineBasicBlock *BB, MachineBasicBlock *LPred,
BlockChain &Chain, BlockFilterSet *BlockFilter,
MachineFunction::iterator &PrevUnplacedBlockIt,
bool &DuplicatedToPred);
bool hasBetterLayoutPredecessor(
const MachineBasicBlock *BB, const MachineBasicBlock *Succ,
const BlockChain &SuccChain, BranchProbability SuccProb,
BranchProbability RealSuccProb, const BlockChain &Chain,
const BlockFilterSet *BlockFilter);
BlockAndTailDupResult selectBestSuccessor(
const MachineBasicBlock *BB, const BlockChain &Chain,
const BlockFilterSet *BlockFilter);
MachineBasicBlock *selectBestCandidateBlock(
const BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList);
MachineBasicBlock *getFirstUnplacedBlock(
const BlockChain &PlacedChain,
MachineFunction::iterator &PrevUnplacedBlockIt,
const BlockFilterSet *BlockFilter);
/// \brief Add a basic block to the work list if it is appropriate.
/// If the optional parameter BlockFilter is provided, only MBB
/// present in the set will be added to the worklist. If nullptr
/// is provided, no filtering occurs.
void fillWorkLists(const MachineBasicBlock *MBB,
SmallPtrSetImpl<BlockChain *> &UpdatedPreds,
const BlockFilterSet *BlockFilter);
void buildChain(const MachineBasicBlock *BB, BlockChain &Chain,
BlockFilterSet *BlockFilter = nullptr);
MachineBasicBlock *findBestLoopTop(
const MachineLoop &L, const BlockFilterSet &LoopBlockSet);
MachineBasicBlock *findBestLoopExit(
const MachineLoop &L, const BlockFilterSet &LoopBlockSet);
BlockFilterSet collectLoopBlockSet(const MachineLoop &L);
void buildLoopChains(const MachineLoop &L);
void rotateLoop(
BlockChain &LoopChain, const MachineBasicBlock *ExitingBB,
const BlockFilterSet &LoopBlockSet);
void rotateLoopWithProfile(
BlockChain &LoopChain, const MachineLoop &L,
const BlockFilterSet &LoopBlockSet);
void buildCFGChains();
void optimizeBranches();
void alignBlocks();
/// Returns true if a block should be tail-duplicated to increase fallthrough
/// opportunities.
bool shouldTailDuplicate(MachineBasicBlock *BB);
/// Check the edge frequencies to see if tail duplication will increase
/// fallthroughs.
bool isProfitableToTailDup(
const MachineBasicBlock *BB, const MachineBasicBlock *Succ,
BranchProbability AdjustedSumProb,
const BlockChain &Chain, const BlockFilterSet *BlockFilter);
/// Check for a trellis layout.
bool isTrellis(const MachineBasicBlock *BB,
const SmallVectorImpl<MachineBasicBlock *> &ViableSuccs,
const BlockChain &Chain, const BlockFilterSet *BlockFilter);
/// Get the best successor given a trellis layout.
BlockAndTailDupResult getBestTrellisSuccessor(
const MachineBasicBlock *BB,
const SmallVectorImpl<MachineBasicBlock *> &ViableSuccs,
BranchProbability AdjustedSumProb, const BlockChain &Chain,
const BlockFilterSet *BlockFilter);
/// Get the best pair of non-conflicting edges.
static std::pair<WeightedEdge, WeightedEdge> getBestNonConflictingEdges(
const MachineBasicBlock *BB,
MutableArrayRef<SmallVector<WeightedEdge, 8>> Edges);
/// Returns true if a block can tail duplicate into all unplaced
/// predecessors. Filters based on loop.
bool canTailDuplicateUnplacedPreds(
const MachineBasicBlock *BB, MachineBasicBlock *Succ,
const BlockChain &Chain, const BlockFilterSet *BlockFilter);
/// Find chains of triangles to tail-duplicate where a global analysis works,
/// but a local analysis would not find them.
void precomputeTriangleChains();
static char ID; // Pass identification, replacement for typeid
MachineBlockPlacement() : MachineFunctionPass(ID) {
bool runOnMachineFunction(MachineFunction &F) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
if (TailDupPlacement)
} // end anonymous namespace
char MachineBlockPlacement::ID = 0;
char &llvm::MachineBlockPlacementID = MachineBlockPlacement::ID;
"Branch Probability Basic Block Placement", false, false)
"Branch Probability Basic Block Placement", false, false)
#ifndef NDEBUG
/// \brief Helper to print the name of a MBB.
/// Only used by debug logging.
static std::string getBlockName(const MachineBasicBlock *BB) {
std::string Result;
raw_string_ostream OS(Result);
OS << "BB#" << BB->getNumber();
OS << " ('" << BB->getName() << "')";
return Result;
/// \brief Mark a chain's successors as having one fewer preds.
/// When a chain is being merged into the "placed" chain, this routine will
/// quickly walk the successors of each block in the chain and mark them as
/// having one fewer active predecessor. It also adds any successors of this
/// chain which reach the zero-predecessor state to the appropriate worklist.
void MachineBlockPlacement::markChainSuccessors(
const BlockChain &Chain, const MachineBasicBlock *LoopHeaderBB,
const BlockFilterSet *BlockFilter) {
// Walk all the blocks in this chain, marking their successors as having
// a predecessor placed.
for (MachineBasicBlock *MBB : Chain) {
markBlockSuccessors(Chain, MBB, LoopHeaderBB, BlockFilter);
/// \brief Mark a single block's successors as having one fewer preds.
/// Under normal circumstances, this is only called by markChainSuccessors,
/// but if a block that was to be placed is completely tail-duplicated away,
/// and was duplicated into the chain end, we need to redo markBlockSuccessors
/// for just that block.
void MachineBlockPlacement::markBlockSuccessors(
const BlockChain &Chain, const MachineBasicBlock *MBB,
const MachineBasicBlock *LoopHeaderBB, const BlockFilterSet *BlockFilter) {
// Add any successors for which this is the only un-placed in-loop
// predecessor to the worklist as a viable candidate for CFG-neutral
// placement. No subsequent placement of this block will violate the CFG
// shape, so we get to use heuristics to choose a favorable placement.
for (MachineBasicBlock *Succ : MBB->successors()) {
if (BlockFilter && !BlockFilter->count(Succ))
BlockChain &SuccChain = *BlockToChain[Succ];
// Disregard edges within a fixed chain, or edges to the loop header.
if (&Chain == &SuccChain || Succ == LoopHeaderBB)
// This is a cross-chain edge that is within the loop, so decrement the
// loop predecessor count of the destination chain.
if (SuccChain.UnscheduledPredecessors == 0 ||
--SuccChain.UnscheduledPredecessors > 0)
auto *NewBB = *SuccChain.begin();
if (NewBB->isEHPad())
/// This helper function collects the set of successors of block
/// \p BB that are allowed to be its layout successors, and return
/// the total branch probability of edges from \p BB to those
/// blocks.
BranchProbability MachineBlockPlacement::collectViableSuccessors(
const MachineBasicBlock *BB, const BlockChain &Chain,
const BlockFilterSet *BlockFilter,
SmallVector<MachineBasicBlock *, 4> &Successors) {
// Adjust edge probabilities by excluding edges pointing to blocks that is
// either not in BlockFilter or is already in the current chain. Consider the
// following CFG:
// --->A
// | / \
// | B C
// | \ / \
// ----D E
// Assume A->C is very hot (>90%), and C->D has a 50% probability, then after
// A->C is chosen as a fall-through, D won't be selected as a successor of C
// due to CFG constraint (the probability of C->D is not greater than
// HotProb to break topo-order). If we exclude E that is not in BlockFilter
// when calculating the probability of C->D, D will be selected and we
// will get A C D B as the layout of this loop.
auto AdjustedSumProb = BranchProbability::getOne();
for (MachineBasicBlock *Succ : BB->successors()) {
bool SkipSucc = false;
if (Succ->isEHPad() || (BlockFilter && !BlockFilter->count(Succ))) {
SkipSucc = true;
} else {
BlockChain *SuccChain = BlockToChain[Succ];
if (SuccChain == &Chain) {
SkipSucc = true;
} else if (Succ != *SuccChain->begin()) {
DEBUG(dbgs() << " " << getBlockName(Succ) << " -> Mid chain!\n");
if (SkipSucc)
AdjustedSumProb -= MBPI->getEdgeProbability(BB, Succ);
return AdjustedSumProb;
/// The helper function returns the branch probability that is adjusted
/// or normalized over the new total \p AdjustedSumProb.
static BranchProbability
getAdjustedProbability(BranchProbability OrigProb,
BranchProbability AdjustedSumProb) {
BranchProbability SuccProb;
uint32_t SuccProbN = OrigProb.getNumerator();
uint32_t SuccProbD = AdjustedSumProb.getNumerator();
if (SuccProbN >= SuccProbD)
SuccProb = BranchProbability::getOne();
SuccProb = BranchProbability(SuccProbN, SuccProbD);
return SuccProb;
/// Check if \p BB has exactly the successors in \p Successors.
static bool
hasSameSuccessors(MachineBasicBlock &BB,
SmallPtrSetImpl<const MachineBasicBlock *> &Successors) {
if (BB.succ_size() != Successors.size())
return false;
// We don't want to count self-loops
if (Successors.count(&BB))
return false;
for (MachineBasicBlock *Succ : BB.successors())
if (!Successors.count(Succ))
return false;
return true;
/// Check if a block should be tail duplicated to increase fallthrough
/// opportunities.
/// \p BB Block to check.
bool MachineBlockPlacement::shouldTailDuplicate(MachineBasicBlock *BB) {
// Blocks with single successors don't create additional fallthrough
// opportunities. Don't duplicate them. TODO: When conditional exits are
// analyzable, allow them to be duplicated.
bool IsSimple = TailDup.isSimpleBB(BB);
if (BB->succ_size() == 1)
return false;
return TailDup.shouldTailDuplicate(IsSimple, *BB);
/// Compare 2 BlockFrequency's with a small penalty for \p A.
/// In order to be conservative, we apply a X% penalty to account for
/// increased icache pressure and static heuristics. For small frequencies
/// we use only the numerators to improve accuracy. For simplicity, we assume the
/// penalty is less than 100%
/// TODO(iteratee): Use 64-bit fixed point edge frequencies everywhere.
static bool greaterWithBias(BlockFrequency A, BlockFrequency B,
uint64_t EntryFreq) {
BranchProbability ThresholdProb(TailDupPlacementPenalty, 100);
BlockFrequency Gain = A - B;
return (Gain / ThresholdProb).getFrequency() >= EntryFreq;
/// Check the edge frequencies to see if tail duplication will increase
/// fallthroughs. It only makes sense to call this function when
/// \p Succ would not be chosen otherwise. Tail duplication of \p Succ is
/// always locally profitable if we would have picked \p Succ without
/// considering duplication.
bool MachineBlockPlacement::isProfitableToTailDup(
const MachineBasicBlock *BB, const MachineBasicBlock *Succ,
BranchProbability QProb,
const BlockChain &Chain, const BlockFilterSet *BlockFilter) {
// We need to do a probability calculation to make sure this is profitable.
// First: does succ have a successor that post-dominates? This affects the
// calculation. The 2 relevant cases are:
// BB BB
// | \Qout | \Qout
// P| C |P C
// = C' = C'
// | /Qin | /Qin
// | / | /
// Succ Succ
// / \ | \ V
// U/ =V |U \
// / \ = D
// D E | /
// | /
// |/
// PDom
// '=' : Branch taken for that CFG edge
// In the second case, Placing Succ while duplicating it into C prevents the
// fallthrough of Succ into either D or PDom, because they now have C as an
// unplaced predecessor
// Start by figuring out which case we fall into
MachineBasicBlock *PDom = nullptr;
SmallVector<MachineBasicBlock *, 4> SuccSuccs;
// Only scan the relevant successors
auto AdjustedSuccSumProb =
collectViableSuccessors(Succ, Chain, BlockFilter, SuccSuccs);
BranchProbability PProb = MBPI->getEdgeProbability(BB, Succ);
auto BBFreq = MBFI->getBlockFreq(BB);
auto SuccFreq = MBFI->getBlockFreq(Succ);
BlockFrequency P = BBFreq * PProb;
BlockFrequency Qout = BBFreq * QProb;
uint64_t EntryFreq = MBFI->getEntryFreq();
// If there are no more successors, it is profitable to copy, as it strictly
// increases fallthrough.
if (SuccSuccs.size() == 0)
return greaterWithBias(P, Qout, EntryFreq);
auto BestSuccSucc = BranchProbability::getZero();
// Find the PDom or the best Succ if no PDom exists.
for (MachineBasicBlock *SuccSucc : SuccSuccs) {
auto Prob = MBPI->getEdgeProbability(Succ, SuccSucc);
if (Prob > BestSuccSucc)
BestSuccSucc = Prob;
if (PDom == nullptr)
if (MPDT->dominates(SuccSucc, Succ)) {
PDom = SuccSucc;
// For the comparisons, we need to know Succ's best incoming edge that isn't
// from BB.
auto SuccBestPred = BlockFrequency(0);
for (MachineBasicBlock *SuccPred : Succ->predecessors()) {
if (SuccPred == Succ || SuccPred == BB
|| BlockToChain[SuccPred] == &Chain
|| (BlockFilter && !BlockFilter->count(SuccPred)))
auto Freq = MBFI->getBlockFreq(SuccPred)
* MBPI->getEdgeProbability(SuccPred, Succ);
if (Freq > SuccBestPred)
SuccBestPred = Freq;
// Qin is Succ's best unplaced incoming edge that isn't BB
BlockFrequency Qin = SuccBestPred;
// If it doesn't have a post-dominating successor, here is the calculation:
// BB BB
// | \Qout | \
// P| C | =
// = C' | C
// | /Qin | |
// | / | C' (+Succ)
// Succ Succ /|
// / \ | \/ |
// U/ =V | == |
// / \ | / \|
// D E D E
// '=' : Branch taken for that CFG edge
// Cost in the first case is: P + V
// For this calculation, we always assume P > Qout. If Qout > P
// The result of this function will be ignored at the caller.
// Let F = SuccFreq - Qin
// Cost in the second case is: Qout + min(Qin, F) * U + max(Qin, F) * V
if (PDom == nullptr || !Succ->isSuccessor(PDom)) {
BranchProbability UProb = BestSuccSucc;
BranchProbability VProb = AdjustedSuccSumProb - UProb;
BlockFrequency F = SuccFreq - Qin;
BlockFrequency V = SuccFreq * VProb;
BlockFrequency QinU = std::min(Qin, F) * UProb;
BlockFrequency BaseCost = P + V;
BlockFrequency DupCost = Qout + QinU + std::max(Qin, F) * VProb;
return greaterWithBias(BaseCost, DupCost, EntryFreq);
BranchProbability UProb = MBPI->getEdgeProbability(Succ, PDom);
BranchProbability VProb = AdjustedSuccSumProb - UProb;
BlockFrequency U = SuccFreq * UProb;
BlockFrequency V = SuccFreq * VProb;
BlockFrequency F = SuccFreq - Qin;
// If there is a post-dominating successor, here is the calculation:
// | \Qout | \ | \Qout | \
// |P C | = |P C | =
// = C' |P C = C' |P C
// | /Qin | | | /Qin | |
// | / | C' (+Succ) | / | C' (+Succ)
// Succ Succ /| Succ Succ /|
// | \ V | \/ | | \ V | \/ |
// |U \ |U /\ =? |U = |U /\ |
// = D = = =?| | D | = =|
// | / |/ D | / |/ D
// | / | / | = | /
// |/ | / |/ | =
// Dom Dom Dom Dom
// '=' : Branch taken for that CFG edge
// The cost for taken branches in the first case is P + U
// Let F = SuccFreq - Qin
// The cost in the second case (assuming independence), given the layout:
// BB, Succ, (C+Succ), D, Dom or the layout:
// BB, Succ, D, Dom, (C+Succ)
// is Qout + max(F, Qin) * U + min(F, Qin)
// compare P + U vs Qout + P * U + Qin.
// The 3rd and 4th cases cover when Dom would be chosen to follow Succ.
// For the 3rd case, the cost is P + 2 * V
// For the 4th case, the cost is Qout + min(Qin, F) * U + max(Qin, F) * V + V
// We choose 4 over 3 when (P + V) > Qout + min(Qin, F) * U + max(Qin, F) * V
if (UProb > AdjustedSuccSumProb / 2 &&
!hasBetterLayoutPredecessor(Succ, PDom, *BlockToChain[PDom], UProb, UProb,
Chain, BlockFilter))
// Cases 3 & 4
return greaterWithBias(
(P + V), (Qout + std::max(Qin, F) * VProb + std::min(Qin, F) * UProb),
// Cases 1 & 2
return greaterWithBias((P + U),
(Qout + std::min(Qin, F) * AdjustedSuccSumProb +
std::max(Qin, F) * UProb),
/// Check for a trellis layout. \p BB is the upper part of a trellis if its
/// successors form the lower part of a trellis. A successor set S forms the
/// lower part of a trellis if all of the predecessors of S are either in S or
/// have all of S as successors. We ignore trellises where BB doesn't have 2
/// successors because for fewer than 2, it's trivial, and for 3 or greater they
/// are very uncommon and complex to compute optimally. Allowing edges within S
/// is not strictly a trellis, but the same algorithm works, so we allow it.
bool MachineBlockPlacement::isTrellis(
const MachineBasicBlock *BB,
const SmallVectorImpl<MachineBasicBlock *> &ViableSuccs,
const BlockChain &Chain, const BlockFilterSet *BlockFilter) {
// Technically BB could form a trellis with branching factor higher than 2.
// But that's extremely uncommon.
if (BB->succ_size() != 2 || ViableSuccs.size() != 2)
return false;
SmallPtrSet<const MachineBasicBlock *, 2> Successors(BB->succ_begin(),
// To avoid reviewing the same predecessors twice.
SmallPtrSet<const MachineBasicBlock *, 8> SeenPreds;
for (MachineBasicBlock *Succ : ViableSuccs) {
int PredCount = 0;
for (auto SuccPred : Succ->predecessors()) {
// Allow triangle successors, but don't count them.
if (Successors.count(SuccPred)) {
// Make sure that it is actually a triangle.
for (MachineBasicBlock *CheckSucc : SuccPred->successors())
if (!Successors.count(CheckSucc))
return false;
const BlockChain *PredChain = BlockToChain[SuccPred];
if (SuccPred == BB || (BlockFilter && !BlockFilter->count(SuccPred)) ||
PredChain == &Chain || PredChain == BlockToChain[Succ])
// Perform the successor check only once.
if (!SeenPreds.insert(SuccPred).second)
if (!hasSameSuccessors(*SuccPred, Successors))
return false;
// If one of the successors has only BB as a predecessor, it is not a
// trellis.
if (PredCount < 1)
return false;
return true;
/// Pick the highest total weight pair of edges that can both be laid out.
/// The edges in \p Edges[0] are assumed to have a different destination than
/// the edges in \p Edges[1]. Simple counting shows that the best pair is either
/// the individual highest weight edges to the 2 different destinations, or in
/// case of a conflict, one of them should be replaced with a 2nd best edge.
const MachineBasicBlock *BB,
MutableArrayRef<SmallVector<MachineBlockPlacement::WeightedEdge, 8>>
Edges) {
// Sort the edges, and then for each successor, find the best incoming
// predecessor. If the best incoming predecessors aren't the same,
// then that is clearly the best layout. If there is a conflict, one of the
// successors will have to fallthrough from the second best predecessor. We
// compare which combination is better overall.
// Sort for highest frequency.
auto Cmp = [](WeightedEdge A, WeightedEdge B) { return A.Weight > B.Weight; };
std::stable_sort(Edges[0].begin(), Edges[0].end(), Cmp);
std::stable_sort(Edges[1].begin(), Edges[1].end(), Cmp);
auto BestA = Edges[0].begin();
auto BestB = Edges[1].begin();
// Arrange for the correct answer to be in BestA and BestB
// If the 2 best edges don't conflict, the answer is already there.
if (BestA->Src == BestB->Src) {
// Compare the total fallthrough of (Best + Second Best) for both pairs
auto SecondBestA = std::next(BestA);
auto SecondBestB = std::next(BestB);
BlockFrequency BestAScore = BestA->Weight + SecondBestB->Weight;
BlockFrequency BestBScore = BestB->Weight + SecondBestA->Weight;
if (BestAScore < BestBScore)
BestA = SecondBestA;
BestB = SecondBestB;
// Arrange for the BB edge to be in BestA if it exists.
if (BestB->Src == BB)
std::swap(BestA, BestB);
return std::make_pair(*BestA, *BestB);
/// Get the best successor from \p BB based on \p BB being part of a trellis.
/// We only handle trellises with 2 successors, so the algorithm is
/// straightforward: Find the best pair of edges that don't conflict. We find
/// the best incoming edge for each successor in the trellis. If those conflict,
/// we consider which of them should be replaced with the second best.
/// Upon return the two best edges will be in \p BestEdges. If one of the edges
/// comes from \p BB, it will be in \p BestEdges[0]
const MachineBasicBlock *BB,
const SmallVectorImpl<MachineBasicBlock *> &ViableSuccs,
BranchProbability AdjustedSumProb, const BlockChain &Chain,
const BlockFilterSet *BlockFilter) {
BlockAndTailDupResult Result = {nullptr, false};
SmallPtrSet<const MachineBasicBlock *, 4> Successors(BB->succ_begin(),
// We assume size 2 because it's common. For general n, we would have to do
// the Hungarian algorithm, but it's not worth the complexity because more
// than 2 successors is fairly uncommon, and a trellis even more so.
if (Successors.size() != 2 || ViableSuccs.size() != 2)
return Result;
// Collect the edge frequencies of all edges that form the trellis.
SmallVector<WeightedEdge, 8> Edges[2];
int SuccIndex = 0;
for (auto Succ : ViableSuccs) {
for (MachineBasicBlock *SuccPred : Succ->predecessors()) {
// Skip any placed predecessors that are not BB
if (SuccPred != BB)
if ((BlockFilter && !BlockFilter->count(SuccPred)) ||
BlockToChain[SuccPred] == &Chain ||
BlockToChain[SuccPred] == BlockToChain[Succ])
BlockFrequency EdgeFreq = MBFI->getBlockFreq(SuccPred) *
MBPI->getEdgeProbability(SuccPred, Succ);
Edges[SuccIndex].push_back({EdgeFreq, SuccPred, Succ});
// Pick the best combination of 2 edges from all the edges in the trellis.
WeightedEdge BestA, BestB;
std::tie(BestA, BestB) = getBestNonConflictingEdges(BB, Edges);
if (BestA.Src != BB) {
// If we have a trellis, and BB doesn't have the best fallthrough edges,
// we shouldn't choose any successor. We've already looked and there's a
// better fallthrough edge for all the successors.
DEBUG(dbgs() << "Trellis, but not one of the chosen edges.\n");
return Result;
// Did we pick the triangle edge? If tail-duplication is profitable, do
// that instead. Otherwise merge the triangle edge now while we know it is
// optimal.
if (BestA.Dest == BestB.Src) {
// The edges are BB->Succ1->Succ2, and we're looking to see if BB->Succ2
// would be better.
MachineBasicBlock *Succ1 = BestA.Dest;
MachineBasicBlock *Succ2 = BestB.Dest;
// Check to see if tail-duplication would be profitable.
if (TailDupPlacement && shouldTailDuplicate(Succ2) &&
canTailDuplicateUnplacedPreds(BB, Succ2, Chain, BlockFilter) &&
isProfitableToTailDup(BB, Succ2, MBPI->getEdgeProbability(BB, Succ1),
Chain, BlockFilter)) {
DEBUG(BranchProbability Succ2Prob = getAdjustedProbability(
MBPI->getEdgeProbability(BB, Succ2), AdjustedSumProb);
dbgs() << " Selected: " << getBlockName(Succ2)
<< ", probability: " << Succ2Prob << " (Tail Duplicate)\n");
Result.BB = Succ2;
Result.ShouldTailDup = true;
return Result;
// We have already computed the optimal edge for the other side of the
// trellis.
ComputedEdges[BestB.Src] = { BestB.Dest, false };
auto TrellisSucc = BestA.Dest;
DEBUG(BranchProbability SuccProb = getAdjustedProbability(
MBPI->getEdgeProbability(BB, TrellisSucc), AdjustedSumProb);
dbgs() << " Selected: " << getBlockName(TrellisSucc)
<< ", probability: " << SuccProb << " (Trellis)\n");
Result.BB = TrellisSucc;
return Result;
/// When the option TailDupPlacement is on, this method checks if the
/// fallthrough candidate block \p Succ (of block \p BB) can be tail-duplicated
/// into all of its unplaced, unfiltered predecessors, that are not BB.
bool MachineBlockPlacement::canTailDuplicateUnplacedPreds(
const MachineBasicBlock *BB, MachineBasicBlock *Succ,
const BlockChain &Chain, const BlockFilterSet *BlockFilter) {
if (!shouldTailDuplicate(Succ))
return false;
// For CFG checking.
SmallPtrSet<const MachineBasicBlock *, 4> Successors(BB->succ_begin(),
for (MachineBasicBlock *Pred : Succ->predecessors()) {
// Make sure all unplaced and unfiltered predecessors can be
// tail-duplicated into.
// Skip any blocks that are already placed or not in this loop.
if (Pred == BB || (BlockFilter && !BlockFilter->count(Pred))
|| BlockToChain[Pred] == &Chain)
if (!TailDup.canTailDuplicate(Succ, Pred)) {
if (Successors.size() > 1 && hasSameSuccessors(*Pred, Successors))
// This will result in a trellis after tail duplication, so we don't
// need to copy Succ into this predecessor. In the presence
// of a trellis tail duplication can continue to be profitable.
// For example:
// A A
// |\ |\
// | \ | \
// | C | C+BB
// | / | |
// |/ | |
// BB => BB |
// |\ |\/|
// | \ |/\|
// | D | D
// | / | /
// |/ |/
// Succ Succ
// After BB was duplicated into C, the layout looks like the one on the
// right. BB and C now have the same successors. When considering
// whether Succ can be duplicated into all its unplaced predecessors, we
// ignore C.
// We can do this because C already has a profitable fallthrough, namely
// D. TODO(iteratee): ignore sufficiently cold predecessors for
// duplication and for this test.
// This allows trellises to be laid out in 2 separate chains
// (A,B,Succ,...) and later (C,D,...) This is a reasonable heuristic
// because it allows the creation of 2 fallthrough paths with links
// between them, and we correctly identify the best layout for these
// CFGs. We want to extend trellises that the user created in addition
// to trellises created by tail-duplication, so we just look for the
// CFG.
return false;
return true;
/// Find chains of triangles where we believe it would be profitable to
/// tail-duplicate them all, but a local analysis would not find them.
/// There are 3 ways this can be profitable:
/// 1) The post-dominators marked 50% are actually taken 55% (This shrinks with
/// longer chains)
/// 2) The chains are statically correlated. Branch probabilities have a very
/// U-shaped distribution.
/// []
/// If the branches in a chain are likely to be from the same side of the
/// distribution as their predecessor, but are independent at runtime, this
/// transformation is profitable. (Because the cost of being wrong is a small
/// fixed cost, unlike the standard triangle layout where the cost of being
/// wrong scales with the # of triangles.)
/// 3) The chains are dynamically correlated. If the probability that a previous
/// branch was taken positively influences whether the next branch will be
/// taken
/// We believe that 2 and 3 are common enough to justify the small margin in 1.
void MachineBlockPlacement::precomputeTriangleChains() {
struct TriangleChain {
std::vector<MachineBasicBlock *> Edges;
TriangleChain(MachineBasicBlock *src, MachineBasicBlock *dst)
: Edges({src, dst}) {}
void append(MachineBasicBlock *dst) {
assert(getKey()->isSuccessor(dst) &&
"Attempting to append a block that is not a successor.");
unsigned count() const { return Edges.size() - 1; }
MachineBasicBlock *getKey() const {
return Edges.back();
if (TriangleChainCount == 0)
DEBUG(dbgs() << "Pre-computing triangle chains.\n");
// Map from last block to the chain that contains it. This allows us to extend
// chains as we find new triangles.
DenseMap<const MachineBasicBlock *, TriangleChain> TriangleChainMap;
for (MachineBasicBlock &BB : *F) {
// If BB doesn't have 2 successors, it doesn't start a triangle.
if (BB.succ_size() != 2)
MachineBasicBlock *PDom = nullptr;
for (MachineBasicBlock *Succ : BB.successors()) {
if (!MPDT->dominates(Succ, &BB))
PDom = Succ;
// If BB doesn't have a post-dominating successor, it doesn't form a
// triangle.
if (PDom == nullptr)
// If PDom has a hint that it is low probability, skip this triangle.
if (MBPI->getEdgeProbability(&BB, PDom) < BranchProbability(50, 100))
// If PDom isn't eligible for duplication, this isn't the kind of triangle
// we're looking for.
if (!shouldTailDuplicate(PDom))
bool CanTailDuplicate = true;
// If PDom can't tail-duplicate into it's non-BB predecessors, then this
// isn't the kind of triangle we're looking for.
for (MachineBasicBlock* Pred : PDom->predecessors()) {
if (Pred == &BB)
if (!TailDup.canTailDuplicate(PDom, Pred)) {
CanTailDuplicate = false;
// If we can't tail-duplicate PDom to its predecessors, then skip this
// triangle.
if (!CanTailDuplicate)
// Now we have an interesting triangle. Insert it if it's not part of an
// existing chain.
// Note: This cannot be replaced with a call insert() or emplace() because
// the find key is BB, but the insert/emplace key is PDom.
auto Found = TriangleChainMap.find(&BB);
// If it is, remove the chain from the map, grow it, and put it back in the
// map with the end as the new key.
if (Found != TriangleChainMap.end()) {
TriangleChain Chain = std::move(Found->second);
TriangleChainMap.insert(std::make_pair(Chain.getKey(), std::move(Chain)));
} else {
auto InsertResult = TriangleChainMap.try_emplace(PDom, &BB, PDom);
assert(InsertResult.second && "Block seen twice.");
// Iterating over a DenseMap is safe here, because the only thing in the body
// of the loop is inserting into another DenseMap (ComputedEdges).
// ComputedEdges is never iterated, so this doesn't lead to non-determinism.
for (auto &ChainPair : TriangleChainMap) {
TriangleChain &Chain = ChainPair.second;
// Benchmarking has shown that due to branch correlation duplicating 2 or
// more triangles is profitable, despite the calculations assuming
// independence.
if (Chain.count() < TriangleChainCount)
MachineBasicBlock *dst = Chain.Edges.back();
for (MachineBasicBlock *src : reverse(Chain.Edges)) {
DEBUG(dbgs() << "Marking edge: " << getBlockName(src) << "->" <<
getBlockName(dst) << " as pre-computed based on triangles.\n");
auto InsertResult = ComputedEdges.insert({src, {dst, true}});
assert(InsertResult.second && "Block seen twice.");
dst = src;
// When profile is not present, return the StaticLikelyProb.
