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llvm-mirror/lib/CodeGen/BranchFolding.cpp
Haicheng Wu 4b9f9313ec Reapply "[MBP] Reduce code size by running tail merging in MBP."" 
This reapplies commit r271930, r271915, r271923.  They hit a bug in
Thumb which is fixed in r272258 now.

The original message:

The code layout that TailMerging (inside BranchFolding) works on is not the
final layout optimized based on the branch probability. Generally, after
BlockPlacement, many new merging opportunities emerge.

This patch calls Tail Merging after MBP and calls MBP again if Tail Merging
merges anything.

llvm-svn: 272267
2016-06-09 15:24:29 +00:00

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//===-- BranchFolding.cpp - Fold machine code branch instructions ---------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass forwards branches to unconditional branches to make them branch
// directly to the target block. This pass often results in dead MBB's, which
// it then removes.
//
// Note that this pass must be run after register allocation, it cannot handle
// SSA form. It also must handle virtual registers for targets that emit virtual
// ISA (e.g. NVPTX).
//
//===----------------------------------------------------------------------===//
#include "BranchFolding.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/Analysis.h"
#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineJumpTableInfo.h"
#include "llvm/CodeGen/MachineMemOperand.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetPassConfig.h"
#include "llvm/CodeGen/RegisterScavenging.h"
#include "llvm/IR/Function.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetRegisterInfo.h"
#include "llvm/Target/TargetSubtargetInfo.h"
#include <algorithm>
using namespace llvm;
#define DEBUG_TYPE "branchfolding"
STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
STATISTIC(NumBranchOpts, "Number of branches optimized");
STATISTIC(NumTailMerge , "Number of block tails merged");
STATISTIC(NumHoist , "Number of times common instructions are hoisted");
static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge",
cl::init(cl::BOU_UNSET), cl::Hidden);
// Throttle for huge numbers of predecessors (compile speed problems)
static cl::opt<unsigned>
TailMergeThreshold("tail-merge-threshold",
cl::desc("Max number of predecessors to consider tail merging"),
cl::init(150), cl::Hidden);
// Heuristic for tail merging (and, inversely, tail duplication).
// TODO: This should be replaced with a target query.
static cl::opt<unsigned>
TailMergeSize("tail-merge-size",
cl::desc("Min number of instructions to consider tail merging"),
cl::init(3), cl::Hidden);
namespace {
/// BranchFolderPass - Wrap branch folder in a machine function pass.
class BranchFolderPass : public MachineFunctionPass {
public:
static char ID;
explicit BranchFolderPass(): MachineFunctionPass(ID) {}
bool runOnMachineFunction(MachineFunction &MF) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<MachineBlockFrequencyInfo>();
AU.addRequired<MachineBranchProbabilityInfo>();
AU.addRequired<TargetPassConfig>();
MachineFunctionPass::getAnalysisUsage(AU);
}
};
}
char BranchFolderPass::ID = 0;
char &llvm::BranchFolderPassID = BranchFolderPass::ID;
INITIALIZE_PASS(BranchFolderPass, "branch-folder",
"Control Flow Optimizer", false, false)
bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
if (skipFunction(*MF.getFunction()))
return false;
TargetPassConfig *PassConfig = &getAnalysis<TargetPassConfig>();
// TailMerge can create jump into if branches that make CFG irreducible for
// HW that requires structurized CFG.
bool EnableTailMerge = !MF.getTarget().requiresStructuredCFG() &&
PassConfig->getEnableTailMerge();
BranchFolder::MBFIWrapper MBBFreqInfo(
getAnalysis<MachineBlockFrequencyInfo>());
BranchFolder Folder(EnableTailMerge, /*CommonHoist=*/true, MBBFreqInfo,
getAnalysis<MachineBranchProbabilityInfo>());
return Folder.OptimizeFunction(MF, MF.getSubtarget().getInstrInfo(),
MF.getSubtarget().getRegisterInfo(),
getAnalysisIfAvailable<MachineModuleInfo>());
}
BranchFolder::BranchFolder(bool defaultEnableTailMerge, bool CommonHoist,
MBFIWrapper &FreqInfo,
const MachineBranchProbabilityInfo &ProbInfo)
: EnableHoistCommonCode(CommonHoist), MBBFreqInfo(FreqInfo),
MBPI(ProbInfo) {
switch (FlagEnableTailMerge) {
case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break;
case cl::BOU_TRUE: EnableTailMerge = true; break;
case cl::BOU_FALSE: EnableTailMerge = false; break;
}
}
/// RemoveDeadBlock - Remove the specified dead machine basic block from the
/// function, updating the CFG.
void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
assert(MBB->pred_empty() && "MBB must be dead!");
DEBUG(dbgs() << "\nRemoving MBB: " << *MBB);
MachineFunction *MF = MBB->getParent();
// drop all successors.
while (!MBB->succ_empty())
MBB->removeSuccessor(MBB->succ_end()-1);
// Avoid matching if this pointer gets reused.
TriedMerging.erase(MBB);
// Remove the block.
MF->erase(MBB);
FuncletMembership.erase(MBB);
if (MLI)
MLI->removeBlock(MBB);
}
/// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def
/// followed by terminators, and if the implicitly defined registers are not
/// used by the terminators, remove those implicit_def's. e.g.
/// BB1:
/// r0 = implicit_def
/// r1 = implicit_def
/// br
/// This block can be optimized away later if the implicit instructions are
/// removed.
bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) {
SmallSet<unsigned, 4> ImpDefRegs;
MachineBasicBlock::iterator I = MBB->begin();
while (I != MBB->end()) {
if (!I->isImplicitDef())
break;
unsigned Reg = I->getOperand(0).getReg();
if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
for (MCSubRegIterator SubRegs(Reg, TRI, /*IncludeSelf=*/true);
SubRegs.isValid(); ++SubRegs)
ImpDefRegs.insert(*SubRegs);
} else {
ImpDefRegs.insert(Reg);
}
++I;
}
if (ImpDefRegs.empty())
return false;
MachineBasicBlock::iterator FirstTerm = I;
while (I != MBB->end()) {
if (!TII->isUnpredicatedTerminator(*I))
return false;
// See if it uses any of the implicitly defined registers.
for (const MachineOperand &MO : I->operands()) {
if (!MO.isReg() || !MO.isUse())
continue;
unsigned Reg = MO.getReg();
if (ImpDefRegs.count(Reg))
return false;
}
++I;
}
I = MBB->begin();
while (I != FirstTerm) {
MachineInstr *ImpDefMI = &*I;
++I;
MBB->erase(ImpDefMI);
}
return true;
}
/// OptimizeFunction - Perhaps branch folding, tail merging and other
/// CFG optimizations on the given function. Block placement changes the layout
/// and may create new tail merging opportunities.
bool BranchFolder::OptimizeFunction(MachineFunction &MF,
const TargetInstrInfo *tii,
const TargetRegisterInfo *tri,
MachineModuleInfo *mmi,
MachineLoopInfo *mli, bool AfterPlacement) {
if (!tii) return false;
TriedMerging.clear();
AfterBlockPlacement = AfterPlacement;
TII = tii;
TRI = tri;
MMI = mmi;
MLI = mli;
RS = nullptr;
// Use a RegScavenger to help update liveness when required.
MachineRegisterInfo &MRI = MF.getRegInfo();
if (MRI.tracksLiveness() && TRI->trackLivenessAfterRegAlloc(MF))
RS = new RegScavenger();
else
MRI.invalidateLiveness();
// Fix CFG. The later algorithms expect it to be right.
bool MadeChange = false;
for (MachineBasicBlock &MBB : MF) {
MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
SmallVector<MachineOperand, 4> Cond;
if (!TII->AnalyzeBranch(MBB, TBB, FBB, Cond, true))
MadeChange |= MBB.CorrectExtraCFGEdges(TBB, FBB, !Cond.empty());
MadeChange |= OptimizeImpDefsBlock(&MBB);
}
// Recalculate funclet membership.
FuncletMembership = getFuncletMembership(MF);
bool MadeChangeThisIteration = true;
while (MadeChangeThisIteration) {
MadeChangeThisIteration = TailMergeBlocks(MF);
// No need to clean up if tail merging does not change anything after the
// block placement.
if (!AfterBlockPlacement || MadeChangeThisIteration)
MadeChangeThisIteration |= OptimizeBranches(MF);
if (EnableHoistCommonCode)
MadeChangeThisIteration |= HoistCommonCode(MF);
MadeChange |= MadeChangeThisIteration;
}
// See if any jump tables have become dead as the code generator
// did its thing.
MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
if (!JTI) {
delete RS;
return MadeChange;
}
// Walk the function to find jump tables that are live.
BitVector JTIsLive(JTI->getJumpTables().size());
for (const MachineBasicBlock &BB : MF) {
for (const MachineInstr &I : BB)
for (const MachineOperand &Op : I.operands()) {
if (!Op.isJTI()) continue;
// Remember that this JT is live.
JTIsLive.set(Op.getIndex());
}
}
// Finally, remove dead jump tables. This happens when the
// indirect jump was unreachable (and thus deleted).
for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
if (!JTIsLive.test(i)) {
JTI->RemoveJumpTable(i);
MadeChange = true;
}
delete RS;
return MadeChange;
}
//===----------------------------------------------------------------------===//
// Tail Merging of Blocks
//===----------------------------------------------------------------------===//
/// HashMachineInstr - Compute a hash value for MI and its operands.
static unsigned HashMachineInstr(const MachineInstr &MI) {
unsigned Hash = MI.getOpcode();
for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
const MachineOperand &Op = MI.getOperand(i);
// Merge in bits from the operand if easy. We can't use MachineOperand's
// hash_code here because it's not deterministic and we sort by hash value
// later.
unsigned OperandHash = 0;
switch (Op.getType()) {
case MachineOperand::MO_Register:
OperandHash = Op.getReg();
break;
case MachineOperand::MO_Immediate:
OperandHash = Op.getImm();
break;
case MachineOperand::MO_MachineBasicBlock:
OperandHash = Op.getMBB()->getNumber();
break;
case MachineOperand::MO_FrameIndex:
case MachineOperand::MO_ConstantPoolIndex:
case MachineOperand::MO_JumpTableIndex:
OperandHash = Op.getIndex();
break;
case MachineOperand::MO_GlobalAddress:
case MachineOperand::MO_ExternalSymbol:
// Global address / external symbol are too hard, don't bother, but do
// pull in the offset.
OperandHash = Op.getOffset();
break;
default:
break;
}
Hash += ((OperandHash << 3) | Op.getType()) << (i & 31);
}
return Hash;
}
/// HashEndOfMBB - Hash the last instruction in the MBB.
static unsigned HashEndOfMBB(const MachineBasicBlock &MBB) {
MachineBasicBlock::const_iterator I = MBB.getLastNonDebugInstr();
if (I == MBB.end())
return 0;
return HashMachineInstr(*I);
}
/// ComputeCommonTailLength - Given two machine basic blocks, compute the number
/// of instructions they actually have in common together at their end. Return
/// iterators for the first shared instruction in each block.
static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
MachineBasicBlock *MBB2,
MachineBasicBlock::iterator &I1,
MachineBasicBlock::iterator &I2) {
I1 = MBB1->end();
I2 = MBB2->end();
unsigned TailLen = 0;
while (I1 != MBB1->begin() && I2 != MBB2->begin()) {
--I1; --I2;
// Skip debugging pseudos; necessary to avoid changing the code.
while (I1->isDebugValue()) {
if (I1==MBB1->begin()) {
while (I2->isDebugValue()) {
if (I2==MBB2->begin())
// I1==DBG at begin; I2==DBG at begin
return TailLen;
--I2;
}
++I2;
// I1==DBG at begin; I2==non-DBG, or first of DBGs not at begin
return TailLen;
}
--I1;
}
// I1==first (untested) non-DBG preceding known match
while (I2->isDebugValue()) {
if (I2==MBB2->begin()) {
++I1;
// I1==non-DBG, or first of DBGs not at begin; I2==DBG at begin
return TailLen;
}
--I2;
}
// I1, I2==first (untested) non-DBGs preceding known match
if (!I1->isIdenticalTo(*I2) ||
// FIXME: This check is dubious. It's used to get around a problem where
// people incorrectly expect inline asm directives to remain in the same
// relative order. This is untenable because normal compiler
// optimizations (like this one) may reorder and/or merge these
// directives.
I1->isInlineAsm()) {
++I1; ++I2;
break;
}
++TailLen;
}
// Back past possible debugging pseudos at beginning of block. This matters
// when one block differs from the other only by whether debugging pseudos
// are present at the beginning. (This way, the various checks later for
// I1==MBB1->begin() work as expected.)
if (I1 == MBB1->begin() && I2 != MBB2->begin()) {
--I2;
while (I2->isDebugValue()) {
if (I2 == MBB2->begin())
return TailLen;
--I2;
}
++I2;
}
if (I2 == MBB2->begin() && I1 != MBB1->begin()) {
--I1;
while (I1->isDebugValue()) {
if (I1 == MBB1->begin())
return TailLen;
--I1;
}
++I1;
}
return TailLen;
}
void BranchFolder::MaintainLiveIns(MachineBasicBlock *CurMBB,
MachineBasicBlock *NewMBB) {
if (RS) {
RS->enterBasicBlock(*CurMBB);
if (!CurMBB->empty())
RS->forward(std::prev(CurMBB->end()));
for (unsigned int i = 1, e = TRI->getNumRegs(); i != e; i++)
if (RS->isRegUsed(i, false))
NewMBB->addLiveIn(i);
}
}
/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything
/// after it, replacing it with an unconditional branch to NewDest.
void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
MachineBasicBlock *NewDest) {
MachineBasicBlock *CurMBB = OldInst->getParent();
TII->ReplaceTailWithBranchTo(OldInst, NewDest);
// For targets that use the register scavenger, we must maintain LiveIns.
MaintainLiveIns(CurMBB, NewDest);
++NumTailMerge;
}
/// SplitMBBAt - Given a machine basic block and an iterator into it, split the
/// MBB so that the part before the iterator falls into the part starting at the
/// iterator. This returns the new MBB.
MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
MachineBasicBlock::iterator BBI1,
const BasicBlock *BB) {
if (!TII->isLegalToSplitMBBAt(CurMBB, BBI1))
return nullptr;
MachineFunction &MF = *CurMBB.getParent();
// Create the fall-through block.
MachineFunction::iterator MBBI = CurMBB.getIterator();
MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(BB);
CurMBB.getParent()->insert(++MBBI, NewMBB);
// Move all the successors of this block to the specified block.
NewMBB->transferSuccessors(&CurMBB);
// Add an edge from CurMBB to NewMBB for the fall-through.
CurMBB.addSuccessor(NewMBB);
// Splice the code over.
NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end());
// NewMBB belongs to the same loop as CurMBB.
if (MLI)
if (MachineLoop *ML = MLI->getLoopFor(&CurMBB))
ML->addBasicBlockToLoop(NewMBB, MLI->getBase());
// NewMBB inherits CurMBB's block frequency.
MBBFreqInfo.setBlockFreq(NewMBB, MBBFreqInfo.getBlockFreq(&CurMBB));
// For targets that use the register scavenger, we must maintain LiveIns.
MaintainLiveIns(&CurMBB, NewMBB);
// Add the new block to the funclet.
const auto &FuncletI = FuncletMembership.find(&CurMBB);
if (FuncletI != FuncletMembership.end()) {
auto n = FuncletI->second;
FuncletMembership[NewMBB] = n;
}
return NewMBB;
}
/// EstimateRuntime - Make a rough estimate for how long it will take to run
/// the specified code.
static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
MachineBasicBlock::iterator E) {
unsigned Time = 0;
for (; I != E; ++I) {
if (I->isDebugValue())
continue;
if (I->isCall())
Time += 10;
else if (I->mayLoad() || I->mayStore())
Time += 2;
else
++Time;
}
return Time;
}
// CurMBB needs to add an unconditional branch to SuccMBB (we removed these
// branches temporarily for tail merging). In the case where CurMBB ends
// with a conditional branch to the next block, optimize by reversing the
// test and conditionally branching to SuccMBB instead.
static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB,
const TargetInstrInfo *TII) {
MachineFunction *MF = CurMBB->getParent();
MachineFunction::iterator I = std::next(MachineFunction::iterator(CurMBB));
MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
SmallVector<MachineOperand, 4> Cond;
DebugLoc dl; // FIXME: this is nowhere
if (I != MF->end() &&
!TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) {
MachineBasicBlock *NextBB = &*I;
if (TBB == NextBB && !Cond.empty() && !FBB) {
if (!TII->ReverseBranchCondition(Cond)) {
TII->RemoveBranch(*CurMBB);
TII->InsertBranch(*CurMBB, SuccBB, nullptr, Cond, dl);
return;
}
}
}
TII->InsertBranch(*CurMBB, SuccBB, nullptr,
SmallVector<MachineOperand, 0>(), dl);
}
bool
BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const {
if (getHash() < o.getHash())
return true;
if (getHash() > o.getHash())
return false;
if (getBlock()->getNumber() < o.getBlock()->getNumber())
return true;
if (getBlock()->getNumber() > o.getBlock()->getNumber())
return false;
// _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
// an object with itself.
#ifndef _GLIBCXX_DEBUG
llvm_unreachable("Predecessor appears twice");
#else
return false;
#endif
}
BlockFrequency
BranchFolder::MBFIWrapper::getBlockFreq(const MachineBasicBlock *MBB) const {
auto I = MergedBBFreq.find(MBB);
if (I != MergedBBFreq.end())
return I->second;
return MBFI.getBlockFreq(MBB);
}
void BranchFolder::MBFIWrapper::setBlockFreq(const MachineBasicBlock *MBB,
BlockFrequency F) {
MergedBBFreq[MBB] = F;
}
raw_ostream &
BranchFolder::MBFIWrapper::printBlockFreq(raw_ostream &OS,
const MachineBasicBlock *MBB) const {
return MBFI.printBlockFreq(OS, getBlockFreq(MBB));
}
raw_ostream &
BranchFolder::MBFIWrapper::printBlockFreq(raw_ostream &OS,
const BlockFrequency Freq) const {
return MBFI.printBlockFreq(OS, Freq);
}
/// CountTerminators - Count the number of terminators in the given
/// block and set I to the position of the first non-terminator, if there
/// is one, or MBB->end() otherwise.
static unsigned CountTerminators(MachineBasicBlock *MBB,
MachineBasicBlock::iterator &I) {
I = MBB->end();
unsigned NumTerms = 0;
for (;;) {
if (I == MBB->begin()) {
I = MBB->end();
break;
}
--I;
if (!I->isTerminator()) break;
++NumTerms;
}
return NumTerms;
}
/// ProfitableToMerge - Check if two machine basic blocks have a common tail
/// and decide if it would be profitable to merge those tails. Return the
/// length of the common tail and iterators to the first common instruction
/// in each block.
static bool
ProfitableToMerge(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2,
unsigned minCommonTailLength, unsigned &CommonTailLen,
MachineBasicBlock::iterator &I1,
MachineBasicBlock::iterator &I2, MachineBasicBlock *SuccBB,
MachineBasicBlock *PredBB,
DenseMap<const MachineBasicBlock *, int> &FuncletMembership) {
// It is never profitable to tail-merge blocks from two different funclets.
if (!FuncletMembership.empty()) {
auto Funclet1 = FuncletMembership.find(MBB1);
assert(Funclet1 != FuncletMembership.end());
auto Funclet2 = FuncletMembership.find(MBB2);
assert(Funclet2 != FuncletMembership.end());
if (Funclet1->second != Funclet2->second)
return false;
}
CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2);
if (CommonTailLen == 0)
return false;
DEBUG(dbgs() << "Common tail length of BB#" << MBB1->getNumber()
<< " and BB#" << MBB2->getNumber() << " is " << CommonTailLen
<< '\n');
// It's almost always profitable to merge any number of non-terminator
// instructions with the block that falls through into the common successor.
if (MBB1 == PredBB || MBB2 == PredBB) {
MachineBasicBlock::iterator I;
unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I);
if (CommonTailLen > NumTerms)
return true;
}
// If one of the blocks can be completely merged and happens to be in
// a position where the other could fall through into it, merge any number
// of instructions, because it can be done without a branch.
// TODO: If the blocks are not adjacent, move one of them so that they are?
if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin())
return true;
if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin())
return true;
// If both blocks have an unconditional branch temporarily stripped out,
// count that as an additional common instruction for the following
// heuristics.
unsigned EffectiveTailLen = CommonTailLen;
if (SuccBB && MBB1 != PredBB && MBB2 != PredBB &&
!MBB1->back().isBarrier() &&
!MBB2->back().isBarrier())
++EffectiveTailLen;
// Check if the common tail is long enough to be worthwhile.
if (EffectiveTailLen >= minCommonTailLength)
return true;
// If we are optimizing for code size, 2 instructions in common is enough if
// we don't have to split a block. At worst we will be introducing 1 new
// branch instruction, which is likely to be smaller than the 2
// instructions that would be deleted in the merge.
MachineFunction *MF = MBB1->getParent();
return EffectiveTailLen >= 2 && MF->getFunction()->optForSize() &&
(I1 == MBB1->begin() || I2 == MBB2->begin());
}
/// ComputeSameTails - Look through all the blocks in MergePotentials that have
/// hash CurHash (guaranteed to match the last element). Build the vector
/// SameTails of all those that have the (same) largest number of instructions
/// in common of any pair of these blocks. SameTails entries contain an
/// iterator into MergePotentials (from which the MachineBasicBlock can be
/// found) and a MachineBasicBlock::iterator into that MBB indicating the
/// instruction where the matching code sequence begins.
/// Order of elements in SameTails is the reverse of the order in which
/// those blocks appear in MergePotentials (where they are not necessarily
/// consecutive).
unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
unsigned minCommonTailLength,
MachineBasicBlock *SuccBB,
MachineBasicBlock *PredBB) {
unsigned maxCommonTailLength = 0U;
SameTails.clear();
MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
MPIterator HighestMPIter = std::prev(MergePotentials.end());
for (MPIterator CurMPIter = std::prev(MergePotentials.end()),
B = MergePotentials.begin();
CurMPIter != B && CurMPIter->getHash() == CurHash; --CurMPIter) {
for (MPIterator I = std::prev(CurMPIter); I->getHash() == CurHash; --I) {
unsigned CommonTailLen;
if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(),
minCommonTailLength,
CommonTailLen, TrialBBI1, TrialBBI2,
SuccBB, PredBB,
FuncletMembership)) {
if (CommonTailLen > maxCommonTailLength) {
SameTails.clear();
maxCommonTailLength = CommonTailLen;
HighestMPIter = CurMPIter;
SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1));
}
if (HighestMPIter == CurMPIter &&
CommonTailLen == maxCommonTailLength)
SameTails.push_back(SameTailElt(I, TrialBBI2));
}
if (I == B)
break;
}
}
return maxCommonTailLength;
}
/// RemoveBlocksWithHash - Remove all blocks with hash CurHash from
/// MergePotentials, restoring branches at ends of blocks as appropriate.
void BranchFolder::RemoveBlocksWithHash(unsigned CurHash,
MachineBasicBlock *SuccBB,
MachineBasicBlock *PredBB) {
MPIterator CurMPIter, B;
for (CurMPIter = std::prev(MergePotentials.end()),
B = MergePotentials.begin();
CurMPIter->getHash() == CurHash; --CurMPIter) {
// Put the unconditional branch back, if we need one.
MachineBasicBlock *CurMBB = CurMPIter->getBlock();
if (SuccBB && CurMBB != PredBB)
FixTail(CurMBB, SuccBB, TII);
if (CurMPIter == B)
break;
}
if (CurMPIter->getHash() != CurHash)
CurMPIter++;
MergePotentials.erase(CurMPIter, MergePotentials.end());
}
/// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist
/// only of the common tail. Create a block that does by splitting one.
bool BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
MachineBasicBlock *SuccBB,
unsigned maxCommonTailLength,
unsigned &commonTailIndex) {
commonTailIndex = 0;
unsigned TimeEstimate = ~0U;
for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
// Use PredBB if possible; that doesn't require a new branch.
if (SameTails[i].getBlock() == PredBB) {
commonTailIndex = i;
break;
}
// Otherwise, make a (fairly bogus) choice based on estimate of
// how long it will take the various blocks to execute.
unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(),
SameTails[i].getTailStartPos());
if (t <= TimeEstimate) {
TimeEstimate = t;
commonTailIndex = i;
}
}
MachineBasicBlock::iterator BBI =
SameTails[commonTailIndex].getTailStartPos();
MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
// If the common tail includes any debug info we will take it pretty
// randomly from one of the inputs. Might be better to remove it?
DEBUG(dbgs() << "\nSplitting BB#" << MBB->getNumber() << ", size "
<< maxCommonTailLength);
// If the split block unconditionally falls-thru to SuccBB, it will be
// merged. In control flow terms it should then take SuccBB's name. e.g. If
// SuccBB is an inner loop, the common tail is still part of the inner loop.
const BasicBlock *BB = (SuccBB && MBB->succ_size() == 1) ?
SuccBB->getBasicBlock() : MBB->getBasicBlock();
MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI, BB);
if (!newMBB) {
DEBUG(dbgs() << "... failed!");
return false;
}
SameTails[commonTailIndex].setBlock(newMBB);
SameTails[commonTailIndex].setTailStartPos(newMBB->begin());
// If we split PredBB, newMBB is the new predecessor.
if (PredBB == MBB)
PredBB = newMBB;
return true;
}
static void
mergeMMOsFromMemoryOperations(MachineBasicBlock::iterator MBBIStartPos,
MachineBasicBlock &MBBCommon) {
// Merge MMOs from memory operations in the common block.
MachineBasicBlock *MBB = MBBIStartPos->getParent();
// Note CommonTailLen does not necessarily matches the size of
// the common BB nor all its instructions because of debug
// instructions differences.
unsigned CommonTailLen = 0;
for (auto E = MBB->end(); MBBIStartPos != E; ++MBBIStartPos)
++CommonTailLen;
MachineBasicBlock::reverse_iterator MBBI = MBB->rbegin();
MachineBasicBlock::reverse_iterator MBBIE = MBB->rend();
MachineBasicBlock::reverse_iterator MBBICommon = MBBCommon.rbegin();
MachineBasicBlock::reverse_iterator MBBIECommon = MBBCommon.rend();
while (CommonTailLen--) {
assert(MBBI != MBBIE && "Reached BB end within common tail length!");
(void)MBBIE;
if (MBBI->isDebugValue()) {
++MBBI;
continue;
}
while ((MBBICommon != MBBIECommon) && MBBICommon->isDebugValue())
++MBBICommon;
assert(MBBICommon != MBBIECommon &&
"Reached BB end within common tail length!");
assert(MBBICommon->isIdenticalTo(*MBBI) && "Expected matching MIIs!");
if (MBBICommon->mayLoad() || MBBICommon->mayStore())
MBBICommon->setMemRefs(MBBICommon->mergeMemRefsWith(*MBBI));
++MBBI;
++MBBICommon;
}
}
// See if any of the blocks in MergePotentials (which all have a common single
// successor, or all have no successor) can be tail-merged. If there is a
// successor, any blocks in MergePotentials that are not tail-merged and
// are not immediately before Succ must have an unconditional branch to
// Succ added (but the predecessor/successor lists need no adjustment).
// The lone predecessor of Succ that falls through into Succ,
// if any, is given in PredBB.
bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
MachineBasicBlock *PredBB) {
bool MadeChange = false;
// Except for the special cases below, tail-merge if there are at least
// this many instructions in common.
unsigned minCommonTailLength = TailMergeSize;
DEBUG(dbgs() << "\nTryTailMergeBlocks: ";
for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
dbgs() << "BB#" << MergePotentials[i].getBlock()->getNumber()
<< (i == e-1 ? "" : ", ");
dbgs() << "\n";
if (SuccBB) {
dbgs() << " with successor BB#" << SuccBB->getNumber() << '\n';
if (PredBB)
dbgs() << " which has fall-through from BB#"
<< PredBB->getNumber() << "\n";
}
dbgs() << "Looking for common tails of at least "
<< minCommonTailLength << " instruction"
<< (minCommonTailLength == 1 ? "" : "s") << '\n';
);
// Sort by hash value so that blocks with identical end sequences sort
// together.
array_pod_sort(MergePotentials.begin(), MergePotentials.end());
// Walk through equivalence sets looking for actual exact matches.
while (MergePotentials.size() > 1) {
unsigned CurHash = MergePotentials.back().getHash();
// Build SameTails, identifying the set of blocks with this hash code
// and with the maximum number of instructions in common.
unsigned maxCommonTailLength = ComputeSameTails(CurHash,
minCommonTailLength,
SuccBB, PredBB);
// If we didn't find any pair that has at least minCommonTailLength
// instructions in common, remove all blocks with this hash code and retry.
if (SameTails.empty()) {
RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
continue;
}
// If one of the blocks is the entire common tail (and not the entry
// block, which we can't jump to), we can treat all blocks with this same
// tail at once. Use PredBB if that is one of the possibilities, as that
// will not introduce any extra branches.
MachineBasicBlock *EntryBB =
&MergePotentials.front().getBlock()->getParent()->front();
unsigned commonTailIndex = SameTails.size();
// If there are two blocks, check to see if one can be made to fall through
// into the other.
if (SameTails.size() == 2 &&
SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) &&
SameTails[1].tailIsWholeBlock())
commonTailIndex = 1;
else if (SameTails.size() == 2 &&
SameTails[1].getBlock()->isLayoutSuccessor(
SameTails[0].getBlock()) &&
SameTails[0].tailIsWholeBlock())
commonTailIndex = 0;
else {
// Otherwise just pick one, favoring the fall-through predecessor if
// there is one.
for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
MachineBasicBlock *MBB = SameTails[i].getBlock();
if (MBB == EntryBB && SameTails[i].tailIsWholeBlock())
continue;
if (MBB == PredBB) {
commonTailIndex = i;
break;
}
if (SameTails[i].tailIsWholeBlock())
commonTailIndex = i;
}
}
if (commonTailIndex == SameTails.size() ||
(SameTails[commonTailIndex].getBlock() == PredBB &&
!SameTails[commonTailIndex].tailIsWholeBlock())) {
// None of the blocks consist entirely of the common tail.
// Split a block so that one does.
if (!CreateCommonTailOnlyBlock(PredBB, SuccBB,
maxCommonTailLength, commonTailIndex)) {
RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
continue;
}
}
MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
// Recompute common tail MBB's edge weights and block frequency.
setCommonTailEdgeWeights(*MBB);
// MBB is common tail. Adjust all other BB's to jump to this one.
// Traversal must be forwards so erases work.
DEBUG(dbgs() << "\nUsing common tail in BB#" << MBB->getNumber()
<< " for ");
for (unsigned int i=0, e = SameTails.size(); i != e; ++i) {
if (commonTailIndex == i)
continue;
DEBUG(dbgs() << "BB#" << SameTails[i].getBlock()->getNumber()
<< (i == e-1 ? "" : ", "));
// Merge MMOs from memory operations as needed.
mergeMMOsFromMemoryOperations(SameTails[i].getTailStartPos(), *MBB);
// Hack the end off BB i, making it jump to BB commonTailIndex instead.
ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB);
// BB i is no longer a predecessor of SuccBB; remove it from the worklist.
MergePotentials.erase(SameTails[i].getMPIter());
}
DEBUG(dbgs() << "\n");
// We leave commonTailIndex in the worklist in case there are other blocks
// that match it with a smaller number of instructions.
MadeChange = true;
}
return MadeChange;
}
bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
bool MadeChange = false;
if (!EnableTailMerge) return MadeChange;
// First find blocks with no successors.
// Block placement does not create new tail merging opportunities for these
// blocks.
if (!AfterBlockPlacement) {
MergePotentials.clear();
for (MachineBasicBlock &MBB : MF) {
if (MergePotentials.size() == TailMergeThreshold)
break;
if (!TriedMerging.count(&MBB) && MBB.succ_empty())
MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(MBB), &MBB));
}
// If this is a large problem, avoid visiting the same basic blocks
// multiple times.
if (MergePotentials.size() == TailMergeThreshold)
for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
TriedMerging.insert(MergePotentials[i].getBlock());
// See if we can do any tail merging on those.
if (MergePotentials.size() >= 2)
MadeChange |= TryTailMergeBlocks(nullptr, nullptr);
}
// Look at blocks (IBB) with multiple predecessors (PBB).
// We change each predecessor to a canonical form, by
// (1) temporarily removing any unconditional branch from the predecessor
// to IBB, and
// (2) alter conditional branches so they branch to the other block
// not IBB; this may require adding back an unconditional branch to IBB
// later, where there wasn't one coming in. E.g.
// Bcc IBB
// fallthrough to QBB
// here becomes
// Bncc QBB
// with a conceptual B to IBB after that, which never actually exists.
// With those changes, we see whether the predecessors' tails match,
// and merge them if so. We change things out of canonical form and
// back to the way they were later in the process. (OptimizeBranches
// would undo some of this, but we can't use it, because we'd get into
// a compile-time infinite loop repeatedly doing and undoing the same
// transformations.)
for (MachineFunction::iterator I = std::next(MF.begin()), E = MF.end();
I != E; ++I) {
if (I->pred_size() < 2) continue;
SmallPtrSet<MachineBasicBlock *, 8> UniquePreds;
MachineBasicBlock *IBB = &*I;
MachineBasicBlock *PredBB = &*std::prev(I);
MergePotentials.clear();
for (MachineBasicBlock *PBB : I->predecessors()) {
if (MergePotentials.size() == TailMergeThreshold)
break;
if (TriedMerging.count(PBB))
continue;
// Skip blocks that loop to themselves, can't tail merge these.
if (PBB == IBB)
continue;
// Visit each predecessor only once.
if (!UniquePreds.insert(PBB).second)
continue;
// Skip blocks which may jump to a landing pad. Can't tail merge these.
if (PBB->hasEHPadSuccessor())
continue;
// Bail out if the loop header (IBB) is not the top of the loop chain
// after the block placement. Otherwise, the common tail of IBB's
// predecessors may become the loop top if block placement is called again
// and the predecessors may branch to this common tail.
// FIXME: Relaxed this check if the algorithm of finding loop top is
// changed in MBP.
if (AfterBlockPlacement && MLI)
if (MachineLoop *ML = MLI->getLoopFor(IBB))
if (IBB == ML->getHeader() && ML == MLI->getLoopFor(PBB))
continue;
MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
SmallVector<MachineOperand, 4> Cond;
if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) {
// Failing case: IBB is the target of a cbr, and we cannot reverse the
// branch.
SmallVector<MachineOperand, 4> NewCond(Cond);
if (!Cond.empty() && TBB == IBB) {
if (TII->ReverseBranchCondition(NewCond))
continue;
// This is the QBB case described above
if (!FBB) {
auto Next = ++PBB->getIterator();
if (Next != MF.end())
FBB = &*Next;
}
}
// Failing case: the only way IBB can be reached from PBB is via
// exception handling. Happens for landing pads. Would be nice to have
// a bit in the edge so we didn't have to do all this.
if (IBB->isEHPad()) {
MachineFunction::iterator IP = ++PBB->getIterator();
MachineBasicBlock *PredNextBB = nullptr;
if (IP != MF.end())
PredNextBB = &*IP;
if (!TBB) {
if (IBB != PredNextBB) // fallthrough
continue;
} else if (FBB) {
if (TBB != IBB && FBB != IBB) // cbr then ubr
continue;
} else if (Cond.empty()) {
if (TBB != IBB) // ubr
continue;
} else {
if (TBB != IBB && IBB != PredNextBB) // cbr
continue;
}
}
// Remove the unconditional branch at the end, if any.
if (TBB && (Cond.empty() || FBB)) {
DebugLoc dl; // FIXME: this is nowhere
TII->RemoveBranch(*PBB);
if (!Cond.empty())
// reinsert conditional branch only, for now
TII->InsertBranch(*PBB, (TBB == IBB) ? FBB : TBB, nullptr,
NewCond, dl);
}
MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(*PBB), PBB));
}
}
// If this is a large problem, avoid visiting the same basic blocks multiple
// times.
if (MergePotentials.size() == TailMergeThreshold)
for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
TriedMerging.insert(MergePotentials[i].getBlock());
if (MergePotentials.size() >= 2)
MadeChange |= TryTailMergeBlocks(IBB, PredBB);
// Reinsert an unconditional branch if needed. The 1 below can occur as a
// result of removing blocks in TryTailMergeBlocks.
PredBB = &*std::prev(I); // this may have been changed in TryTailMergeBlocks
if (MergePotentials.size() == 1 &&
MergePotentials.begin()->getBlock() != PredBB)
FixTail(MergePotentials.begin()->getBlock(), IBB, TII);
}
return MadeChange;
}
void BranchFolder::setCommonTailEdgeWeights(MachineBasicBlock &TailMBB) {
SmallVector<BlockFrequency, 2> EdgeFreqLs(TailMBB.succ_size());
BlockFrequency AccumulatedMBBFreq;
// Aggregate edge frequency of successor edge j:
// edgeFreq(j) = sum (freq(bb) * edgeProb(bb, j)),
// where bb is a basic block that is in SameTails.
for (const auto &Src : SameTails) {
const MachineBasicBlock *SrcMBB = Src.getBlock();
BlockFrequency BlockFreq = MBBFreqInfo.getBlockFreq(SrcMBB);
AccumulatedMBBFreq += BlockFreq;
// It is not necessary to recompute edge weights if TailBB has less than two
// successors.
if (TailMBB.succ_size() <= 1)
continue;
auto EdgeFreq = EdgeFreqLs.begin();
for (auto SuccI = TailMBB.succ_begin(), SuccE = TailMBB.succ_end();
SuccI != SuccE; ++SuccI, ++EdgeFreq)
*EdgeFreq += BlockFreq * MBPI.getEdgeProbability(SrcMBB, *SuccI);
}
MBBFreqInfo.setBlockFreq(&TailMBB, AccumulatedMBBFreq);
if (TailMBB.succ_size() <= 1)
return;
auto SumEdgeFreq =
std::accumulate(EdgeFreqLs.begin(), EdgeFreqLs.end(), BlockFrequency(0))
.getFrequency();
auto EdgeFreq = EdgeFreqLs.begin();
if (SumEdgeFreq > 0) {
for (auto SuccI = TailMBB.succ_begin(), SuccE = TailMBB.succ_end();
SuccI != SuccE; ++SuccI, ++EdgeFreq) {
auto Prob = BranchProbability::getBranchProbability(
EdgeFreq->getFrequency(), SumEdgeFreq);
TailMBB.setSuccProbability(SuccI, Prob);
}
}
}
//===----------------------------------------------------------------------===//
// Branch Optimization
//===----------------------------------------------------------------------===//
bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
bool MadeChange = false;
// Make sure blocks are numbered in order
MF.RenumberBlocks();
// Renumbering blocks alters funclet membership, recalculate it.
FuncletMembership = getFuncletMembership(MF);
for (MachineFunction::iterator I = std::next(MF.begin()), E = MF.end();
I != E; ) {
MachineBasicBlock *MBB = &*I++;
MadeChange |= OptimizeBlock(MBB);
// If it is dead, remove it.
if (MBB->pred_empty()) {
RemoveDeadBlock(MBB);
MadeChange = true;
++NumDeadBlocks;
}
}
return MadeChange;
}
// Blocks should be considered empty if they contain only debug info;
// else the debug info would affect codegen.
static bool IsEmptyBlock(MachineBasicBlock *MBB) {
return MBB->getFirstNonDebugInstr() == MBB->end();
}
// Blocks with only debug info and branches should be considered the same
// as blocks with only branches.
static bool IsBranchOnlyBlock(MachineBasicBlock *MBB) {
MachineBasicBlock::iterator I = MBB->getFirstNonDebugInstr();
assert(I != MBB->end() && "empty block!");
return I->isBranch();
}
/// IsBetterFallthrough - Return true if it would be clearly better to
/// fall-through to MBB1 than to fall through into MBB2. This has to return
/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will
/// result in infinite loops.
static bool IsBetterFallthrough(MachineBasicBlock *MBB1,
MachineBasicBlock *MBB2) {
// Right now, we use a simple heuristic. If MBB2 ends with a call, and
// MBB1 doesn't, we prefer to fall through into MBB1. This allows us to
// optimize branches that branch to either a return block or an assert block
// into a fallthrough to the return.
MachineBasicBlock::iterator MBB1I = MBB1->getLastNonDebugInstr();
MachineBasicBlock::iterator MBB2I = MBB2->getLastNonDebugInstr();
if (MBB1I == MBB1->end() || MBB2I == MBB2->end())
return false;
// If there is a clear successor ordering we make sure that one block
// will fall through to the next
if (MBB1->isSuccessor(MBB2)) return true;
if (MBB2->isSuccessor(MBB1)) return false;
return MBB2I->isCall() && !MBB1I->isCall();
}
/// getBranchDebugLoc - Find and return, if any, the DebugLoc of the branch
/// instructions on the block.
static DebugLoc getBranchDebugLoc(MachineBasicBlock &MBB) {
MachineBasicBlock::iterator I = MBB.getLastNonDebugInstr();
if (I != MBB.end() && I->isBranch())
return I->getDebugLoc();
return DebugLoc();
}
/// OptimizeBlock - Analyze and optimize control flow related to the specified
/// block. This is never called on the entry block.
bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
bool MadeChange = false;
MachineFunction &MF = *MBB->getParent();
ReoptimizeBlock:
MachineFunction::iterator FallThrough = MBB->getIterator();
++FallThrough;
// Make sure MBB and FallThrough belong to the same funclet.
bool SameFunclet = true;
if (!FuncletMembership.empty() && FallThrough != MF.end()) {
auto MBBFunclet = FuncletMembership.find(MBB);
assert(MBBFunclet != FuncletMembership.end());
auto FallThroughFunclet = FuncletMembership.find(&*FallThrough);
assert(FallThroughFunclet != FuncletMembership.end());
SameFunclet = MBBFunclet->second == FallThroughFunclet->second;
}
// If this block is empty, make everyone use its fall-through, not the block
// explicitly. Landing pads should not do this since the landing-pad table
// points to this block. Blocks with their addresses taken shouldn't be
// optimized away.
if (IsEmptyBlock(MBB) && !MBB->isEHPad() && !MBB->hasAddressTaken() &&
SameFunclet) {
// Dead block? Leave for cleanup later.
if (MBB->pred_empty()) return MadeChange;
if (FallThrough == MF.end()) {
// TODO: Simplify preds to not branch here if possible!
} else if (FallThrough->isEHPad()) {
// Don't rewrite to a landing pad fallthough. That could lead to the case
// where a BB jumps to more than one landing pad.
// TODO: Is it ever worth rewriting predecessors which don't already
// jump to a landing pad, and so can safely jump to the fallthrough?
} else {
// Rewrite all predecessors of the old block to go to the fallthrough
// instead.
while (!MBB->pred_empty()) {
MachineBasicBlock *Pred = *(MBB->pred_end()-1);
Pred->ReplaceUsesOfBlockWith(MBB, &*FallThrough);
}
// If MBB was the target of a jump table, update jump tables to go to the
// fallthrough instead.
if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
MJTI->ReplaceMBBInJumpTables(MBB, &*FallThrough);
MadeChange = true;
}
return MadeChange;
}
// Check to see if we can simplify the terminator of the block before this
// one.
MachineBasicBlock &PrevBB = *std::prev(MachineFunction::iterator(MBB));
MachineBasicBlock *PriorTBB = nullptr, *PriorFBB = nullptr;
SmallVector<MachineOperand, 4> PriorCond;
bool PriorUnAnalyzable =
TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
if (!PriorUnAnalyzable) {
// If the CFG for the prior block has extra edges, remove them.
MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB,
!PriorCond.empty());
// If the previous branch is conditional and both conditions go to the same
// destination, remove the branch, replacing it with an unconditional one or
// a fall-through.
if (PriorTBB && PriorTBB == PriorFBB) {
DebugLoc dl = getBranchDebugLoc(PrevBB);
TII->RemoveBranch(PrevBB);
PriorCond.clear();
if (PriorTBB != MBB)
TII->InsertBranch(PrevBB, PriorTBB, nullptr, PriorCond, dl);
MadeChange = true;
++NumBranchOpts;
goto ReoptimizeBlock;
}
// If the previous block unconditionally falls through to this block and
// this block has no other predecessors, move the contents of this block
// into the prior block. This doesn't usually happen when SimplifyCFG
// has been used, but it can happen if tail merging splits a fall-through
// predecessor of a block.
// This has to check PrevBB->succ_size() because EH edges are ignored by
// AnalyzeBranch.
if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 &&
PrevBB.succ_size() == 1 &&
!MBB->hasAddressTaken() && !MBB->isEHPad()) {
DEBUG(dbgs() << "\nMerging into block: " << PrevBB
<< "From MBB: " << *MBB);
// Remove redundant DBG_VALUEs first.
if (PrevBB.begin() != PrevBB.end()) {
MachineBasicBlock::iterator PrevBBIter = PrevBB.end();
--PrevBBIter;
MachineBasicBlock::iterator MBBIter = MBB->begin();
// Check if DBG_VALUE at the end of PrevBB is identical to the
// DBG_VALUE at the beginning of MBB.
while (PrevBBIter != PrevBB.begin() && MBBIter != MBB->end()
&& PrevBBIter->isDebugValue() && MBBIter->isDebugValue()) {
if (!MBBIter->isIdenticalTo(*PrevBBIter))
break;
MachineInstr &DuplicateDbg = *MBBIter;
++MBBIter; -- PrevBBIter;
DuplicateDbg.eraseFromParent();
}
}
PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end());
PrevBB.removeSuccessor(PrevBB.succ_begin());
assert(PrevBB.succ_empty());
PrevBB.transferSuccessors(MBB);
MadeChange = true;
return MadeChange;
}
// If the previous branch *only* branches to *this* block (conditional or
// not) remove the branch.
if (PriorTBB == MBB && !PriorFBB) {
TII->RemoveBranch(PrevBB);
MadeChange = true;
++NumBranchOpts;
goto ReoptimizeBlock;
}
// If the prior block branches somewhere else on the condition and here if
// the condition is false, remove the uncond second branch.
if (PriorFBB == MBB) {
DebugLoc dl = getBranchDebugLoc(PrevBB);
TII->RemoveBranch(PrevBB);
TII->InsertBranch(PrevBB, PriorTBB, nullptr, PriorCond, dl);
MadeChange = true;
++NumBranchOpts;
goto ReoptimizeBlock;
}
// If the prior block branches here on true and somewhere else on false, and
// if the branch condition is reversible, reverse the branch to create a
// fall-through.
if (PriorTBB == MBB) {
SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
if (!TII->ReverseBranchCondition(NewPriorCond)) {
DebugLoc dl = getBranchDebugLoc(PrevBB);
TII->RemoveBranch(PrevBB);
TII->InsertBranch(PrevBB, PriorFBB, nullptr, NewPriorCond, dl);
MadeChange = true;
++NumBranchOpts;
goto ReoptimizeBlock;
}
}
// If this block has no successors (e.g. it is a return block or ends with
// a call to a no-return function like abort or __cxa_throw) and if the pred
// falls through into this block, and if it would otherwise fall through
// into the block after this, move this block to the end of the function.
//
// We consider it more likely that execution will stay in the function (e.g.
// due to loops) than it is to exit it. This asserts in loops etc, moving
// the assert condition out of the loop body.
if (MBB->succ_empty() && !PriorCond.empty() && !PriorFBB &&
MachineFunction::iterator(PriorTBB) == FallThrough &&
!MBB->canFallThrough()) {
bool DoTransform = true;
// We have to be careful that the succs of PredBB aren't both no-successor
// blocks. If neither have successors and if PredBB is the second from
// last block in the function, we'd just keep swapping the two blocks for
// last. Only do the swap if one is clearly better to fall through than
// the other.
if (FallThrough == --MF.end() &&
!IsBetterFallthrough(PriorTBB, MBB))
DoTransform = false;
if (DoTransform) {
// Reverse the branch so we will fall through on the previous true cond.
SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
if (!TII->ReverseBranchCondition(NewPriorCond)) {
DEBUG(dbgs() << "\nMoving MBB: " << *MBB
<< "To make fallthrough to: " << *PriorTBB << "\n");
DebugLoc dl = getBranchDebugLoc(PrevBB);
TII->RemoveBranch(PrevBB);
TII->InsertBranch(PrevBB, MBB, nullptr, NewPriorCond, dl);
// Move this block to the end of the function.
MBB->moveAfter(&MF.back());
MadeChange = true;
++NumBranchOpts;
return MadeChange;
}
}
}
}
// Analyze the branch in the current block.
MachineBasicBlock *CurTBB = nullptr, *CurFBB = nullptr;
SmallVector<MachineOperand, 4> CurCond;
bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true);
if (!CurUnAnalyzable) {
// If the CFG for the prior block has extra edges, remove them.
MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty());
// If this is a two-way branch, and the FBB branches to this block, reverse
// the condition so the single-basic-block loop is faster. Instead of:
// Loop: xxx; jcc Out; jmp Loop
// we want:
// Loop: xxx; jncc Loop; jmp Out
if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
SmallVector<MachineOperand, 4> NewCond(CurCond);
if (!TII->ReverseBranchCondition(NewCond)) {
DebugLoc dl = getBranchDebugLoc(*MBB);
TII->RemoveBranch(*MBB);
TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond, dl);
MadeChange = true;
++NumBranchOpts;
goto ReoptimizeBlock;
}
}
// If this branch is the only thing in its block, see if we can forward
// other blocks across it.
if (CurTBB && CurCond.empty() && !CurFBB &&
IsBranchOnlyBlock(MBB) && CurTBB != MBB &&
!MBB->hasAddressTaken() && !MBB->isEHPad()) {
DebugLoc dl = getBranchDebugLoc(*MBB);
// This block may contain just an unconditional branch. Because there can
// be 'non-branch terminators' in the block, try removing the branch and
// then seeing if the block is empty.
TII->RemoveBranch(*MBB);
// If the only things remaining in the block are debug info, remove these
// as well, so this will behave the same as an empty block in non-debug
// mode.
if (IsEmptyBlock(MBB)) {
// Make the block empty, losing the debug info (we could probably
// improve this in some cases.)
MBB->erase(MBB->begin(), MBB->end());
}
// If this block is just an unconditional branch to CurTBB, we can
// usually completely eliminate the block. The only case we cannot
// completely eliminate the block is when the block before this one
// falls through into MBB and we can't understand the prior block's branch
// condition.
if (MBB->empty()) {
bool PredHasNoFallThrough = !PrevBB.canFallThrough();
if (PredHasNoFallThrough || !PriorUnAnalyzable ||
!PrevBB.isSuccessor(MBB)) {
// If the prior block falls through into us, turn it into an
// explicit branch to us to make updates simpler.
if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) &&
PriorTBB != MBB && PriorFBB != MBB) {
if (!PriorTBB) {
assert(PriorCond.empty() && !PriorFBB &&
"Bad branch analysis");
PriorTBB = MBB;
} else {
assert(!PriorFBB && "Machine CFG out of date!");
PriorFBB = MBB;
}
DebugLoc pdl = getBranchDebugLoc(PrevBB);
TII->RemoveBranch(PrevBB);
TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, pdl);
}
// Iterate through all the predecessors, revectoring each in-turn.
size_t PI = 0;
bool DidChange = false;
bool HasBranchToSelf = false;
while(PI != MBB->pred_size()) {
MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI);
if (PMBB == MBB) {
// If this block has an uncond branch to itself, leave it.
++PI;
HasBranchToSelf = true;
} else {
DidChange = true;
PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
// If this change resulted in PMBB ending in a conditional
// branch where both conditions go to the same destination,
// change this to an unconditional branch (and fix the CFG).
MachineBasicBlock *NewCurTBB = nullptr, *NewCurFBB = nullptr;
SmallVector<MachineOperand, 4> NewCurCond;
bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB,
NewCurFBB, NewCurCond, true);
if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) {
DebugLoc pdl = getBranchDebugLoc(*PMBB);
TII->RemoveBranch(*PMBB);
NewCurCond.clear();
TII->InsertBranch(*PMBB, NewCurTBB, nullptr, NewCurCond, pdl);
MadeChange = true;
++NumBranchOpts;
PMBB->CorrectExtraCFGEdges(NewCurTBB, nullptr, false);
}
}
}
// Change any jumptables to go to the new MBB.
if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
MJTI->ReplaceMBBInJumpTables(MBB, CurTBB);
if (DidChange) {
++NumBranchOpts;
MadeChange = true;
if (!HasBranchToSelf) return MadeChange;
}
}
}
// Add the branch back if the block is more than just an uncond branch.
TII->InsertBranch(*MBB, CurTBB, nullptr, CurCond, dl);
}
}
// If the prior block doesn't fall through into this block, and if this
// block doesn't fall through into some other block, see if we can find a
// place to move this block where a fall-through will happen.
if (!PrevBB.canFallThrough()) {
// Now we know that there was no fall-through into this block, check to
// see if it has a fall-through into its successor.
bool CurFallsThru = MBB->canFallThrough();
if (!MBB->isEHPad()) {
// Check all the predecessors of this block. If one of them has no fall
// throughs, move this block right after it.
for (MachineBasicBlock *PredBB : MBB->predecessors()) {
// Analyze the branch at the end of the pred.
MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
SmallVector<MachineOperand, 4> PredCond;
if (PredBB != MBB && !PredBB->canFallThrough() &&
!TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true)
&& (!CurFallsThru || !CurTBB || !CurFBB)
&& (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
// If the current block doesn't fall through, just move it.
// If the current block can fall through and does not end with a
// conditional branch, we need to append an unconditional jump to
// the (current) next block. To avoid a possible compile-time
// infinite loop, move blocks only backward in this case.
// Also, if there are already 2 branches here, we cannot add a third;
// this means we have the case
// Bcc next
// B elsewhere
// next:
if (CurFallsThru) {
MachineBasicBlock *NextBB = &*std::next(MBB->getIterator());
CurCond.clear();
TII->InsertBranch(*MBB, NextBB, nullptr, CurCond, DebugLoc());
}
MBB->moveAfter(PredBB);
MadeChange = true;
goto ReoptimizeBlock;
}
}
}
if (!CurFallsThru) {
// Check all successors to see if we can move this block before it.
for (MachineBasicBlock *SuccBB : MBB->successors()) {
// Analyze the branch at the end of the block before the succ.
MachineFunction::iterator SuccPrev = --SuccBB->getIterator();
// If this block doesn't already fall-through to that successor, and if
// the succ doesn't already have a block that can fall through into it,
// and if the successor isn't an EH destination, we can arrange for the
// fallthrough to happen.
if (SuccBB != MBB && &*SuccPrev != MBB &&
!SuccPrev->canFallThrough() && !CurUnAnalyzable &&
!SuccBB->isEHPad()) {
MBB->moveBefore(SuccBB);
MadeChange = true;
goto ReoptimizeBlock;
}
}
// Okay, there is no really great place to put this block. If, however,
// the block before this one would be a fall-through if this block were
// removed, move this block to the end of the function.
MachineBasicBlock *PrevTBB = nullptr, *PrevFBB = nullptr;
SmallVector<MachineOperand, 4> PrevCond;
// We're looking for cases where PrevBB could possibly fall through to
// FallThrough, but if FallThrough is an EH pad that wouldn't be useful
// so here we skip over any EH pads so we might have a chance to find
// a branch target from PrevBB.
while (FallThrough != MF.end() && FallThrough->isEHPad())
++FallThrough;
// Now check to see if the current block is sitting between PrevBB and
// a block to which it could fall through.
if (FallThrough != MF.end() &&
!TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
PrevBB.isSuccessor(&*FallThrough)) {
MBB->moveAfter(&MF.back());
MadeChange = true;
return MadeChange;
}
}
}
return MadeChange;
}
//===----------------------------------------------------------------------===//
// Hoist Common Code
//===----------------------------------------------------------------------===//
/// HoistCommonCode - Hoist common instruction sequences at the start of basic
/// blocks to their common predecessor.
bool BranchFolder::HoistCommonCode(MachineFunction &MF) {
bool MadeChange = false;
for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ) {
MachineBasicBlock *MBB = &*I++;
MadeChange |= HoistCommonCodeInSuccs(MBB);
}
return MadeChange;
}
/// findFalseBlock - BB has a fallthrough. Find its 'false' successor given
/// its 'true' successor.
