| //===-- MachineSink.cpp - Sinking for machine instructions ----------------===// |
| // |
| // The LLVM Compiler Infrastructure |
| // |
| // This file is distributed under the University of Illinois Open Source |
| // License. See LICENSE.TXT for details. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This pass moves instructions into successor blocks when possible, so that |
| // they aren't executed on paths where their results aren't needed. |
| // |
| // This pass is not intended to be a replacement or a complete alternative |
| // for an LLVM-IR-level sinking pass. It is only designed to sink simple |
| // constructs that are not exposed before lowering and instruction selection. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #define DEBUG_TYPE "machine-sink" |
| #include "llvm/CodeGen/Passes.h" |
| #include "llvm/CodeGen/MachineRegisterInfo.h" |
| #include "llvm/CodeGen/MachineDominators.h" |
| #include "llvm/CodeGen/MachineLoopInfo.h" |
| #include "llvm/Analysis/AliasAnalysis.h" |
| #include "llvm/Target/TargetRegisterInfo.h" |
| #include "llvm/Target/TargetInstrInfo.h" |
| #include "llvm/Target/TargetMachine.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/Support/CommandLine.h" |
| #include "llvm/Support/Debug.h" |
| #include "llvm/Support/raw_ostream.h" |
| using namespace llvm; |
| |
| static cl::opt<bool> |
| SplitEdges("machine-sink-split", |
| cl::desc("Split critical edges during machine sinking"), |
| cl::init(false), cl::Hidden); |
| static cl::opt<unsigned> |
| SplitLimit("split-limit", |
| cl::init(~0u), cl::Hidden); |
| |
| STATISTIC(NumSunk, "Number of machine instructions sunk"); |
| STATISTIC(NumSplit, "Number of critical edges split"); |
| |
| namespace { |
| class MachineSinking : public MachineFunctionPass { |
| const TargetInstrInfo *TII; |
| const TargetRegisterInfo *TRI; |
| MachineRegisterInfo *RegInfo; // Machine register information |
| MachineDominatorTree *DT; // Machine dominator tree |
| MachineLoopInfo *LI; |
| AliasAnalysis *AA; |
| BitVector AllocatableSet; // Which physregs are allocatable? |
| |
| public: |
| static char ID; // Pass identification |
| MachineSinking() : MachineFunctionPass(ID) {} |
| |
| virtual bool runOnMachineFunction(MachineFunction &MF); |
| |
| virtual void getAnalysisUsage(AnalysisUsage &AU) const { |
| AU.setPreservesCFG(); |
| MachineFunctionPass::getAnalysisUsage(AU); |
| AU.addRequired<AliasAnalysis>(); |
| AU.addRequired<MachineDominatorTree>(); |
| AU.addRequired<MachineLoopInfo>(); |
| AU.addPreserved<MachineDominatorTree>(); |
| AU.addPreserved<MachineLoopInfo>(); |
| } |
| private: |
| bool ProcessBlock(MachineBasicBlock &MBB); |
| MachineBasicBlock *SplitCriticalEdge(MachineBasicBlock *From, |
| MachineBasicBlock *To); |
| bool SinkInstruction(MachineInstr *MI, bool &SawStore); |
| bool AllUsesDominatedByBlock(unsigned Reg, MachineBasicBlock *MBB, |
| MachineBasicBlock *DefMBB, bool &LocalUse) const; |
| }; |
| } // end anonymous namespace |
| |
| char MachineSinking::ID = 0; |
| INITIALIZE_PASS(MachineSinking, "machine-sink", |
| "Machine code sinking", false, false); |
| |
| FunctionPass *llvm::createMachineSinkingPass() { return new MachineSinking(); } |
| |
| /// AllUsesDominatedByBlock - Return true if all uses of the specified register |
| /// occur in blocks dominated by the specified block. If any use is in the |
| /// definition block, then return false since it is never legal to move def |
| /// after uses. |
| bool MachineSinking::AllUsesDominatedByBlock(unsigned Reg, |
| MachineBasicBlock *MBB, |
| MachineBasicBlock *DefMBB, |
| bool &LocalUse) const { |
| assert(TargetRegisterInfo::isVirtualRegister(Reg) && |
| "Only makes sense for vregs"); |
| // Ignoring debug uses is necessary so debug info doesn't affect the code. |
| // This may leave a referencing dbg_value in the original block, before |
| // the definition of the vreg. Dwarf generator handles this although the |
