| //===- TailDuplicator.cpp - Duplicate blocks into predecessors' tails -----===// |
| // |
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
| // See https://llvm.org/LICENSE.txt for license information. |
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This utility class duplicates basic blocks ending in unconditional branches |
| // into the tails of their predecessors. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "llvm/CodeGen/TailDuplicator.h" |
| #include "llvm/ADT/DenseMap.h" |
| #include "llvm/ADT/DenseSet.h" |
| #include "llvm/ADT/STLExtras.h" |
| #include "llvm/ADT/SetVector.h" |
| #include "llvm/ADT/SmallPtrSet.h" |
| #include "llvm/ADT/SmallVector.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/Analysis/ProfileSummaryInfo.h" |
| #include "llvm/CodeGen/MachineBasicBlock.h" |
| #include "llvm/CodeGen/MachineBranchProbabilityInfo.h" |
| #include "llvm/CodeGen/MachineBlockFrequencyInfo.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/MachineSizeOpts.h" |
| #include "llvm/CodeGen/MachineSSAUpdater.h" |
| #include "llvm/CodeGen/TargetInstrInfo.h" |
| #include "llvm/CodeGen/TargetRegisterInfo.h" |
| #include "llvm/CodeGen/TargetSubtargetInfo.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/TargetMachine.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, bool PreRegAlloc, |
| const MachineBranchProbabilityInfo *MBPIin, |
| const MachineBlockFrequencyInfo *MBFIin, |
| ProfileSummaryInfo *PSIin, |
| bool LayoutModeIn, unsigned TailDupSizeIn) { |
| MF = &MFin; |
| TII = MF->getSubtarget().getInstrInfo(); |
| TRI = MF->getSubtarget().getRegisterInfo(); |
| MRI = &MF->getRegInfo(); |
| MMI = &MF->getMMI(); |
| MBPI = MBPIin; |
| MBFI = MBFIin; |
| PSI = PSIin; |
| TailDupSize = TailDupSizeIn; |
| |
| assert(MBPI != nullptr && "Machine Branch Probability Info required"); |
| |
| LayoutMode = LayoutModeIn; |
| this->PreRegAlloc = PreRegAlloc; |
| } |
| |
| 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 " << printMBBReference(*MBB) << ": " |
| << *MI; |
| dbgs() << " missing input from predecessor " |
| << printMBBReference(*PredBB) << '\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 " << printMBBReference(*MBB) |
| << ": " << *MI; |
| dbgs() << " extra input from predecessor " |
| << printMBBReference(*PHIBB) << '\n'; |
| llvm_unreachable(nullptr); |
| } |
| if (PHIBB->getNumber() < 0) { |
| dbgs() << "Malformed PHI in " << printMBBReference(*MBB) << ": " |
| << *MI; |
| dbgs() << " non-existing " << printMBBReference(*PHIBB) << '\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; |
| Register Dst = Copy->getOperand(0).getReg(); |
| Register 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) { |
| LLVM_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) { |
| Register 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) { |
| Register DefReg = MI->getOperand(0).getReg(); |
| unsigned SrcOpIdx = getPHISrcRegOpIdx(MI, PredBB); |
| assert(SrcOpIdx && "Unable to find matching PHI source?"); |
| Register 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. |
| Register 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) { |
| // Allow duplication of CFI instructions. |
| if (MI->isCFIInstruction()) { |
| BuildMI(*PredBB, PredBB->end(), PredBB->findDebugLoc(PredBB->begin()), |
| TII->get(TargetOpcode::CFI_INSTRUCTION)).addCFIIndex( |
| MI->getOperand(0).getCFIIndex()); |
| return; |
| } |
| MachineInstr &NewMI = TII->duplicate(*PredBB, PredBB->end(), *MI); |
| if (PreRegAlloc) { |
| for (unsigned i = 0, e = NewMI.getNumOperands(); i != e; ++i) { |
| MachineOperand &MO = NewMI.getOperand(i); |
| if (!MO.isReg()) |
| continue; |
| Register Reg = MO.getReg(); |
| if (!Register::isVirtualRegister(Reg)) |
| continue; |
| if (MO.isDef()) { |
| const TargetRegisterClass *RC = MRI->getRegClass(Reg); |
| Register 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; |
| Register NewReg = MRI->createVirtualRegister(NewRC); |
| BuildMI(*PredBB, NewMI, NewMI.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); |
| } |
| } |
| } |
| } |
| } |
| |
| /// 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); |
| Register 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; |
| bool OptForSize = MF->getFunction().hasOptSize() || |
| llvm::shouldOptimizeForSize(&TailBB, PSI, MBFI); |
| if (TailDupSize == 0) |
| MaxDuplicateCount = TailDuplicateSize; |
| else |
| MaxDuplicateCount = TailDupSize; |
| if (OptForSize) |
| MaxDuplicateCount = 1; |
| |
| // 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. |
| // CFI instructions are marked as non-duplicable, because Darwin compact |
| // unwind info emission can't handle multiple prologue setups. In case of |
| // DWARF, allow them be duplicated, so that their existence doesn't prevent |
| // tail duplication of some basic blocks, that would be duplicated otherwise. |
| if (MI.isNotDuplicable() && |
| (TailBB.getParent()->getTarget().getTargetTriple().isOSDarwin() || |
| !MI.isCFIInstruction())) |
| 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.isMetaInstruction()) |
| 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; |
| LLVM_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) { |
| LLVM_DEBUG(dbgs() << "\n*** Tail-duplicating " << printMBBReference(*TailBB) |
| << '\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; |
| |
| LLVM_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; |
| for (MachineBasicBlock::iterator I = TailBB->begin(), E = TailBB->end(); |
| I != E; /* empty */) { |
| 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); |
| |
| 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()) { |
| LLVM_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!"); |
| LLVM_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(); |
| } |