static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
MachineBasicBlock *TrueBB) {
for (MachineBasicBlock *SuccBB : BB->successors())
if (SuccBB != TrueBB)
return SuccBB;
return nullptr;
}
template <class Container>
static void addRegAndItsAliases(unsigned Reg, const TargetRegisterInfo *TRI,
Container &Set) {
if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
Set.insert(*AI);
} else {
Set.insert(Reg);
}
}
/// findHoistingInsertPosAndDeps - Find the location to move common instructions
/// in successors to. The location is usually just before the terminator,
/// however if the terminator is a conditional branch and its previous
/// instruction is the flag setting instruction, the previous instruction is
/// the preferred location. This function also gathers uses and defs of the
/// instructions from the insertion point to the end of the block. The data is
/// used by HoistCommonCodeInSuccs to ensure safety.
static
MachineBasicBlock::iterator findHoistingInsertPosAndDeps(MachineBasicBlock *MBB,
const TargetInstrInfo *TII,
const TargetRegisterInfo *TRI,
SmallSet<unsigned,4> &Uses,
SmallSet<unsigned,4> &Defs) {
MachineBasicBlock::iterator Loc = MBB->getFirstTerminator();
if (!TII->isUnpredicatedTerminator(*Loc))
return MBB->end();
for (const MachineOperand &MO : Loc->operands()) {
if (!MO.isReg())
continue;
unsigned Reg = MO.getReg();
if (!Reg)
continue;
if (MO.isUse()) {
addRegAndItsAliases(Reg, TRI, Uses);
} else {
if (!MO.isDead())
// Don't try to hoist code in the rare case the terminator defines a
// register that is later used.
return MBB->end();
// If the terminator defines a register, make sure we don't hoist
// the instruction whose def might be clobbered by the terminator.
addRegAndItsAliases(Reg, TRI, Defs);
}
}
if (Uses.empty())
return Loc;
if (Loc == MBB->begin())
return MBB->end();
// The terminator is probably a conditional branch, try not to separate the
// branch from condition setting instruction.
MachineBasicBlock::iterator PI = Loc;
--PI;
while (PI != MBB->begin() && PI->isDebugValue())
--PI;
bool IsDef = false;
for (const MachineOperand &MO : PI->operands()) {
// If PI has a regmask operand, it is probably a call. Separate away.
if (MO.isRegMask())
return Loc;
if (!MO.isReg() || MO.isUse())
continue;
unsigned Reg = MO.getReg();
if (!Reg)
continue;
if (Uses.count(Reg)) {
IsDef = true;
break;
}
}
if (!IsDef)
// The condition setting instruction is not just before the conditional
// branch.
return Loc;
// Be conservative, don't insert instruction above something that may have
// side-effects. And since it's potentially bad to separate flag setting
// instruction from the conditional branch, just abort the optimization
// completely.
// Also avoid moving code above predicated instruction since it's hard to
// reason about register liveness with predicated instruction.
bool DontMoveAcrossStore = true;
if (!PI->isSafeToMove(nullptr, DontMoveAcrossStore) || TII->isPredicated(*PI))
return MBB->end();
// Find out what registers are live. Note this routine is ignoring other live
// registers which are only used by instructions in successor blocks.
for (const MachineOperand &MO : PI->operands()) {
if (!MO.isReg())
continue;
unsigned Reg = MO.getReg();
if (!Reg)
continue;
if (MO.isUse()) {
addRegAndItsAliases(Reg, TRI, Uses);
} else {
if (Uses.erase(Reg)) {
if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
for (MCSubRegIterator SubRegs(Reg, TRI); SubRegs.isValid(); ++SubRegs)
Uses.erase(*SubRegs); // Use sub-registers to be conservative
}
}
addRegAndItsAliases(Reg, TRI, Defs);
}
}
return PI;
}
/// HoistCommonCodeInSuccs - If the successors of MBB has common instruction
/// sequence at the start of the function, move the instructions before MBB
/// terminator if it's legal.
bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
SmallVector<MachineOperand, 4> Cond;
if (TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true) || !TBB || Cond.empty())
return false;
if (!FBB) FBB = findFalseBlock(MBB, TBB);
if (!FBB)
// Malformed bcc? True and false blocks are the same?
return false;
// Restrict the optimization to cases where MBB is the only predecessor,
// it is an obvious win.
if (TBB->pred_size() > 1 || FBB->pred_size() > 1)
return false;
// Find a suitable position to hoist the common instructions to. Also figure
// out which registers are used or defined by instructions from the insertion
// point to the end of the block.
SmallSet<unsigned, 4> Uses, Defs;
MachineBasicBlock::iterator Loc =
findHoistingInsertPosAndDeps(MBB, TII, TRI, Uses, Defs);
if (Loc == MBB->end())
return false;
bool HasDups = false;
SmallVector<unsigned, 4> LocalDefs;
SmallSet<unsigned, 4> LocalDefsSet;
MachineBasicBlock::iterator TIB = TBB->begin();
MachineBasicBlock::iterator FIB = FBB->begin();
MachineBasicBlock::iterator TIE = TBB->end();
MachineBasicBlock::iterator FIE = FBB->end();
while (TIB != TIE && FIB != FIE) {
// Skip dbg_value instructions. These do not count.
if (TIB->isDebugValue()) {
while (TIB != TIE && TIB->isDebugValue())
++TIB;
if (TIB == TIE)
break;
}
if (FIB->isDebugValue()) {
while (FIB != FIE && FIB->isDebugValue())
++FIB;
if (FIB == FIE)
break;
}
if (!TIB->isIdenticalTo(*FIB, MachineInstr::CheckKillDead))
break;
if (TII->isPredicated(*TIB))
// Hard to reason about register liveness with predicated instruction.
break;
bool IsSafe = true;
for (MachineOperand &MO : TIB->operands()) {
// Don't attempt to hoist instructions with register masks.
if (MO.isRegMask()) {
IsSafe = false;
break;
}
if (!MO.isReg())
continue;
unsigned Reg = MO.getReg();
if (!Reg)
continue;
if (MO.isDef()) {
if (Uses.count(Reg)) {
// Avoid clobbering a register that's used by the instruction at
// the point of insertion.
IsSafe = false;
break;
}
if (Defs.count(Reg) && !MO.isDead()) {
// Don't hoist the instruction if the def would be clobber by the
// instruction at the point insertion. FIXME: This is overly
// conservative. It should be possible to hoist the instructions
// in BB2 in the following example:
// BB1:
// r1, eflag = op1 r2, r3
// brcc eflag
//
// BB2:
// r1 = op2, ...
// = op3, r1<kill>
IsSafe = false;
break;
}
} else if (!LocalDefsSet.count(Reg)) {
if (Defs.count(Reg)) {
// Use is defined by the instruction at the point of insertion.
IsSafe = false;
break;
}
if (MO.isKill() && Uses.count(Reg))
// Kills a register that's read by the instruction at the point of
// insertion. Remove the kill marker.
MO.setIsKill(false);
}
}
if (!IsSafe)
break;
bool DontMoveAcrossStore = true;
if (!TIB->isSafeToMove(nullptr, DontMoveAcrossStore))
break;
// Remove kills from LocalDefsSet, these registers had short live ranges.
for (const MachineOperand &MO : TIB->operands()) {
if (!MO.isReg() || !MO.isUse() || !MO.isKill())
continue;
unsigned Reg = MO.getReg();
if (!Reg || !LocalDefsSet.count(Reg))
continue;
if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
LocalDefsSet.erase(*AI);
} else {
LocalDefsSet.erase(Reg);
}
}
// Track local defs so we can update liveins.
for (const MachineOperand &MO : TIB->operands()) {
if (!MO.isReg() || !MO.isDef() || MO.isDead())
continue;
unsigned Reg = MO.getReg();
if (!Reg || TargetRegisterInfo::isVirtualRegister(Reg))
continue;
LocalDefs.push_back(Reg);
addRegAndItsAliases(Reg, TRI, LocalDefsSet);
}
HasDups = true;
++TIB;
++FIB;
}
if (!HasDups)
return false;
MBB->splice(Loc, TBB, TBB->begin(), TIB);
FBB->erase(FBB->begin(), FIB);
// Update livein's.
for (unsigned i = 0, e = LocalDefs.size(); i != e; ++i) {
unsigned Def = LocalDefs[i];
if (LocalDefsSet.count(Def)) {
TBB->addLiveIn(Def);
FBB->addLiveIn(Def);
}
}
++NumHoist;
return true;
}
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