| //===- CriticalAntiDepBreaker.cpp - Anti-dep breaker ----------------------===// |
| // |
| // 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 file implements the CriticalAntiDepBreaker class, which |
| // implements register anti-dependence breaking along a blocks |
| // critical path during post-RA scheduler. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "CriticalAntiDepBreaker.h" |
| #include "llvm/ADT/ArrayRef.h" |
| #include "llvm/ADT/BitVector.h" |
| #include "llvm/ADT/DenseMap.h" |
| #include "llvm/ADT/SmallVector.h" |
| #include "llvm/CodeGen/MachineBasicBlock.h" |
| #include "llvm/CodeGen/MachineFrameInfo.h" |
| #include "llvm/CodeGen/MachineFunction.h" |
| #include "llvm/CodeGen/MachineInstr.h" |
| #include "llvm/CodeGen/MachineOperand.h" |
| #include "llvm/CodeGen/MachineRegisterInfo.h" |
| #include "llvm/CodeGen/RegisterClassInfo.h" |
| #include "llvm/CodeGen/ScheduleDAG.h" |
| #include "llvm/CodeGen/TargetInstrInfo.h" |
| #include "llvm/CodeGen/TargetRegisterInfo.h" |
| #include "llvm/CodeGen/TargetSubtargetInfo.h" |
| #include "llvm/MC/MCInstrDesc.h" |
| #include "llvm/MC/MCRegisterInfo.h" |
| #include "llvm/Support/Debug.h" |
| #include "llvm/Support/raw_ostream.h" |
| #include <cassert> |
| #include <map> |
| #include <utility> |
| #include <vector> |
| |
| using namespace llvm; |
| |
| #define DEBUG_TYPE "post-RA-sched" |
| |
| CriticalAntiDepBreaker::CriticalAntiDepBreaker(MachineFunction &MFi, |
| const RegisterClassInfo &RCI) |
| : AntiDepBreaker(), MF(MFi), MRI(MF.getRegInfo()), |
| TII(MF.getSubtarget().getInstrInfo()), |
| TRI(MF.getSubtarget().getRegisterInfo()), RegClassInfo(RCI), |
| Classes(TRI->getNumRegs(), nullptr), KillIndices(TRI->getNumRegs(), 0), |
| DefIndices(TRI->getNumRegs(), 0), KeepRegs(TRI->getNumRegs(), false) {} |
| |
| CriticalAntiDepBreaker::~CriticalAntiDepBreaker() = default; |
| |
| void CriticalAntiDepBreaker::StartBlock(MachineBasicBlock *BB) { |
| const unsigned BBSize = BB->size(); |
| for (unsigned i = 0, e = TRI->getNumRegs(); i != e; ++i) { |
| // Clear out the register class data. |
| Classes[i] = nullptr; |
| |
| // Initialize the indices to indicate that no registers are live. |
| KillIndices[i] = ~0u; |
| DefIndices[i] = BBSize; |
| } |
| |
| // Clear "do not change" set. |
| KeepRegs.reset(); |
| |
| bool IsReturnBlock = BB->isReturnBlock(); |
| |
| // Examine the live-in regs of all successors. |
| for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(), |
| SE = BB->succ_end(); SI != SE; ++SI) |
| for (const auto &LI : (*SI)->liveins()) { |
| for (MCRegAliasIterator AI(LI.PhysReg, TRI, true); AI.isValid(); ++AI) { |
| unsigned Reg = *AI; |
| Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1); |
| KillIndices[Reg] = BBSize; |
| DefIndices[Reg] = ~0u; |
| } |
| } |
| |
| // Mark live-out callee-saved registers. In a return block this is |
| // all callee-saved registers. In non-return this is any |
| // callee-saved register that is not saved in the prolog. |
| const MachineFrameInfo &MFI = MF.getFrameInfo(); |
| BitVector Pristine = MFI.getPristineRegs(MF); |
| for (const MCPhysReg *I = MF.getRegInfo().getCalleeSavedRegs(); *I; |
| ++I) { |
| unsigned Reg = *I; |
| if (!IsReturnBlock && !Pristine.test(Reg)) |
| continue; |
| for (MCRegAliasIterator AI(*I, TRI, true); AI.isValid(); ++AI) { |
| unsigned Reg = *AI; |
| Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1); |
