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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 absense 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.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "block-placement2"
#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/Passes.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/SCCIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetLowering.h"
#include <algorithm>
using namespace llvm;
namespace {
/// \brief A structure for storing a weighted edge.
///
/// This stores an edge and its weight, computed as the product of the
/// frequency that the starting block is entered with the probability of
/// a particular exit block.
struct WeightedEdge {
BlockFrequency EdgeFrequency;
MachineBasicBlock *From, *To;
bool operator<(const WeightedEdge &RHS) const {
return EdgeFrequency < RHS.EdgeFrequency;
}
};
}
namespace {
class BlockChain;
/// \brief Type for our function-wide basic block -> block chain mapping.
typedef DenseMap<MachineBasicBlock *, BlockChain *> BlockToChainMapType;
}
namespace {
/// \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. We also can use a block chain to represent a sequence of
/// basic blocks which have some external (correctness) requirement for
/// sequential layout.
///
/// Eventually, the block chains will form a directed graph over the function.
/// We provide an SCC-supporting-iterator in order to quicky build and walk the
/// SCCs of block chains within a function.
///
/// The block chains also have support for calculating and caching probability
/// information related to the chain itself versus other chains. This is used
/// for ranking during the final layout of block chains.
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;
public:
/// \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.
typedef SmallVectorImpl<MachineBasicBlock *>::const_iterator iterator;
/// \brief Beginning of blocks within the chain.
iterator begin() const { return Blocks.begin(); }
/// \brief End of blocks within the chain.
iterator end() const { return Blocks.end(); }
/// \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);
assert(!Blocks.empty());
assert(Blocks.back()->isSuccessor(BB));
// Fast path in case we don't have a chain already.
if (!Chain) {
assert(!BlockToChain[BB]);
Blocks.push_back(BB);
BlockToChain[BB] = this;
return;
}
assert(BB == *Chain->begin());
assert(Chain->begin() != Chain->end());
// Update the incoming blocks to point to this chain, and add them to the
// chain structure.
for (BlockChain::iterator BI = Chain->begin(), BE = Chain->end();
BI != BE; ++BI) {
Blocks.push_back(*BI);
assert(BlockToChain[*BI] == Chain && "Incoming blocks not in chain");
BlockToChain[*BI] = this;
}
}
};
}
namespace {
class MachineBlockPlacement : public MachineFunctionPass {
/// \brief A typedef for a block filter set.
typedef SmallPtrSet<MachineBasicBlock *, 16> BlockFilterSet;
/// \brief A handle to the branch probability pass.
const MachineBranchProbabilityInfo *MBPI;
/// \brief A handle to the function-wide block frequency pass.
const MachineBlockFrequencyInfo *MBFI;
/// \brief A handle to the loop info.
const MachineLoopInfo *MLI;
/// \brief A handle to the target's instruction info.
const TargetInstrInfo *TII;
/// \brief A handle to the target's lowering info.
const TargetLowering *TLI;
/// \brief Allocator and owner of BlockChain structures.
///
/// We build BlockChains lazily by merging together high probability BB
/// sequences acording to the "Algo2" in the paper mentioned at the top of
/// the file. To reduce malloc traffic, we allocate them using this slab-like
/// allocator, and destroy them after the pass completes.
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<MachineBasicBlock *, BlockChain *> BlockToChain;
BlockChain *CreateChain(MachineBasicBlock *BB);
void mergeSuccessor(MachineBasicBlock *BB, BlockChain *Chain,
BlockFilterSet *Filter = 0);
void buildLoopChains(MachineFunction &F, MachineLoop &L);
void buildCFGChains(MachineFunction &F);
void placeChainsTopologically(MachineFunction &F);
void AlignLoops(MachineFunction &F);
public:
static char ID; // Pass identification, replacement for typeid
MachineBlockPlacement() : MachineFunctionPass(ID) {
initializeMachineBlockPlacementPass(*PassRegistry::getPassRegistry());
}
bool runOnMachineFunction(MachineFunction &F);
void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<MachineBranchProbabilityInfo>();
AU.addRequired<MachineBlockFrequencyInfo>();
AU.addRequired<MachineLoopInfo>();
MachineFunctionPass::getAnalysisUsage(AU);
}
const char *getPassName() const { return "Block Placement"; }
};
}
char MachineBlockPlacement::ID = 0;
INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement2",
"Branch Probability Basic Block Placement", false, false)
INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement2",
"Branch Probability Basic Block Placement", false, false)
FunctionPass *llvm::createMachineBlockPlacementPass() {
return new MachineBlockPlacement();
}
#ifndef NDEBUG
/// \brief Helper to print the name of a MBB.
