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

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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 "llvm/CodeGen/Passes.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetLowering.h"
#include "llvm/Target/TargetSubtargetInfo.h"
#include <algorithm>
using namespace llvm;
#define DEBUG_TYPE "block-placement"
STATISTIC(NumCondBranches, "Number of conditional branches");
STATISTIC(NumUncondBranches, "Number of uncondittional branches");
STATISTIC(CondBranchTakenFreq,
"Potential frequency of taking conditional branches");
STATISTIC(UncondBranchTakenFreq,
"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);
// FIXME: Find a good default for this flag and remove the flag.
static cl::opt<unsigned> ExitBlockBias(
"block-placement-exit-block-bias",
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);
static cl::opt<bool> OutlineOptionalBranches(
"outline-optional-branches",
cl::desc("Put completely optional branches, i.e. branches with a common "
"post dominator, out of line."),
cl::init(false), cl::Hidden);
static cl::opt<unsigned> OutlineOptionalThreshold(
"outline-optional-threshold",
cl::desc("Don't outline optional branches that are a single block with an "
"instruction count below this threshold"),
cl::init(4), cl::Hidden);
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 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;
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), LoopPredecessors(0) {
assert(BB && "Cannot create a chain with a null basic block");
BlockToChain[BB] = this;
}
/// \brief Iterator over blocks within the chain.
typedef SmallVectorImpl<MachineBasicBlock *>::iterator iterator;
/// \brief Beginning of blocks within the chain.
iterator begin() { return Blocks.begin(); }
/// \brief End of blocks within the chain.
iterator end() { 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());
// 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 (MachineBasicBlock *ChainBB : *Chain) {
Blocks.push_back(ChainBB);
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)
MBB->dump();
}
#endif // NDEBUG
/// \brief Count of predecessors within the loop currently being processed.
///
/// This count is updated at each loop we process to represent the number of
/// in-loop predecessors of this chain.
unsigned LoopPredecessors;
};
} // namespace
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 TargetLoweringBase *TLI;
/// \brief A handle to the post dominator tree.
MachineDominatorTree *MDT;
/// \brief A set of blocks that are unavoidably execute, i.e. they dominate
/// all terminators of the MachineFunction.
SmallPtrSet<MachineBasicBlock *, 4> UnavoidableBlocks;
/// \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<MachineBasicBlock *, BlockChain *> BlockToChain;
void markChainSuccessors(BlockChain &Chain, MachineBasicBlock *LoopHeaderBB,
SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
const BlockFilterSet *BlockFilter = nullptr);
MachineBasicBlock *selectBestSuccessor(MachineBasicBlock *BB,
BlockChain &Chain,
const BlockFilterSet *BlockFilter);
MachineBasicBlock *
selectBestCandidateBlock(BlockChain &Chain,
SmallVectorImpl<MachineBasicBlock *> &WorkList,
const BlockFilterSet *BlockFilter);
MachineBasicBlock *
getFirstUnplacedBlock(MachineFunction &F, const BlockChain &PlacedChain,
MachineFunction::iterator &PrevUnplacedBlockIt,
const BlockFilterSet *BlockFilter);
void buildChain(MachineBasicBlock *BB, BlockChain &Chain,
SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
const BlockFilterSet *BlockFilter = nullptr);
MachineBasicBlock *findBestLoopTop(MachineLoop &L,
const BlockFilterSet &LoopBlockSet);
MachineBasicBlock *findBestLoopExit(MachineFunction &F, MachineLoop &L,
const BlockFilterSet &LoopBlockSet);
void buildLoopChains(MachineFunction &F, MachineLoop &L);
void rotateLoop(BlockChain &LoopChain, MachineBasicBlock *ExitingBB,
const BlockFilterSet &LoopBlockSet);
void buildCFGChains(MachineFunction &F);
public:
static char ID; // Pass identification, replacement for typeid
MachineBlockPlacement() : MachineFunctionPass(ID) {
initializeMachineBlockPlacementPass(*PassRegistry::getPassRegistry());
}
bool runOnMachineFunction(MachineFunction &F) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<MachineBranchProbabilityInfo>();
AU.addRequired<MachineBlockFrequencyInfo>();
AU.addRequired<MachineDominatorTree>();
AU.addRequired<MachineLoopInfo>();
MachineFunctionPass::getAnalysisUsage(AU);
}
};
} // namespace
char MachineBlockPlacement::ID = 0;
char &llvm::MachineBlockPlacementID = MachineBlockPlacement::ID;
INITIALIZE_PASS_BEGIN(MachineBlockPlacement, "block-placement",
"Branch Probability Basic Block Placement", false, false)
INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
INITIALIZE_PASS_END(MachineBlockPlacement, "block-placement",
"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(MachineBasicBlock *BB) {
std::string Result;
raw_string_ostream OS(Result);
OS << "BB#" << BB->getNumber();
OS << " (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 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 worklist passed in.