| KillIndices[Reg] = BBSize; |
| DefIndices[Reg] = ~0u; |
| } |
| } |
| } |
| |
| void CriticalAntiDepBreaker::FinishBlock() { |
| RegRefs.clear(); |
| KeepRegs.reset(); |
| } |
| |
| void CriticalAntiDepBreaker::Observe(MachineInstr &MI, unsigned Count, |
| unsigned InsertPosIndex) { |
| // Kill instructions can define registers but are really nops, and there might |
| // be a real definition earlier that needs to be paired with uses dominated by |
| // this kill. |
| |
| // FIXME: It may be possible to remove the isKill() restriction once PR18663 |
| // has been properly fixed. There can be value in processing kills as seen in |
| // the AggressiveAntiDepBreaker class. |
| if (MI.isDebugInstr() || MI.isKill()) |
| return; |
| assert(Count < InsertPosIndex && "Instruction index out of expected range!"); |
| |
| for (unsigned Reg = 0; Reg != TRI->getNumRegs(); ++Reg) { |
| if (KillIndices[Reg] != ~0u) { |
| // If Reg is currently live, then mark that it can't be renamed as |
| // we don't know the extent of its live-range anymore (now that it |
| // has been scheduled). |
| Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1); |
| KillIndices[Reg] = Count; |
| } else if (DefIndices[Reg] < InsertPosIndex && DefIndices[Reg] >= Count) { |
| // Any register which was defined within the previous scheduling region |
| // may have been rescheduled and its lifetime may overlap with registers |
| // in ways not reflected in our current liveness state. For each such |
| // register, adjust the liveness state to be conservatively correct. |
| Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1); |
| |
| // Move the def index to the end of the previous region, to reflect |
| // that the def could theoretically have been scheduled at the end. |
| DefIndices[Reg] = InsertPosIndex; |
| } |
| } |
| |
| PrescanInstruction(MI); |
| ScanInstruction(MI, Count); |
| } |
| |
| /// CriticalPathStep - Return the next SUnit after SU on the bottom-up |
| /// critical path. |
| static const SDep *CriticalPathStep(const SUnit *SU) { |
| const SDep *Next = nullptr; |
| unsigned NextDepth = 0; |
| // Find the predecessor edge with the greatest depth. |
| for (SUnit::const_pred_iterator P = SU->Preds.begin(), PE = SU->Preds.end(); |
| P != PE; ++P) { |
| const SUnit *PredSU = P->getSUnit(); |
| unsigned PredLatency = P->getLatency(); |
| unsigned PredTotalLatency = PredSU->getDepth() + PredLatency; |
| // In the case of a latency tie, prefer an anti-dependency edge over |
| // other types of edges. |
| if (NextDepth < PredTotalLatency || |
| (NextDepth == PredTotalLatency && P->getKind() == SDep::Anti)) { |
| NextDepth = PredTotalLatency; |
| Next = &*P; |
| } |
| } |
| return Next; |
| } |
| |
| void CriticalAntiDepBreaker::PrescanInstruction(MachineInstr &MI) { |
| // It's not safe to change register allocation for source operands of |
| // instructions that have special allocation requirements. Also assume all |
| // registers used in a call must not be changed (ABI). |
| // FIXME: The issue with predicated instruction is more complex. We are being |
| // conservative here because the kill markers cannot be trusted after |
| // if-conversion: |
| // %r6 = LDR %sp, %reg0, 92, 14, %reg0; mem:LD4[FixedStack14] |
| // ... |
| // STR %r0, killed %r6, %reg0, 0, 0, %cpsr; mem:ST4[%395] |
| // %r6 = LDR %sp, %reg0, 100, 0, %cpsr; mem:LD4[FixedStack12] |
