Break anti-dependence breaking out into its own class.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@85127 91177308-0d34-0410-b5e6-96231b3b80d8

7 files changed, 737 insertions, 545 deletions

diff --git a/include/llvm/CodeGen/AntiDepBreaker.h b/include/llvm/CodeGen/AntiDepBreaker.h
new file mode 100644
index 0000000000..31fdb24ec4
--- /dev/null
+++ b/include/llvm/CodeGen/AntiDepBreaker.h
@@ -0,0 +1,56 @@
+//=- llvm/CodeGen/AntiDepBreaker.h - Anti-Dependence Breaking -*- C++ -*-=//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the AntiDepBreaker class, which implements
+// anti-dependence breaking heuristics for post-register-allocation scheduling.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CODEGEN_ANTIDEPBREAKER_H
+#define LLVM_CODEGEN_ANTIDEPBREAKER_H
+
+#include "llvm/CodeGen/MachineBasicBlock.h"
+#include "llvm/CodeGen/MachineFrameInfo.h"
+#include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/ScheduleDAG.h"
+#include "llvm/Target/TargetRegisterInfo.h"
+
+namespace llvm {
+
+/// AntiDepBreaker - This class works into conjunction with the
+/// post-RA scheduler to rename registers to break register
+/// anti-dependencies.
+class AntiDepBreaker {
+public:
+  /// Start - Initialize anti-dep breaking for a new basic block.
+  virtual void StartBlock(MachineBasicBlock *BB) =0;
+
+  /// BreakAntiDependencies - Identifiy anti-dependencies within a
+  /// basic-block region and break them by renaming registers. Return
+  /// the number of anti-dependencies broken.
+  ///
+  virtual unsigned BreakAntiDependencies(std::vector<SUnit>& SUnits,
+                                         MachineBasicBlock::iterator& Begin,
+                                         MachineBasicBlock::iterator& End,
+                                         unsigned InsertPosIndex) =0;
+  
+  /// Observe - Update liveness information to account for the current
+  /// instruction, which will not be scheduled.
+  ///
+  virtual void Observe(MachineInstr *MI, unsigned Count,
+                       unsigned InsertPosIndex) =0;
+  
+  /// Finish - Finish anti-dep breaking for a basic block.
+  virtual void FinishBlock() =0;
+};
+
+}
+
+#endif
diff --git a/lib/CodeGen/CMakeLists.txt b/lib/CodeGen/CMakeLists.txt
index 713c30c7d4..14b215ec07 100644
--- a/lib/CodeGen/CMakeLists.txt
+++ b/lib/CodeGen/CMakeLists.txt
@@ -1,6 +1,7 @@
 add_llvm_library(LLVMCodeGen
   BranchFolding.cpp
   CodePlacementOpt.cpp
+  CriticalAntiDepBreaker.cpp
   DeadMachineInstructionElim.cpp
   DwarfEHPrepare.cpp
   ELFCodeEmitter.cpp
diff --git a/lib/CodeGen/CriticalAntiDepBreaker.cpp b/lib/CodeGen/CriticalAntiDepBreaker.cpp
new file mode 100644
index 0000000000..ceb48d7845
--- /dev/null
+++ b/lib/CodeGen/CriticalAntiDepBreaker.cpp
@@ -0,0 +1,539 @@
+//===----- CriticalAntiDepBreaker.cpp - Anti-dep breaker -------- ---------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the CriticalAntiDepBreaker class, which
+// implements register anti-dependence breaking along a blocks
+// critical path during post-RA scheduler.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "critical-antidep"
+#include "CriticalAntiDepBreaker.h"
+#include "llvm/CodeGen/MachineBasicBlock.h"
+#include "llvm/CodeGen/MachineFrameInfo.h"
+#include "llvm/Target/TargetMachine.h"
+#include "llvm/Target/TargetRegisterInfo.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/raw_ostream.h"
+
+using namespace llvm;
+
+CriticalAntiDepBreaker::
+CriticalAntiDepBreaker(MachineFunction& MFi) : 
+  AntiDepBreaker(), MF(MFi),
+  MRI(MF.getRegInfo()),
+  TRI(MF.getTarget().getRegisterInfo()),
+  AllocatableSet(TRI->getAllocatableSet(MF))
+{
+}
+
+CriticalAntiDepBreaker::~CriticalAntiDepBreaker() {
+}
+
+void CriticalAntiDepBreaker::StartBlock(MachineBasicBlock *BB) {
+  // Clear out the register class data.
+  std::fill(Classes, array_endof(Classes),
+            static_cast<const TargetRegisterClass *>(0));
+
+  // Initialize the indices to indicate that no registers are live.
+  std::fill(KillIndices, array_endof(KillIndices), ~0u);
+  std::fill(DefIndices, array_endof(DefIndices), BB->size());
+
+  // