RPCS3/llvm-mirror
1
0
Fork 0
mirror of https://github.com/RPCS3/llvm-mirror.git synced 2024-10-21 12:02:58 +02:00
Code Issues Projects Releases Wiki Activity
71644ee1e5
llvm-mirror/lib/CodeGen/MachineBlockPlacement.cpp
Kyle Butt 71644ee1e5 Codegen: Tail-duplicate during placement. 
The tail duplication pass uses an assumed layout when making duplication
decisions. This is fine, but passes up duplication opportunities that
may arise when blocks are outlined. Because we want the updated CFG to
affect subsequent placement decisions, this change must occur during
placement.

In order to achieve this goal, TailDuplicationPass is split into a
utility class, TailDuplicator, and the pass itself. The pass delegates
nearly everything to the TailDuplicator object, except for looping over
the blocks in a function. This allows the same code to be used for tail
duplication in both places.

This change, in concert with outlining optional branches, allows
triangle shaped code to perform much better, esepecially when the
taken/untaken branches are correlated, as it creates a second spine when
the tests are small enough.

Issue from previous rollback fixed, and a new test was added for that
case as well. Issue was worklist/scheduling/taildup issue in layout.

Issue from 2nd rollback fixed, with 2 additional tests. Issue was
tail merging/loop info/tail-duplication causing issue with loops that share
a header block.

Differential revision: https://reviews.llvm.org/D18226

llvm-svn: 283619
2016-10-07 22:33:20 +00:00

2110 lines
86 KiB
C++
Raw Blame History

//===-- 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/CodeGen/TargetPassConfig.h"
#include "BranchFolding.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/CodeGen/TailDuplicator.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 unconditional 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);
static cl::opt<unsigned> AlignAllNonFallThruBlocks(
"align-all-nofallthru-blocks",
cl::desc("Force the alignment of all "
"blocks that have no fall-through predecessors (i.e. don't add "
"nops that are executed)."),
cl::init(0), cl::Hidden);
// FIXME: Find a good default for this flag and remove the flag.
static cl::opt<unsigned> ExitBlockBias(
"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);
// Definition:
// - Outlining: placement of a basic block outside the chain or hot path.
static cl::opt<bool> OutlineOptionalBranches(
"outline-optional-branches",
cl::desc("Outlining optional branches will place blocks that are optional "
"branches, i.e. branches with a common post dominator, outside "
"the hot path or chain"),
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);
static cl::opt<unsigned> LoopToColdBlockRatio(
"loop-to-cold-block-ratio",
cl::desc("Outline loop blocks from loop chain if (frequency of loop) / "
"(frequency of block) is greater than this ratio"),
cl::init(5), cl::Hidden);
static cl::opt<bool>
PreciseRotationCost("precise-rotation-cost",
cl::desc("Model the cost of loop rotation more "
"precisely by using profile data."),
cl::init(false), cl::Hidden);
static cl::opt<bool>
ForcePreciseRotationCost("force-precise-rotation-cost",
cl::desc("Force the use of precise cost "
"loop rotation strategy."),
cl::init(false), cl::Hidden);
static cl::opt<unsigned> MisfetchCost(
"misfetch-cost",
cl::desc("Cost that models the probabilistic risk of an instruction "
"misfetch due to a jump comparing to falling through, whose cost "
"is zero."),
cl::init(1), cl::Hidden);
static cl::opt<unsigned> JumpInstCost("jump-inst-cost",
cl::desc("Cost of jump instructions."),
cl::init(1), cl::Hidden);
static cl::opt<bool>
TailDupPlacement("tail-dup-placement",
cl::desc("Perform tail duplication during placement. "
"Creates more fallthrough opportunites in "
"outline branches."),
cl::init(true), cl::Hidden);
static cl::opt<bool>
BranchFoldPlacement("branch-fold-placement",
cl::desc("Perform branch folding during placement. "
"Reduces code size."),
cl::init(true), cl::Hidden);
// Heuristic for tail duplication.
static cl::opt<unsigned> TailDuplicatePlacementThreshold(
"tail-dup-placement-threshold",
cl::desc("Instruction cutoff for tail duplication during layout. "
"Tail merging during layout is forced to have a threshold "
"that won't conflict."), cl::init(2),
cl::Hidden);
extern cl::opt<unsigned> StaticLikelyProb;
extern cl::opt<unsigned> ProfileLikelyProb;
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), UnscheduledPredecessors(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(); }
bool remove(MachineBasicBlock* BB) {
for(iterator i = begin(); i != end(); ++i) {
if (*i == BB) {
Blocks.erase(i);
return true;
}
}
return false;
}
/// \brief Merge a block chain into this one.
///
/// This routine merges a block chain into this one. It takes care of forming
/// a contiguous sequence of basic blocks, updating the edge list, and
/// updating the block -> chain mapping. It does not free or tear down the
/// old chain, but the old chain's block list is no longer valid.
void merge(MachineBasicBlock *BB, BlockChain *Chain) {
assert(BB);
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 of any block within the chain which have not
/// yet been scheduled. In general, we will delay scheduling this chain
/// until those predecessors are scheduled (or we find a sufficiently good
/// reason to override this heuristic.) Note that when forming loop chains,
/// blocks outside the loop are ignored and treated as if they were already
/// scheduled.
///
/// Note: This field is reinitialized multiple times - once for each loop,
/// and then once for the function as a whole.
unsigned UnscheduledPredecessors;
};
}
namespace {
class MachineBlockPlacement : public MachineFunctionPass {
/// \brief A typedef for a block filter set.
typedef SmallPtrSet<MachineBasicBlock *, 16> BlockFilterSet;
/// \brief work lists of blocks that are ready to be laid out
SmallVector<MachineBasicBlock *, 16> BlockWorkList;
SmallVector<MachineBasicBlock *, 16> EHPadWorkList;
/// \brief Machine Function
MachineFunction *F;
/// \brief A handle to the branch probability pass.
const MachineBranchProbabilityInfo *MBPI;
/// \brief A handle to the function-wide block frequency pass.
std::unique_ptr<BranchFolder::MBFIWrapper> MBFI;
/// \brief A handle to the loop info.
MachineLoopInfo *MLI;
/// \brief 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 Duplicator used to duplicate tails during placement.
///
/// Placement decisions can open up new tail duplication opportunities, but
/// since tail duplication affects placement decisions of later blocks, it
/// must be done inline.
TailDuplicator TailDup;
/// \brief 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;
/// Decrease the UnscheduledPredecessors count for all blocks in chain, and
/// if the count goes to 0, add them to the appropriate work list.
void markChainSuccessors(BlockChain &Chain, MachineBasicBlock *LoopHeaderBB,
const BlockFilterSet *BlockFilter = nullptr);
/// Decrease the UnscheduledPredecessors count for a single block, and
/// if the count goes to 0, add them to the appropriate work list.
void markBlockSuccessors(
BlockChain &Chain, MachineBasicBlock *BB, MachineBasicBlock *LoopHeaderBB,
const BlockFilterSet *BlockFilter = nullptr);
BranchProbability
collectViableSuccessors(MachineBasicBlock *BB, BlockChain &Chain,
const BlockFilterSet *BlockFilter,
SmallVector<MachineBasicBlock *, 4> &Successors);
bool shouldPredBlockBeOutlined(MachineBasicBlock *BB, MachineBasicBlock *Succ,
BlockChain &Chain,
const BlockFilterSet *BlockFilter,
BranchProbability SuccProb,
BranchProbability HotProb);
bool repeatedlyTailDuplicateBlock(
MachineBasicBlock *BB, MachineBasicBlock *&LPred,
MachineBasicBlock *LoopHeaderBB,
BlockChain &Chain, BlockFilterSet *BlockFilter,
MachineFunction::iterator &PrevUnplacedBlockIt);
bool maybeTailDuplicateBlock(MachineBasicBlock *BB, MachineBasicBlock *LPred,
const BlockChain &Chain,
BlockFilterSet *BlockFilter,
MachineFunction::iterator &PrevUnplacedBlockIt,
bool &DuplicatedToPred);
bool
hasBetterLayoutPredecessor(MachineBasicBlock *BB, MachineBasicBlock *Succ,
BlockChain &SuccChain, BranchProbability SuccProb,
BranchProbability RealSuccProb, BlockChain &Chain,
const BlockFilterSet *BlockFilter);
MachineBasicBlock *selectBestSuccessor(MachineBasicBlock *BB,
BlockChain &Chain,
const BlockFilterSet *BlockFilter);
MachineBasicBlock *
selectBestCandidateBlock(BlockChain &Chain,
SmallVectorImpl<MachineBasicBlock *> &WorkList);
MachineBasicBlock *
getFirstUnplacedBlock(const BlockChain &PlacedChain,
MachineFunction::iterator &PrevUnplacedBlockIt,
const BlockFilterSet *BlockFilter);
/// \brief Add a basic block to the work list if it is appropriate.