///
/// Only used by debug logging.
static std::string getBlockName(MachineBasicBlock *BB) {
std::string Result;
raw_string_ostream OS(Result);
OS << "BB#" << BB->getNumber()
<< " (derived from LLVM BB '" << BB->getName() << "')";
OS.flush();
return Result;
}
/// \brief Helper to print the number of a MBB.
///
/// Only used by debug logging.
static std::string getBlockNum(MachineBasicBlock *BB) {
std::string Result;
raw_string_ostream OS(Result);
OS << "BB#" << BB->getNumber();
OS.flush();
return Result;
}
#endif
/// \brief Helper to create a new chain for a single BB.
///
/// Takes care of growing the Chains, setting up the BlockChain object, and any
/// debug checking logic.
/// \returns A pointer to the new BlockChain.
BlockChain *MachineBlockPlacement::CreateChain(MachineBasicBlock *BB) {
BlockChain *Chain =
new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB);
return Chain;
}
/// \brief Merge a chain with any viable successor.
///
/// This routine walks the predecessors of the current block, looking for
/// viable merge candidates. It has strict rules it uses to determine when
/// a predecessor can be merged with the current block, which center around
/// preserving the CFG structure. It performs the merge if any viable candidate
/// is found.
void MachineBlockPlacement::mergeSuccessor(MachineBasicBlock *BB,
BlockChain *Chain,
BlockFilterSet *Filter) {
assert(BB);
assert(Chain);
// If this block is not at the end of its chain, it cannot merge with any
// other chain.
if (Chain && *llvm::prior(Chain->end()) != BB)
return;
// Walk through the successors looking for the highest probability edge.
MachineBasicBlock *Successor = 0;
BranchProbability BestProb = BranchProbability::getZero();
DEBUG(dbgs() << "Attempting merge from: " << getBlockName(BB) << "\n");
for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
SE = BB->succ_end();
SI != SE; ++SI) {
if (BB == *SI || (Filter && !Filter->count(*SI)))
continue;
BranchProbability SuccProb = MBPI->getEdgeProbability(BB, *SI);
DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb << "\n");
if (!Successor || SuccProb > BestProb || (!(SuccProb < BestProb) &&
BB->isLayoutSuccessor(*SI))) {
Successor = *SI;
BestProb = SuccProb;
}
}
if (!Successor)
return;
// Grab a chain if it exists already for this successor and make sure the
// successor is at the start of the chain as we can't merge mid-chain. Also,
// if the successor chain is the same as our chain, we're already merged.
BlockChain *SuccChain = BlockToChain[Successor];
if (SuccChain && (SuccChain == Chain || Successor != *SuccChain->begin()))
return;
// We only merge chains across a CFG merge when the desired merge path is
// significantly hotter than the incoming edge. We define a hot edge more
// strictly than the BranchProbabilityInfo does, as the two predecessor
// blocks may have dramatically different incoming probabilities we need to
// account for. Therefor we use the "global" edge weight which is the
// branch's probability times the block frequency of the predecessor.
BlockFrequency MergeWeight = MBFI->getBlockFreq(BB);
MergeWeight *= MBPI->getEdgeProbability(BB, Successor);
// We only want to consider breaking the CFG when the merge weight is much
// higher (80% vs. 20%), so multiply it by 1/4. This will require the merged
// edge to be 4x more likely before we disrupt the CFG. This number matches
// the definition of "hot" in BranchProbabilityAnalysis (80% vs. 20%).
MergeWeight *= BranchProbability(1, 4);
for (MachineBasicBlock::pred_iterator PI = Successor->pred_begin(),
PE = Successor->pred_end();
PI != PE; ++PI) {
if (BB == *PI || Successor == *PI) continue;
BlockFrequency PredWeight = MBFI->getBlockFreq(*PI);
PredWeight *= MBPI->getEdgeProbability(*PI, Successor);
// Return on the first predecessor we find which outstrips our merge weight.
if (MergeWeight < PredWeight)
return;
DEBUG(dbgs() << "Breaking CFG edge!\n"
<< " Edge from " << getBlockNum(BB) << " to "
<< getBlockNum(Successor) << ": " << MergeWeight << "\n"
<< " vs. " << getBlockNum(BB) << " to "
<< getBlockNum(*PI) << ": " << PredWeight << "\n");
}
DEBUG(dbgs() << "Merging from " << getBlockNum(BB) << " to "
<< getBlockNum(Successor) << "\n");
Chain->merge(Successor, SuccChain);
}
/// \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(MachineFunction &F, MachineLoop &L) {
// First recurse through any nested loops, building chains for those inner
// loops.