void MachineBlockPlacement::markChainSuccessors(
BlockChain &Chain, MachineBasicBlock *LoopHeaderBB,
SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
const BlockFilterSet *BlockFilter) {
// Walk all the blocks in this chain, marking their successors as having
// a predecessor placed.
for (MachineBasicBlock *MBB : Chain) {
// 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))
continue;
BlockChain &SuccChain = *BlockToChain[Succ];
// Disregard edges within a fixed chain, or edges to the loop header.
if (&Chain == &SuccChain || Succ == LoopHeaderBB)
continue;
// This is a cross-chain edge that is within the loop, so decrement the
// loop predecessor count of the destination chain.
if (SuccChain.LoopPredecessors > 0 && --SuccChain.LoopPredecessors == 0)
BlockWorkList.push_back(*SuccChain.begin());
}
}
}
/// \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.
MachineBasicBlock *
MachineBlockPlacement::selectBestSuccessor(MachineBasicBlock *BB,
BlockChain &Chain,
const BlockFilterSet *BlockFilter) {
const BranchProbability HotProb(4, 5); // 80%
MachineBasicBlock *BestSucc = nullptr;
// FIXME: Due to the performance of the probability and weight routines in
// the MBPI analysis, we manually compute probabilities using the edge
// weights. This is suboptimal as it means that the somewhat subtle
// definition of edge weight semantics is encoded here as well. We should
// improve the MBPI interface to efficiently support query patterns such as
// this.
uint32_t BestWeight = 0;
uint32_t WeightScale = 0;
uint32_t SumWeight = MBPI->getSumForBlock(BB, WeightScale);
DEBUG(dbgs() << "Attempting merge from: " << getBlockName(BB) << "\n");
for (MachineBasicBlock *Succ : BB->successors()) {
if (BlockFilter && !BlockFilter->count(Succ))
continue;
BlockChain &SuccChain = *BlockToChain[Succ];
if (&SuccChain == &Chain) {
DEBUG(dbgs() << " " << getBlockName(Succ) << " -> Already merged!\n");
continue;
}
if (Succ != *SuccChain.begin()) {
DEBUG(dbgs() << " " << getBlockName(Succ) << " -> Mid chain!\n");
continue;
}
uint32_t SuccWeight = MBPI->getEdgeWeight(BB, Succ);
BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
// If we outline optional branches, look whether Succ is unavoidable, i.e.
// dominates all terminators of the MachineFunction. If it does, other
// successors must be optional. Don't do this for cold branches.
if (OutlineOptionalBranches && SuccProb > HotProb.getCompl() &&
UnavoidableBlocks.count(Succ) > 0) {
auto HasShortOptionalBranch = [&]() {
for (MachineBasicBlock *Pred : Succ->predecessors()) {
// Check whether there is an unplaced optional branch.
if (Pred == Succ || (BlockFilter && !BlockFilter->count(Pred)) ||
BlockToChain[Pred] == &Chain)
continue;
// Check whether the optional branch has exactly one BB.
if (Pred->pred_size() > 1 || *Pred->pred_begin() != BB)
continue;
// Check whether the optional branch is small.
if (Pred->size() < OutlineOptionalThreshold)
return true;
}
return false;
};
if (!HasShortOptionalBranch())
return Succ;
}
// Only consider successors which are either "hot", or wouldn't violate
// any CFG constraints.
if (SuccChain.LoopPredecessors != 0) {
if (SuccProb < HotProb) {
DEBUG(dbgs() << " " << getBlockName(Succ) << " -> " << SuccProb
<< " (prob) (CFG conflict)\n");
continue;
}
// Make sure that a hot successor doesn't have a globally more
// important predecessor.