///
/// If the optional parameter BlockFilter is provided, only MBB
/// present in the set will be added to the worklist. If nullptr
/// is provided, no filtering occurs.
void fillWorkLists(MachineBasicBlock *MBB,
SmallPtrSetImpl<BlockChain *> &UpdatedPreds,
const BlockFilterSet *BlockFilter);
void buildChain(MachineBasicBlock *BB, BlockChain &Chain,
BlockFilterSet *BlockFilter = nullptr);
MachineBasicBlock *findBestLoopTop(MachineLoop &L,
const BlockFilterSet &LoopBlockSet);
MachineBasicBlock *findBestLoopExit(MachineLoop &L,
const BlockFilterSet &LoopBlockSet);
BlockFilterSet collectLoopBlockSet(MachineLoop &L);
void buildLoopChains(MachineLoop &L);
void rotateLoop(BlockChain &LoopChain, MachineBasicBlock *ExitingBB,
const BlockFilterSet &LoopBlockSet);
void rotateLoopWithProfile(BlockChain &LoopChain, MachineLoop &L,
const BlockFilterSet &LoopBlockSet);
void collectMustExecuteBBs();
void buildCFGChains();
void optimizeBranches();
void alignBlocks();
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>();
AU.addRequired<TargetPassConfig>();
MachineFunctionPass::getAnalysisUsage(AU);
}
};
}
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 << " ('" << BB->getName() << "')";
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 appropriate worklist.
void MachineBlockPlacement::markChainSuccessors(
BlockChain &Chain, MachineBasicBlock *LoopHeaderBB,
const BlockFilterSet *BlockFilter) {
// Walk all the blocks in this chain, marking their successors as having
// a predecessor placed.
for (MachineBasicBlock *MBB : Chain) {
markBlockSuccessors(Chain, MBB, LoopHeaderBB, BlockFilter);
}
}
/// \brief Mark a single block's successors as having one fewer preds.
///
/// Under normal circumstances, this is only called by markChainSuccessors,
/// but if a block that was to be placed is completely tail-duplicated away,
/// and was duplicated into the chain end, we need to redo markBlockSuccessors
/// for just that block.
void MachineBlockPlacement::markBlockSuccessors(
BlockChain &Chain, MachineBasicBlock *MBB, MachineBasicBlock *LoopHeaderBB,
const BlockFilterSet *BlockFilter) {
// Add any successors for which this is the only un-placed in-loop
// predecessor to the worklist as a viable candidate for CFG-neutral
// placement. No subsequent placement of this block will violate the CFG
// shape, so we get to use heuristics to choose a favorable placement.
for (MachineBasicBlock *Succ : MBB->successors()) {
if (BlockFilter && !BlockFilter->count(Succ))
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.UnscheduledPredecessors == 0 ||
--SuccChain.UnscheduledPredecessors > 0)
continue;
auto *NewBB = *SuccChain.begin();
if (NewBB->isEHPad())
EHPadWorkList.push_back(NewBB);
else
BlockWorkList.push_back(NewBB);
}
}
/// This helper function collects the set of successors of block
/// \p BB that are allowed to be its layout successors, and return
/// the total branch probability of edges from \p BB to those
/// blocks.
BranchProbability MachineBlockPlacement::collectViableSuccessors(
MachineBasicBlock *BB, BlockChain &Chain, const BlockFilterSet *BlockFilter,
SmallVector<MachineBasicBlock *, 4> &Successors) {
// Adjust edge probabilities by excluding edges pointing to blocks that is
// either not in BlockFilter or is already in the current chain. Consider the
// following CFG:
//
// --->A
// | / \
// | B C
// | \ / \
// ----D E
//
// Assume A->C is very hot (>90%), and C->D has a 50% probability, then after
// A->C is chosen as a fall-through, D won't be selected as a successor of C
// due to CFG constraint (the probability of C->D is not greater than
// HotProb to break top-order). If we exclude E that is not in BlockFilter
// when calculating the probability of C->D, D will be selected and we
// will get A C D B as the layout of this loop.
auto AdjustedSumProb = BranchProbability::getOne();
for (MachineBasicBlock *Succ : BB->successors()) {
bool SkipSucc = false;
if (Succ->isEHPad() || (BlockFilter && !BlockFilter->count(Succ))) {
SkipSucc = true;
} else {
BlockChain *SuccChain = BlockToChain[Succ];
if (SuccChain == &Chain) {
SkipSucc = true;
} else if (Succ != *SuccChain->begin()) {
DEBUG(dbgs() << " " << getBlockName(Succ) << " -> Mid chain!\n");
continue;
}
}
if (SkipSucc)
AdjustedSumProb -= MBPI->getEdgeProbability(BB, Succ);
else
Successors.push_back(Succ);
}
return AdjustedSumProb;
}
/// The helper function returns the branch probability that is adjusted
/// or normalized over the new total \p AdjustedSumProb.
static BranchProbability
getAdjustedProbability(BranchProbability OrigProb,
BranchProbability AdjustedSumProb) {
BranchProbability SuccProb;
uint32_t SuccProbN = OrigProb.getNumerator();
uint32_t SuccProbD = AdjustedSumProb.getNumerator();
if (SuccProbN >= SuccProbD)
SuccProb = BranchProbability::getOne();
else
SuccProb = BranchProbability(SuccProbN, SuccProbD);
return SuccProb;
}
/// When the option OutlineOptionalBranches is on, this method
/// checks if the fallthrough candidate block \p Succ (of block
/// \p BB) also has other unscheduled predecessor blocks which
/// are also successors of \p BB (forming triangular shape CFG).
/// If none of such predecessors are small, it returns true.
/// The caller can choose to select \p Succ as the layout successors
/// so that \p Succ's predecessors (optional branches) can be
/// outlined.
/// FIXME: fold this with more general layout cost analysis.
bool MachineBlockPlacement::shouldPredBlockBeOutlined(
MachineBasicBlock *BB, MachineBasicBlock *Succ, BlockChain &Chain,
const BlockFilterSet *BlockFilter, BranchProbability SuccProb,
BranchProbability HotProb) {
if (!OutlineOptionalBranches)
return false;
// 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 (SuccProb > HotProb.getCompl() && UnavoidableBlocks.count(Succ) > 0) {
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 false;
}
return true;
} else
return false;
}
// When profile is not present, return the StaticLikelyProb.
// When profile is available, we need to handle the triangle-shape CFG.
static BranchProbability getLayoutSuccessorProbThreshold(
MachineBasicBlock *BB) {
if (!BB->getParent()->getFunction()->getEntryCount())
return BranchProbability(StaticLikelyProb, 100);
if (BB->succ_size() == 2) {
const MachineBasicBlock *Succ1 = *BB->succ_begin();
const MachineBasicBlock *Succ2 = *(BB->succ_begin() + 1);
if (Succ1->isSuccessor(Succ2) || Succ2->isSuccessor(Succ1)) {
/* See case 1 below for the cost analysis. For BB->Succ to
* be taken with smaller cost, the following needs to hold:
* Prob(BB->Succ) > 2* Prob(BB->Pred)
* So the threshold T
* T = 2 * (1-Prob(BB->Pred). Since T + Prob(BB->Pred) == 1,
* We have T + T/2 = 1, i.e. T = 2/3. Also adding user specified
* branch bias, we have
* T = (2/3)*(ProfileLikelyProb/50)
* = (2*ProfileLikelyProb)/150)
*/
return BranchProbability(2 * ProfileLikelyProb, 150);
}
}
return BranchProbability(ProfileLikelyProb, 100);
}
/// Checks to see if the layout candidate block \p Succ has a better layout
/// predecessor than \c BB. If yes, returns true.
bool MachineBlockPlacement::hasBetterLayoutPredecessor(
MachineBasicBlock *BB, MachineBasicBlock *Succ, BlockChain &SuccChain,
BranchProbability SuccProb, BranchProbability RealSuccProb,
BlockChain &Chain, const BlockFilterSet *BlockFilter) {
// There isn't a better layout when there are no unscheduled predecessors.
if (SuccChain.UnscheduledPredecessors == 0)
return false;
// There are two basic scenarios here:
// -------------------------------------
// Case 1: triangular shape CFG (if-then):
// BB
// | \
// | \
// | Pred
// | /
// Succ
// In this case, we are evaluating whether to select edge -> Succ, e.g.
// set Succ as the layout successor of BB. Picking Succ as BB's
// successor breaks the CFG constraints (FIXME: define these constraints).
// With this layout, Pred BB
// is forced to be outlined, so the overall cost will be cost of the
// branch taken from BB to Pred, plus the cost of back taken branch
// from Pred to Succ, as well as the additional cost associated
// with the needed unconditional jump instruction from Pred To Succ.
// The cost of the topological order layout is the taken branch cost
// from BB to Succ, so to make BB->Succ a viable candidate, the following
// must hold:
// 2 * freq(BB->Pred) * taken_branch_cost + unconditional_jump_cost
// < freq(BB->Succ) * taken_branch_cost.
// Ignoring unconditional jump cost, we get
// freq(BB->Succ) > 2 * freq(BB->Pred), i.e.,
// prob(BB->Succ) > 2 * prob(BB->Pred)
//
// When real profile data is available, we can precisely compute the
// probability threshold that is needed for edge BB->Succ to be considered.
// Without profile data, the heuristic requires the branch bias to be
// a lot larger to make sure the signal is very strong (e.g. 80% default).
// -----------------------------------------------------------------
// Case 2: diamond like CFG (if-then-else):
// S
// / \
// | \
// BB Pred
// \ /
// Succ
// ..
//
// The current block is BB and edge BB->Succ is now being evaluated.
// Note that edge S->BB was previously already selected because
// prob(S->BB) > prob(S->Pred).
// At this point, 2 blocks can be placed after BB: Pred or Succ. If we
// choose Pred, we will have a topological ordering as shown on the left
// in the picture below. If we choose Succ, we have the solution as shown
// on the right:
//
// topo-order:
//
// S----- ---S
// | | | |
// ---BB | | BB
// | | | |
// | pred-- | Succ--
// | | | |
// ---succ ---pred--
//
// cost = freq(S->Pred) + freq(BB->Succ) cost = 2 * freq (S->Pred)
// = freq(S->Pred) + freq(S->BB)
//
// If we have profile data (i.e, branch probabilities can be trusted), the
// cost (number of taken branches) with layout S->BB->Succ->Pred is 2 *
// freq(S->Pred) while the cost of topo order is freq(S->Pred) + freq(S->BB).
// We know Prob(S->BB) > Prob(S->Pred), so freq(S->BB) > freq(S->Pred), which
// means the cost of topological order is greater.
// When profile data is not available, however, we need to be more
// conservative. If the branch prediction is wrong, breaking the topo-order
// will actually yield a layout with large cost. For this reason, we need
// strong biased branch at block S with Prob(S->BB) in order to select
// BB->Succ. This is equivalent to looking the CFG backward with backward
// edge: Prob(Succ->BB) needs to >= HotProb in order to be selected (without
// profile data).
// --------------------------------------------------------------------------
// Case 3: forked diamond
// S
// / \
// / \
// BB Pred
// | \ / |
// | \ / |
// | X |
// | / \ |
// | / \ |
// S1 S2
//
// The current block is BB and edge BB->S1 is now being evaluated.
// As above S->BB was already selected because
// prob(S->BB) > prob(S->Pred). Assume that prob(BB->S1) >= prob(BB->S2).
//
// topo-order:
//
// S-------| ---S
// | | | |
// ---BB | | BB
// | | | |
// | Pred----| | S1----
// | | | |
// --(S1 or S2) ---Pred--
//
// topo-cost = freq(S->Pred) + freq(BB->S1) + freq(BB->S2)
// + min(freq(Pred->S1), freq(Pred->S2))
// Non-topo-order cost:
// In the worst case, S2 will not get laid out after Pred.
// non-topo-cost = 2 * freq(S->Pred) + freq(BB->S2).
// To be conservative, we can assume that min(freq(Pred->S1), freq(Pred->S2))
// is 0. Then the non topo layout is better when
// freq(S->Pred) < freq(BB->S1).
// This is exactly what is checked below.
// Note there are other shapes that apply (Pred may not be a single block,
// but they all fit this general pattern.)
BranchProbability HotProb = getLayoutSuccessorProbThreshold(BB);
// Make sure that a hot successor doesn't have a globally more
// important predecessor.
BlockFrequency CandidateEdgeFreq = MBFI->getBlockFreq(BB) * RealSuccProb;
bool BadCFGConflict = false;
for (MachineBasicBlock *Pred : Succ->predecessors()) {
if (Pred == Succ || BlockToChain[Pred] == &SuccChain ||
(BlockFilter && !BlockFilter->count(Pred)) ||
BlockToChain[Pred] == &Chain)
continue;
// Do backward checking.
// For all cases above, we need a backward checking to filter out edges that
// are not 'strongly' biased. With profile data available, the check is
// mostly redundant for case 2 (when threshold prob is set at 50%) unless S
// has more than two successors.
// BB Pred
// \ /
// Succ
// We select edge BB->Succ if
// freq(BB->Succ) > freq(Succ) * HotProb
// i.e. freq(BB->Succ) > freq(BB->Succ) * HotProb + freq(Pred->Succ) *
// HotProb
// i.e. freq((BB->Succ) * (1 - HotProb) > freq(Pred->Succ) * HotProb
// Case 1 is covered too, because the first equation reduces to:
// prob(BB->Succ) > HotProb. (freq(Succ) = freq(BB) for a triangle)
BlockFrequency PredEdgeFreq =
MBFI->getBlockFreq(Pred) * MBPI->getEdgeProbability(Pred, Succ);
if (PredEdgeFreq * HotProb >= CandidateEdgeFreq * HotProb.getCompl()) {
BadCFGConflict = true;
break;
}
}
if (BadCFGConflict) {
DEBUG(dbgs() << " Not a candidate: " << getBlockName(Succ) << " -> " << SuccProb
<< " (prob) (non-cold CFG conflict)\n");
return true;
}
return false;
}
/// \brief Select the best successor for a block.
///
/// This looks across all successors of a particular block and attempts to
/// select the "best" one to be the layout successor. It only considers direct
/// successors which also pass the block filter. It will attempt to avoid
/// breaking CFG structure, but cave and break such structures in the case of
/// very hot successor edges.
///
/// \returns The best successor block found, or null if none are viable.