// When profile is available, we need to handle the triangle-shape CFG.
static BranchProbability getLayoutSuccessorProbThreshold(
const MachineBasicBlock *BB) {
if (!BB->getParent()->getFunction()->getEntryCount())
return BranchProbability(StaticLikelyProb, 100);
if (BB->succ_size() == 2) {
const MachineBasicBlock *Succ1 = *BB->succ_begin();
const MachineBasicBlock *Succ2 = *(BB->succ_begin() + 1);
if (Succ1->isSuccessor(Succ2) || Succ2->isSuccessor(Succ1)) {
/* See case 1 below for the cost analysis. For BB->Succ to
* be taken with smaller cost, the following needs to hold:
* Prob(BB->Succ) > 2 * Prob(BB->Pred)
* So the threshold T in the calculation below
* (1-T) * Prob(BB->Succ) > T * Prob(BB->Pred)
* So T / (1 - T) = 2, Yielding T = 2/3
* Also adding user specified branch bias, we have
* T = (2/3)*(ProfileLikelyProb/50)
* = (2*ProfileLikelyProb)/150)
return BranchProbability(2 * ProfileLikelyProb, 150);
return BranchProbability(ProfileLikelyProb, 100);
/// Checks to see if the layout candidate block \p Succ has a better layout
/// predecessor than \c BB. If yes, returns true.
/// \p SuccProb: The probability adjusted for only remaining blocks.
/// Only used for logging
/// \p RealSuccProb: The un-adjusted probability.
/// \p Chain: The chain that BB belongs to and Succ is being considered for.
/// \p BlockFilter: if non-null, the set of blocks that make up the loop being
/// considered
bool MachineBlockPlacement::hasBetterLayoutPredecessor(
const MachineBasicBlock *BB, const MachineBasicBlock *Succ,
const BlockChain &SuccChain, BranchProbability SuccProb,
BranchProbability RealSuccProb, const BlockChain &Chain,
const BlockFilterSet *BlockFilter) {
// There isn't a better layout when there are no unscheduled predecessors.
if (SuccChain.UnscheduledPredecessors == 0)
return false;
// There are two basic scenarios here:
// -------------------------------------
// Case 1: triangular shape CFG (if-then):
// BB
// | \
// | \
// | Pred
// | /
// Succ
// In this case, we are evaluating whether to select edge -> Succ, e.g.
// set Succ as the layout successor of BB. Picking Succ as BB's
// successor breaks the CFG constraints (FIXME: define these constraints).
// With this layout, Pred BB
// is forced to be outlined, so the overall cost will be cost of the
// branch taken from BB to Pred, plus the cost of back taken branch
// from Pred to Succ, as well as the additional cost associated
// with the needed unconditional jump instruction from Pred To Succ.
// The cost of the topological order layout is the taken branch cost
// from BB to Succ, so to make BB->Succ a viable candidate, the following
// must hold:
// 2 * freq(BB->Pred) * taken_branch_cost + unconditional_jump_cost
// < freq(BB->Succ) * taken_branch_cost.
// Ignoring unconditional jump cost, we get
// freq(BB->Succ) > 2 * freq(BB->Pred), i.e.,
// prob(BB->Succ) > 2 * prob(BB->Pred)
// When real profile data is available, we can precisely compute the
// probability threshold that is needed for edge BB->Succ to be considered.
// Without profile data, the heuristic requires the branch bias to be
// a lot larger to make sure the signal is very strong (e.g. 80% default).
// -----------------------------------------------------------------
// Case 2: diamond like CFG (if-then-else):
// S
// / \
// | \
// BB Pred
// \ /
// Succ
// ..
// The current block is BB and edge BB->Succ is now being evaluated.
// Note that edge S->BB was previously already selected because
// prob(S->BB) > prob(S->Pred).
// At this point, 2 blocks can be placed after BB: Pred or Succ. If we
// choose Pred, we will have a topological ordering as shown on the left
// in the picture below. If we choose Succ, we have the solution as shown
// on the right:
// topo-order:
// S----- ---S
// | | | |
// ---BB | | BB
// | | | |
// | Pred-- | Succ--
// | | | |
// ---Succ ---Pred--
// cost = freq(S->Pred) + freq(BB->Succ) cost = 2 * freq (S->Pred)
// = freq(S->Pred) + freq(S->BB)
// If we have profile data (i.e, branch probabilities can be trusted), the
// cost (number of taken branches) with layout S->BB->Succ->Pred is 2 *
// freq(S->Pred) while the cost of topo order is freq(S->Pred) + freq(S->BB).
// We know Prob(S->BB) > Prob(S->Pred), so freq(S->BB) > freq(S->Pred), which
// means the cost of topological order is greater.
// When profile data is not available, however, we need to be more
// conservative. If the branch prediction is wrong, breaking the topo-order
// will actually yield a layout with large cost. For this reason, we need
// strong biased branch at block S with Prob(S->BB) in order to select
// BB->Succ. This is equivalent to looking the CFG backward with backward
// edge: Prob(Succ->BB) needs to >= HotProb in order to be selected (without
// profile data).
// --------------------------------------------------------------------------
// Case 3: forked diamond
// S
// / \
// / \
// BB Pred
// | \ / |
// | \ / |
// | X |
// | / \ |
// | / \ |
// S1 S2
// The current block is BB and edge BB->S1 is now being evaluated.
// As above S->BB was already selected because
// prob(S->BB) > prob(S->Pred). Assume that prob(BB->S1) >= prob(BB->S2).
// topo-order:
// S-------| ---S
// | | | |
// ---BB | | BB
// | | | |
// | Pred----| | S1----
// | | | |
// --(S1 or S2) ---Pred--
// |
// S2
// topo-cost = freq(S->Pred) + freq(BB->S1) + freq(BB->S2)
// + min(freq(Pred->S1), freq(Pred->S2))
// Non-topo-order cost:
// non-topo-cost = 2 * freq(S->Pred) + freq(BB->S2).
// To be conservative, we can assume that min(freq(Pred->S1), freq(Pred->S2))
// is 0. Then the non topo layout is better when
// freq(S->Pred) < freq(BB->S1).
// This is exactly what is checked below.
// Note there are other shapes that apply (Pred may not be a single block,
// but they all fit this general pattern.)
BranchProbability HotProb = getLayoutSuccessorProbThreshold(BB);
// Make sure that a hot successor doesn't have a globally more
// important predecessor.
BlockFrequency CandidateEdgeFreq = MBFI->getBlockFreq(BB) * RealSuccProb;
bool BadCFGConflict = false;
for (MachineBasicBlock *Pred : Succ->predecessors()) {
if (Pred == Succ || BlockToChain[Pred] == &SuccChain ||
(BlockFilter && !BlockFilter->count(Pred)) ||
BlockToChain[Pred] == &Chain ||
// This check is redundant except for look ahead. This function is
// called for lookahead by isProfitableToTailDup when BB hasn't been
// placed yet.
(Pred == BB))
// Do backward checking.
// For all cases above, we need a backward checking to filter out edges that
// are not 'strongly' biased.
// BB Pred
// \ /
// Succ
// We select edge BB->Succ if
// freq(BB->Succ) > freq(Succ) * HotProb
// i.e. freq(BB->Succ) > freq(BB->Succ) * HotProb + freq(Pred->Succ) *
// HotProb
// i.e. freq((BB->Succ) * (1 - HotProb) > freq(Pred->Succ) * HotProb
// Case 1 is covered too, because the first equation reduces to:
// prob(BB->Succ) > HotProb. (freq(Succ) = freq(BB) for a triangle)
BlockFrequency PredEdgeFreq =
MBFI->getBlockFreq(Pred) * MBPI->getEdgeProbability(Pred, Succ);
if (PredEdgeFreq * HotProb >= CandidateEdgeFreq * HotProb.getCompl()) {
BadCFGConflict = true;
if (BadCFGConflict) {
DEBUG(dbgs() << " Not a candidate: " << getBlockName(Succ) << " -> " << SuccProb
<< " (prob) (non-cold CFG conflict)\n");
return true;
return false;
/// \brief Select the best successor for a block.
/// This looks across all successors of a particular block and attempts to
/// select the "best" one to be the layout successor. It only considers direct
/// successors which also pass the block filter. It will attempt to avoid
/// breaking CFG structure, but cave and break such structures in the case of
/// very hot successor edges.
/// \returns The best successor block found, or null if none are viable, along
/// with a boolean indicating if tail duplication is necessary.
const MachineBasicBlock *BB, const BlockChain &Chain,
const BlockFilterSet *BlockFilter) {
const BranchProbability HotProb(StaticLikelyProb, 100);
BlockAndTailDupResult BestSucc = { nullptr, false };
auto BestProb = BranchProbability::getZero();
SmallVector<MachineBasicBlock *, 4> Successors;
auto AdjustedSumProb =
collectViableSuccessors(BB, Chain, BlockFilter, Successors);
DEBUG(dbgs() << "Selecting best successor for: " << getBlockName(BB) << "\n");
// if we already precomputed the best successor for BB, return that if still
// applicable.
auto FoundEdge = ComputedEdges.find(BB);
if (FoundEdge != ComputedEdges.end()) {
MachineBasicBlock *Succ = FoundEdge->second.BB;
BlockChain *SuccChain = BlockToChain[Succ];
if (BB->isSuccessor(Succ) && (!BlockFilter || BlockFilter->count(Succ)) &&
SuccChain != &Chain && Succ == *SuccChain->begin())
return FoundEdge->second;
// if BB is part of a trellis, Use the trellis to determine the optimal
// fallthrough edges
if (isTrellis(BB, Successors, Chain, BlockFilter))
return getBestTrellisSuccessor(BB, Successors, AdjustedSumProb, Chain,
// For blocks with CFG violations, we may be able to lay them out anyway with
// tail-duplication. We keep this vector so we can perform the probability
// calculations the minimum number of times.
SmallVector<std::tuple<BranchProbability, MachineBasicBlock *>, 4>
for (MachineBasicBlock *Succ : Successors) {
auto RealSuccProb = MBPI->getEdgeProbability(BB, Succ);
BranchProbability SuccProb =
getAdjustedProbability(RealSuccProb, AdjustedSumProb);
BlockChain &SuccChain = *BlockToChain[Succ];
// Skip the edge \c BB->Succ if block \c Succ has a better layout
// predecessor that yields lower global cost.
if (hasBetterLayoutPredecessor(BB, Succ, SuccChain, SuccProb, RealSuccProb,
Chain, BlockFilter)) {
// If tail duplication would make Succ profitable, place it.
if (TailDupPlacement && shouldTailDuplicate(Succ))
DupCandidates.push_back(std::make_tuple(SuccProb, Succ));
dbgs() << " Candidate: " << getBlockName(Succ) << ", probability: "
<< SuccProb
<< (SuccChain.UnscheduledPredecessors != 0 ? " (CFG break)" : "")
<< "\n");
if (BestSucc.BB && BestProb >= SuccProb) {
DEBUG(dbgs() << " Not the best candidate, continuing\n");
DEBUG(dbgs() << " Setting it as best candidate\n");
BestSucc.BB = Succ;
BestProb = SuccProb;
// Handle the tail duplication candidates in order of decreasing probability.