| // STR %r0, killed %r6, %reg0, 0, 14, %reg0; mem:ST4[%396](align=8) |
| // |
| // The first R6 kill is not really a kill since it's killed by a predicated |
| // instruction which may not be executed. The second R6 def may or may not |
| // re-define R6 so it's not safe to change it since the last R6 use cannot be |
| // changed. |
| bool Special = |
| MI.isCall() || MI.hasExtraSrcRegAllocReq() || TII->isPredicated(MI); |
| |
| // Scan the register operands for this instruction and update |
| // Classes and RegRefs. |
| for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) { |
| MachineOperand &MO = MI.getOperand(i); |
| if (!MO.isReg()) continue; |
| Register Reg = MO.getReg(); |
| if (Reg == 0) continue; |
| const TargetRegisterClass *NewRC = nullptr; |
| |
| if (i < MI.getDesc().getNumOperands()) |
| NewRC = TII->getRegClass(MI.getDesc(), i, TRI, MF); |
| |
| // For now, only allow the register to be changed if its register |
| // class is consistent across all uses. |
| if (!Classes[Reg] && NewRC) |
| Classes[Reg] = NewRC; |
| else if (!NewRC || Classes[Reg] != NewRC) |
| Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1); |
| |
| // Now check for aliases. |
| for (MCRegAliasIterator AI(Reg, TRI, false); AI.isValid(); ++AI) { |
| // If an alias of the reg is used during the live range, give up. |
| // Note that this allows us to skip checking if AntiDepReg |
| // overlaps with any of the aliases, among other things. |
| unsigned AliasReg = *AI; |
| if (Classes[AliasReg]) { |
| Classes[AliasReg] = reinterpret_cast<TargetRegisterClass *>(-1); |
| Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1); |
| } |
| } |
| |
| // If we're still willing to consider this register, note the reference. |
| if (Classes[Reg] != reinterpret_cast<TargetRegisterClass *>(-1)) |
| RegRefs.insert(std::make_pair(Reg, &MO)); |
| |
| // If this reg is tied and live (Classes[Reg] is set to -1), we can't change |
| // it or any of its sub or super regs. We need to use KeepRegs to mark the |
| // reg because not all uses of the same reg within an instruction are |
| // necessarily tagged as tied. |
| // Example: an x86 "xor %eax, %eax" will have one source operand tied to the |
| // def register but not the second (see PR20020 for details). |
| // FIXME: can this check be relaxed to account for undef uses |
| // of a register? In the above 'xor' example, the uses of %eax are undef, so |
| // earlier instructions could still replace %eax even though the 'xor' |
| // itself can't be changed. |
| if (MI.isRegTiedToUseOperand(i) && |
| Classes[Reg] == reinterpret_cast<TargetRegisterClass *>(-1)) { |
| for (MCSubRegIterator SubRegs(Reg, TRI, /*IncludeSelf=*/true); |
| SubRegs.isValid(); ++SubRegs) { |
| KeepRegs.set(*SubRegs); |
| } |
| for (MCSuperRegIterator SuperRegs(Reg, TRI); |
| SuperRegs.isValid(); ++SuperRegs) { |
| KeepRegs.set(*SuperRegs); |
| } |
| } |
| |
| if (MO.isUse() && Special) { |
| if (!KeepRegs.test(Reg)) { |
| for (MCSubRegIterator SubRegs(Reg, TRI, /*IncludeSelf=*/true); |
| SubRegs.isValid(); ++SubRegs) |
| KeepRegs.set(*SubRegs); |
| } |
| } |
| } |
| } |
| |
| void CriticalAntiDepBreaker::ScanInstruction(MachineInstr &MI, unsigned Count) { |
| // Update liveness. |
| // Proceeding upwards, registers that are defed but not used in this |
| // instruction are now dead. |
| assert(!MI.isKill() && "Attempting to scan a kill instruction"); |
| |
| if (!TII->isPredicated(MI)) { |
| // Predicated defs are modeled as read + write, i.e. similar to two |
| // address updates. |
| for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) { |
| MachineOperand &MO = MI.getOperand(i); |
| |
| if (MO.isRegMask()) { |
| auto ClobbersPhysRegAndSubRegs = [&](unsigned PhysReg) { |
| for (MCSubRegIterator SRI(PhysReg, TRI, true); SRI.isValid(); ++SRI) |
| if (!MO.clobbersPhysReg(*SRI)) |
| return false; |
| |
| return true; |
| }; |
| |
| for (unsigned i = 0, e = TRI->getNumRegs(); i != e; ++i) { |
| if (ClobbersPhysRegAndSubRegs(i)) { |
| DefIndices[i] = Count; |
| KillIndices[i] = ~0u; |
| KeepRegs.reset(i); |
| Classes[i] = nullptr; |
| RegRefs.erase(i); |
| } |
| } |
| } |
| |
| if (!MO.isReg()) continue; |
| Register Reg = MO.getReg(); |
| if (Reg == 0) continue; |
| if (!MO.isDef()) continue; |
| |
| // Ignore two-addr defs. |
| if (MI.isRegTiedToUseOperand(i)) |
| continue; |
| |
| // If we've already marked this reg as unchangeable, don't remove |
| // it or any of its subregs from KeepRegs. |
| bool Keep = KeepRegs.test(Reg); |
| |
| // For the reg itself and all subregs: update the def to current; |
| // reset the kill state, any restrictions, and references. |
| for (MCSubRegIterator SRI(Reg, TRI, true); SRI.isValid(); ++SRI) { |
| unsigned SubregReg = *SRI; |
| DefIndices[SubregReg] = Count; |
| KillIndices[SubregReg] = ~0u; |
| Classes[SubregReg] = nullptr; |
| RegRefs.erase(SubregReg); |
| if (!Keep) |
| KeepRegs.reset(SubregReg); |
| } |
| // Conservatively mark super-registers as unusable. |
| for (MCSuperRegIterator SR(Reg, TRI); SR.isValid(); ++SR) |
| Classes[*SR] = reinterpret_cast<TargetRegisterClass *>(-1); |
| } |
| } |
| for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) { |
| MachineOperand &MO = MI.getOperand(i); |
| if (!MO.isReg()) continue; |
| Register Reg = MO.getReg(); |
| if (Reg == 0) continue; |
| if (!MO.isUse()) continue; |
| |
| const TargetRegisterClass *NewRC = nullptr; |
| if (i < MI.getDesc().getNumOperands()) |
| NewRC = TII->getRegClass(MI.getDesc(), i, TRI, MF); |
| |
| // For now, only allow the register to be changed if its register |
| // class is consistent across all uses. |
| if (!Classes[Reg] && NewRC) |
| Classes[Reg] = NewRC; |
| else if (!NewRC || Classes[Reg] != NewRC) |
| Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1); |
| |
| RegRefs.insert(std::make_pair(Reg, &MO)); |
| |
| // It wasn't previously live but now it is, this is a kill. |
| // Repeat for all aliases. |
| for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) { |
| unsigned AliasReg = *AI; |
| if (KillIndices[AliasReg] == ~0u) { |
| KillIndices[AliasReg] = Count; |
| DefIndices[AliasReg] = ~0u; |
| } |
| } |
| } |
| } |
| |
| // Check all machine operands that reference the antidependent register and must |
| // be replaced by NewReg. Return true if any of their parent instructions may |
| // clobber the new register. |
| // |
| // Note: AntiDepReg may be referenced by a two-address instruction such that |
| // it's use operand is tied to a def operand. We guard against the case in which |
| // the two-address instruction also defines NewReg, as may happen with |
| // pre/postincrement loads. In this case, both the use and def operands are in |
| // RegRefs because the def is inserted by PrescanInstruction and not erased |