for (MachineLoop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
buildLoopChains(F, **LI);
SmallPtrSet<MachineBasicBlock *, 16> LoopBlockSet(L.block_begin(),
L.block_end());
// Begin building up a set of chains of blocks within this loop which should
// remain contiguous. Some of the blocks already belong to a chain which
// represents an inner loop.
for (MachineLoop::block_iterator BI = L.block_begin(), BE = L.block_end();
BI != BE; ++BI) {
MachineBasicBlock *BB = *BI;
BlockChain *Chain = BlockToChain[BB];
if (!Chain) Chain = CreateChain(BB);
mergeSuccessor(BB, Chain, &LoopBlockSet);
}
}
void MachineBlockPlacement::buildCFGChains(MachineFunction &F) {
// First build any loop-based chains.
for (MachineLoopInfo::iterator LI = MLI->begin(), LE = MLI->end(); LI != LE;
++LI)
buildLoopChains(F, **LI);
// Now walk the blocks of the function forming chains where they don't
// violate any CFG structure.
for (MachineFunction::iterator BI = F.begin(), BE = F.end();
BI != BE; ++BI) {
MachineBasicBlock *BB = BI;
BlockChain *Chain = BlockToChain[BB];
if (!Chain) Chain = CreateChain(BB);
mergeSuccessor(BB, Chain);
}
}
void MachineBlockPlacement::placeChainsTopologically(MachineFunction &F) {
MachineBasicBlock *EntryB = &F.front();
assert(BlockToChain[EntryB] && "Missing chain for entry block");
assert(*BlockToChain[EntryB]->begin() == EntryB &&
"Entry block is not the head of the entry block chain");
// Walk the blocks in RPO, and insert each block for a chain in order the
// first time we see that chain.
MachineFunction::iterator InsertPos = F.begin();
SmallPtrSet<BlockChain *, 16> VisitedChains;
ReversePostOrderTraversal<MachineBasicBlock *> RPOT(EntryB);
typedef ReversePostOrderTraversal<MachineBasicBlock *>::rpo_iterator
rpo_iterator;
for (rpo_iterator I = RPOT.begin(), E = RPOT.end(); I != E; ++I) {
BlockChain *Chain = BlockToChain[*I];
assert(Chain);
if(!VisitedChains.insert(Chain))
continue;
for (BlockChain::iterator BI = Chain->begin(), BE = Chain->end(); BI != BE;
++BI) {
DEBUG(dbgs() << (BI == Chain->begin() ? "Placing chain "
: " ... ")
<< getBlockName(*BI) << "\n");
if (InsertPos != MachineFunction::iterator(*BI))
F.splice(InsertPos, *BI);
else
++InsertPos;
}
}
// Now that every block is in its final position, update all of the
// terminators.
SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
// 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.
Cond.clear();
MachineBasicBlock *TBB = 0, *FBB = 0; // For AnalyzeBranch.
if (!TII->AnalyzeBranch(*FI, TBB, FBB, Cond))
FI->updateTerminator();
}
}
/// \brief Recursive helper to align a loop and any nested loops.
static void AlignLoop(MachineFunction &F, MachineLoop *L, unsigned Align) {
// Recurse through nested loops.
for (MachineLoop::iterator I = L->begin(), E = L->end(); I != E; ++I)
AlignLoop(F, *I, Align);
L->getTopBlock()->setAlignment(Align);
}
/// \brief Align loop headers to target preferred alignments.
void MachineBlockPlacement::AlignLoops(MachineFunction &F) {
if (F.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
return;
unsigned Align = TLI->getPrefLoopAlignment();
if (!Align)
return; // Don't care about loop alignment.
for (MachineLoopInfo::iterator I = MLI->begin(), E = MLI->end(); I != E; ++I)
AlignLoop(F, *I, Align);
}
bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
// Check for single-block functions and skip them.
if (llvm::next(F.begin()) == F.end())
return false;
MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
MLI = &getAnalysis<MachineLoopInfo>();
TII = F.getTarget().getInstrInfo();
TLI = F.getTarget().getTargetLowering();
assert(BlockToChain.empty());
buildCFGChains(F);
placeChainsTopologically(F);
AlignLoops(F);
BlockToChain.clear();
// We always return true as we have no way to track whether the final order
// differs from the original order.
return true;
}
|