| // user might not get the right info at runtime. |
| for (MachineRegisterInfo::use_nodbg_iterator |
| I = RegInfo->use_nodbg_begin(Reg), E = RegInfo->use_nodbg_end(); |
| I != E; ++I) { |
| // Determine the block of the use. |
| MachineInstr *UseInst = &*I; |
| MachineBasicBlock *UseBlock = UseInst->getParent(); |
| |
| if (UseInst->isPHI()) { |
| // PHI nodes use the operand in the predecessor block, not the block with |
| // the PHI. |
| UseBlock = UseInst->getOperand(I.getOperandNo()+1).getMBB(); |
| } else if (UseBlock == DefMBB) { |
| LocalUse = true; |
| return false; |
| } |
| |
| // Check that it dominates. |
| if (!DT->dominates(MBB, UseBlock)) |
| return false; |
| } |
| |
| return true; |
| } |
| |
| bool MachineSinking::runOnMachineFunction(MachineFunction &MF) { |
| DEBUG(dbgs() << "******** Machine Sinking ********\n"); |
| |
| const TargetMachine &TM = MF.getTarget(); |
| TII = TM.getInstrInfo(); |
| TRI = TM.getRegisterInfo(); |
| RegInfo = &MF.getRegInfo(); |
| DT = &getAnalysis<MachineDominatorTree>(); |
| LI = &getAnalysis<MachineLoopInfo>(); |
| AA = &getAnalysis<AliasAnalysis>(); |
| AllocatableSet = TRI->getAllocatableSet(MF); |
| |
| bool EverMadeChange = false; |
| |
| while (1) { |
| bool MadeChange = false; |
| |
| // Process all basic blocks. |
| for (MachineFunction::iterator I = MF.begin(), E = MF.end(); |
| I != E; ++I) |
| MadeChange |= ProcessBlock(*I); |
| |
| // If this iteration over the code changed anything, keep iterating. |
| if (!MadeChange) break; |
| EverMadeChange = true; |
| } |
| return EverMadeChange; |
| } |
| |
| bool MachineSinking::ProcessBlock(MachineBasicBlock &MBB) { |
| // Can't sink anything out of a block that has less than two successors. |
| if (MBB.succ_size() <= 1 || MBB.empty()) return false; |
| |
| // Don't bother sinking code out of unreachable blocks. In addition to being |
| // unprofitable, it can also lead to infinite looping, because in an |
| // unreachable loop there may be nowhere to stop. |
| if (!DT->isReachableFromEntry(&MBB)) return false; |
| |
| bool MadeChange = false; |
| |
| // Walk the basic block bottom-up. Remember if we saw a store. |
| MachineBasicBlock::iterator I = MBB.end(); |
| --I; |
| bool ProcessedBegin, SawStore = false; |
| do { |
| MachineInstr *MI = I; // The instruction to sink. |
| |
| // Predecrement I (if it's not begin) so that it isn't invalidated by |
| // sinking. |
| ProcessedBegin = I == MBB.begin(); |
| if (!ProcessedBegin) |
| --I; |
| |
| if (MI->isDebugValue()) |
| continue; |
| |
| if (SinkInstruction(MI, SawStore)) |
| ++NumSunk, MadeChange = true; |
| |
| // If we just processed the first instruction in the block, we're done. |
| } while (!ProcessedBegin); |
| |
| return MadeChange; |
| } |
| |
| MachineBasicBlock *MachineSinking::SplitCriticalEdge(MachineBasicBlock *FromBB, |
| MachineBasicBlock *ToBB) { |
| // Avoid breaking back edge. From == To means backedge for single BB loop. |
| if (!SplitEdges || NumSplit == SplitLimit || FromBB == ToBB) |
| return 0; |
| |
| // Check for more "complex" loops. |
| if (LI->getLoopFor(FromBB) != LI->getLoopFor(ToBB) || |
| !LI->isLoopHeader(ToBB)) { |
| // It's not always legal to break critical edges and sink the computation |
| // to the edge. |
| // |
| // BB#1: |
| // v1024 |
| // Beq BB#3 |
| // <fallthrough> |
| // BB#2: |
| // ... no uses of v1024 |
| // <fallthrough> |
| // BB#3: |
| // ... |
| // = v1024 |
| // |
| // If BB#1 -> BB#3 edge is broken and computation of v1024 is inserted: |
| // |
| // BB#1: |
| // ... |
| // Bne BB#2 |
| // BB#4: |
| // v1024 = |
| // B BB#3 |
| // BB#2: |
| // ... no uses of v1024 |
| // <fallthrough> |
| // BB#3: |
| // ... |
| // = v1024 |
| // |
| // This is incorrect since v1024 is not computed along the BB#1->BB#2->BB#3 |
| // flow. We need to ensure the new basic block where the computation is |
| // sunk to dominates all the uses. |
