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llvm/lib/CodeGen/TailDuplicator.cpp

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//===- TailDuplicator.cpp - Duplicate blocks into predecessors' tails -----===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This utility class duplicates basic blocks ending in unconditional branches
// into the tails of their predecessors.
//
//===----------------------------------------------------------------------===//
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/MachineSSAUpdater.h"
#include "llvm/CodeGen/TailDuplicator.h"
#include "llvm/IR/DebugLoc.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>
#include <cassert>
#include <iterator>
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "tailduplication"
STATISTIC(NumTails, "Number of tails duplicated");
STATISTIC(NumTailDups, "Number of tail duplicated blocks");
STATISTIC(NumTailDupAdded,
"Number of instructions added due to tail duplication");
STATISTIC(NumTailDupRemoved,
"Number of instructions removed due to tail duplication");
STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
STATISTIC(NumAddedPHIs, "Number of phis added");
// Heuristic for tail duplication.
static cl::opt<unsigned> TailDuplicateSize(
"tail-dup-size",
cl::desc("Maximum instructions to consider tail duplicating"), cl::init(2),
cl::Hidden);
static cl::opt<unsigned> TailDupIndirectBranchSize(
"tail-dup-indirect-size",
cl::desc("Maximum instructions to consider tail duplicating blocks that "
"end with indirect branches."), cl::init(20),
cl::Hidden);
static cl::opt<bool>
TailDupVerify("tail-dup-verify",
cl::desc("Verify sanity of PHI instructions during taildup"),
cl::init(false), cl::Hidden);
static cl::opt<unsigned> TailDupLimit("tail-dup-limit", cl::init(~0U),
cl::Hidden);
void TailDuplicator::initMF(MachineFunction &MFin,
const MachineBranchProbabilityInfo *MBPIin,
bool LayoutModeIn, unsigned TailDupSizeIn) {
MF = &MFin;
TII = MF->getSubtarget().getInstrInfo();
TRI = MF->getSubtarget().getRegisterInfo();
MRI = &MF->getRegInfo();
MMI = &MF->getMMI();
MBPI = MBPIin;
TailDupSize = TailDupSizeIn;
assert(MBPI != nullptr && "Machine Branch Probability Info required");
LayoutMode = LayoutModeIn;
PreRegAlloc = MRI->isSSA();
}
static void VerifyPHIs(MachineFunction &MF, bool CheckExtra) {
for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ++I) {
MachineBasicBlock *MBB = &*I;
SmallSetVector<MachineBasicBlock *, 8> Preds(MBB->pred_begin(),
MBB->pred_end());
MachineBasicBlock::iterator MI = MBB->begin();
while (MI != MBB->end()) {
if (!MI->isPHI())
break;
for (MachineBasicBlock *PredBB : Preds) {
bool Found = false;
for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
MachineBasicBlock *PHIBB = MI->getOperand(i + 1).getMBB();
if (PHIBB == PredBB) {
Found = true;
break;
}
}
if (!Found) {
dbgs() << "Malformed PHI in BB#" << MBB->getNumber() << ": " << *MI;
dbgs() << " missing input from predecessor BB#"
<< PredBB->getNumber() << '\n';
llvm_unreachable(nullptr);
}
}
for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
MachineBasicBlock *PHIBB = MI->getOperand(i + 1).getMBB();
if (CheckExtra && !Preds.count(PHIBB)) {
dbgs() << "Warning: malformed PHI in BB#" << MBB->getNumber() << ": "
<< *MI;
dbgs() << " extra input from predecessor BB#" << PHIBB->getNumber()
<< '\n';
llvm_unreachable(nullptr);
}
if (PHIBB->getNumber() < 0) {
dbgs() << "Malformed PHI in BB#" << MBB->getNumber() << ": " << *MI;
dbgs() << " non-existing BB#" << PHIBB->getNumber() << '\n';
llvm_unreachable(nullptr);
}
}
++MI;
}
}
}
/// Tail duplicate the block and cleanup.
/// \p IsSimple - return value of isSimpleBB
/// \p MBB - block to be duplicated
/// \p ForcedLayoutPred - If non-null, treat this block as the layout
/// predecessor, instead of using the ordering in MF
/// \p DuplicatedPreds - if non-null, \p DuplicatedPreds will contain a list of
/// all Preds that received a copy of \p MBB.
/// \p RemovalCallback - if non-null, called just before MBB is deleted.
bool TailDuplicator::tailDuplicateAndUpdate(
bool IsSimple, MachineBasicBlock *MBB,
MachineBasicBlock *ForcedLayoutPred,
SmallVectorImpl<MachineBasicBlock*> *DuplicatedPreds,
function_ref<void(MachineBasicBlock *)> *RemovalCallback) {
// Save the successors list.
SmallSetVector<MachineBasicBlock *, 8> Succs(MBB->succ_begin(),
MBB->succ_end());
SmallVector<MachineBasicBlock *, 8> TDBBs;
SmallVector<MachineInstr *, 16> Copies;
if (!tailDuplicate(IsSimple, MBB, ForcedLayoutPred, TDBBs, Copies))
return false;
++NumTails;
SmallVector<MachineInstr *, 8> NewPHIs;
MachineSSAUpdater SSAUpdate(*MF, &NewPHIs);
// TailBB's immediate successors are now successors of those predecessors
// which duplicated TailBB. Add the predecessors as sources to the PHI
// instructions.
bool isDead = MBB->pred_empty() && !MBB->hasAddressTaken();
if (PreRegAlloc)
updateSuccessorsPHIs(MBB, isDead, TDBBs, Succs);
// If it is dead, remove it.
if (isDead) {
NumTailDupRemoved += MBB->size();
removeDeadBlock(MBB, RemovalCallback);
++NumDeadBlocks;
}
// Update SSA form.
if (!SSAUpdateVRs.empty()) {
for (unsigned i = 0, e = SSAUpdateVRs.size(); i != e; ++i) {
unsigned VReg = SSAUpdateVRs[i];
SSAUpdate.Initialize(VReg);
// If the original definition is still around, add it as an available
// value.
MachineInstr *DefMI = MRI->getVRegDef(VReg);
MachineBasicBlock *DefBB = nullptr;
if (DefMI) {
DefBB = DefMI->getParent();
SSAUpdate.AddAvailableValue(DefBB, VReg);
}
// Add the new vregs as available values.
DenseMap<unsigned, AvailableValsTy>::iterator LI =
SSAUpdateVals.find(VReg);
for (unsigned j = 0, ee = LI->second.size(); j != ee; ++j) {
MachineBasicBlock *SrcBB = LI->second[j].first;
unsigned SrcReg = LI->second[j].second;
SSAUpdate.AddAvailableValue(SrcBB, SrcReg);
}
// Rewrite uses that are outside of the original def's block.
MachineRegisterInfo::use_iterator UI = MRI->use_begin(VReg);
while (UI != MRI->use_end()) {
MachineOperand &UseMO = *UI;
MachineInstr *UseMI = UseMO.getParent();
++UI;
if (UseMI->isDebugValue()) {
// SSAUpdate can replace the use with an undef. That creates
// a debug instruction that is a kill.
// FIXME: Should it SSAUpdate job to delete debug instructions
// instead of replacing the use with undef?
UseMI->eraseFromParent();
continue;
}
if (UseMI->getParent() == DefBB && !UseMI->isPHI())
continue;
SSAUpdate.RewriteUse(UseMO);
}
}
SSAUpdateVRs.clear();
SSAUpdateVals.clear();
}
// Eliminate some of the copies inserted by tail duplication to maintain
// SSA form.
for (unsigned i = 0, e = Copies.size(); i != e; ++i) {
MachineInstr *Copy = Copies[i];
if (!Copy->isCopy())
continue;
unsigned Dst = Copy->getOperand(0).getReg();
unsigned Src = Copy->getOperand(1).getReg();
if (MRI->hasOneNonDBGUse(Src) &&
MRI->constrainRegClass(Src, MRI->getRegClass(Dst))) {
// Copy is the only use. Do trivial copy propagation here.
MRI->replaceRegWith(Dst, Src);
Copy->eraseFromParent();
}
}
if (NewPHIs.size())
NumAddedPHIs += NewPHIs.size();
if (DuplicatedPreds)
*DuplicatedPreds = std::move(TDBBs);
return true;
}
/// Look for small blocks that are unconditionally branched to and do not fall
/// through. Tail-duplicate their instructions into their predecessors to
/// eliminate (dynamic) branches.
bool TailDuplicator::tailDuplicateBlocks() {
bool MadeChange = false;
if (PreRegAlloc && TailDupVerify) {
DEBUG(dbgs() << "\n*** Before tail-duplicating\n");
VerifyPHIs(*MF, true);
}
for (MachineFunction::iterator I = ++MF->begin(), E = MF->end(); I != E;) {
MachineBasicBlock *MBB = &*I++;
if (NumTails == TailDupLimit)
break;
bool IsSimple = isSimpleBB(MBB);
if (!shouldTailDuplicate(IsSimple, *MBB))
continue;
MadeChange |= tailDuplicateAndUpdate(IsSimple, MBB, nullptr);
}
if (PreRegAlloc && TailDupVerify)
VerifyPHIs(*MF, false);
return MadeChange;
}
static bool isDefLiveOut(unsigned Reg, MachineBasicBlock *BB,
const MachineRegisterInfo *MRI) {
for (MachineInstr &UseMI : MRI->use_instructions(Reg)) {
if (UseMI.isDebugValue())
continue;
if (UseMI.getParent() != BB)
return true;
}
return false;
}
static unsigned getPHISrcRegOpIdx(MachineInstr *MI, MachineBasicBlock *SrcBB) {
for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2)
if (MI->getOperand(i + 1).getMBB() == SrcBB)
return i;
return 0;
}
// Remember which registers are used by phis in this block. This is
// used to determine which registers are liveout while modifying the
// block (which is why we need to copy the information).
static void getRegsUsedByPHIs(const MachineBasicBlock &BB,
DenseSet<unsigned> *UsedByPhi) {
for (const auto &MI : BB) {
if (!MI.isPHI())
break;
for (unsigned i = 1, e = MI.getNumOperands(); i != e; i += 2) {
unsigned SrcReg = MI.getOperand(i).getReg();
UsedByPhi->insert(SrcReg);
}
}
}
/// Add a definition and source virtual registers pair for SSA update.
void TailDuplicator::addSSAUpdateEntry(unsigned OrigReg, unsigned NewReg,
MachineBasicBlock *BB) {
DenseMap<unsigned, AvailableValsTy>::iterator LI =
SSAUpdateVals.find(OrigReg);
if (LI != SSAUpdateVals.end())
LI->second.push_back(std::make_pair(BB, NewReg));
else {
AvailableValsTy Vals;
Vals.push_back(std::make_pair(BB, NewReg));
SSAUpdateVals.insert(std::make_pair(OrigReg, Vals));
SSAUpdateVRs.push_back(OrigReg);
}
}
/// Process PHI node in TailBB by turning it into a copy in PredBB. Remember the
/// source register that's contributed by PredBB and update SSA update map.
void TailDuplicator::processPHI(
MachineInstr *MI, MachineBasicBlock *TailBB, MachineBasicBlock *PredBB,
DenseMap<unsigned, RegSubRegPair> &LocalVRMap,
SmallVectorImpl<std::pair<unsigned, RegSubRegPair>> &Copies,
const DenseSet<unsigned> &RegsUsedByPhi, bool Remove) {
unsigned DefReg = MI->getOperand(0).getReg();
unsigned SrcOpIdx = getPHISrcRegOpIdx(MI, PredBB);
assert(SrcOpIdx && "Unable to find matching PHI source?");
unsigned SrcReg = MI->getOperand(SrcOpIdx).getReg();
unsigned SrcSubReg = MI->getOperand(SrcOpIdx).getSubReg();
const TargetRegisterClass *RC = MRI->getRegClass(DefReg);
LocalVRMap.insert(std::make_pair(DefReg, RegSubRegPair(SrcReg, SrcSubReg)));
// Insert a copy from source to the end of the block. The def register is the
// available value liveout of the block.
unsigned NewDef = MRI->createVirtualRegister(RC);
Copies.push_back(std::make_pair(NewDef, RegSubRegPair(SrcReg, SrcSubReg)));
if (isDefLiveOut(DefReg, TailBB, MRI) || RegsUsedByPhi.count(DefReg))
addSSAUpdateEntry(DefReg, NewDef, PredBB);
if (!Remove)
return;
// Remove PredBB from the PHI node.
MI->RemoveOperand(SrcOpIdx + 1);
MI->RemoveOperand(SrcOpIdx);
if (MI->getNumOperands() == 1)
MI->eraseFromParent();
}
/// Duplicate a TailBB instruction to PredBB and update
/// the source operands due to earlier PHI translation.
void TailDuplicator::duplicateInstruction(
MachineInstr *MI, MachineBasicBlock *TailBB, MachineBasicBlock *PredBB,
DenseMap<unsigned, RegSubRegPair> &LocalVRMap,
const DenseSet<unsigned> &UsedByPhi) {
MachineInstr *NewMI = TII->duplicate(*MI, *MF);
if (PreRegAlloc) {
for (unsigned i = 0, e = NewMI->getNumOperands(); i != e; ++i) {
MachineOperand &MO = NewMI->getOperand(i);
if (!MO.isReg())
continue;
unsigned Reg = MO.getReg();
if (!TargetRegisterInfo::isVirtualRegister(Reg))
continue;
if (MO.isDef()) {
const TargetRegisterClass *RC = MRI->getRegClass(Reg);
unsigned NewReg = MRI->createVirtualRegister(RC);
MO.setReg(NewReg);
LocalVRMap.insert(std::make_pair(Reg, RegSubRegPair(NewReg, 0)));
if (isDefLiveOut(Reg, TailBB, MRI) || UsedByPhi.count(Reg))
addSSAUpdateEntry(Reg, NewReg, PredBB);
} else {
auto VI = LocalVRMap.find(Reg);
if (VI != LocalVRMap.end()) {
// Need to make sure that the register class of the mapped register
// will satisfy the constraints of the class of the register being
// replaced.
auto *OrigRC = MRI->getRegClass(Reg);
auto *MappedRC = MRI->getRegClass(VI->second.Reg);
const TargetRegisterClass *ConstrRC;
if (VI->second.SubReg != 0) {
ConstrRC = TRI->getMatchingSuperRegClass(MappedRC, OrigRC,
VI->second.SubReg);
if (ConstrRC) {
// The actual constraining (as in "find appropriate new class")
// is done by getMatchingSuperRegClass, so now we only need to
// change the class of the mapped register.
MRI->setRegClass(VI->second.Reg, ConstrRC);
}
} else {
// For mapped registers that do not have sub-registers, simply
// restrict their class to match the original one.
ConstrRC = MRI->constrainRegClass(VI->second.Reg, OrigRC);
}
if (ConstrRC) {
// If the class constraining succeeded, we can simply replace
// the old register with the mapped one.
MO.setReg(VI->second.Reg);
// We have Reg -> VI.Reg:VI.SubReg, so if Reg is used with a
// sub-register, we need to compose the sub-register indices.
MO.setSubReg(TRI->composeSubRegIndices(MO.getSubReg(),
VI->second.SubReg));
} else {
// The direct replacement is not possible, due to failing register
// class constraints. An explicit COPY is necessary. Create one
// that can be reused
auto *NewRC = MI->getRegClassConstraint(i, TII, TRI);
if (NewRC == nullptr)
NewRC = OrigRC;
unsigned NewReg = MRI->createVirtualRegister(NewRC);
BuildMI(*PredBB, MI, MI->getDebugLoc(),
TII->get(TargetOpcode::COPY), NewReg)
.addReg(VI->second.Reg, 0, VI->second.SubReg);
LocalVRMap.erase(VI);
LocalVRMap.insert(std::make_pair(Reg, RegSubRegPair(NewReg, 0)));
MO.setReg(NewReg);
// The composed VI.Reg:VI.SubReg is replaced with NewReg, which
// is equivalent to the whole register Reg. Hence, Reg:subreg
// is same as NewReg:subreg, so keep the sub-register index
// unchanged.
}
// Clear any kill flags from this operand. The new register could
// have uses after this one, so kills are not valid here.
MO.setIsKill(false);
}
}
}
}
PredBB->insert(PredBB->instr_end(), NewMI);
}
/// After FromBB is tail duplicated into its predecessor blocks, the successors
/// have gained new predecessors. Update the PHI instructions in them
/// accordingly.
void TailDuplicator::updateSuccessorsPHIs(
MachineBasicBlock *FromBB, bool isDead,
SmallVectorImpl<MachineBasicBlock *> &TDBBs,
SmallSetVector<MachineBasicBlock *, 8> &Succs) {
for (MachineBasicBlock *SuccBB : Succs) {
for (MachineInstr &MI : *SuccBB) {
if (!MI.isPHI())
break;
MachineInstrBuilder MIB(*FromBB->getParent(), MI);
unsigned Idx = 0;
for (unsigned i = 1, e = MI.getNumOperands(); i != e; i += 2) {
MachineOperand &MO = MI.getOperand(i + 1);
if (MO.getMBB() == FromBB) {
Idx = i;
break;
}
}
assert(Idx != 0);
MachineOperand &MO0 = MI.getOperand(Idx);
unsigned Reg = MO0.getReg();
if (isDead) {
// Folded into the previous BB.
// There could be duplicate phi source entries. FIXME: Should sdisel
// or earlier pass fixed this?
for (unsigned i = MI.getNumOperands() - 2; i != Idx; i -= 2) {
MachineOperand &MO = MI.getOperand(i + 1);
if (MO.getMBB() == FromBB) {
MI.RemoveOperand(i + 1);
MI.RemoveOperand(i);
}
}
} else
Idx = 0;
// If Idx is set, the operands at Idx and Idx+1 must be removed.
// We reuse the location to avoid expensive RemoveOperand calls.
DenseMap<unsigned, AvailableValsTy>::iterator LI =
SSAUpdateVals.find(Reg);
if (LI != SSAUpdateVals.end()) {
// This register is defined in the tail block.
for (unsigned j = 0, ee = LI->second.size(); j != ee; ++j) {
MachineBasicBlock *SrcBB = LI->second[j].first;
// If we didn't duplicate a bb into a particular predecessor, we
// might still have added an entry to SSAUpdateVals to correcly
// recompute SSA. If that case, avoid adding a dummy extra argument
// this PHI.
if (!SrcBB->isSuccessor(SuccBB))
continue;
unsigned SrcReg = LI->second[j].second;
if (Idx != 0) {
MI.getOperand(Idx).setReg(SrcReg);
MI.getOperand(Idx + 1).setMBB(SrcBB);
Idx = 0;
} else {
MIB.addReg(SrcReg).addMBB(SrcBB);
}
}
} else {
// Live in tail block, must also be live in predecessors.
for (unsigned j = 0, ee = TDBBs.size(); j != ee; ++j) {
MachineBasicBlock *SrcBB = TDBBs[j];
if (Idx != 0) {
MI.getOperand(Idx).setReg(Reg);
MI.getOperand(Idx + 1).setMBB(SrcBB);
Idx = 0;
} else {
MIB.addReg(Reg).addMBB(SrcBB);
}
}
}
if (Idx != 0) {
MI.RemoveOperand(Idx + 1);
MI.RemoveOperand(Idx);
}
}
}
}
/// Determine if it is profitable to duplicate this block.
bool TailDuplicator::shouldTailDuplicate(bool IsSimple,
MachineBasicBlock &TailBB) {
// When doing tail-duplication during layout, the block ordering is in flux,
// so canFallThrough returns a result based on incorrect information and
// should just be ignored.
if (!LayoutMode && TailBB.canFallThrough())
return false;
// Don't try to tail-duplicate single-block loops.
if (TailBB.isSuccessor(&TailBB))
return false;
// Set the limit on the cost to duplicate. When optimizing for size,
// duplicate only one, because one branch instruction can be eliminated to
// compensate for the duplication.
unsigned MaxDuplicateCount;
if (TailDupSize == 0 &&
TailDuplicateSize.getNumOccurrences() == 0 &&
MF->getFunction()->optForSize())
MaxDuplicateCount = 1;
else if (TailDupSize == 0)
MaxDuplicateCount = TailDuplicateSize;
else
MaxDuplicateCount = TailDupSize;
// If the block to be duplicated ends in an unanalyzable fallthrough, don't
// duplicate it.
// A similar check is necessary in MachineBlockPlacement to make sure pairs of
// blocks with unanalyzable fallthrough get layed out contiguously.
MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
SmallVector<MachineOperand, 4> PredCond;
if (TII->analyzeBranch(TailBB, PredTBB, PredFBB, PredCond) &&
TailBB.canFallThrough())
return false;
// If the target has hardware branch prediction that can handle indirect
// branches, duplicating them can often make them predictable when there
// are common paths through the code. The limit needs to be high enough
// to allow undoing the effects of tail merging and other optimizations
// that rearrange the predecessors of the indirect branch.
bool HasIndirectbr = false;
if (!TailBB.empty())
HasIndirectbr = TailBB.back().isIndirectBranch();
if (HasIndirectbr && PreRegAlloc)
MaxDuplicateCount = TailDupIndirectBranchSize;
// Check the instructions in the block to determine whether tail-duplication
// is invalid or unlikely to be profitable.
unsigned InstrCount = 0;
for (MachineInstr &MI : TailBB) {
// Non-duplicable things shouldn't be tail-duplicated.
if (MI.isNotDuplicable())
return false;
// Convergent instructions can be duplicated only if doing so doesn't add
// new control dependencies, which is what we're going to do here.
if (MI.isConvergent())
return false;
// Do not duplicate 'return' instructions if this is a pre-regalloc run.
// A return may expand into a lot more instructions (e.g. reload of callee
// saved registers) after PEI.
if (PreRegAlloc && MI.isReturn())
return false;
// Avoid duplicating calls before register allocation. Calls presents a
// barrier to register allocation so duplicating them may end up increasing
// spills.
if (PreRegAlloc && MI.isCall())
return false;
if (!MI.isPHI() && !MI.isDebugValue())
InstrCount += 1;
if (InstrCount > MaxDuplicateCount)
return false;
}
// Check if any of the successors of TailBB has a PHI node in which the
// value corresponding to TailBB uses a subregister.
// If a phi node uses a register paired with a subregister, the actual
// "value type" of the phi may differ from the type of the register without
// any subregisters. Due to a bug, tail duplication may add a new operand
// without a necessary subregister, producing an invalid code. This is
// demonstrated by test/CodeGen/Hexagon/tail-dup-subreg-abort.ll.