BlockFrequency CandidateEdgeFreq =
MBFI->getBlockFreq(BB) * SuccProb * HotProb.getCompl();
bool BadCFGConflict = false;
for (MachineBasicBlock *Pred : Succ->predecessors()) {
if (Pred == Succ || (BlockFilter && !BlockFilter->count(Pred)) ||
BlockToChain[Pred] == &Chain)
continue;
BlockFrequency PredEdgeFreq =
MBFI->getBlockFreq(Pred) * MBPI->getEdgeProbability(Pred, Succ);
if (PredEdgeFreq >= CandidateEdgeFreq) {
BadCFGConflict = true;
break;
}
}
if (BadCFGConflict) {
DEBUG(dbgs() << " " << getBlockName(Succ) << " -> " << SuccProb
<< " (prob) (non-cold CFG conflict)\n");
continue;
}
}
DEBUG(dbgs() << " " << getBlockName(Succ) << " -> " << SuccProb
<< " (prob)"
<< (SuccChain.LoopPredecessors != 0 ? " (CFG break)" : "")
<< "\n");
if (BestSucc && BestWeight >= SuccWeight)
continue;
BestSucc = Succ;
BestWeight = SuccWeight;
}
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 icache behavior.
///
/// \returns The best block found, or null if none are viable.
MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock(
BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList,
const BlockFilterSet *BlockFilter) {
// 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.
WorkList.erase(std::remove_if(WorkList.begin(), WorkList.end(),
[&](MachineBasicBlock *BB) {
return BlockToChain.lookup(BB) == &Chain;
}),
WorkList.end());
MachineBasicBlock *BestBlock = nullptr;
BlockFrequency BestFreq;
for (MachineBasicBlock *MBB : WorkList) {
BlockChain &SuccChain = *BlockToChain[MBB];
if (&SuccChain == &Chain) {
DEBUG(dbgs() << " " << getBlockName(MBB) << " -> Already merged!\n");
continue;
}
assert(SuccChain.LoopPredecessors == 0 && "Found CFG-violating block");
BlockFrequency CandidateFreq = MBFI->getBlockFreq(MBB);
DEBUG(dbgs() << " " << getBlockName(MBB) << " -> ";
MBFI->printBlockFreq(dbgs(), CandidateFreq) << " (freq)\n");
if (BestBlock && BestFreq >= CandidateFreq)
continue;
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(
MachineFunction &F, 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))
continue;
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::buildChain(
MachineBasicBlock *BB, BlockChain &Chain,
SmallVectorImpl<MachineBasicBlock *> &BlockWorkList,
const BlockFilterSet *BlockFilter) {
assert(BB);
assert(BlockToChain[BB] == &Chain);
MachineFunction &F = *BB->getParent();
MachineFunction::iterator PrevUnplacedBlockIt = F.begin();
MachineBasicBlock *LoopHeaderBB = BB;
markChainSuccessors(Chain, LoopHeaderBB, BlockWorkList, BlockFilter);
BB = *std::prev(Chain.end());
for (;;) {
assert(BB);
assert(BlockToChain[BB] == &Chain);
assert(*std::prev(Chain.end()) == BB);
// Look for the best viable successor if there is one to place immediately
// after this block.
MachineBasicBlock *BestSucc = selectBestSuccessor(BB, Chain, BlockFilter);
// 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, BlockFilter);
if (!BestSucc) {
BestSucc =
getFirstUnplacedBlock(F, Chain, PrevUnplacedBlockIt, BlockFilter);
if (!BestSucc)
break;
DEBUG(dbgs() << "Unnatural loop CFG detected, forcibly merging the "
"layout successor until the CFG reduces\n");
}
// Place this block, updating the datastructures to reflect its placement.
BlockChain &SuccChain = *BlockToChain[BestSucc];
// Zero out LoopPredecessors for the successor we're about to merge in case
// we selected a successor that didn't fit naturally into the CFG.