// Stop at the first one that is profitable. Also stop if they are less
// profitable than BestSucc. Position is important because we preserve it and
// prefer first best match. Here we aren't comparing in order, so we capture
// the position instead.
if (DupCandidates.size() != 0) {
auto cmp =
[](const std::tuple<BranchProbability, MachineBasicBlock *> &a,
const std::tuple<BranchProbability, MachineBasicBlock *> &b) {
return std::get<0>(a) > std::get<0>(b);
std::stable_sort(DupCandidates.begin(), DupCandidates.end(), cmp);
for(auto &Tup : DupCandidates) {
BranchProbability DupProb;
MachineBasicBlock *Succ;
std::tie(DupProb, Succ) = Tup;
if (DupProb < BestProb)
if (canTailDuplicateUnplacedPreds(BB, Succ, Chain, BlockFilter)
&& (isProfitableToTailDup(BB, Succ, BestProb, Chain, BlockFilter))) {
dbgs() << " Candidate: " << getBlockName(Succ) << ", probability: "
<< DupProb
<< " (Tail Duplicate)\n");
BestSucc.BB = Succ;
BestSucc.ShouldTailDup = true;
if (BestSucc.BB)
DEBUG(dbgs() << " Selected: " << getBlockName(BestSucc.BB) << "\n");
return BestSucc;
/// \brief Select the best block from a worklist.
/// This looks through the provided worklist as a list of candidate basic
/// blocks and select the most profitable one to place. The definition of
/// profitable only really makes sense in the context of a loop. This returns
/// the most frequently visited block in the worklist, which in the case of
/// a loop, is the one most desirable to be physically close to the rest of the
/// loop body in order to improve i-cache behavior.
/// \returns The best block found, or null if none are viable.
MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock(
const BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList) {
// Once we need to walk the worklist looking for a candidate, cleanup the
// worklist of already placed entries.
// FIXME: If this shows up on profiles, it could be folded (at the cost of
// some code complexity) into the loop below.
[&](MachineBasicBlock *BB) {
return BlockToChain.lookup(BB) == &Chain;
if (WorkList.empty())
return nullptr;
bool IsEHPad = WorkList[0]->isEHPad();
MachineBasicBlock *BestBlock = nullptr;
BlockFrequency BestFreq;
for (MachineBasicBlock *MBB : WorkList) {
assert(MBB->isEHPad() == IsEHPad &&
"EHPad mismatch between block and work list.");
BlockChain &SuccChain = *BlockToChain[MBB];
if (&SuccChain == &Chain)
assert(SuccChain.UnscheduledPredecessors == 0 &&
"Found CFG-violating block");
BlockFrequency CandidateFreq = MBFI->getBlockFreq(MBB);
DEBUG(dbgs() << " " << getBlockName(MBB) << " -> ";
MBFI->printBlockFreq(dbgs(), CandidateFreq) << " (freq)\n");
// For ehpad, we layout the least probable first as to avoid jumping back
// from least probable landingpads to more probable ones.
// FIXME: Using probability is probably (!) not the best way to achieve
// this. We should probably have a more principled approach to layout
// cleanup code.
// The goal is to get:
// +--------------------------+
// | V
// InnerLp -> InnerCleanup OuterLp -> OuterCleanup -> Resume
// Rather than:
// +-------------------------------------+
// V |
// OuterLp -> OuterCleanup -> Resume InnerLp -> InnerCleanup
if (BestBlock && (IsEHPad ^ (BestFreq >= CandidateFreq)))
BestBlock = MBB;
BestFreq = CandidateFreq;
return BestBlock;
/// \brief Retrieve the first unplaced basic block.
/// This routine is called when we are unable to use the CFG to walk through
/// all of the basic blocks and form a chain due to unnatural loops in the CFG.
/// We walk through the function's blocks in order, starting from the
/// LastUnplacedBlockIt. We update this iterator on each call to avoid
/// re-scanning the entire sequence on repeated calls to this routine.
MachineBasicBlock *MachineBlockPlacement::getFirstUnplacedBlock(
const BlockChain &PlacedChain,
MachineFunction::iterator &PrevUnplacedBlockIt,
const BlockFilterSet *BlockFilter) {
for (MachineFunction::iterator I = PrevUnplacedBlockIt, E = F->end(); I != E;
++I) {
if (BlockFilter && !BlockFilter->count(&*I))
if (BlockToChain[&*I] != &PlacedChain) {
PrevUnplacedBlockIt = I;
// Now select the head of the chain to which the unplaced block belongs
// as the block to place. This will force the entire chain to be placed,
// and satisfies the requirements of merging chains.
return *BlockToChain[&*I]->begin();
return nullptr;
void MachineBlockPlacement::fillWorkLists(
const MachineBasicBlock *MBB,
SmallPtrSetImpl<BlockChain *> &UpdatedPreds,
const BlockFilterSet *BlockFilter = nullptr) {
BlockChain &Chain = *BlockToChain[MBB];
if (!UpdatedPreds.insert(&Chain).second)
Chain.UnscheduledPredecessors == 0 &&
"Attempting to place block with unscheduled predecessors in worklist.");
for (MachineBasicBlock *ChainBB : Chain) {
assert(BlockToChain[ChainBB] == &Chain &&
"Block in chain doesn't match BlockToChain map.");
for (MachineBasicBlock *Pred : ChainBB->predecessors()) {
if (BlockFilter && !BlockFilter->count(Pred))
if (BlockToChain[Pred] == &Chain)
if (Chain.UnscheduledPredecessors != 0)
MachineBasicBlock *BB = *Chain.begin();
if (BB->isEHPad())
void MachineBlockPlacement::buildChain(
const MachineBasicBlock *HeadBB, BlockChain &Chain,
BlockFilterSet *BlockFilter) {
assert(HeadBB && "BB must not be null.\n");
assert(BlockToChain[HeadBB] == &Chain && "BlockToChainMap mis-match.\n");
MachineFunction::iterator PrevUnplacedBlockIt = F->begin();
const MachineBasicBlock *LoopHeaderBB = HeadBB;
markChainSuccessors(Chain, LoopHeaderBB, BlockFilter);
MachineBasicBlock *BB = *std::prev(Chain.end());
while (true) {
assert(BB && "null block found at end of chain in loop.");
assert(BlockToChain[BB] == &Chain && "BlockToChainMap mis-match in loop.");
assert(*std::prev(Chain.end()) == BB && "BB Not found at end of chain.");
// Look for the best viable successor if there is one to place immediately
// after this block.
auto Result = selectBestSuccessor(BB, Chain, BlockFilter);
MachineBasicBlock* BestSucc = Result.BB;
bool ShouldTailDup = Result.ShouldTailDup;
if (TailDupPlacement)
ShouldTailDup |= (BestSucc && shouldTailDuplicate(BestSucc));
// If an immediate successor isn't available, look for the best viable
// block among those we've identified as not violating the loop's CFG at
// this point. This won't be a fallthrough, but it will increase locality.
if (!BestSucc)
BestSucc = selectBestCandidateBlock(Chain, BlockWorkList);
if (!BestSucc)
BestSucc = selectBestCandidateBlock(Chain, EHPadWorkList);
if (!BestSucc) {
BestSucc = getFirstUnplacedBlock(Chain, PrevUnplacedBlockIt, BlockFilter);
if (!BestSucc)
DEBUG(dbgs() << "Unnatural loop CFG detected, forcibly merging the "
"layout successor until the CFG reduces\n");
// Placement may have changed tail duplication opportunities.
// Check for that now.
if (TailDupPlacement && BestSucc && ShouldTailDup) {
// If the chosen successor was duplicated into all its predecessors,
// don't bother laying it out, just go round the loop again with BB as
// the chain end.
if (repeatedlyTailDuplicateBlock(BestSucc, BB, LoopHeaderBB, Chain,
BlockFilter, PrevUnplacedBlockIt))
// Place this block, updating the datastructures to reflect its placement.
BlockChain &SuccChain = *BlockToChain[BestSucc];
// Zero out UnscheduledPredecessors for the successor we're about to merge in case
// we selected a successor that didn't fit naturally into the CFG.
SuccChain.UnscheduledPredecessors = 0;
DEBUG(dbgs() << "Merging from " << getBlockName(BB) << " to "
<< getBlockName(BestSucc) << "\n");
markChainSuccessors(SuccChain, LoopHeaderBB, BlockFilter);
Chain.merge(BestSucc, &SuccChain);
BB = *std::prev(Chain.end());
DEBUG(dbgs() << "Finished forming chain for header block "
<< getBlockName(*Chain.begin()) << "\n");
/// \brief Find the best loop top block for layout.
/// Look for a block which is strictly better than the loop header for laying
/// out at the top of the loop. This looks for one and only one pattern:
/// a latch block with no conditional exit. This block will cause a conditional
/// jump around it or will be the bottom of the loop if we lay it out in place,
/// but if it it doesn't end up at the bottom of the loop for any reason,
/// rotation alone won't fix it. Because such a block will always result in an
/// unconditional jump (for the backedge) rotating it in front of the loop
/// header is always profitable.
MachineBasicBlock *
MachineBlockPlacement::findBestLoopTop(const MachineLoop &L,
const BlockFilterSet &LoopBlockSet) {
// Placing the latch block before the header may introduce an extra branch
// that skips this block the first time the loop is executed, which we want
// to avoid when optimising for size.
// FIXME: in theory there is a case that does not introduce a new branch,
// i.e. when the layout predecessor does not fallthrough to the loop header.
// In practice this never happens though: there always seems to be a preheader
// that can fallthrough and that is also placed before the header.
if (F->getFunction()->optForSize())
return L.getHeader();
// Check that the header hasn't been fused with a preheader block due to
// crazy branches. If it has, we need to start with the header at the top to
// prevent pulling the preheader into the loop body.