// Disable tail duplication for this case for now, until the problem is
// fixed.
for (auto SB : TailBB.successors()) {
for (auto &I : *SB) {
if (!I.isPHI())
break;
unsigned Idx = getPHISrcRegOpIdx(&I, &TailBB);
assert(Idx != 0);
MachineOperand &PU = I.getOperand(Idx);
if (PU.getSubReg() != 0)
return false;
}
}
if (HasIndirectbr && PreRegAlloc)
return true;
if (IsSimple)
return true;
if (!PreRegAlloc)
return true;
return canCompletelyDuplicateBB(TailBB);
}
/// True if this BB has only one unconditional jump.
bool TailDuplicator::isSimpleBB(MachineBasicBlock *TailBB) {
if (TailBB->succ_size() != 1)
return false;
if (TailBB->pred_empty())
return false;
MachineBasicBlock::iterator I = TailBB->getFirstNonDebugInstr();
if (I == TailBB->end())
return true;
return I->isUnconditionalBranch();
}
static bool bothUsedInPHI(const MachineBasicBlock &A,
const SmallPtrSet<MachineBasicBlock *, 8> &SuccsB) {
for (MachineBasicBlock *BB : A.successors())
if (SuccsB.count(BB) && !BB->empty() && BB->begin()->isPHI())
return true;
return false;
}
bool TailDuplicator::canCompletelyDuplicateBB(MachineBasicBlock &BB) {
for (MachineBasicBlock *PredBB : BB.predecessors()) {
if (PredBB->succ_size() > 1)
return false;
MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
SmallVector<MachineOperand, 4> PredCond;
if (TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond))
return false;
if (!PredCond.empty())
return false;
}
return true;
}
bool TailDuplicator::duplicateSimpleBB(
MachineBasicBlock *TailBB, SmallVectorImpl<MachineBasicBlock *> &TDBBs,
const DenseSet<unsigned> &UsedByPhi,
SmallVectorImpl<MachineInstr *> &Copies) {
SmallPtrSet<MachineBasicBlock *, 8> Succs(TailBB->succ_begin(),
TailBB->succ_end());
SmallVector<MachineBasicBlock *, 8> Preds(TailBB->pred_begin(),
TailBB->pred_end());
bool Changed = false;
for (MachineBasicBlock *PredBB : Preds) {
if (PredBB->hasEHPadSuccessor())
continue;
if (bothUsedInPHI(*PredBB, Succs))
continue;
MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
SmallVector<MachineOperand, 4> PredCond;
if (TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond))
continue;
Changed = true;
DEBUG(dbgs() << "\nTail-duplicating into PredBB: " << *PredBB
<< "From simple Succ: " << *TailBB);
MachineBasicBlock *NewTarget = *TailBB->succ_begin();
MachineBasicBlock *NextBB = PredBB->getNextNode();
// Make PredFBB explicit.
if (PredCond.empty())
PredFBB = PredTBB;
// Make fall through explicit.
if (!PredTBB)
PredTBB = NextBB;
if (!PredFBB)
PredFBB = NextBB;
// Redirect
if (PredFBB == TailBB)
PredFBB = NewTarget;
if (PredTBB == TailBB)
PredTBB = NewTarget;
// Make the branch unconditional if possible
if (PredTBB == PredFBB) {
PredCond.clear();
PredFBB = nullptr;
}
// Avoid adding fall through branches.
if (PredFBB == NextBB)
PredFBB = nullptr;
if (PredTBB == NextBB && PredFBB == nullptr)
PredTBB = nullptr;
auto DL = PredBB->findBranchDebugLoc();
TII->removeBranch(*PredBB);
if (!PredBB->isSuccessor(NewTarget))
PredBB->replaceSuccessor(TailBB, NewTarget);
else {
PredBB->removeSuccessor(TailBB, true);
assert(PredBB->succ_size() <= 1);
}
if (PredTBB)
TII->insertBranch(*PredBB, PredTBB, PredFBB, PredCond, DL);
TDBBs.push_back(PredBB);
}
return Changed;
}
bool TailDuplicator::canTailDuplicate(MachineBasicBlock *TailBB,
MachineBasicBlock *PredBB) {
// EH edges are ignored by analyzeBranch.
if (PredBB->succ_size() > 1)
return false;
MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
SmallVector<MachineOperand, 4> PredCond;
if (TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond))
return false;
if (!PredCond.empty())
return false;
return true;
}
/// If it is profitable, duplicate TailBB's contents in each
/// of its predecessors.
/// \p IsSimple result of isSimpleBB
/// \p TailBB Block to be duplicated.
/// \p ForcedLayoutPred When non-null, use this block as the layout predecessor
/// instead of the previous block in MF's order.