| // during ScanInstruction. So checking for an instruction with definitions of |
| // both NewReg and AntiDepReg covers it. |
| bool |
| CriticalAntiDepBreaker::isNewRegClobberedByRefs(RegRefIter RegRefBegin, |
| RegRefIter RegRefEnd, |
| unsigned NewReg) { |
| for (RegRefIter I = RegRefBegin; I != RegRefEnd; ++I ) { |
| MachineOperand *RefOper = I->second; |
| |
| // Don't allow the instruction defining AntiDepReg to earlyclobber its |
| // operands, in case they may be assigned to NewReg. In this case antidep |
| // breaking must fail, but it's too rare to bother optimizing. |
| if (RefOper->isDef() && RefOper->isEarlyClobber()) |
| return true; |
| |
| // Handle cases in which this instruction defines NewReg. |
| MachineInstr *MI = RefOper->getParent(); |
| for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { |
| const MachineOperand &CheckOper = MI->getOperand(i); |
| |
| if (CheckOper.isRegMask() && CheckOper.clobbersPhysReg(NewReg)) |
| return true; |
| |
| if (!CheckOper.isReg() || !CheckOper.isDef() || |
| CheckOper.getReg() != NewReg) |
| continue; |
| |
| // Don't allow the instruction to define NewReg and AntiDepReg. |
| // When AntiDepReg is renamed it will be an illegal op. |
| if (RefOper->isDef()) |
| return true; |
| |
| // Don't allow an instruction using AntiDepReg to be earlyclobbered by |
| // NewReg. |
| if (CheckOper.isEarlyClobber()) |
| return true; |
| |
| // Don't allow inline asm to define NewReg at all. Who knows what it's |
| // doing with it. |
| if (MI->isInlineAsm()) |
| return true; |
| } |
| } |
| return false; |
| } |
| |
| unsigned CriticalAntiDepBreaker:: |
| findSuitableFreeRegister(RegRefIter RegRefBegin, |
| RegRefIter RegRefEnd, |
| unsigned AntiDepReg, |
| unsigned LastNewReg, |
| const TargetRegisterClass *RC, |
| SmallVectorImpl<unsigned> &Forbid) { |
| ArrayRef<MCPhysReg> Order = RegClassInfo.getOrder(RC); |
| for (unsigned i = 0; i != Order.size(); ++i) { |
| unsigned NewReg = Order[i]; |
| // Don't replace a register with itself. |
| if (NewReg == AntiDepReg) continue; |
| // Don't replace a register with one that was recently used to repair |
| // an anti-dependence with this AntiDepReg, because that would |
| // re-introduce that anti-dependence. |
| if (NewReg == LastNewReg) continue; |
| // If any instructions that define AntiDepReg also define the NewReg, it's |
| // not suitable. For example, Instruction with multiple definitions can |
| // result in this condition. |
| if (isNewRegClobberedByRefs(RegRefBegin, RegRefEnd, NewReg)) continue; |
| // If NewReg is dead and NewReg's most recent def is not before |
| // AntiDepReg's kill, it's safe to replace AntiDepReg with NewReg. |
| assert(((KillIndices[AntiDepReg] == ~0u) != (DefIndices[AntiDepReg] == ~0u)) |
| && "Kill and Def maps aren't consistent for AntiDepReg!"); |
| assert(((KillIndices[NewReg] == ~0u) != (DefIndices[NewReg] == ~0u)) |
| && "Kill and Def maps aren't consistent for NewReg!"); |
| if (KillIndices[NewReg] != ~0u || |
| Classes[NewReg] == reinterpret_cast<TargetRegisterClass *>(-1) || |
| KillIndices[AntiDepReg] > DefIndices[NewReg]) |
| continue; |
| // If NewReg overlaps any of the forbidden registers, we can't use it. |
| bool Forbidden = false; |
| for (SmallVectorImpl<unsigned>::iterator it = Forbid.begin(), |
| ite = Forbid.end(); it != ite; ++it) |
| if (TRI->regsOverlap(NewReg, *it)) { |
| Forbidden = true; |
| break; |
