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Diffstat (limited to 'lib/CodeGen/TailDuplicator.cpp')
| -rw-r--r-- | lib/CodeGen/TailDuplicator.cpp | 932 |
1 files changed, 932 insertions, 0 deletions
diff --git a/lib/CodeGen/TailDuplicator.cpp b/lib/CodeGen/TailDuplicator.cpp new file mode 100644 index 000000000000..847a09349a59 --- /dev/null +++ b/lib/CodeGen/TailDuplicator.cpp @@ -0,0 +1,932 @@ +//===-- 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/CodeGen/TailDuplicator.h" +#include "llvm/ADT/DenseSet.h" +#include "llvm/ADT/SetVector.h" +#include "llvm/ADT/SmallSet.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/CodeGen/MachineBranchProbabilityInfo.h" +#include "llvm/CodeGen/MachineFunctionPass.h" +#include "llvm/CodeGen/MachineInstrBuilder.h" +#include "llvm/CodeGen/MachineModuleInfo.h" +#include "llvm/CodeGen/Passes.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" +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<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); + +namespace llvm { + +void TailDuplicator::initMF(MachineFunction &MF, const MachineModuleInfo *MMIin, + const MachineBranchProbabilityInfo *MBPIin) { + TII = MF.getSubtarget().getInstrInfo(); + TRI = MF.getSubtarget().getRegisterInfo(); + MRI = &MF.getRegInfo(); + MMI = MMIin; + MBPI = MBPIin; + + assert(MBPI != nullptr && "Machine Branch Probability Info required"); + + 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 (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(), + PE = Preds.end(); + PI != PE; ++PI) { + MachineBasicBlock *PredBB = *PI; + 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. +bool TailDuplicator::tailDuplicateAndUpdate(MachineFunction &MF, bool IsSimple, + MachineBasicBlock *MBB) { + // Save the successors list. + SmallSetVector<MachineBasicBlock *, 8> Succs(MBB->succ_begin(), + MBB->succ_end()); + + SmallVector<MachineBasicBlock *, 8> TDBBs; + SmallVector<MachineInstr *, 16> Copies; + if (!tailDuplicate(MF, IsSimple, MBB, 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); + ++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(); + + 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(MachineFunction &MF) { + 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(MF, IsSimple, *MBB)) + continue; + + MadeChange |= tailDuplicateAndUpdate(MF, IsSimple, MBB); + } + + 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, + MachineFunction &MF, + 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 (SmallSetVector<MachineBasicBlock *, 8>::iterator SI = Succs.begin(), + SE = Succs.end(); + SI != SE; ++SI) { + MachineBasicBlock *SuccBB = *SI; + for (MachineBasicBlock::iterator II = SuccBB->begin(), EE = SuccBB->end(); + II != EE; ++II) { + if (!II->isPHI()) + break; + MachineInstrBuilder MIB(*FromBB->getParent(), II); + unsigned Idx = 0; + for (unsigned i = 1, e = II->getNumOperands(); i != e; i += 2) { + MachineOperand &MO = II->getOperand(i + 1); + if (MO.getMBB() == FromBB) { + Idx = i; + break; + } + } + + assert(Idx != 0); + MachineOperand &MO0 = II->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 = II->getNumOperands() - 2; i != Idx; i -= 2) { + MachineOperand &MO = II->getOperand(i + 1); + if (MO.getMBB() == FromBB) { + II->RemoveOperand(i + 1); + II->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) { + II->getOperand(Idx).setReg(SrcReg); + II->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) { + II->getOperand(Idx).setReg(Reg); + II->getOperand(Idx + 1).setMBB(SrcBB); + Idx = 0; + } else { + MIB.addReg(Reg).addMBB(SrcBB); + } + } + } + if (Idx != 0) { + II->RemoveOperand(Idx + 1); + II->RemoveOperand(Idx); + } + } + } +} + +/// Determine if it is profitable to duplicate this block. +bool TailDuplicator::shouldTailDuplicate(const MachineFunction &MF, + bool IsSimple, + MachineBasicBlock &TailBB) { + // Only duplicate blocks that end with unconditional branches. + if (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 (TailDuplicateSize.getNumOccurrences() == 0 && + // FIXME: Use Function::optForSize(). + MF.getFunction()->hasFnAttribute(Attribute::OptimizeForSize)) + MaxDuplicateCount = 1; + else + MaxDuplicateCount = TailDuplicateSize; + + // 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 = 20; + + // 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, true)) + 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 (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(), + PE = Preds.end(); + PI != PE; ++PI) { + MachineBasicBlock *PredBB = *PI; + + 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, true)) + continue; + + Changed = true; + DEBUG(dbgs() << "\nTail-duplicating into PredBB: " << *PredBB + << "From simple Succ: " << *TailBB); + + MachineBasicBlock *NewTarget = *TailBB->succ_begin(); + MachineBasicBlock *NextBB = &*std::next(PredBB->getIterator()); + + // 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; + + 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, DebugLoc()); + + TDBBs.push_back(PredBB); + } + return Changed; +} + +/// If it is profitable, duplicate TailBB's contents in each +/// of its predecessors. +bool TailDuplicator::tailDuplicate(MachineFunction &MF, bool IsSimple, + MachineBasicBlock *TailBB, + 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 (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(), + PE = Preds.end(); + PI != PE; ++PI) { + MachineBasicBlock *PredBB = *PI; + + assert(TailBB != PredBB && + "Single-block loop should have been rejected earlier!"); + // EH edges are ignored by AnalyzeBranch. + if (PredBB->succ_size() > 1) + continue; + + MachineBasicBlock *PredTBB, *PredFBB; + SmallVector<MachineOperand, 4> PredCond; + if (TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true)) + continue; + if (!PredCond.empty()) + continue; + // Don't duplicate into a fall-through predecessor (at least for now). + if (PredBB->isLayoutSuccessor(TailBB) && PredBB->canFallThrough()) + 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, MF, LocalVRMap, UsedByPhi); + } + } + appendCopies(PredBB, CopyInfos, Copies); + + // Simplify + TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true); + + 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_iterator I = TailBB->succ_begin(), + E = TailBB->succ_end(); + I != E; ++I) + PredBB->addSuccessor(*I, MBPI->getEdgeProbability(TailBB, I)); + + 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 = &*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 && + !TII->analyzeBranch(*PrevBB, PriorTBB, PriorFBB, PriorCond, true) && + PriorCond.empty() && !PriorTBB && TailBB->pred_size() == 1 && + !TailBB->hasAddressTaken()) { + DEBUG(dbgs() << "\nMerging into block: " << *PrevBB + << "From MBB: " << *TailBB); + 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, MF, LocalVRMap, UsedByPhi); + MI->eraseFromParent(); + } + appendCopies(PrevBB, CopyInfos, Copies); + } else { + // 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 (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(), + PE = Preds.end(); + PI != PE; ++PI) { + MachineBasicBlock *PredBB = *PI; + if (std::find(TDBBs.begin(), TDBBs.end(), PredBB) != TDBBs.end()) + 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) { + assert(MBB->pred_empty() && "MBB must be dead!"); + DEBUG(dbgs() << "\nRemoving MBB: " << *MBB); + + // Remove all successors. + while (!MBB->succ_empty()) + MBB->removeSuccessor(MBB->succ_end() - 1); + + // Remove the block. + MBB->eraseFromParent(); +} + +} // End llvm namespace |