/// \p TDBBs A vector to keep track of all blocks tail-duplicated
/// into.
/// \p Copies A vector of copy instructions inserted. Used later to
/// walk all the inserted copies and remove redundant ones.
bool TailDuplicator::tailDuplicate(bool IsSimple, MachineBasicBlock *TailBB,
MachineBasicBlock *ForcedLayoutPred,
SmallVectorImpl<MachineBasicBlock *> &TDBBs,
SmallVectorImpl<MachineInstr *> &Copies) {
DEBUG(dbgs() << "\n*** Tail-duplicating BB#" << TailBB->getNumber() << '\n');
DenseSet<unsigned> UsedByPhi;
getRegsUsedByPHIs(*TailBB, &UsedByPhi);
if (IsSimple)
return duplicateSimpleBB(TailBB, TDBBs, UsedByPhi, Copies);
// Iterate through all the unique predecessors and tail-duplicate this
// block into them, if possible. Copying the list ahead of time also
// avoids trouble with the predecessor list reallocating.
bool Changed = false;
SmallSetVector<MachineBasicBlock *, 8> Preds(TailBB->pred_begin(),
TailBB->pred_end());
for (MachineBasicBlock *PredBB : Preds) {
assert(TailBB != PredBB &&
"Single-block loop should have been rejected earlier!");
if (!canTailDuplicate(TailBB, PredBB))
continue;
// Don't duplicate into a fall-through predecessor (at least for now).
bool IsLayoutSuccessor = false;
if (ForcedLayoutPred)
IsLayoutSuccessor = (ForcedLayoutPred == PredBB);
else if (PredBB->isLayoutSuccessor(TailBB) && PredBB->canFallThrough())
IsLayoutSuccessor = true;
if (IsLayoutSuccessor)
continue;
DEBUG(dbgs() << "\nTail-duplicating into PredBB: " << *PredBB
<< "From Succ: " << *TailBB);
TDBBs.push_back(PredBB);
// Remove PredBB's unconditional branch.
TII->removeBranch(*PredBB);
// Clone the contents of TailBB into PredBB.
DenseMap<unsigned, RegSubRegPair> LocalVRMap;
SmallVector<std::pair<unsigned, RegSubRegPair>, 4> CopyInfos;
// Use instr_iterator here to properly handle bundles, e.g.
// ARM Thumb2 IT block.
MachineBasicBlock::instr_iterator I = TailBB->instr_begin();
while (I != TailBB->instr_end()) {
MachineInstr *MI = &*I;
++I;
if (MI->isPHI()) {
// Replace the uses of the def of the PHI with the register coming
// from PredBB.
processPHI(MI, TailBB, PredBB, LocalVRMap, CopyInfos, UsedByPhi, true);
} else {
// Replace def of virtual registers with new registers, and update
// uses with PHI source register or the new registers.
duplicateInstruction(MI, TailBB, PredBB, LocalVRMap, UsedByPhi);
}
}
appendCopies(PredBB, CopyInfos, Copies);
// Simplify
MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
SmallVector<MachineOperand, 4> PredCond;
TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond);
NumTailDupAdded += TailBB->size() - 1; // subtract one for removed branch
// Update the CFG.
PredBB->removeSuccessor(PredBB->succ_begin());
assert(PredBB->succ_empty() &&
"TailDuplicate called on block with multiple successors!");
for (MachineBasicBlock *Succ : TailBB->successors())
PredBB->addSuccessor(Succ, MBPI->getEdgeProbability(TailBB, Succ));
Changed = true;
++NumTailDups;
}
// If TailBB was duplicated into all its predecessors except for the prior
// block, which falls through unconditionally, move the contents of this
// block into the prior block.
MachineBasicBlock *PrevBB = ForcedLayoutPred;
if (!PrevBB)
PrevBB = &*std::prev(TailBB->getIterator());
MachineBasicBlock *PriorTBB = nullptr, *PriorFBB = nullptr;
SmallVector<MachineOperand, 4> PriorCond;
// This has to check PrevBB->succ_size() because EH edges are ignored by
// analyzeBranch.
if (PrevBB->succ_size() == 1 &&
// Layout preds are not always CFG preds. Check.
*PrevBB->succ_begin() == TailBB &&
!TII->analyzeBranch(*PrevBB, PriorTBB, PriorFBB, PriorCond) &&
PriorCond.empty() &&
(!PriorTBB || PriorTBB == TailBB) &&
TailBB->pred_size() == 1 &&
!TailBB->hasAddressTaken()) {
DEBUG(dbgs() << "\nMerging into block: " << *PrevBB
<< "From MBB: " << *TailBB);
// There may be a branch to the layout successor. This is unlikely but it
// happens. The correct thing to do is to remove the branch before
// duplicating the instructions in all cases.