| } |
| if (Forbidden) continue; |
| return NewReg; |
| } |
| |
| // No registers are free and available! |
| return 0; |
| } |
| |
| unsigned CriticalAntiDepBreaker:: |
| BreakAntiDependencies(const std::vector<SUnit> &SUnits, |
| MachineBasicBlock::iterator Begin, |
| MachineBasicBlock::iterator End, |
| unsigned InsertPosIndex, |
| DbgValueVector &DbgValues) { |
| // The code below assumes that there is at least one instruction, |
| // so just duck out immediately if the block is empty. |
| if (SUnits.empty()) return 0; |
| |
| // Keep a map of the MachineInstr*'s back to the SUnit representing them. |
| // This is used for updating debug information. |
| // |
| // FIXME: Replace this with the existing map in ScheduleDAGInstrs::MISUnitMap |
| DenseMap<MachineInstr *, const SUnit *> MISUnitMap; |
| |
| // Find the node at the bottom of the critical path. |
| const SUnit *Max = nullptr; |
| for (unsigned i = 0, e = SUnits.size(); i != e; ++i) { |
| const SUnit *SU = &SUnits[i]; |
| MISUnitMap[SU->getInstr()] = SU; |
| if (!Max || SU->getDepth() + SU->Latency > Max->getDepth() + Max->Latency) |
| Max = SU; |
| } |
| assert(Max && "Failed to find bottom of the critical path"); |
| |
| #ifndef NDEBUG |
| { |
| LLVM_DEBUG(dbgs() << "Critical path has total latency " |
| << (Max->getDepth() + Max->Latency) << "\n"); |
| LLVM_DEBUG(dbgs() << "Available regs:"); |
| for (unsigned Reg = 0; Reg < TRI->getNumRegs(); ++Reg) { |
| if (KillIndices[Reg] == ~0u) |
| LLVM_DEBUG(dbgs() << " " << printReg(Reg, TRI)); |
| } |
| LLVM_DEBUG(dbgs() << '\n'); |
| } |
| #endif |
| |
| // Track progress along the critical path through the SUnit graph as we walk |
| // the instructions. |
| const SUnit *CriticalPathSU = Max; |
| MachineInstr *CriticalPathMI = CriticalPathSU->getInstr(); |
| |
| // Consider this pattern: |
| // A = ... |
| // ... = A |
| // A = ... |
| // ... = A |
| // A = ... |
| // ... = A |
| // A = ... |
| // ... = A |
| // There are three anti-dependencies here, and without special care, |
| // we'd break all of them using the same register: |
| // A = ... |
| // ... = A |
| // B = ... |
| // ... = B |
| // B = ... |
| // ... = B |
| // B = ... |
| // ... = B |
| // because at each anti-dependence, B is the first register that |
| // isn't A which is free. This re-introduces anti-dependencies |
| // at all but one of the original anti-dependencies that we were |
| // trying to break. To avoid this, keep track of the most recent |
| // register that each register was replaced with, avoid |
| // using it to repair an anti-dependence on the same register. |
| // This lets us produce this: |
| // A = ... |
| // ... = A |
| // B = ... |
| // ... = B |
| // C = ... |
| // ... = C |
| // B = ... |
| // ... = B |
| // This still has an anti-dependence on B, but at least it isn't on the |
| // original critical path. |
| // |
| // TODO: If we tracked more than one register here, we could potentially |
| // fix that remaining critical edge too. This is a little more involved, |
| // because unlike the most recent register, less recent registers should |
| // still be considered, though only if no other registers are available. |
| std::vector<unsigned> LastNewReg(TRI->getNumRegs(), 0); |
| |
| // Attempt to break anti-dependence edges on the critical path. Walk the |
| // instructions from the bottom up, tracking information about liveness |
