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llvm-mirror/lib/CodeGen/MachineBlockPlacement.cpp
Eric Christopher 67c04e77e5 Have MachineFunction cache a pointer to the subtarget to make lookups 
shorter/easier and have the DAG use that to do the same lookup. This
can be used in the future for TargetMachine based caching lookups from
the MachineFunction easily.

Update the MIPS subtarget switching machinery to update this pointer
at the same time it runs.

llvm-svn: 214838
2014-08-05 02:39:49 +00:00

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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/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/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetLowering.h"
#include "llvm/Target/TargetSubtargetInfo.h"
#include <algorithm>
using namespace llvm;
#define DEBUG_TYPE "block-placement2"
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);
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 (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;
}
}
#ifndef NDEBUG
/// \brief Dump the blocks in this chain.
LLVM_DUMP_METHOD void dump() {
for (iterator I = begin(), E = end(); I != E; ++I)
(*I)->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 {
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 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<MachineLoopInfo>();
MachineFunctionPass::getAnalysisUsage(AU);
}
};
}
char MachineBlockPlacement::ID = 0;
char &llvm::MachineBlockPlacementID = MachineBlockPlacement::ID;
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)
#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 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 (BlockChain::iterator CBI = Chain.begin(), CBE = Chain.end();
CBI != CBE; ++CBI) {
// 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_iterator SI = (*CBI)->succ_begin(),
SE = (*CBI)->succ_end();
SI != SE; ++SI) {
if (BlockFilter && !BlockFilter->count(*SI))
continue;
BlockChain &SuccChain = *BlockToChain[*SI];
// Disregard edges within a fixed chain, or edges to the loop header.
if (&Chain == &SuccChain || *SI == 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_iterator SI = BB->succ_begin(),
SE = BB->succ_end();
SI != SE; ++SI) {
if (BlockFilter && !BlockFilter->count(*SI))
continue;
BlockChain &SuccChain = *BlockToChain[*SI];
if (&SuccChain == &Chain) {
DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Already merged!\n");
continue;
}
if (*SI != *SuccChain.begin()) {
DEBUG(dbgs() << " " << getBlockName(*SI) << " -> Mid chain!\n");
continue;
}
uint32_t SuccWeight = MBPI->getEdgeWeight(BB, *SI);
BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
// Only consider successors which are either "hot", or wouldn't violate
// any CFG constraints.
if (SuccChain.LoopPredecessors != 0) {
if (SuccProb < HotProb) {
DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << 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_iterator PI = (*SI)->pred_begin(),
PE = (*SI)->pred_end();
PI != PE; ++PI) {
if (*PI == *SI || (BlockFilter && !BlockFilter->count(*PI)) ||
BlockToChain[*PI] == &Chain)
continue;
BlockFrequency PredEdgeFreq
= MBFI->getBlockFreq(*PI) * MBPI->getEdgeProbability(*PI, *SI);
if (PredEdgeFreq >= CandidateEdgeFreq) {
BadCFGConflict = true;
break;
}
}
if (BadCFGConflict) {
DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
<< " (prob) (non-cold CFG conflict)\n");
continue;
}
}
DEBUG(dbgs() << " " << getBlockName(*SI) << " -> " << SuccProb
<< " (prob)"
<< (SuccChain.LoopPredecessors != 0 ? " (CFG break)" : "")
<< "\n");
if (BestSucc && BestWeight >= SuccWeight)
continue;
BestSucc = *SI;
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 (SmallVectorImpl<MachineBasicBlock *>::iterator WBI = WorkList.begin(),
WBE = WorkList.end();
WBI != WBE; ++WBI) {
BlockChain &SuccChain = *BlockToChain[*WBI];
if (&SuccChain == &Chain) {
DEBUG(dbgs() << " " << getBlockName(*WBI)
<< " -> Already merged!\n");
continue;
}
assert(SuccChain.LoopPredecessors == 0 && "Found CFG-violating block");
BlockFrequency CandidateFreq = MBFI->getBlockFreq(*WBI);
DEBUG(dbgs() << " " << getBlockName(*WBI) << " -> ";
MBFI->printBlockFreq(dbgs(), CandidateFreq) << " (freq)\n");
if (BestBlock && BestFreq >= CandidateFreq)
continue;
BestBlock = *WBI;
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_iterator PI = L.getHeader()->pred_begin(),
PE = L.getHeader()->pred_end();
PI != PE; ++PI) {
MachineBasicBlock *Pred = *PI;
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 (MachineLoop::block_iterator I = L.block_begin(),
E = L.block_end();
I != E; ++I) {
BlockChain &Chain = *BlockToChain[*I];
// 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 analyzable branch.