TII->removeBranch(*PrevBB);
if (PreRegAlloc) {
DenseMap<unsigned, RegSubRegPair> LocalVRMap;
SmallVector<std::pair<unsigned, RegSubRegPair>, 4> CopyInfos;
MachineBasicBlock::iterator I = TailBB->begin();
// Process PHI instructions first.
while (I != TailBB->end() && I->isPHI()) {
// Replace the uses of the def of the PHI with the register coming
// from PredBB.
MachineInstr *MI = &*I++;
processPHI(MI, TailBB, PrevBB, LocalVRMap, CopyInfos, UsedByPhi, true);
}
// Now copy the non-PHI instructions.
while (I != TailBB->end()) {
// Replace def of virtual registers with new registers, and update
// uses with PHI source register or the new registers.
MachineInstr *MI = &*I++;
assert(!MI->isBundle() && "Not expecting bundles before regalloc!");
duplicateInstruction(MI, TailBB, PrevBB, LocalVRMap, UsedByPhi);
MI->eraseFromParent();
}
appendCopies(PrevBB, CopyInfos, Copies);
} else {
TII->removeBranch(*PrevBB);
// No PHIs to worry about, just splice the instructions over.
PrevBB->splice(PrevBB->end(), TailBB, TailBB->begin(), TailBB->end());
}
PrevBB->removeSuccessor(PrevBB->succ_begin());
assert(PrevBB->succ_empty());
PrevBB->transferSuccessors(TailBB);
TDBBs.push_back(PrevBB);
Changed = true;
}
// If this is after register allocation, there are no phis to fix.
if (!PreRegAlloc)
return Changed;
// If we made no changes so far, we are safe.
if (!Changed)
return Changed;
// Handle the nasty case in that we duplicated a block that is part of a loop
// into some but not all of its predecessors. For example:
// 1 -> 2 <-> 3 |
// \ |
// \---> rest |
// if we duplicate 2 into 1 but not into 3, we end up with
// 12 -> 3 <-> 2 -> rest |
// \ / |
// \----->-----/ |
// If there was a "var = phi(1, 3)" in 2, it has to be ultimately replaced
// with a phi in 3 (which now dominates 2).
// What we do here is introduce a copy in 3 of the register defined by the
// phi, just like when we are duplicating 2 into 3, but we don't copy any
// real instructions or remove the 3 -> 2 edge from the phi in 2.
for (MachineBasicBlock *PredBB : Preds) {
if (is_contained(TDBBs, PredBB))
continue;
// EH edges
if (PredBB->succ_size() != 1)
continue;
DenseMap<unsigned, RegSubRegPair> LocalVRMap;
SmallVector<std::pair<unsigned, RegSubRegPair>, 4> CopyInfos;
MachineBasicBlock::iterator I = TailBB->begin();
// Process PHI instructions first.
while (I != TailBB->end() && I->isPHI()) {
// Replace the uses of the def of the PHI with the register coming
// from PredBB.
MachineInstr *MI = &*I++;
processPHI(MI, TailBB, PredBB, LocalVRMap, CopyInfos, UsedByPhi, false);
}
appendCopies(PredBB, CopyInfos, Copies);
}
return Changed;
}
/// At the end of the block \p MBB generate COPY instructions between registers
/// described by \p CopyInfos. Append resulting instructions to \p Copies.
void TailDuplicator::appendCopies(MachineBasicBlock *MBB,
SmallVectorImpl<std::pair<unsigned,RegSubRegPair>> &CopyInfos,
SmallVectorImpl<MachineInstr*> &Copies) {
MachineBasicBlock::iterator Loc = MBB->getFirstTerminator();
const MCInstrDesc &CopyD = TII->get(TargetOpcode::COPY);
for (auto &CI : CopyInfos) {
auto C = BuildMI(*MBB, Loc, DebugLoc(), CopyD, CI.first)
.addReg(CI.second.Reg, 0, CI.second.SubReg);
Copies.push_back(C);
}
}
/// Remove the specified dead machine basic block from the function, updating
/// the CFG.
void TailDuplicator::removeDeadBlock(
MachineBasicBlock *MBB,
function_ref<void(MachineBasicBlock *)> *RemovalCallback) {
assert(MBB->pred_empty() && "MBB must be dead!");
DEBUG(dbgs() << "\nRemoving MBB: " << *MBB);
if (RemovalCallback)
(*RemovalCallback)(MBB);
// Remove all successors.
while (!MBB->succ_empty())
MBB->removeSuccessor(MBB->succ_end() - 1);
// Remove the block.
MBB->eraseFromParent();
}
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