| // as we go to help determine which registers are available. |
| unsigned Broken = 0; |
| unsigned Count = InsertPosIndex - 1; |
| for (MachineBasicBlock::iterator I = End, E = Begin; I != E; --Count) { |
| MachineInstr &MI = *--I; |
| // Kill instructions can define registers but are really nops, and there |
| // might be a real definition earlier that needs to be paired with uses |
| // dominated by this kill. |
| |
| // FIXME: It may be possible to remove the isKill() restriction once PR18663 |
| // has been properly fixed. There can be value in processing kills as seen |
| // in the AggressiveAntiDepBreaker class. |
| if (MI.isDebugInstr() || MI.isKill()) |
| continue; |
| |
| // Check if this instruction has a dependence on the critical path that |
| // is an anti-dependence that we may be able to break. If it is, set |
| // AntiDepReg to the non-zero register associated with the anti-dependence. |
| // |
| // We limit our attention to the critical path as a heuristic to avoid |
| // breaking anti-dependence edges that aren't going to significantly |
| // impact the overall schedule. There are a limited number of registers |
| // and we want to save them for the important edges. |
| // |
| // TODO: Instructions with multiple defs could have multiple |
| // anti-dependencies. The current code here only knows how to break one |
| // edge per instruction. Note that we'd have to be able to break all of |
| // the anti-dependencies in an instruction in order to be effective. |
| unsigned AntiDepReg = 0; |
| if (&MI == CriticalPathMI) { |
| if (const SDep *Edge = CriticalPathStep(CriticalPathSU)) { |
| const SUnit *NextSU = Edge->getSUnit(); |
| |
| // Only consider anti-dependence edges. |
| if (Edge->getKind() == SDep::Anti) { |
| AntiDepReg = Edge->getReg(); |
| assert(AntiDepReg != 0 && "Anti-dependence on reg0?"); |
| if (!MRI.isAllocatable(AntiDepReg)) |
| // Don't break anti-dependencies on non-allocatable registers. |
| AntiDepReg = 0; |
| else if (KeepRegs.test(AntiDepReg)) |
| // Don't break anti-dependencies if a use down below requires |
| // this exact register. |
| AntiDepReg = 0; |
| else { |
| // If the SUnit has other dependencies on the SUnit that it |
| // anti-depends on, don't bother breaking the anti-dependency |
| // since those edges would prevent such units from being |
| // scheduled past each other regardless. |
| // |
| // Also, if there are dependencies on other SUnits with the |
| // same register as the anti-dependency, don't attempt to |
| // break it. |
| for (SUnit::const_pred_iterator P = CriticalPathSU->Preds.begin(), |
| PE = CriticalPathSU->Preds.end(); P != PE; ++P) |
| if (P->getSUnit() == NextSU ? |
| (P->getKind() != SDep::Anti || P->getReg() != AntiDepReg) : |
| (P->getKind() == SDep::Data && P->getReg() == AntiDepReg)) { |
| AntiDepReg = 0; |
| break; |
| } |
| } |
| } |
| CriticalPathSU = NextSU; |
| CriticalPathMI = CriticalPathSU->getInstr(); |
| } else { |
| // We've reached the end of the critical path. |
| CriticalPathSU = nullptr; |
| CriticalPathMI = nullptr; |
| } |
| } |
| |
| PrescanInstruction(MI); |
| |
| SmallVector<unsigned, 2> ForbidRegs; |
| |
| // If MI's defs have a special allocation requirement, don't allow |
| // any def registers to be changed. Also assume all registers |
| // defined in a call must not be changed (ABI). |
| if (MI.isCall() || MI.hasExtraDefRegAllocReq() || TII->isPredicated(MI)) |