SuccChain.LoopPredecessors = 0;
DEBUG(dbgs() << "Merging from " << getBlockNum(BB) << " to "
<< getBlockNum(BestSucc) << "\n");
markChainSuccessors(SuccChain, LoopHeaderBB, BlockWorkList, BlockFilter);
Chain.merge(BestSucc, &SuccChain);
BB = *std::prev(Chain.end());
}
DEBUG(dbgs() << "Finished forming chain for header block "
<< getBlockNum(*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(MachineLoop &L,
const BlockFilterSet &LoopBlockSet) {
// 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))
continue;
DEBUG(dbgs() << " header pred: " << getBlockName(Pred) << ", "
<< Pred->succ_size() << " successors, ";
MBFI->printBlockFreq(dbgs(), Pred) << " freq\n");
if (Pred->succ_size() > 1)
continue;
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)
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(MachineFunction &F, 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 opportunites 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()))
continue;
// 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;
// FIXME: Due to the performance of the probability and weight routines in
// the MBPI analysis, we use the internal weights and manually compute the
// probabilities to avoid quadratic behavior.
uint32_t WeightScale = 0;
uint32_t SumWeight = MBPI->getSumForBlock(MBB, WeightScale);
for (MachineBasicBlock *Succ : MBB->successors()) {
if (Succ->isLandingPad())
continue;
if (Succ == MBB)
continue;
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");
continue;
}
uint32_t SuccWeight = MBPI->getEdgeWeight(MBB, Succ);
if (LoopBlockSet.count(Succ)) {
DEBUG(dbgs() << " looping: " << getBlockName(MBB) << " -> "
<< getBlockName(Succ) << " (" << SuccWeight << ")\n");
HasLoopingSucc = true;
continue;
}
unsigned SuccLoopDepth = 0;
if (MachineLoop *ExitLoop = MLI->getLoopFor(Succ)) {
SuccLoopDepth = ExitLoop->getLoopDepth();
if (ExitLoop->contains(&L))
BlocksExitingToOuterLoop.insert(MBB);
}
BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
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;
continue;
}
}
// 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 || L.getNumBlocks() == 1)
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() &&
!BlocksExitingToOuterLoop.count(ExitingBB))
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,
MachineBasicBlock *ExitingBB,
const BlockFilterSet &LoopBlockSet) {
if (!ExitingBB)
return;
MachineBasicBlock *Top = *LoopChain.begin();
bool ViableTopFallthrough = false;
for (MachineBasicBlock *Pred : Top->predecessors()) {
BlockChain *PredChain = BlockToChain[Pred];
if (!LoopBlockSet.count(Pred) &&
(!PredChain || Pred == *std::prev(PredChain->end()))) {
ViableTopFallthrough = true;
break;
}
}
// 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) {
MachineBasicBlock *Bottom = *std::prev(LoopChain.end());
for (MachineBasicBlock *Succ : Bottom->successors()) {
BlockChain *SuccChain = BlockToChain[Succ];
if (!LoopBlockSet.count(Succ) &&
(!SuccChain || Succ == *SuccChain->begin()))
return;
}
}
BlockChain::iterator ExitIt =
std::find(LoopChain.begin(), LoopChain.end(), ExitingBB);
if (ExitIt == LoopChain.end())
return;
std::rotate(LoopChain.begin(), std::next(ExitIt), LoopChain.end());
}
/// \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 *InnerLoop : L)
buildLoopChains(F, *InnerLoop);
SmallVector<MachineBasicBlock *, 16> BlockWorkList;
BlockFilterSet LoopBlockSet(L.block_begin(), L.block_end());
// 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.
MachineBasicBlock *LoopTop = 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.
MachineBasicBlock *ExitingBB = nullptr;
if (LoopTop == L.getHeader())
ExitingBB = findBestLoopExit(F, 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.LoopPredecessors == 0);
UpdatedPreds.insert(&LoopChain);
for (MachineBasicBlock *LoopBB : L.getBlocks()) {
BlockChain &Chain = *BlockToChain[LoopBB];
if (!UpdatedPreds.insert(&Chain).second)
continue;
assert(Chain.LoopPredecessors == 0);
for (MachineBasicBlock *ChainBB : Chain) {
assert(BlockToChain[ChainBB] == &Chain);
for (MachineBasicBlock *Pred : ChainBB->predecessors()) {
if (BlockToChain[Pred] == &Chain || !LoopBlockSet.count(Pred))
continue;
++Chain.LoopPredecessors;
}
}
if (Chain.LoopPredecessors == 0)
BlockWorkList.push_back(*Chain.begin());
}
buildChain(LoopTop, LoopChain, BlockWorkList, &LoopBlockSet);
rotateLoop(LoopChain, ExitingBB, LoopBlockSet);
DEBUG({
// Crash at the end so we get all of the debugging output first.
bool BadLoop = false;
if (LoopChain.LoopPredecessors) {
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.erase(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 (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(MachineFunction &F) {
// 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.
for (;;) {
Cond.clear();
MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
if (!TII->AnalyzeBranch(*BB, TBB, FBB, Cond) || !FI->canFallThrough())
break;
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);
FI = NextFI;
BB = NextBB;
}
}
if (OutlineOptionalBranches) {
// Find the nearest common dominator of all of F's terminators.