MachineBasicBlock *
MachineBlockPlacement::selectBestSuccessor(MachineBasicBlock *BB,
BlockChain &Chain,
const BlockFilterSet *BlockFilter) {
const BranchProbability HotProb(StaticLikelyProb, 100);
MachineBasicBlock *BestSucc = nullptr;
auto BestProb = BranchProbability::getZero();
SmallVector<MachineBasicBlock *, 4> Successors;
auto AdjustedSumProb =
collectViableSuccessors(BB, Chain, BlockFilter, Successors);
DEBUG(dbgs() << "Selecting best successor for: " << getBlockName(BB) << "\n");
for (MachineBasicBlock *Succ : Successors) {
auto RealSuccProb = MBPI->getEdgeProbability(BB, Succ);
BranchProbability SuccProb =
getAdjustedProbability(RealSuccProb, AdjustedSumProb);
// This heuristic is off by default.
if (shouldPredBlockBeOutlined(BB, Succ, Chain, BlockFilter, SuccProb,
HotProb))
return Succ;
BlockChain &SuccChain = *BlockToChain[Succ];
// Skip the edge \c BB->Succ if block \c Succ has a better layout
// predecessor that yields lower global cost.
if (hasBetterLayoutPredecessor(BB, Succ, SuccChain, SuccProb, RealSuccProb,
Chain, BlockFilter))
continue;
DEBUG(
dbgs() << " Candidate: " << getBlockName(Succ) << ", probability: "
<< SuccProb
<< (SuccChain.UnscheduledPredecessors != 0 ? " (CFG break)" : "")
<< "\n");
if (BestSucc && BestProb >= SuccProb) {
DEBUG(dbgs() << " Not the best candidate, continuing\n");
continue;
}
DEBUG(dbgs() << " Setting it as best candidate\n");
BestSucc = Succ;
BestProb = SuccProb;
}
if (BestSucc)
DEBUG(dbgs() << " Selected: " << getBlockName(BestSucc) << "\n");
return BestSucc;
}
/// \brief Select the best block from a worklist.
///
/// This looks through the provided worklist as a list of candidate basic
/// blocks and select the most profitable one to place. The definition of
/// profitable only really makes sense in the context of a loop. This returns
/// the most frequently visited block in the worklist, which in the case of
/// a loop, is the one most desirable to be physically close to the rest of the
/// loop body in order to improve i-cache behavior.
///
/// \returns The best block found, or null if none are viable.
MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock(
BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList) {
// Once we need to walk the worklist looking for a candidate, cleanup the
// worklist of already placed entries.
// FIXME: If this shows up on profiles, it could be folded (at the cost of
// some code complexity) into the loop below.
WorkList.erase(remove_if(WorkList,
[&](MachineBasicBlock *BB) {
return BlockToChain.lookup(BB) == &Chain;
}),
WorkList.end());
if (WorkList.empty())
return nullptr;
bool IsEHPad = WorkList[0]->isEHPad();
MachineBasicBlock *BestBlock = nullptr;
BlockFrequency BestFreq;
for (MachineBasicBlock *MBB : WorkList) {
assert(MBB->isEHPad() == IsEHPad);
BlockChain &SuccChain = *BlockToChain[MBB];
if (&SuccChain == &Chain)
continue;
assert(SuccChain.UnscheduledPredecessors == 0 && "Found CFG-violating block");
BlockFrequency CandidateFreq = MBFI->getBlockFreq(MBB);
DEBUG(dbgs() << " " << getBlockName(MBB) << " -> ";
MBFI->printBlockFreq(dbgs(), CandidateFreq) << " (freq)\n");
// For ehpad, we layout the least probable first as to avoid jumping back
// from least probable landingpads to more probable ones.
//
// FIXME: Using probability is probably (!) not the best way to achieve
// this. We should probably have a more principled approach to layout
// cleanup code.
//
// The goal is to get:
//
// +--------------------------+
// | V
// InnerLp -> InnerCleanup OuterLp -> OuterCleanup -> Resume
//
// Rather than:
//
// +-------------------------------------+
// V |
// OuterLp -> OuterCleanup -> Resume InnerLp -> InnerCleanup
if (BestBlock && (IsEHPad ^ (BestFreq >= CandidateFreq)))
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(
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::fillWorkLists(
MachineBasicBlock *MBB,
SmallPtrSetImpl<BlockChain *> &UpdatedPreds,
const BlockFilterSet *BlockFilter = nullptr) {
BlockChain &Chain = *BlockToChain[MBB];
if (!UpdatedPreds.insert(&Chain).second)
return;
assert(Chain.UnscheduledPredecessors == 0);
for (MachineBasicBlock *ChainBB : Chain) {
assert(BlockToChain[ChainBB] == &Chain);
for (MachineBasicBlock *Pred : ChainBB->predecessors()) {
if (BlockFilter && !BlockFilter->count(Pred))
continue;
if (BlockToChain[Pred] == &Chain)
continue;
++Chain.UnscheduledPredecessors;
}
}
if (Chain.UnscheduledPredecessors != 0)
return;
MBB = *Chain.begin();
if (MBB->isEHPad())
EHPadWorkList.push_back(MBB);
else
BlockWorkList.push_back(MBB);
}
void MachineBlockPlacement::buildChain(
MachineBasicBlock *BB, BlockChain &Chain,
BlockFilterSet *BlockFilter) {
assert(BB && "BB must not be null.\n");
assert(BlockToChain[BB] == &Chain && "BlockToChainMap mis-match.\n");
MachineFunction::iterator PrevUnplacedBlockIt = F->begin();
MachineBasicBlock *LoopHeaderBB = BB;
markChainSuccessors(Chain, LoopHeaderBB, BlockFilter);
BB = *std::prev(Chain.end());
for (;;) {
assert(BB && "null block found at end of chain in loop.");
assert(BlockToChain[BB] == &Chain && "BlockToChainMap mis-match in loop.");
assert(*std::prev(Chain.end()) == BB && "BB Not found at end of chain.");
// Look for the best viable successor if there is one to place immediately
// after this block.
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);
if (!BestSucc)
BestSucc = selectBestCandidateBlock(Chain, EHPadWorkList);
if (!BestSucc) {
BestSucc = getFirstUnplacedBlock(Chain, PrevUnplacedBlockIt, BlockFilter);
if (!BestSucc)
break;
DEBUG(dbgs() << "Unnatural loop CFG detected, forcibly merging the "
"layout successor until the CFG reduces\n");
}
// Placement may have changed tail duplication opportunities.
// Check for that now.
if (TailDupPlacement && BestSucc) {
// If the chosen successor was duplicated into all its predecessors,
// don't bother laying it out, just go round the loop again with BB as
// the chain end.
if (repeatedlyTailDuplicateBlock(BestSucc, BB, LoopHeaderBB, Chain,
BlockFilter, PrevUnplacedBlockIt))
continue;
}
// Place this block, updating the datastructures to reflect its placement.
BlockChain &SuccChain = *BlockToChain[BestSucc];
// Zero out UnscheduledPredecessors for the successor we're about to merge in case
// we selected a successor that didn't fit naturally into the CFG.
SuccChain.UnscheduledPredecessors = 0;
DEBUG(dbgs() << "Merging from " << getBlockName(BB) << " to "
<< getBlockName(BestSucc) << "\n");
markChainSuccessors(SuccChain, LoopHeaderBB, BlockFilter);
Chain.merge(BestSucc, &SuccChain);
BB = *std::prev(Chain.end());
}
DEBUG(dbgs() << "Finished forming chain for header block "
<< getBlockName(*Chain.begin()) << "\n");
}
/// \brief Find the best loop top block for layout.
///
/// Look for a block which is strictly better than the loop header for laying
/// out at the top of the loop. This looks for one and only one pattern:
/// a latch block with no conditional exit. This block will cause a conditional
/// jump around it or will be the bottom of the loop if we lay it out in place,
/// but if it it doesn't end up at the bottom of the loop for any reason,
/// rotation alone won't fix it. Because such a block will always result in an
/// unconditional jump (for the backedge) rotating it in front of the loop
/// header is always profitable.
MachineBasicBlock *
MachineBlockPlacement::findBestLoopTop(MachineLoop &L,
const BlockFilterSet &LoopBlockSet) {
// Placing the latch block before the header may introduce an extra branch
// that skips this block the first time the loop is executed, which we want
// to avoid when optimising for size.
// FIXME: in theory there is a case that does not introduce a new branch,
// i.e. when the layout predecessor does not fallthrough to the loop header.
// In practice this never happens though: there always seems to be a preheader
// that can fallthrough and that is also placed before the header.
if (F->getFunction()->optForSize())
return L.getHeader();
// Check that the header hasn't been fused with a preheader block due to
// crazy branches. If it has, we need to start with the header at the top to
// prevent pulling the preheader into the loop body.