| // If this instruction's defs have special allocation requirement, don't |
| // break this anti-dependency. |
| AntiDepReg = 0; |
| else if (AntiDepReg) { |
| // If this instruction has a use of AntiDepReg, breaking it |
| // is invalid. If the instruction defines other registers, |
| // save a list of them so that we don't pick a new register |
| // that overlaps any of them. |
| for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) { |
| MachineOperand &MO = MI.getOperand(i); |
| if (!MO.isReg()) continue; |
| Register Reg = MO.getReg(); |
| if (Reg == 0) continue; |
| if (MO.isUse() && TRI->regsOverlap(AntiDepReg, Reg)) { |
| AntiDepReg = 0; |
| break; |
| } |
| if (MO.isDef() && Reg != AntiDepReg) |
| ForbidRegs.push_back(Reg); |
| } |
| } |
| |
| // Determine AntiDepReg's register class, if it is live and is |
| // consistently used within a single class. |
| const TargetRegisterClass *RC = AntiDepReg != 0 ? Classes[AntiDepReg] |
| : nullptr; |
| assert((AntiDepReg == 0 || RC != nullptr) && |
| "Register should be live if it's causing an anti-dependence!"); |
| if (RC == reinterpret_cast<TargetRegisterClass *>(-1)) |
| AntiDepReg = 0; |
| |
| // Look for a suitable register to use to break the anti-dependence. |
| // |
| // TODO: Instead of picking the first free register, consider which might |
| // be the best. |
| if (AntiDepReg != 0) { |
| std::pair<std::multimap<unsigned, MachineOperand *>::iterator, |
| std::multimap<unsigned, MachineOperand *>::iterator> |
| Range = RegRefs.equal_range(AntiDepReg); |
| if (unsigned NewReg = findSuitableFreeRegister(Range.first, Range.second, |
| AntiDepReg, |
| LastNewReg[AntiDepReg], |
| RC, ForbidRegs)) { |
| LLVM_DEBUG(dbgs() << "Breaking anti-dependence edge on " |
| << printReg(AntiDepReg, TRI) << " with " |
| << RegRefs.count(AntiDepReg) << " references" |
| << " using " << printReg(NewReg, TRI) << "!\n"); |
| |
| // Update the references to the old register to refer to the new |
| // register. |
| for (std::multimap<unsigned, MachineOperand *>::iterator |
| Q = Range.first, QE = Range.second; Q != QE; ++Q) { |
| Q->second->setReg(NewReg); |
| // If the SU for the instruction being updated has debug information |
| // related to the anti-dependency register, make sure to update that |
| // as well. |
| const SUnit *SU = MISUnitMap[Q->second->getParent()]; |
| if (!SU) continue; |
| UpdateDbgValues(DbgValues, Q->second->getParent(), |
| AntiDepReg, NewReg); |
| } |
| |
| // We just went back in time and modified history; the |
| // liveness information for the anti-dependence reg is now |
| // inconsistent. Set the state as if it were dead. |
| Classes[NewReg] = Classes[AntiDepReg]; |
| DefIndices[NewReg] = DefIndices[AntiDepReg]; |
| KillIndices[NewReg] = KillIndices[AntiDepReg]; |
| assert(((KillIndices[NewReg] == ~0u) != |
| (DefIndices[NewReg] == ~0u)) && |
| "Kill and Def maps aren't consistent for NewReg!"); |
| |
| Classes[AntiDepReg] = nullptr; |
| DefIndices[AntiDepReg] = KillIndices[AntiDepReg]; |
| KillIndices[AntiDepReg] = ~0u; |
| assert(((KillIndices[AntiDepReg] == ~0u) != |
| (DefIndices[AntiDepReg] == ~0u)) && |
| "Kill and Def maps aren't consistent for AntiDepReg!"); |
| |
| RegRefs.erase(AntiDepReg); |
| LastNewReg[AntiDepReg] = NewReg; |
| ++Broken; |
| } |
| } |
| |
| ScanInstruction(MI, Count); |
| } |
| |
| return Broken; |
| } |