MachineBasicBlock *Terminator = nullptr;
for (MachineBasicBlock &MBB : F) {
if (MBB.succ_size() == 0) {
if (Terminator == nullptr)
Terminator = &MBB;
else
Terminator = MDT->findNearestCommonDominator(Terminator, &MBB);
}
}
// MBBs dominating this common dominator are unavoidable.
UnavoidableBlocks.clear();
for (MachineBasicBlock &MBB : F) {
if (MDT->dominates(&MBB, Terminator)) {
UnavoidableBlocks.insert(&MBB);
}
}
}
// Build any loop-based chains.
for (MachineLoop *L : *MLI)
buildLoopChains(F, *L);
SmallVector<MachineBasicBlock *, 16> BlockWorkList;
SmallPtrSet<BlockChain *, 4> UpdatedPreds;
for (MachineBasicBlock &MBB : F) {
BlockChain &Chain = *BlockToChain[&MBB];
if (!UpdatedPreds.insert(&Chain).second)
continue;
assert(Chain.LoopPredecessors == 0);
for (MachineBasicBlock *ChainBB : Chain) {
assert(BlockToChain[ChainBB] == &Chain);
for (MachineBasicBlock *Pred : ChainBB->predecessors()) {
if (BlockToChain[Pred] == &Chain)
continue;
++Chain.LoopPredecessors;
}
}
if (Chain.LoopPredecessors == 0)
BlockWorkList.push_back(*Chain.begin());
}
BlockChain &FunctionChain = *BlockToChain[&F.front()];
buildChain(&F.front(), FunctionChain, BlockWorkList);
#ifndef NDEBUG
typedef SmallPtrSet<MachineBasicBlock *, 16> FunctionBlockSetType;
#endif
DEBUG({
// Crash at the end so we get all of the debugging output first.
bool BadFunc = false;
FunctionBlockSetType FunctionBlockSet;
for (MachineBasicBlock &MBB : F)
FunctionBlockSet.insert(&MBB);
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();
for (MachineBasicBlock *ChainBB : FunctionChain) {
DEBUG(dbgs() << (ChainBB == *FunctionChain.begin() ? "Placing chain "
: " ... ")
<< getBlockName(ChainBB) << "\n");
if (InsertPos != MachineFunction::iterator(ChainBB))
F.splice(InsertPos, ChainBB);
else
++InsertPos;
// Update the terminator of the previous block.
if (ChainBB == *FunctionChain.begin())
continue;
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.
Cond.clear();
MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
if (!TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond)) {
// The "PrevBB" is not yet updated to reflect current code layout, so,
// o. it may fall-through to a block without explict "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".
//
bool needUpdateBr = true;
if (!Cond.empty() && (!FBB || FBB == ChainBB)) {
PrevBB->updateTerminator();
needUpdateBr = false;
Cond.clear();
TBB = FBB = nullptr;
if (TII->AnalyzeBranch(*PrevBB, TBB, FBB, Cond)) {
// FIXME: This should never take place.
TBB = FBB = nullptr;
}
}
// If PrevBB has a two-way branch, try to re-order the branches
// such that we branch to the successor with higher weight first.
if (TBB && !Cond.empty() && FBB &&
MBPI->getEdgeWeight(PrevBB, FBB) > MBPI->getEdgeWeight(PrevBB, TBB) &&
!TII->ReverseBranchCondition(Cond)) {
DEBUG(dbgs() << "Reverse order of the two branches: "
<< getBlockName(PrevBB) << "\n");
DEBUG(dbgs() << " Edge weight: " << MBPI->getEdgeWeight(PrevBB, FBB)
<< " vs " << MBPI->getEdgeWeight(PrevBB, TBB) << "\n");
DebugLoc dl; // FIXME: this is nowhere
TII->RemoveBranch(*PrevBB);
TII->InsertBranch(*PrevBB, FBB, TBB, Cond, dl);
needUpdateBr = true;
}
if (needUpdateBr)
PrevBB->updateTerminator();
}
}
// Fixup the last block.