BlockChain &HeaderChain = *BlockToChain[L.getHeader()];
if (!LoopBlockSet.count(*HeaderChain.begin()))
return L.getHeader();
DEBUG(dbgs() << "Finding best loop top for: " << getBlockName(L.getHeader())
<< "\n");
BlockFrequency BestPredFreq;
MachineBasicBlock *BestPred = nullptr;
for (MachineBasicBlock *Pred : L.getHeader()->predecessors()) {
if (!LoopBlockSet.count(Pred))
DEBUG(dbgs() << " header pred: " << getBlockName(Pred) << ", has "
<< Pred->succ_size() << " successors, ";
MBFI->printBlockFreq(dbgs(), Pred) << " freq\n");
if (Pred->succ_size() > 1)
BlockFrequency PredFreq = MBFI->getBlockFreq(Pred);
if (!BestPred || PredFreq > BestPredFreq ||
(!(PredFreq < BestPredFreq) &&
Pred->isLayoutSuccessor(L.getHeader()))) {
BestPred = Pred;
BestPredFreq = PredFreq;
// If no direct predecessor is fine, just use the loop header.
if (!BestPred) {
DEBUG(dbgs() << " final top unchanged\n");
return L.getHeader();
// Walk backwards through any straight line of predecessors.
while (BestPred->pred_size() == 1 &&
(*BestPred->pred_begin())->succ_size() == 1 &&
*BestPred->pred_begin() != L.getHeader())
BestPred = *BestPred->pred_begin();
DEBUG(dbgs() << " final top: " << getBlockName(BestPred) << "\n");
return BestPred;
/// \brief Find the best loop exiting block for layout.
/// This routine implements the logic to analyze the loop looking for the best
/// block to layout at the top of the loop. Typically this is done to maximize
/// fallthrough opportunities.
MachineBasicBlock *
MachineBlockPlacement::findBestLoopExit(const MachineLoop &L,
const BlockFilterSet &LoopBlockSet) {
// We don't want to layout the loop linearly in all cases. If the loop header
// is just a normal basic block in the loop, we want to look for what block
// within the loop is the best one to layout at the top. However, if the loop
// header has be pre-merged into a chain due to predecessors not having
// analyzable branches, *and* the predecessor it is merged with is *not* part
// of the loop, rotating the header into the middle of the loop will create
// a non-contiguous range of blocks which is Very Bad. So start with the
// header and only rotate if safe.
BlockChain &HeaderChain = *BlockToChain[L.getHeader()];
if (!LoopBlockSet.count(*HeaderChain.begin()))
return nullptr;
BlockFrequency BestExitEdgeFreq;
unsigned BestExitLoopDepth = 0;
MachineBasicBlock *ExitingBB = nullptr;
// If there are exits to outer loops, loop rotation can severely limit
// fallthrough opportunities unless it selects such an exit. Keep a set of
// blocks where rotating to exit with that block will reach an outer loop.
SmallPtrSet<MachineBasicBlock *, 4> BlocksExitingToOuterLoop;
DEBUG(dbgs() << "Finding best loop exit for: " << getBlockName(L.getHeader())
<< "\n");
for (MachineBasicBlock *MBB : L.getBlocks()) {
BlockChain &Chain = *BlockToChain[MBB];
// Ensure that this block is at the end of a chain; otherwise it could be
// mid-way through an inner loop or a successor of an unanalyzable branch.
if (MBB != *std::prev(Chain.end()))
// Now walk the successors. We need to establish whether this has a viable
// exiting successor and whether it has a viable non-exiting successor.
// We store the old exiting state and restore it if a viable looping
// successor isn't found.
MachineBasicBlock *OldExitingBB = ExitingBB;
BlockFrequency OldBestExitEdgeFreq = BestExitEdgeFreq;
bool HasLoopingSucc = false;
for (MachineBasicBlock *Succ : MBB->successors()) {
if (Succ->isEHPad())
if (Succ == MBB)
BlockChain &SuccChain = *BlockToChain[Succ];
// Don't split chains, either this chain or the successor's chain.
if (&Chain == &SuccChain) {
DEBUG(dbgs() << " exiting: " << getBlockName(MBB) << " -> "
<< getBlockName(Succ) << " (chain conflict)\n");
auto SuccProb = MBPI->getEdgeProbability(MBB, Succ);
if (LoopBlockSet.count(Succ)) {
DEBUG(dbgs() << " looping: " << getBlockName(MBB) << " -> "
<< getBlockName(Succ) << " (" << SuccProb << ")\n");
HasLoopingSucc = true;
unsigned SuccLoopDepth = 0;
if (MachineLoop *ExitLoop = MLI->getLoopFor(Succ)) {
SuccLoopDepth = ExitLoop->getLoopDepth();
if (ExitLoop->contains(&L))
BlockFrequency ExitEdgeFreq = MBFI->getBlockFreq(MBB) * SuccProb;
DEBUG(dbgs() << " exiting: " << getBlockName(MBB) << " -> "
<< getBlockName(Succ) << " [L:" << SuccLoopDepth << "] (";
MBFI->printBlockFreq(dbgs(), ExitEdgeFreq) << ")\n");
// Note that we bias this toward an existing layout successor to retain
// incoming order in the absence of better information. The exit must have
// a frequency higher than the current exit before we consider breaking
// the layout.
BranchProbability Bias(100 - ExitBlockBias, 100);
if (!ExitingBB || SuccLoopDepth > BestExitLoopDepth ||
ExitEdgeFreq > BestExitEdgeFreq ||
(MBB->isLayoutSuccessor(Succ) &&
!(ExitEdgeFreq < BestExitEdgeFreq * Bias))) {
BestExitEdgeFreq = ExitEdgeFreq;
ExitingBB = MBB;
if (!HasLoopingSucc) {
// Restore the old exiting state, no viable looping successor was found.
ExitingBB = OldExitingBB;
BestExitEdgeFreq = OldBestExitEdgeFreq;
// Without a candidate exiting block or with only a single block in the
// loop, just use the loop header to layout the loop.
if (!ExitingBB) {
DEBUG(dbgs() << " No other candidate exit blocks, using loop header\n");
return nullptr;
if (L.getNumBlocks() == 1) {
DEBUG(dbgs() << " Loop has 1 block, using loop header as exit\n");
return nullptr;
// Also, if we have exit blocks which lead to outer loops but didn't select
// one of them as the exiting block we are rotating toward, disable loop
// rotation altogether.
if (!BlocksExitingToOuterLoop.empty() &&
return nullptr;
DEBUG(dbgs() << " Best exiting block: " << getBlockName(ExitingBB) << "\n");
return ExitingBB;
/// \brief Attempt to rotate an exiting block to the bottom of the loop.
/// Once we have built a chain, try to rotate it to line up the hot exit block
/// with fallthrough out of the loop if doing so doesn't introduce unnecessary
/// branches. For example, if the loop has fallthrough into its header and out
/// of its bottom already, don't rotate it.
void MachineBlockPlacement::rotateLoop(BlockChain &LoopChain,
const MachineBasicBlock *ExitingBB,
const BlockFilterSet &LoopBlockSet) {
if (!ExitingBB)
MachineBasicBlock *Top = *LoopChain.begin();
MachineBasicBlock *Bottom = *std::prev(LoopChain.end());
// If ExitingBB is already the last one in a chain then nothing to do.
if (Bottom == ExitingBB)
bool ViableTopFallthrough = false;
for (MachineBasicBlock *Pred : Top->predecessors()) {
BlockChain *PredChain = BlockToChain[Pred];
if (!LoopBlockSet.count(Pred) &&
(!PredChain || Pred == *std::prev(PredChain->end()))) {
ViableTopFallthrough = true;
// If the header has viable fallthrough, check whether the current loop
// bottom is a viable exiting block. If so, bail out as rotating will
// introduce an unnecessary branch.
if (ViableTopFallthrough) {
for (MachineBasicBlock *Succ : Bottom->successors()) {
BlockChain *SuccChain = BlockToChain[Succ];
if (!LoopBlockSet.count(Succ) &&
(!SuccChain || Succ == *SuccChain->begin()))
BlockChain::iterator ExitIt = llvm::find(LoopChain, ExitingBB);
if (ExitIt == LoopChain.end())
// Rotating a loop exit to the bottom when there is a fallthrough to top
// trades the entry fallthrough for an exit fallthrough.
// If there is no bottom->top edge, but the chosen exit block does have
// a fallthrough, we break that fallthrough for nothing in return.
// Let's consider an example. We have a built chain of basic blocks
// B1, B2, ..., Bn, where Bk is a ExitingBB - chosen exit block.
// By doing a rotation we get
// Bk+1, ..., Bn, B1, ..., Bk
// Break of fallthrough to B1 is compensated by a fallthrough from Bk.
// If we had a fallthrough Bk -> Bk+1 it is broken now.
// It might be compensated by fallthrough Bn -> B1.
// So we have a condition to avoid creation of extra branch by loop rotation.
// All below must be true to avoid loop rotation:
// If there is a fallthrough to top (B1)
// There was fallthrough from chosen exit block (Bk) to next one (Bk+1)
// There is no fallthrough from bottom (Bn) to top (B1).
// Please note that there is no exit fallthrough from Bn because we checked it
// above.
if (ViableTopFallthrough) {
assert(std::next(ExitIt) != LoopChain.end() &&
"Exit should not be last BB");
MachineBasicBlock *NextBlockInChain = *std::next(ExitIt);
if (ExitingBB->isSuccessor(NextBlockInChain))
if (!Bottom->isSuccessor(Top))
DEBUG(dbgs() << "Rotating loop to put exit " << getBlockName(ExitingBB)
<< " at bottom\n");
std::rotate(LoopChain.begin(), std::next(ExitIt), LoopChain.end());
/// \brief Attempt to rotate a loop based on profile data to reduce branch cost.
/// With profile data, we can determine the cost in terms of missed fall through
/// opportunities when rotating a loop chain and select the best rotation.
/// Basically, there are three kinds of cost to consider for each rotation:
/// 1. The possibly missed fall through edge (if it exists) from BB out of
/// the loop to the loop header.
/// 2. The possibly missed fall through edges (if they exist) from the loop
/// exits to BB out of the loop.
/// 3. The missed fall through edge (if it exists) from the last BB to the
/// first BB in the loop chain.
/// Therefore, the cost for a given rotation is the sum of costs listed above.