if (*I != *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(*I, WeightScale);
for (MachineBasicBlock::succ_iterator SI = (*I)->succ_begin(),
SE = (*I)->succ_end();
SI != SE; ++SI) {
if ((*SI)->isLandingPad())
continue;
if (*SI == *I)
continue;
BlockChain &SuccChain = *BlockToChain[*SI];
// Don't split chains, either this chain or the successor's chain.
if (&Chain == &SuccChain) {
DEBUG(dbgs() << " exiting: " << getBlockName(*I) << " -> "
<< getBlockName(*SI) << " (chain conflict)\n");
continue;
}
uint32_t SuccWeight = MBPI->getEdgeWeight(*I, *SI);
if (LoopBlockSet.count(*SI)) {
DEBUG(dbgs() << " looping: " << getBlockName(*I) << " -> "
<< getBlockName(*SI) << " (" << SuccWeight << ")\n");
HasLoopingSucc = true;
continue;
}
unsigned SuccLoopDepth = 0;
if (MachineLoop *ExitLoop = MLI->getLoopFor(*SI)) {
SuccLoopDepth = ExitLoop->getLoopDepth();
if (ExitLoop->contains(&L))
BlocksExitingToOuterLoop.insert(*I);
}
BranchProbability SuccProb(SuccWeight / WeightScale, SumWeight);
BlockFrequency ExitEdgeFreq = MBFI->getBlockFreq(*I) * SuccProb;
DEBUG(dbgs() << " exiting: " << getBlockName(*I) << " -> "
<< getBlockName(*SI) << " [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 || BestExitLoopDepth < SuccLoopDepth ||
ExitEdgeFreq > BestExitEdgeFreq ||
((*I)->isLayoutSuccessor(*SI) &&
!(ExitEdgeFreq < BestExitEdgeFreq * Bias))) {
BestExitEdgeFreq = ExitEdgeFreq;
ExitingBB = *I;
}
}
// Restore the old exiting state, no viable looping successor was found.
if (!HasLoopingSucc) {
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_iterator PI = Top->pred_begin(),
PE = Top->pred_end();
PI != PE; ++PI) {
BlockChain *PredChain = BlockToChain[*PI];
if (!LoopBlockSet.count(*PI) &&
(!PredChain || *PI == *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_iterator SI = Bottom->succ_begin(),
SE = Bottom->succ_end();
SI != SE; ++SI) {
BlockChain *SuccChain = BlockToChain[*SI];
if (!LoopBlockSet.count(*SI) &&
(!SuccChain || *SI == *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::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
buildLoopChains(F, **LI);
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 (MachineLoop::block_iterator BI = L.block_begin(),
BE = L.block_end();
BI != BE; ++BI) {
BlockChain &Chain = *BlockToChain[*BI];
if (!UpdatedPreds.insert(&Chain))
continue;
assert(Chain.LoopPredecessors == 0);
for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
BCI != BCE; ++BCI) {
assert(BlockToChain[*BCI] == &Chain);
for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
PE = (*BCI)->pred_end();
PI != PE; ++PI) {
if (BlockToChain[*PI] == &Chain || !LoopBlockSet.count(*PI))
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 (BlockChain::iterator BCI = LoopChain.begin(), BCE = LoopChain.end();
BCI != BCE; ++BCI) {
dbgs() << " ... " << getBlockName(*BCI) << "\n";
if (!LoopBlockSet.erase(*BCI)) {
// 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(*BCI) << "\n";
}
}
if (!LoopBlockSet.empty()) {
BadLoop = true;
for (BlockFilterSet::iterator LBI = LoopBlockSet.begin(),
LBE = LoopBlockSet.end();
LBI != LBE; ++LBI)
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(*LBI) << "\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;
}
}
// Build any loop-based chains.