| // It's only legal to break critical edge and sink the computation to the |
| // new block if all the predecessors of "To", except for "From", are |
| // not dominated by "From". Given SSA property, this means these |
| // predecessors are dominated by "To". |
| for (MachineBasicBlock::pred_iterator PI = ToBB->pred_begin(), |
| E = ToBB->pred_end(); PI != E; ++PI) { |
| if (*PI == FromBB) |
| continue; |
| if (!DT->dominates(ToBB, *PI)) |
| return 0; |
| } |
| |
| // FIXME: Determine if it's cost effective to break this edge. |
| return FromBB->SplitCriticalEdge(ToBB, this); |
| } |
| |
| return 0; |
| } |
| |
| /// SinkInstruction - Determine whether it is safe to sink the specified machine |
| /// instruction out of its current block into a successor. |
| bool MachineSinking::SinkInstruction(MachineInstr *MI, bool &SawStore) { |
| // Check if it's safe to move the instruction. |
| if (!MI->isSafeToMove(TII, AA, SawStore)) |
| return false; |
| |
| // FIXME: This should include support for sinking instructions within the |
| // block they are currently in to shorten the live ranges. We often get |
| // instructions sunk into the top of a large block, but it would be better to |
| // also sink them down before their first use in the block. This xform has to |
| // be careful not to *increase* register pressure though, e.g. sinking |
| // "x = y + z" down if it kills y and z would increase the live ranges of y |
| // and z and only shrink the live range of x. |
| |
| // Loop over all the operands of the specified instruction. If there is |
| // anything we can't handle, bail out. |
| MachineBasicBlock *ParentBlock = MI->getParent(); |
| |
| // SuccToSinkTo - This is the successor to sink this instruction to, once we |
| // decide. |
| MachineBasicBlock *SuccToSinkTo = 0; |
| |
| for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { |
| const MachineOperand &MO = MI->getOperand(i); |
| if (!MO.isReg()) continue; // Ignore non-register operands. |
| |
| unsigned Reg = MO.getReg(); |
| if (Reg == 0) continue; |
| |
| if (TargetRegisterInfo::isPhysicalRegister(Reg)) { |
| if (MO.isUse()) { |
| // If the physreg has no defs anywhere, it's just an ambient register |
| // and we can freely move its uses. Alternatively, if it's allocatable, |
| // it could get allocated to something with a def during allocation. |
| if (!RegInfo->def_empty(Reg)) |
| return false; |
| |
| if (AllocatableSet.test(Reg)) |
| return false; |
| |
| // Check for a def among the register's aliases too. |
| for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) { |
| unsigned AliasReg = *Alias; |
| if (!RegInfo->def_empty(AliasReg)) |
| return false; |
| |
| if (AllocatableSet.test(AliasReg)) |
| return false; |
| } |
| } else if (!MO.isDead()) { |
| // A def that isn't dead. We can't move it. |
| return false; |
| } |
| } else { |
| // Virtual register uses are always safe to sink. |
| if (MO.isUse()) continue; |
| |
| // If it's not safe to move defs of the register class, then abort. |
| if (!TII->isSafeToMoveRegClassDefs(RegInfo->getRegClass(Reg))) |
| return false; |
| |
| // FIXME: This picks a successor to sink into based on having one |
| // successor that dominates all the uses. However, there are cases where |
| // sinking can happen but where the sink point isn't a successor. For |
| // example: |
| // |
| // x = computation |
| // if () {} else {} |
| // use x |
| // |
| // the instruction could be sunk over the whole diamond for the |
| // if/then/else (or loop, etc), allowing it to be sunk into other blocks |
| // after that. |
| |
| // Virtual register defs can only be sunk if all their uses are in blocks |
| // dominated by one of the successors. |
| if (SuccToSinkTo) { |
| // If a previous operand picked a block to sink to, then this operand |
| // must be sinkable to the same block. |
| bool LocalUse = false; |
| if (!AllUsesDominatedByBlock(Reg, SuccToSinkTo, ParentBlock, LocalUse)) |