BlockChain &HeaderChain = *BlockToChain[L.getHeader()];
if (!LoopBlockSet.count(*HeaderChain.begin()))
return L.getHeader();
DEBUG(dbgs() << "Finding best loop top for: " << getBlockName(L.getHeader())
<< "\n");
BlockFrequency BestPredFreq;
MachineBasicBlock *BestPred = nullptr;
for (MachineBasicBlock *Pred : L.getHeader()->predecessors()) {
if (!LoopBlockSet.count(Pred))
continue;
DEBUG(dbgs() << " header pred: " << getBlockName(Pred) << ", has "
<< 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) {
DEBUG(dbgs() << " final top unchanged\n");
return L.getHeader();
}
// Walk backwards through any straight line of predecessors.
while (BestPred->pred_size() == 1 &&
(*BestPred->pred_begin())->succ_size() == 1 &&
*BestPred->pred_begin() != L.getHeader())
BestPred = *BestPred->pred_begin();
DEBUG(dbgs() << " final top: " << getBlockName(BestPred) << "\n");
return BestPred;
}
/// \brief Find the best loop exiting block for layout.
///
/// This routine implements the logic to analyze the loop looking for the best
/// block to layout at the top of the loop. Typically this is done to maximize
/// fallthrough opportunities.
MachineBasicBlock *
MachineBlockPlacement::findBestLoopExit(MachineLoop &L,
const BlockFilterSet &LoopBlockSet) {
// We don't want to layout the loop linearly in all cases. If the loop header
// is just a normal basic block in the loop, we want to look for what block
// within the loop is the best one to layout at the top. However, if the loop
// header has be pre-merged into a chain due to predecessors not having
// analyzable branches, *and* the predecessor it is merged with is *not* part
// of the loop, rotating the header into the middle of the loop will create
// a non-contiguous range of blocks which is Very Bad. So start with the
// header and only rotate if safe.
BlockChain &HeaderChain = *BlockToChain[L.getHeader()];
if (!LoopBlockSet.count(*HeaderChain.begin()))
return nullptr;
BlockFrequency BestExitEdgeFreq;
unsigned BestExitLoopDepth = 0;
MachineBasicBlock *ExitingBB = nullptr;
// If there are exits to outer loops, loop rotation can severely limit
// fallthrough opportunities unless it selects such an exit. Keep a set of
// blocks where rotating to exit with that block will reach an outer loop.
SmallPtrSet<MachineBasicBlock *, 4> BlocksExitingToOuterLoop;
DEBUG(dbgs() << "Finding best loop exit for: " << getBlockName(L.getHeader())
<< "\n");
for (MachineBasicBlock *MBB : L.getBlocks()) {
BlockChain &Chain = *BlockToChain[MBB];
// Ensure that this block is at the end of a chain; otherwise it could be
// mid-way through an inner loop or a successor of an unanalyzable branch.
if (MBB != *std::prev(Chain.end()))
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;
for (MachineBasicBlock *Succ : MBB->successors()) {
if (Succ->isEHPad())
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;
}
auto SuccProb = MBPI->getEdgeProbability(MBB, Succ);
if (LoopBlockSet.count(Succ)) {
DEBUG(dbgs() << " looping: " << getBlockName(MBB) << " -> "
<< getBlockName(Succ) << " (" << SuccProb << ")\n");
HasLoopingSucc = true;
continue;
}
unsigned SuccLoopDepth = 0;
if (MachineLoop *ExitLoop = MLI->getLoopFor(Succ)) {
SuccLoopDepth = ExitLoop->getLoopDepth();
if (ExitLoop->contains(&L))
BlocksExitingToOuterLoop.insert(MBB);
}
BlockFrequency ExitEdgeFreq = MBFI->getBlockFreq(MBB) * SuccProb;
DEBUG(dbgs() << " exiting: " << getBlockName(MBB) << " -> "
<< getBlockName(Succ) << " [L:" << SuccLoopDepth << "] (";
MBFI->printBlockFreq(dbgs(), ExitEdgeFreq) << ")\n");
// Note that we bias this toward an existing layout successor to retain
// incoming order in the absence of better information. The exit must have
// a frequency higher than the current exit before we consider breaking
// the layout.
BranchProbability Bias(100 - ExitBlockBias, 100);
if (!ExitingBB || SuccLoopDepth > BestExitLoopDepth ||
ExitEdgeFreq > BestExitEdgeFreq ||
(MBB->isLayoutSuccessor(Succ) &&
!(ExitEdgeFreq < BestExitEdgeFreq * Bias))) {
BestExitEdgeFreq = ExitEdgeFreq;
ExitingBB = MBB;
}
}
if (!HasLoopingSucc) {
// Restore the old exiting state, no viable looping successor was found.
ExitingBB = OldExitingBB;
BestExitEdgeFreq = OldBestExitEdgeFreq;
}
}
// Without a candidate exiting block or with only a single block in the
// loop, just use the loop header to layout the loop.
if (!ExitingBB) {
DEBUG(dbgs() << " No other candidate exit blocks, using loop header\n");
return nullptr;
}
if (L.getNumBlocks() == 1) {
DEBUG(dbgs() << " Loop has 1 block, using loop header as exit\n");
return nullptr;
}
// Also, if we have exit blocks which lead to outer loops but didn't select
// one of them as the exiting block we are rotating toward, disable loop
// rotation altogether.
if (!BlocksExitingToOuterLoop.empty() &&
!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 = find(LoopChain, ExitingBB);
if (ExitIt == LoopChain.end())
return;
std::rotate(LoopChain.begin(), std::next(ExitIt), LoopChain.end());
}
/// \brief Attempt to rotate a loop based on profile data to reduce branch cost.
///
/// With profile data, we can determine the cost in terms of missed fall through
/// opportunities when rotating a loop chain and select the best rotation.
/// Basically, there are three kinds of cost to consider for each rotation:
/// 1. The possibly missed fall through edge (if it exists) from BB out of
/// the loop to the loop header.
/// 2. The possibly missed fall through edges (if they exist) from the loop
/// exits to BB out of the loop.
/// 3. The missed fall through edge (if it exists) from the last BB to the
/// first BB in the loop chain.
/// Therefore, the cost for a given rotation is the sum of costs listed above.
/// We select the best rotation with the smallest cost.
void MachineBlockPlacement::rotateLoopWithProfile(
BlockChain &LoopChain, MachineLoop &L, const BlockFilterSet &LoopBlockSet) {
auto HeaderBB = L.getHeader();
auto HeaderIter = find(LoopChain, HeaderBB);
auto RotationPos = LoopChain.end();
BlockFrequency SmallestRotationCost = BlockFrequency::getMaxFrequency();
// A utility lambda that scales up a block frequency by dividing it by a
// branch probability which is the reciprocal of the scale.
auto ScaleBlockFrequency = [](BlockFrequency Freq,
unsigned Scale) -> BlockFrequency {
if (Scale == 0)
return 0;
// Use operator / between BlockFrequency and BranchProbability to implement
// saturating multiplication.
return Freq / BranchProbability(1, Scale);
};
// Compute the cost of the missed fall-through edge to the loop header if the
// chain head is not the loop header. As we only consider natural loops with
// single header, this computation can be done only once.
BlockFrequency HeaderFallThroughCost(0);
for (auto *Pred : HeaderBB->predecessors()) {
BlockChain *PredChain = BlockToChain[Pred];
if (!LoopBlockSet.count(Pred) &&
(!PredChain || Pred == *std::prev(PredChain->end()))) {
auto EdgeFreq =
MBFI->getBlockFreq(Pred) * MBPI->getEdgeProbability(Pred, HeaderBB);
auto FallThruCost = ScaleBlockFrequency(EdgeFreq, MisfetchCost);
// If the predecessor has only an unconditional jump to the header, we
// need to consider the cost of this jump.
if (Pred->succ_size() == 1)
FallThruCost += ScaleBlockFrequency(EdgeFreq, JumpInstCost);
HeaderFallThroughCost = std::max(HeaderFallThroughCost, FallThruCost);
}
}
// Here we collect all exit blocks in the loop, and for each exit we find out
// its hottest exit edge. For each loop rotation, we define the loop exit cost
// as the sum of frequencies of exit edges we collect here, excluding the exit
// edge from the tail of the loop chain.