Cond.clear();
MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
if (!TII->AnalyzeBranch(F.back(), TBB, FBB, Cond))
F.back().updateTerminator();
// 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()->hasFnAttribute(Attribute::OptimizeForSize))
return;
if (FunctionChain.begin() == FunctionChain.end())
return; // Empty chain.
const BranchProbability ColdProb(1, 5); // 20%
BlockFrequency EntryFreq = MBFI->getBlockFreq(F.begin());
BlockFrequency WeightedEntryFreq = EntryFreq * ColdProb;
for (MachineBasicBlock *ChainBB : FunctionChain) {
if (ChainBB == *FunctionChain.begin())
continue;
// 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)
continue;
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)
continue;
// 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))
continue;
// Check for the existence of a non-layout predecessor which would benefit
// from aligning this block.
MachineBasicBlock *LayoutPred =
&*std::prev(MachineFunction::iterator(ChainBB));
// Force alignment if all the predecessors are jumps. We already checked
// that the block isn't cold above.
if (!LayoutPred->isSuccessor(ChainBB)) {
ChainBB->setAlignment(Align);
continue;
}
// 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))
ChainBB->setAlignment(Align);
}
}
bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
// Check for single-block functions and skip them.
if (std::next(F.begin()) == F.end())
return false;
if (skipOptnoneFunction(*F.getFunction()))
return false;
MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
MLI = &getAnalysis<MachineLoopInfo>();
TII = F.getSubtarget().getInstrInfo();
TLI = F.getSubtarget().getTargetLowering();
MDT = &getAnalysis<MachineDominatorTree>();
assert(BlockToChain.empty());
buildCFGChains(F);
BlockToChain.clear();
ChainAllocator.DestroyAll();
if (AlignAllBlock)
// Align all of the blocks in the function to a specific alignment.
for (MachineBasicBlock &MBB : F)
MBB.setAlignment(AlignAllBlock);
// We always return true as we have no way to track whether the final order
// differs from the original order.
return true;
}
namespace {
/// \brief A pass to compute block placement statistics.
///
/// A separate pass to compute interesting statistics for evaluating block
/// placement. This is separate from the actual placement pass so that they can
/// be computed in the absence of any placement transformations or when using
/// alternative placement strategies.
class MachineBlockPlacementStats : public MachineFunctionPass {
/// \brief A handle to the branch probability pass.
const MachineBranchProbabilityInfo *MBPI;
/// \brief A handle to the function-wide block frequency pass.
const MachineBlockFrequencyInfo *MBFI;
public:
static char ID; // Pass identification, replacement for typeid
MachineBlockPlacementStats() : MachineFunctionPass(ID) {
initializeMachineBlockPlacementStatsPass(*PassRegistry::getPassRegistry());
}
bool runOnMachineFunction(MachineFunction &F) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<MachineBranchProbabilityInfo>();
AU.addRequired<MachineBlockFrequencyInfo>();
AU.setPreservesAll();
MachineFunctionPass::getAnalysisUsage(AU);
}
};
} // namespace
char MachineBlockPlacementStats::ID = 0;
char &llvm::MachineBlockPlacementStatsID = MachineBlockPlacementStats::ID;
INITIALIZE_PASS_BEGIN(MachineBlockPlacementStats, "block-placement-stats",
"Basic Block Placement Stats", false, false)
INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
INITIALIZE_PASS_END(MachineBlockPlacementStats, "block-placement-stats",
"Basic Block Placement Stats", false, false)
bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) {
// Check for single-block functions and skip them.
if (std::next(F.begin()) == F.end())
return false;
MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
for (MachineBasicBlock &MBB : F) {
BlockFrequency BlockFreq = MBFI->getBlockFreq(&MBB);
Statistic &NumBranches =
(MBB.succ_size() > 1) ? NumCondBranches : NumUncondBranches;
Statistic &BranchTakenFreq =
(MBB.succ_size() > 1) ? CondBranchTakenFreq : UncondBranchTakenFreq;
for (MachineBasicBlock *Succ : MBB.successors()) {
// Skip if this successor is a fallthrough.
if (MBB.isLayoutSuccessor(Succ))
continue;
BlockFrequency EdgeFreq =
BlockFreq * MBPI->getEdgeProbability(&MBB, Succ);
++NumBranches;
BranchTakenFreq += EdgeFreq.getFrequency();
}
}
return false;
}
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