| return false; |
| |
| continue; |
| } |
| |
| // Otherwise, we should look at all the successors and decide which one |
| // we should sink to. |
| for (MachineBasicBlock::succ_iterator SI = ParentBlock->succ_begin(), |
| E = ParentBlock->succ_end(); SI != E; ++SI) { |
| bool LocalUse = false; |
| if (AllUsesDominatedByBlock(Reg, *SI, ParentBlock, LocalUse)) { |
| SuccToSinkTo = *SI; |
| break; |
| } |
| if (LocalUse) |
| // Def is used locally, it's never safe to move this def. |
| return false; |
| } |
| |
| // If we couldn't find a block to sink to, ignore this instruction. |
| if (SuccToSinkTo == 0) |
| return false; |
| } |
| } |
| |
| // If there are no outputs, it must have side-effects. |
| if (SuccToSinkTo == 0) |
| return false; |
| |
| // It's not safe to sink instructions to EH landing pad. Control flow into |
| // landing pad is implicitly defined. |
| if (SuccToSinkTo->isLandingPad()) |
| return false; |
| |
| // It is not possible to sink an instruction into its own block. This can |
| // happen with loops. |
| if (MI->getParent() == SuccToSinkTo) |
| return false; |
| |
| // If the instruction to move defines a dead physical register which is live |
| // when leaving the basic block, don't move it because it could turn into a |
| // "zombie" define of that preg. E.g., EFLAGS. (<rdar://problem/8030636>) |
| for (unsigned I = 0, E = MI->getNumOperands(); I != E; ++I) { |
| const MachineOperand &MO = MI->getOperand(I); |
| if (!MO.isReg()) continue; |
| unsigned Reg = MO.getReg(); |
| if (Reg == 0 || !TargetRegisterInfo::isPhysicalRegister(Reg)) continue; |
| if (SuccToSinkTo->isLiveIn(Reg)) |
| return false; |
| } |
| |
| DEBUG(dbgs() << "Sink instr " << *MI << "\tinto block " << *SuccToSinkTo); |
| |
| // If the block has multiple predecessors, this would introduce computation on |
| // a path that it doesn't already exist. We could split the critical edge, |
| // but for now we just punt. |
| // FIXME: Split critical edges if not backedges. |
| if (SuccToSinkTo->pred_size() > 1) { |
| // We cannot sink a load across a critical edge - there may be stores in |
| // other code paths. |
| bool TryBreak = false; |
| bool store = true; |
| if (!MI->isSafeToMove(TII, AA, store)) { |
| DEBUG(dbgs() << " *** NOTE: Won't sink load along critical edge.\n"); |
| TryBreak = true; |
| } |
| |
| // We don't want to sink across a critical edge if we don't dominate the |
| // successor. We could be introducing calculations to new code paths. |
| if (!TryBreak && !DT->dominates(ParentBlock, SuccToSinkTo)) { |
| DEBUG(dbgs() << " *** NOTE: Critical edge found\n"); |
| TryBreak = true; |
| } |
| |
| // Don't sink instructions into a loop. |
| if (!TryBreak && LI->isLoopHeader(SuccToSinkTo)) { |
| DEBUG(dbgs() << " *** NOTE: Loop header found\n"); |
| TryBreak = true; |
| } |
| |
| // Otherwise we are OK with sinking along a critical edge. |
| if (!TryBreak) |
| DEBUG(dbgs() << "Sinking along critical edge.\n"); |
| else { |
| MachineBasicBlock *NewSucc = SplitCriticalEdge(ParentBlock, SuccToSinkTo); |
| if (!NewSucc) { |
| DEBUG(dbgs() << |
| " *** PUNTING: Not legal or profitable to break critical edge\n"); |
| return false; |
| } else { |
| DEBUG(dbgs() << " *** Splitting critical edge:" |
| " BB#" << ParentBlock->getNumber() |
| << " -- BB#" << NewSucc->getNumber() |
| << " -- BB#" << SuccToSinkTo->getNumber() << '\n'); |
| SuccToSinkTo = NewSucc; |
| ++NumSplit; |
| } |
| } |
| } |
| |
| // Determine where to insert into. Skip phi nodes. |
| MachineBasicBlock::iterator InsertPos = SuccToSinkTo->begin(); |
| while (InsertPos != SuccToSinkTo->end() && InsertPos->isPHI()) |
| ++InsertPos; |
| |
| // Move the instruction. |
| SuccToSinkTo->splice(InsertPos, ParentBlock, MI, |
| ++MachineBasicBlock::iterator(MI)); |
| |
| // Conservatively, clear any kill flags, since it's possible that they are no |
| // longer correct. |
| MI->clearKillInfo(); |
| |
| return true; |
| } |