SmallVector<std::pair<MachineBasicBlock *, BlockFrequency>, 4> ExitsWithFreq;
for (auto BB : LoopChain) {
auto LargestExitEdgeProb = BranchProbability::getZero();
for (auto *Succ : BB->successors()) {
BlockChain *SuccChain = BlockToChain[Succ];
if (!LoopBlockSet.count(Succ) &&
(!SuccChain || Succ == *SuccChain->begin())) {
auto SuccProb = MBPI->getEdgeProbability(BB, Succ);
LargestExitEdgeProb = std::max(LargestExitEdgeProb, SuccProb);
}
}
if (LargestExitEdgeProb > BranchProbability::getZero()) {
auto ExitFreq = MBFI->getBlockFreq(BB) * LargestExitEdgeProb;
ExitsWithFreq.emplace_back(BB, ExitFreq);
}
}
// In this loop we iterate every block in the loop chain and calculate the
// cost assuming the block is the head of the loop chain. When the loop ends,
// we should have found the best candidate as the loop chain's head.
for (auto Iter = LoopChain.begin(), TailIter = std::prev(LoopChain.end()),
EndIter = LoopChain.end();
Iter != EndIter; Iter++, TailIter++) {
// TailIter is used to track the tail of the loop chain if the block we are
// checking (pointed by Iter) is the head of the chain.
if (TailIter == LoopChain.end())
TailIter = LoopChain.begin();
auto TailBB = *TailIter;
// Calculate the cost by putting this BB to the top.
BlockFrequency Cost = 0;
// If the current BB is the loop header, we need to take into account the
// cost of the missed fall through edge from outside of the loop to the
// header.
if (Iter != HeaderIter)
Cost += HeaderFallThroughCost;
// Collect the loop exit cost by summing up frequencies of all exit edges
// except the one from the chain tail.
for (auto &ExitWithFreq : ExitsWithFreq)
if (TailBB != ExitWithFreq.first)
Cost += ExitWithFreq.second;
// The cost of breaking the once fall-through edge from the tail to the top
// of the loop chain. Here we need to consider three cases:
// 1. If the tail node has only one successor, then we will get an
// additional jmp instruction. So the cost here is (MisfetchCost +
// JumpInstCost) * tail node frequency.
// 2. If the tail node has two successors, then we may still get an
// additional jmp instruction if the layout successor after the loop
// chain is not its CFG successor. Note that the more frequently executed
// jmp instruction will be put ahead of the other one. Assume the
// frequency of those two branches are x and y, where x is the frequency
// of the edge to the chain head, then the cost will be
// (x * MisfetechCost + min(x, y) * JumpInstCost) * tail node frequency.
// 3. If the tail node has more than two successors (this rarely happens),
// we won't consider any additional cost.
if (TailBB->isSuccessor(*Iter)) {
auto TailBBFreq = MBFI->getBlockFreq(TailBB);
if (TailBB->succ_size() == 1)
Cost += ScaleBlockFrequency(TailBBFreq.getFrequency(),
MisfetchCost + JumpInstCost);
else if (TailBB->succ_size() == 2) {
auto TailToHeadProb = MBPI->getEdgeProbability(TailBB, *Iter);
auto TailToHeadFreq = TailBBFreq * TailToHeadProb;
auto ColderEdgeFreq = TailToHeadProb > BranchProbability(1, 2)
? TailBBFreq * TailToHeadProb.getCompl()
: TailToHeadFreq;
Cost += ScaleBlockFrequency(TailToHeadFreq, MisfetchCost) +
ScaleBlockFrequency(ColderEdgeFreq, JumpInstCost);
}
}
DEBUG(dbgs() << "The cost of loop rotation by making " << getBlockName(*Iter)
<< " to the top: " << Cost.getFrequency() << "\n");
if (Cost < SmallestRotationCost) {
SmallestRotationCost = Cost;
RotationPos = Iter;
}
}
if (RotationPos != LoopChain.end()) {
DEBUG(dbgs() << "Rotate loop by making " << getBlockName(*RotationPos)
<< " to the top\n");
std::rotate(LoopChain.begin(), RotationPos, LoopChain.end());
}
}
/// \brief Collect blocks in the given loop that are to be placed.
///
/// When profile data is available, exclude cold blocks from the returned set;
/// otherwise, collect all blocks in the loop.
MachineBlockPlacement::BlockFilterSet
MachineBlockPlacement::collectLoopBlockSet(MachineLoop &L) {
BlockFilterSet LoopBlockSet;
// Filter cold blocks off from LoopBlockSet when profile data is available.
// Collect the sum of frequencies of incoming edges to the loop header from
// outside. If we treat the loop as a super block, this is the frequency of
// the loop. Then for each block in the loop, we calculate the ratio between
// its frequency and the frequency of the loop block. When it is too small,
// don't add it to the loop chain. If there are outer loops, then this block
// will be merged into the first outer loop chain for which this block is not
// cold anymore. This needs precise profile data and we only do this when
// profile data is available.
if (F->getFunction()->getEntryCount()) {
BlockFrequency LoopFreq(0);
for (auto LoopPred : L.getHeader()->predecessors())
if (!L.contains(LoopPred))
LoopFreq += MBFI->getBlockFreq(LoopPred) *
MBPI->getEdgeProbability(LoopPred, L.getHeader());
for (MachineBasicBlock *LoopBB : L.getBlocks()) {
auto Freq = MBFI->getBlockFreq(LoopBB).getFrequency();
if (Freq == 0 || LoopFreq.getFrequency() / Freq > LoopToColdBlockRatio)
continue;
LoopBlockSet.insert(LoopBB);
}
} else
LoopBlockSet.insert(L.block_begin(), L.block_end());
return LoopBlockSet;
}
/// \brief Forms basic block chains from the natural loop structures.
///
/// These chains are designed to preserve the existing *structure* of the code
/// as much as possible. We can then stitch the chains together in a way which
/// both preserves the topological structure and minimizes taken conditional
/// branches.
void MachineBlockPlacement::buildLoopChains(MachineLoop &L) {
// First recurse through any nested loops, building chains for those inner
// loops.
for (MachineLoop *InnerLoop : L)
buildLoopChains(*InnerLoop);
assert(BlockWorkList.empty());
assert(EHPadWorkList.empty());
BlockFilterSet LoopBlockSet = collectLoopBlockSet(L);
// Check if we have profile data for this function. If yes, we will rotate
// this loop by modeling costs more precisely which requires the profile data
// for better layout.
bool RotateLoopWithProfile =
ForcePreciseRotationCost ||
(PreciseRotationCost && F->getFunction()->getEntryCount());
// First check to see if there is an obviously preferable top block for the
// loop. This will default to the header, but may end up as one of the
// predecessors to the header if there is one which will result in strictly
// fewer branches in the loop body.
// When we use profile data to rotate the loop, this is unnecessary.
MachineBasicBlock *LoopTop =
RotateLoopWithProfile ? L.getHeader() : findBestLoopTop(L, LoopBlockSet);
// If we selected just the header for the loop top, look for a potentially
// profitable exit block in the event that rotating the loop can eliminate
// branches by placing an exit edge at the bottom.
MachineBasicBlock *ExitingBB = nullptr;
if (!RotateLoopWithProfile && LoopTop == L.getHeader())
ExitingBB = findBestLoopExit(L, LoopBlockSet);
BlockChain &LoopChain = *BlockToChain[LoopTop];
// FIXME: This is a really lame way of walking the chains in the loop: we
// walk the blocks, and use a set to prevent visiting a particular chain
// twice.