/// We select the best rotation with the smallest cost.
void MachineBlockPlacement::rotateLoopWithProfile(
BlockChain &LoopChain, const MachineLoop &L,
const BlockFilterSet &LoopBlockSet) {
auto HeaderBB = L.getHeader();
auto HeaderIter = llvm::find(LoopChain, HeaderBB);
auto RotationPos = LoopChain.end();
BlockFrequency SmallestRotationCost = BlockFrequency::getMaxFrequency();
// A utility lambda that scales up a block frequency by dividing it by a
// branch probability which is the reciprocal of the scale.
auto ScaleBlockFrequency = [](BlockFrequency Freq,
unsigned Scale) -> BlockFrequency {
if (Scale == 0)
return 0;
// Use operator / between BlockFrequency and BranchProbability to implement
// saturating multiplication.
return Freq / BranchProbability(1, Scale);
// Compute the cost of the missed fall-through edge to the loop header if the
// chain head is not the loop header. As we only consider natural loops with
// single header, this computation can be done only once.
BlockFrequency HeaderFallThroughCost(0);
for (auto *Pred : HeaderBB->predecessors()) {
BlockChain *PredChain = BlockToChain[Pred];
if (!LoopBlockSet.count(Pred) &&
(!PredChain || Pred == *std::prev(PredChain->end()))) {
auto EdgeFreq =
MBFI->getBlockFreq(Pred) * MBPI->getEdgeProbability(Pred, HeaderBB);
auto FallThruCost = ScaleBlockFrequency(EdgeFreq, MisfetchCost);
// If the predecessor has only an unconditional jump to the header, we
// need to consider the cost of this jump.
if (Pred->succ_size() == 1)
FallThruCost += ScaleBlockFrequency(EdgeFreq, JumpInstCost);
HeaderFallThroughCost = std::max(HeaderFallThroughCost, FallThruCost);
// Here we collect all exit blocks in the loop, and for each exit we find out
// its hottest exit edge. For each loop rotation, we define the loop exit cost
// as the sum of frequencies of exit edges we collect here, excluding the exit
// edge from the tail of the loop chain.
SmallVector<std::pair<MachineBasicBlock *, BlockFrequency>, 4> ExitsWithFreq;
for (auto BB : LoopChain) {
auto LargestExitEdgeProb = BranchProbability::getZero();
for (auto *Succ : BB->successors()) {
BlockChain *SuccChain = BlockToChain[Succ];
if (!LoopBlockSet.count(Succ) &&
(!SuccChain || Succ == *SuccChain->begin())) {
auto SuccProb = MBPI->getEdgeProbability(BB, Succ);
LargestExitEdgeProb = std::max(LargestExitEdgeProb, SuccProb);
if (LargestExitEdgeProb > BranchProbability::getZero()) {
auto ExitFreq = MBFI->getBlockFreq(BB) * LargestExitEdgeProb;
ExitsWithFreq.emplace_back(BB, ExitFreq);
// In this loop we iterate every block in the loop chain and calculate the
// cost assuming the block is the head of the loop chain. When the loop ends,
// we should have found the best candidate as the loop chain's head.
for (auto Iter = LoopChain.begin(), TailIter = std::prev(LoopChain.end()),
EndIter = LoopChain.end();
Iter != EndIter; Iter++, TailIter++) {
// TailIter is used to track the tail of the loop chain if the block we are
// checking (pointed by Iter) is the head of the chain.
if (TailIter == LoopChain.end())
TailIter = LoopChain.begin();
auto TailBB = *TailIter;
// Calculate the cost by putting this BB to the top.
BlockFrequency Cost = 0;
// If the current BB is the loop header, we need to take into account the
// cost of the missed fall through edge from outside of the loop to the
// header.
if (Iter != HeaderIter)
Cost += HeaderFallThroughCost;
// Collect the loop exit cost by summing up frequencies of all exit edges
// except the one from the chain tail.
for (auto &ExitWithFreq : ExitsWithFreq)
if (TailBB != ExitWithFreq.first)
Cost += ExitWithFreq.second;
// The cost of breaking the once fall-through edge from the tail to the top
// of the loop chain. Here we need to consider three cases:
// 1. If the tail node has only one successor, then we will get an
// additional jmp instruction. So the cost here is (MisfetchCost +
// JumpInstCost) * tail node frequency.
// 2. If the tail node has two successors, then we may still get an
// additional jmp instruction if the layout successor after the loop
// chain is not its CFG successor. Note that the more frequently executed
// jmp instruction will be put ahead of the other one. Assume the
// frequency of those two branches are x and y, where x is the frequency
// of the edge to the chain head, then the cost will be
// (x * MisfetechCost + min(x, y) * JumpInstCost) * tail node frequency.
// 3. If the tail node has more than two successors (this rarely happens),
// we won't consider any additional cost.
if (TailBB->isSuccessor(*Iter)) {
auto TailBBFreq = MBFI->getBlockFreq(TailBB);
if (TailBB->succ_size() == 1)
Cost += ScaleBlockFrequency(TailBBFreq.getFrequency(),
MisfetchCost + JumpInstCost);
else if (TailBB->succ_size() == 2) {
auto TailToHeadProb = MBPI->getEdgeProbability(TailBB, *Iter);
auto TailToHeadFreq = TailBBFreq * TailToHeadProb;
auto ColderEdgeFreq = TailToHeadProb > BranchProbability(1, 2)
? TailBBFreq * TailToHeadProb.getCompl()
: TailToHeadFreq;
Cost += ScaleBlockFrequency(TailToHeadFreq, MisfetchCost) +
ScaleBlockFrequency(ColderEdgeFreq, JumpInstCost);
DEBUG(dbgs() << "The cost of loop rotation by making " << getBlockName(*Iter)
<< " to the top: " << Cost.getFrequency() << "\n");
if (Cost < SmallestRotationCost) {
SmallestRotationCost = Cost;
RotationPos = Iter;
if (RotationPos != LoopChain.end()) {
DEBUG(dbgs() << "Rotate loop by making " << getBlockName(*RotationPos)
<< " to the top\n");
std::rotate(LoopChain.begin(), RotationPos, LoopChain.end());
/// \brief Collect blocks in the given loop that are to be placed.
/// When profile data is available, exclude cold blocks from the returned set;
/// otherwise, collect all blocks in the loop.
MachineBlockPlacement::collectLoopBlockSet(const MachineLoop &L) {
BlockFilterSet LoopBlockSet;
// Filter cold blocks off from LoopBlockSet when profile data is available.
// Collect the sum of frequencies of incoming edges to the loop header from
// outside. If we treat the loop as a super block, this is the frequency of
// the loop. Then for each block in the loop, we calculate the ratio between
// its frequency and the frequency of the loop block. When it is too small,
// don't add it to the loop chain. If there are outer loops, then this block
// will be merged into the first outer loop chain for which this block is not
// cold anymore. This needs precise profile data and we only do this when
// profile data is available.
if (F->getFunction()->getEntryCount() || ForceLoopColdBlock) {
BlockFrequency LoopFreq(0);
for (auto LoopPred : L.getHeader()->predecessors())
if (!L.contains(LoopPred))
LoopFreq += MBFI->getBlockFreq(LoopPred) *
MBPI->getEdgeProbability(LoopPred, L.getHeader());
for (MachineBasicBlock *LoopBB : L.getBlocks()) {
auto Freq = MBFI->getBlockFreq(LoopBB).getFrequency();
if (Freq == 0 || LoopFreq.getFrequency() / Freq > LoopToColdBlockRatio)
} else
LoopBlockSet.insert(L.block_begin(), L.block_end());
return LoopBlockSet;
/// \brief Forms basic block chains from the natural loop structures.
/// These chains are designed to preserve the existing *structure* of the code
/// as much as possible. We can then stitch the chains together in a way which
/// both preserves the topological structure and minimizes taken conditional
/// branches.
void MachineBlockPlacement::buildLoopChains(const MachineLoop &L) {
// First recurse through any nested loops, building chains for those inner
// loops.
for (const MachineLoop *InnerLoop : L)
assert(BlockWorkList.empty() &&
"BlockWorkList not empty when starting to build loop chains.");
assert(EHPadWorkList.empty() &&
"EHPadWorkList not empty when starting to build loop chains.");
BlockFilterSet LoopBlockSet = collectLoopBlockSet(L);
// Check if we have profile data for this function. If yes, we will rotate
// this loop by modeling costs more precisely which requires the profile data
// for better layout.
bool RotateLoopWithProfile =
ForcePreciseRotationCost ||
(PreciseRotationCost && F->getFunction()->getEntryCount());
// First check to see if there is an obviously preferable top block for the
// loop. This will default to the header, but may end up as one of the
// predecessors to the header if there is one which will result in strictly
// fewer branches in the loop body.
// When we use profile data to rotate the loop, this is unnecessary.
MachineBasicBlock *LoopTop =
RotateLoopWithProfile ? L.getHeader() : findBestLoopTop(L, LoopBlockSet);
// If we selected just the header for the loop top, look for a potentially
// profitable exit block in the event that rotating the loop can eliminate
// branches by placing an exit edge at the bottom.
// Loops are processed innermost to uttermost, make sure we clear
// PreferredLoopExit before processing a new loop.
PreferredLoopExit = nullptr;
if (!RotateLoopWithProfile && LoopTop == L.getHeader())
PreferredLoopExit = findBestLoopExit(L, LoopBlockSet);
BlockChain &LoopChain = *BlockToChain[LoopTop];
// FIXME: This is a really lame way of walking the chains in the loop: we
// walk the blocks, and use a set to prevent visiting a particular chain
// twice.
SmallPtrSet<BlockChain *, 4> UpdatedPreds;
assert(LoopChain.UnscheduledPredecessors == 0 &&
"LoopChain should not have unscheduled predecessors.");
for (const MachineBasicBlock *LoopBB : LoopBlockSet)
fillWorkLists(LoopBB, UpdatedPreds, &LoopBlockSet);
buildChain(LoopTop, LoopChain, &LoopBlockSet);
if (RotateLoopWithProfile)
rotateLoopWithProfile(LoopChain, L, LoopBlockSet);
rotateLoop(LoopChain, PreferredLoopExit, LoopBlockSet);
// Crash at the end so we get all of the debugging output first.
bool BadLoop = false;
if (LoopChain.UnscheduledPredecessors) {
BadLoop = true;
dbgs() << "Loop chain contains a block without its preds placed!\n"
<< " Loop header: " << getBlockName(*L.block_begin()) << "\n"
<< " Chain header: " << getBlockName(*LoopChain.begin()) << "\n";
for (MachineBasicBlock *ChainBB : LoopChain) {
dbgs() << " ... " << getBlockName(ChainBB) << "\n";
if (!LoopBlockSet.remove(ChainBB)) {
// We don't mark the loop as bad here because there are real situations
// where this can occur. For example, with an unanalyzable fallthrough
// from a loop block to a non-loop block or vice versa.
dbgs() << "Loop chain contains a block not contained by the loop!\n"
<< " Loop header: " << getBlockName(*L.block_begin()) << "\n"
<< " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
<< " Bad block: " << getBlockName(ChainBB) << "\n";
if (!LoopBlockSet.empty()) {
BadLoop = true;
for (const MachineBasicBlock *LoopBB : LoopBlockSet)
dbgs() << "Loop contains blocks never placed into a chain!\n"
<< " Loop header: " << getBlockName(*L.block_begin()) << "\n"
<< " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
<< " Bad block: " << getBlockName(LoopBB) << "\n";
assert(!BadLoop && "Detected problems with the placement of this loop.");
void MachineBlockPlacement::buildCFGChains() {
// Ensure that every BB in the function has an associated chain to simplify
// the assumptions of the remaining algorithm.
SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
for (MachineFunction::iterator FI = F->begin(), FE = F->end(); FI != FE;
++FI) {
MachineBasicBlock *BB = &*FI;
BlockChain *Chain =
new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB);
// Also, merge any blocks which we cannot reason about and must preserve
// the exact fallthrough behavior for.
while (true) {
MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
if (!TII->analyzeBranch(*BB, TBB, FBB, Cond) || !FI->canFallThrough())
MachineFunction::iterator NextFI = std::next(FI);
MachineBasicBlock *NextBB = &*NextFI;
// Ensure that the layout successor is a viable block, as we know that
// fallthrough is a possibility.
assert(NextFI != FE && "Can't fallthrough past the last block.");
DEBUG(dbgs() << "Pre-merging due to unanalyzable fallthrough: "
<< getBlockName(BB) << " -> " << getBlockName(NextBB)
<< "\n");
Chain->merge(NextBB, nullptr);
#ifndef NDEBUG
FI = NextFI;
BB = NextBB;
// Build any loop-based chains.
PreferredLoopExit = nullptr;
for (MachineLoop *L : *MLI)
assert(BlockWorkList.empty() &&
"BlockWorkList should be empty before building final chain.");
assert(EHPadWorkList.empty() &&
"EHPadWorkList should be empty before building final chain.");
SmallPtrSet<BlockChain *, 4> UpdatedPreds;
for (MachineBasicBlock &MBB : *F)
fillWorkLists(&MBB, UpdatedPreds);
BlockChain &FunctionChain = *BlockToChain[&F->front()];
buildChain(&F->front(), FunctionChain);
#ifndef NDEBUG
using FunctionBlockSetType = SmallPtrSet<MachineBasicBlock *, 16>;
// Crash at the end so we get all of the debugging output first.
bool BadFunc = false;
FunctionBlockSetType FunctionBlockSet;
for (MachineBasicBlock &MBB : *F)
for (MachineBasicBlock *ChainBB : FunctionChain)
if (!FunctionBlockSet.erase(ChainBB)) {
BadFunc = true;
dbgs() << "Function chain contains a block not in the function!\n"
<< " Bad block: " << getBlockName(ChainBB) << "\n";
if (!FunctionBlockSet.empty()) {
BadFunc = true;
for (MachineBasicBlock *RemainingBB : FunctionBlockSet)
dbgs() << "Function contains blocks never placed into a chain!\n"
<< " Bad block: " << getBlockName(RemainingBB) << "\n";
assert(!BadFunc && "Detected problems with the block placement.");
// Splice the blocks into place.
MachineFunction::iterator InsertPos = F->begin();
DEBUG(dbgs() << "[MBP] Function: "<< F->getName() << "\n");
for (MachineBasicBlock *ChainBB : FunctionChain) {
DEBUG(dbgs() << (ChainBB == *FunctionChain.begin() ? "Placing chain "
: " ... ")
<< getBlockName(ChainBB) << "\n");
if (InsertPos != MachineFunction::iterator(ChainBB))
F->splice(InsertPos, ChainBB);
// Update the terminator of the previous block.
if (ChainBB == *FunctionChain.begin())
MachineBasicBlock *PrevBB = &*std::prev(MachineFunction::iterator(ChainBB));
// FIXME: It would be awesome of updateTerminator would just return rather
// than assert when the branch cannot be analyzed in order to remove this
// boiler plate.
MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
#ifndef NDEBUG
if (!BlocksWithUnanalyzableExits.count(PrevBB)) {
// Given the exact block placement we chose, we may actually not _need_ to
// be able to edit PrevBB's terminator sequence, but not being _able_ to
// do that at this point is a bug.
assert((!TII->analyzeBranch(*PrevBB, TBB, FBB, Cond) ||
!PrevBB->canFallThrough()) &&
"Unexpected block with un-analyzable fallthrough!");
TBB = FBB = nullptr;
// The "PrevBB" is not yet updated to reflect current code layout, so,
// o. it may fall-through to a block without explicit "goto" instruction
// before layout, and no longer fall-through it after layout; or
// o. just opposite.
// analyzeBranch() may return erroneous value for FBB when these two
// situations take place. For the first scenario FBB is mistakenly set NULL;
// for the 2nd scenario, the FBB, which is expected to be NULL, is
// mistakenly pointing to "*BI".
// Thus, if the future change needs to use FBB before the layout is set, it
// has to correct FBB first by using the code similar to the following:
// if (!Cond.empty() && (!FBB || FBB == ChainBB)) {
// PrevBB->updateTerminator();
// Cond.clear();
// TBB = FBB = nullptr;
// if (TII->analyzeBranch(*PrevBB, TBB, FBB, Cond)) {
// // FIXME: This should never take place.
// TBB = FBB = nullptr;
// }
// }
if (!TII->analyzeBranch(*PrevBB, TBB, FBB, Cond))
// Fixup the last block.
MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
if (!TII->analyzeBranch(F->back(), TBB, FBB, Cond))
void MachineBlockPlacement::optimizeBranches() {
BlockChain &FunctionChain = *BlockToChain[&F->front()];
SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
// Now that all the basic blocks in the chain have the proper layout,
// make a final call to AnalyzeBranch with AllowModify set.
// Indeed, the target may be able to optimize the branches in a way we
// cannot because all branches may not be analyzable.
// E.g., the target may be able to remove an unconditional branch to
// a fallthrough when it occurs after predicated terminators.
for (MachineBasicBlock *ChainBB : FunctionChain) {
MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
if (!TII->analyzeBranch(*ChainBB, TBB, FBB, Cond, /*AllowModify*/ true)) {
// If PrevBB has a two-way branch, try to re-order the branches
// such that we branch to the successor with higher probability first.
if (TBB && !Cond.empty() && FBB &&
MBPI->getEdgeProbability(ChainBB, FBB) >
MBPI->getEdgeProbability(ChainBB, TBB) &&
!TII->reverseBranchCondition(Cond)) {
DEBUG(dbgs() << "Reverse order of the two branches: "
<< getBlockName(ChainBB) << "\n");
DEBUG(dbgs() << " Edge probability: "
<< MBPI->getEdgeProbability(ChainBB, FBB) << " vs "
<< MBPI->getEdgeProbability(ChainBB, TBB) << "\n");
DebugLoc dl; // FIXME: this is nowhere
TII->insertBranch(*ChainBB, FBB, TBB, Cond, dl);
void MachineBlockPlacement::alignBlocks() {
// Walk through the backedges of the function now that we have fully laid out
// the basic blocks and align the destination of each backedge. We don't rely
// exclusively on the loop info here so that we can align backedges in
// unnatural CFGs and backedges that were introduced purely because of the
// loop rotations done during this layout pass.
if (F->getFunction()->optForSize())
BlockChain &FunctionChain = *BlockToChain[&F->front()];
if (FunctionChain.begin() == FunctionChain.end())
return; // Empty chain.
const BranchProbability ColdProb(1, 5); // 20%
BlockFrequency EntryFreq = MBFI->getBlockFreq(&F->front());
BlockFrequency WeightedEntryFreq = EntryFreq * ColdProb;
for (MachineBasicBlock *ChainBB : FunctionChain) {
if (ChainBB == *FunctionChain.begin())
// Don't align non-looping basic blocks. These are unlikely to execute
// enough times to matter in practice. Note that we'll still handle
// unnatural CFGs inside of a natural outer loop (the common case) and
// rotated loops.
MachineLoop *L = MLI->getLoopFor(ChainBB);
if (!L)
unsigned Align = TLI->getPrefLoopAlignment(L);
if (!Align)
continue; // Don't care about loop alignment.
// If the block is cold relative to the function entry don't waste space
// aligning it.
BlockFrequency Freq = MBFI->getBlockFreq(ChainBB);
if (Freq < WeightedEntryFreq)
// If the block is cold relative to its loop header, don't align it
// regardless of what edges into the block exist.
MachineBasicBlock *LoopHeader = L->getHeader();
BlockFrequency LoopHeaderFreq = MBFI->getBlockFreq(LoopHeader);
if (Freq < (LoopHeaderFreq * ColdProb))
// Check for the existence of a non-layout predecessor which would benefit
// from aligning this block.
MachineBasicBlock *LayoutPred =
// Force alignment if all the predecessors are jumps. We already checked
// that the block isn't cold above.
if (!LayoutPred->isSuccessor(ChainBB)) {
// Align this block if the layout predecessor's edge into this block is
// cold relative to the block. When this is true, other predecessors make up
// all of the hot entries into the block and thus alignment is likely to be
// important.
BranchProbability LayoutProb =
MBPI->getEdgeProbability(LayoutPred, ChainBB);
BlockFrequency LayoutEdgeFreq = MBFI->getBlockFreq(LayoutPred) * LayoutProb;
if (LayoutEdgeFreq <= (Freq * ColdProb))
/// Tail duplicate \p BB into (some) predecessors if profitable, repeating if
/// it was duplicated into its chain predecessor and removed.
/// \p BB - Basic block that may be duplicated.
/// \p LPred - Chosen layout predecessor of \p BB.
/// Updated to be the chain end if LPred is removed.
/// \p Chain - Chain to which \p LPred belongs, and \p BB will belong.
/// \p BlockFilter - Set of blocks that belong to the loop being laid out.
/// Used to identify which blocks to update predecessor
/// counts.
/// \p PrevUnplacedBlockIt - Iterator pointing to the last block that was
/// chosen in the given order due to unnatural CFG
/// only needed if \p BB is removed and
/// \p PrevUnplacedBlockIt pointed to \p BB.
/// @return true if \p BB was removed.
bool MachineBlockPlacement::repeatedlyTailDuplicateBlock(
MachineBasicBlock *BB, MachineBasicBlock *&LPred,
const MachineBasicBlock *LoopHeaderBB,
BlockChain &Chain, Block