for (MachineLoopInfo::iterator LI = MLI->begin(), LE = MLI->end(); LI != LE;
++LI)
buildLoopChains(F, **LI);
SmallVector<MachineBasicBlock *, 16> BlockWorkList;
SmallPtrSet<BlockChain *, 4> UpdatedPreds;
for (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
MachineBasicBlock *BB = &*FI;
BlockChain &Chain = *BlockToChain[BB];
if (!UpdatedPreds.insert(&Chain))
continue;
assert(Chain.LoopPredecessors == 0);
for (BlockChain::iterator BCI = Chain.begin(), BCE = Chain.end();
BCI != BCE; ++BCI) {
assert(BlockToChain[*BCI] == &Chain);
for (MachineBasicBlock::pred_iterator PI = (*BCI)->pred_begin(),
PE = (*BCI)->pred_end();
PI != PE; ++PI) {
if (BlockToChain[*PI] == &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 (MachineFunction::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
FunctionBlockSet.insert(FI);
for (BlockChain::iterator BCI = FunctionChain.begin(),
BCE = FunctionChain.end();
BCI != BCE; ++BCI)
if (!FunctionBlockSet.erase(*BCI)) {
BadFunc = true;
dbgs() << "Function chain contains a block not in the function!\n"
<< " Bad block: " << getBlockName(*BCI) << "\n";
}
if (!FunctionBlockSet.empty()) {
BadFunc = true;
for (FunctionBlockSetType::iterator FBI = FunctionBlockSet.begin(),
FBE = FunctionBlockSet.end();
FBI != FBE; ++FBI)
dbgs() << "Function contains blocks never placed into a chain!\n"
<< " Bad block: " << getBlockName(*FBI) << "\n";
}
assert(!BadFunc && "Detected problems with the block placement.");
});
// Splice the blocks into place.
MachineFunction::iterator InsertPos = F.begin();
for (BlockChain::iterator BI = FunctionChain.begin(),
BE = FunctionChain.end();
BI != BE; ++BI) {
DEBUG(dbgs() << (BI == FunctionChain.begin() ? "Placing chain "
: " ... ")
<< getBlockName(*BI) << "\n");
if (InsertPos != MachineFunction::iterator(*BI))
F.splice(InsertPos, *BI);
else
++InsertPos;
// Update the terminator of the previous block.
if (BI == FunctionChain.begin())
continue;
MachineBasicBlock *PrevBB = std::prev(MachineFunction::iterator(*BI));
// 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 == *BI)) {
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()->getAttributes().
hasAttribute(AttributeSet::FunctionIndex, Attribute::OptimizeForSize))
return;
unsigned Align = TLI->getPrefLoopAlignment();
if (!Align)
return; // Don't care about loop alignment.
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 (BlockChain::iterator BI = std::next(FunctionChain.begin()),
BE = FunctionChain.end();
BI != BE; ++BI) {
// 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(*BI);
if (!L)
continue;
// If the block is cold relative to the function entry don't waste space
// aligning it.
BlockFrequency Freq = MBFI->getBlockFreq(*BI);
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(BI);
// Force alignment if all the predecessors are jumps. We already checked
// that the block isn't cold above.
if (!LayoutPred->isSuccessor(*BI)) {
(*BI)->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, *BI);
BlockFrequency LayoutEdgeFreq = MBFI->getBlockFreq(LayoutPred) * LayoutProb;
if (LayoutEdgeFreq <= (Freq * ColdProb))
(*BI)->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();
assert(BlockToChain.empty());
buildCFGChains(F);
BlockToChain.clear();
ChainAllocator.DestroyAll();
if (AlignAllBlock)
// Align all of the blocks in the function to a specific alignment.
for (MachineFunction::iterator FI = F.begin(), FE = F.end();
FI != FE; ++FI)
FI->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);
}
};
}
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 (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
BlockFrequency BlockFreq = MBFI->getBlockFreq(I);
Statistic &NumBranches = (I->succ_size() > 1) ? NumCondBranches
: NumUncondBranches;
Statistic &BranchTakenFreq = (I->succ_size() > 1) ? CondBranchTakenFreq
: UncondBranchTakenFreq;
for (MachineBasicBlock::succ_iterator SI = I->succ_begin(),
SE = I->succ_end();
SI != SE; ++SI) {
// Skip if this successor is a fallthrough.
if (I->isLayoutSuccessor(*SI))
continue;
BlockFrequency EdgeFreq = BlockFreq * MBPI->getEdgeProbability(I, *SI);
++NumBranches;
BranchTakenFreq += EdgeFreq.getFrequency();
}
}
return false;
}
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