SmallPtrSet<BlockChain *, 4> UpdatedPreds;
assert(LoopChain.UnscheduledPredecessors == 0);
UpdatedPreds.insert(&LoopChain);
for (MachineBasicBlock *LoopBB : LoopBlockSet)
fillWorkLists(LoopBB, UpdatedPreds, &LoopBlockSet);
buildChain(LoopTop, LoopChain, &LoopBlockSet);
if (RotateLoopWithProfile)
rotateLoopWithProfile(LoopChain, L, LoopBlockSet);
else
rotateLoop(LoopChain, ExitingBB, LoopBlockSet);
DEBUG({
// Crash at the end so we get all of the debugging output first.
bool BadLoop = false;
if (LoopChain.UnscheduledPredecessors) {
BadLoop = true;
dbgs() << "Loop chain contains a block without its preds placed!\n"
<< " Loop header: " << getBlockName(*L.block_begin()) << "\n"
<< " Chain header: " << getBlockName(*LoopChain.begin()) << "\n";
}
for (MachineBasicBlock *ChainBB : LoopChain) {
dbgs() << " ... " << getBlockName(ChainBB) << "\n";
if (!LoopBlockSet.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.");
});
BlockWorkList.clear();
EHPadWorkList.clear();
}
/// When OutlineOpitonalBranches is on, this method collects BBs that
/// dominates all terminator blocks of the function \p F.
void MachineBlockPlacement::collectMustExecuteBBs() {
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);
}
}
}
}
void MachineBlockPlacement::buildCFGChains() {
// Ensure that every BB in the function has an associated chain to simplify
// the assumptions of the remaining algorithm.
SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
for (MachineFunction::iterator FI = F->begin(), FE = F->end(); FI != FE;
++FI) {
MachineBasicBlock *BB = &*FI;
BlockChain *Chain =
new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB);
// Also, merge any blocks which we cannot reason about and must preserve
// the exact fallthrough behavior for.
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;
}
}
// Turned on with OutlineOptionalBranches option
collectMustExecuteBBs();
// Build any loop-based chains.
for (MachineLoop *L : *MLI)
buildLoopChains(*L);
assert(BlockWorkList.empty());
assert(EHPadWorkList.empty());
SmallPtrSet<BlockChain *, 4> UpdatedPreds;
for (MachineBasicBlock &MBB : *F)
fillWorkLists(&MBB, UpdatedPreds);
BlockChain &FunctionChain = *BlockToChain[&F->front()];
buildChain(&F->front(), FunctionChain);
#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();
DEBUG(dbgs() << "[MBP] Function: "<< F->getName() << "\n");
for (MachineBasicBlock *ChainBB : FunctionChain) {
DEBUG(dbgs() << (ChainBB == *FunctionChain.begin() ? "Placing chain "
: " ... ")
<< getBlockName(ChainBB) << "\n");
if (InsertPos != MachineFunction::iterator(ChainBB))
F->splice(InsertPos, ChainBB);
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.
// The "PrevBB" is not yet updated to reflect current code layout, so,
// o. it may fall-through to a block without explicit "goto" instruction
// before layout, and no longer fall-through it after layout; or
// o. just opposite.
//
// analyzeBranch() may return erroneous value for FBB when these two
// situations take place. For the first scenario FBB is mistakenly set NULL;
// for the 2nd scenario, the FBB, which is expected to be NULL, is
// mistakenly pointing to "*BI".
// Thus, if the future change needs to use FBB before the layout is set, it
// has to correct FBB first by using the code similar to the following:
//
// if (!Cond.empty() && (!FBB || FBB == ChainBB)) {
// PrevBB->updateTerminator();
// Cond.clear();
// TBB = FBB = nullptr;
// if (TII->analyzeBranch(*PrevBB, TBB, FBB, Cond)) {
// // FIXME: This should never take place.
// TBB = FBB = nullptr;
// }
// }
if (!TII->analyzeBranch(*PrevBB, TBB, FBB, Cond))
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();
BlockWorkList.clear();
EHPadWorkList.clear();
}
void MachineBlockPlacement::optimizeBranches() {
BlockChain &FunctionChain = *BlockToChain[&F->front()];
SmallVector<MachineOperand, 4> Cond; // For AnalyzeBranch.
// Now that all the basic blocks in the chain have the proper layout,
// make a final call to AnalyzeBranch with AllowModify set.
// Indeed, the target may be able to optimize the branches in a way we
// cannot because all branches may not be analyzable.
// E.g., the target may be able to remove an unconditional branch to
// a fallthrough when it occurs after predicated terminators.
for (MachineBasicBlock *ChainBB : FunctionChain) {
Cond.clear();
MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For AnalyzeBranch.
if (!TII->analyzeBranch(*ChainBB, TBB, FBB, Cond, /*AllowModify*/ true)) {
// If PrevBB has a two-way branch, try to re-order the branches
// such that we branch to the successor with higher probability first.
if (TBB && !Cond.empty() && FBB &&
MBPI->getEdgeProbability(ChainBB, FBB) >
MBPI->getEdgeProbability(ChainBB, TBB) &&
!TII->reverseBranchCondition(Cond)) {
DEBUG(dbgs() << "Reverse order of the two branches: "
<< getBlockName(ChainBB) << "\n");
DEBUG(dbgs() << " Edge probability: "
<< MBPI->getEdgeProbability(ChainBB, FBB) << " vs "
<< MBPI->getEdgeProbability(ChainBB, TBB) << "\n");
DebugLoc dl; // FIXME: this is nowhere
TII->removeBranch(*ChainBB);
TII->insertBranch(*ChainBB, FBB, TBB, Cond, dl);
ChainBB->updateTerminator();
}
}
}
}
void MachineBlockPlacement::alignBlocks() {
// Walk through the backedges of the function now that we have fully laid out
// the basic blocks and align the destination of each backedge. We don't rely
// exclusively on the loop info here so that we can align backedges in
// unnatural CFGs and backedges that were introduced purely because of the
// loop rotations done during this layout pass.
if (F->getFunction()->optForSize())
return;
BlockChain &FunctionChain = *BlockToChain[&F->front()];
if (FunctionChain.begin() == FunctionChain.end())
return; // Empty chain.
const BranchProbability ColdProb(1, 5); // 20%
BlockFrequency EntryFreq = MBFI->getBlockFreq(&F->front());
BlockFrequency WeightedEntryFreq = EntryFreq * ColdProb;
for (MachineBasicBlock *ChainBB : FunctionChain) {
if (ChainBB == *FunctionChain.begin())
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);
}
}
/// Tail duplicate \p BB into (some) predecessors if profitable, repeating if
/// it was duplicated into its chain predecessor and removed.
/// \p BB - Basic block that may be duplicated.
///
/// \p LPred - Chosen layout predecessor of \p BB.
/// Updated to be the chain end if LPred is removed.
/// \p Chain - Chain to which \p LPred belongs, and \p BB will belong.
/// \p BlockFilter - Set of blocks that belong to the loop being laid out.
/// Used to identify which blocks to update predecessor
/// counts.
/// \p PrevUnplacedBlockIt - Iterator pointing to the last block that was
/// chosen in the given order due to unnatural CFG
/// only needed if \p BB is removed and
/// \p PrevUnplacedBlockIt pointed to \p BB.
/// @return true if \p BB was removed.
bool MachineBlockPlacement::repeatedlyTailDuplicateBlock(
MachineBasicBlock *BB, MachineBasicBlock *&LPred,
MachineBasicBlock *LoopHeaderBB,
BlockChain &Chain, BlockFilterSet *BlockFilter,
MachineFunction::iterator &PrevUnplacedBlockIt) {
bool Removed, DuplicatedToLPred;
bool DuplicatedToOriginalLPred;
Removed = maybeTailDuplicateBlock(BB, LPred, Chain, BlockFilter,
PrevUnplacedBlockIt,
DuplicatedToLPred);
if (!Removed)
return false;
DuplicatedToOriginalLPred = DuplicatedToLPred;
// Iteratively try to duplicate again. It can happen that a block that is
// duplicated into is still small enough to be duplicated again.
// No need to call markBlockSuccessors in this case, as the blocks being
// duplicated from here on are already scheduled.
// Note that DuplicatedToLPred always implies Removed.
while (DuplicatedToLPred) {
assert (Removed && "Block must have been removed to be duplicated into its "
"layout predecessor.");
MachineBasicBlock *DupBB, *DupPred;
// The removal callback causes Chain.end() to be updated when a block is
// removed. On the first pass through the loop, the chain end should be the
// same as it was on function entry. On subsequent passes, because we are
// duplicating the block at the end of the chain, if it is removed the
// chain will have shrunk by one block.
BlockChain::iterator ChainEnd = Chain.end();
DupBB = *(--ChainEnd);
// Now try to duplicate again.
if (ChainEnd == Chain.begin())
break;
DupPred = *std::prev(ChainEnd);
Removed = maybeTailDuplicateBlock(DupBB, DupPred, Chain, BlockFilter,
PrevUnplacedBlockIt,
DuplicatedToLPred);
}
// If BB was duplicated into LPred, it is now scheduled. But because it was
// removed, markChainSuccessors won't be called for its chain. Instead we
// call markBlockSuccessors for LPred to achieve the same effect. This must go
// at the end because repeating the tail duplication can increase the number
// of unscheduled predecessors.
if (DuplicatedToOriginalLPred)
markBlockSuccessors(Chain, LPred, LoopHeaderBB, BlockFilter);
LPred = *std::prev(Chain.end());
return true;
}
/// Tail duplicate \p BB into (some) predecessors if profitable.
/// \p BB - Basic block that may be duplicated
/// \p LPred - Chosen layout predecessor of \p BB
/// \p Chain - Chain to which \p LPred belongs, and \p BB will belong.
/// \p BlockFilter - Set of blocks that belong to the loop being laid out.
/// Used to identify which blocks to update predecessor
/// counts.
/// \p PrevUnplacedBlockIt - Iterator pointing to the last block that was
/// chosen in the given order due to unnatural CFG
/// only needed if \p BB is removed and
/// \p PrevUnplacedBlockIt pointed to \p BB.
/// \p DuplicatedToLPred - True if the block was duplicated into LPred. Will
/// only be true if the block was removed.
/// \return - True if the block was duplicated into all preds and removed.
bool MachineBlockPlacement::maybeTailDuplicateBlock(
MachineBasicBlock *BB, MachineBasicBlock *LPred,
const BlockChain &Chain, BlockFilterSet *BlockFilter,
MachineFunction::iterator &PrevUnplacedBlockIt,
bool &DuplicatedToLPred) {
DuplicatedToLPred = false;
DEBUG(dbgs() << "Redoing tail duplication for Succ#"
<< BB->getNumber() << "\n");
bool IsSimple = TailDup.isSimpleBB(BB);
// Blocks with single successors don't create additional fallthrough
// opportunities. Don't duplicate them. TODO: When conditional exits are
// analyzable, allow them to be duplicated.
if (!IsSimple && BB->succ_size() == 1)
return false;
if (!TailDup.shouldTailDuplicate(IsSimple, *BB))
return false;
// This has to be a callback because none of it can be done after
// BB is deleted.
bool Removed = false;
auto RemovalCallback =
[&](MachineBasicBlock *RemBB) {
// Signal to outer function
Removed = true;
// Conservative default.
bool InWorkList = true;
// Remove from the Chain and Chain Map
if (BlockToChain.count(RemBB)) {
BlockChain *Chain = BlockToChain[RemBB];
InWorkList = Chain->UnscheduledPredecessors == 0;
Chain->remove(RemBB);
BlockToChain.erase(RemBB);
}
// Handle the unplaced block iterator
if (&(*PrevUnplacedBlockIt) == RemBB) {
PrevUnplacedBlockIt++;
}
// Handle the Work Lists
if (InWorkList) {
SmallVectorImpl<MachineBasicBlock *> &RemoveList = BlockWorkList;
if (RemBB->isEHPad())
RemoveList = EHPadWorkList;
RemoveList.erase(
remove_if(RemoveList,
[RemBB](MachineBasicBlock *BB) {return BB == RemBB;}),
RemoveList.end());
}
// Handle the filter set
if (BlockFilter) {
BlockFilter->erase(RemBB);
}
// Remove the block from loop info.
MLI->removeBlock(RemBB);
// TailDuplicator handles removing it from loops.
DEBUG(dbgs() << "TailDuplicator deleted block: "
<< getBlockName(RemBB) << "\n");
};
auto RemovalCallbackRef =
llvm::function_ref<void(MachineBasicBlock*)>(RemovalCallback);
SmallVector<MachineBasicBlock *, 8> DuplicatedPreds;
TailDup.tailDuplicateAndUpdate(IsSimple, BB, LPred,
&DuplicatedPreds, &RemovalCallbackRef);
// Update UnscheduledPredecessors to reflect tail-duplication.
DuplicatedToLPred = false;
for (MachineBasicBlock *Pred : DuplicatedPreds) {
// We're only looking for unscheduled predecessors that match the filter.
BlockChain* PredChain = BlockToChain[Pred];
if (Pred == LPred)
DuplicatedToLPred = true;
if (Pred == LPred || (BlockFilter && !BlockFilter->count(Pred))
|| PredChain == &Chain)
continue;
for (MachineBasicBlock *NewSucc : Pred->successors()) {
if (BlockFilter && !BlockFilter->count(NewSucc))
continue;
BlockChain *NewChain = BlockToChain[NewSucc];
if (NewChain != &Chain && NewChain != PredChain)
NewChain->UnscheduledPredecessors++;
}
}
return Removed;
}
bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &MF) {
if (skipFunction(*MF.getFunction()))
return false;
// Check for single-block functions and skip them.
if (std::next(MF.begin()) == MF.end())
return false;
F = &MF;
MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
MBFI = llvm::make_unique<BranchFolder::MBFIWrapper>(
getAnalysis<MachineBlockFrequencyInfo>());
MLI = &getAnalysis<MachineLoopInfo>();
TII = MF.getSubtarget().getInstrInfo();
TLI = MF.getSubtarget().getTargetLowering();
MDT = &getAnalysis<MachineDominatorTree>();
if (TailDupPlacement) {
unsigned TailDupSize = TailDuplicatePlacementThreshold;
if (MF.getFunction()->optForSize())
TailDupSize = 1;
TailDup.initMF(MF, MBPI, /* LayoutMode */ true, TailDupSize);
}
assert(BlockToChain.empty());
buildCFGChains();
// Changing the layout can create new tail merging opportunities.
TargetPassConfig *PassConfig = &getAnalysis<TargetPassConfig>();
// TailMerge can create jump into if branches that make CFG irreducible for
// HW that requires structured CFG.
bool EnableTailMerge = !MF.getTarget().requiresStructuredCFG() &&
PassConfig->getEnableTailMerge() &&
BranchFoldPlacement;
// No tail merging opportunities if the block number is less than four.
if (MF.size() > 3 && EnableTailMerge) {
unsigned TailMergeSize = TailDuplicatePlacementThreshold + 1;
BranchFolder BF(/*EnableTailMerge=*/true, /*CommonHoist=*/false, *MBFI,
*MBPI, TailMergeSize);
if (BF.OptimizeFunction(MF, TII, MF.getSubtarget().getRegisterInfo(),
getAnalysisIfAvailable<MachineModuleInfo>(), MLI,
/*AfterBlockPlacement=*/true)) {
// Redo the layout if tail merging creates/removes/moves blocks.
BlockToChain.clear();
// Must redo the dominator tree if blocks were changed.
MDT->runOnMachineFunction(MF);
ChainAllocator.DestroyAll();
buildCFGChains();
}
}
optimizeBranches();
alignBlocks();
BlockToChain.clear();
ChainAllocator.DestroyAll();
if (AlignAllBlock)
// Align all of the blocks in the function to a specific alignment.
for (MachineBasicBlock &MBB : MF)
MBB.setAlignment(AlignAllBlock);
else if (AlignAllNonFallThruBlocks) {
// Align all of the blocks that have no fall-through predecessors to a
// specific alignment.
for (auto MBI = std::next(MF.begin()), MBE = MF.end(); MBI != MBE; ++MBI) {
auto LayoutPred = std::prev(MBI);
if (!LayoutPred->isSuccessor(&*MBI))
MBI->setAlignment(AlignAllNonFallThruBlocks);
}
}
// 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 (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;
}
Reference in New Issue View Git Blame Copy Permalink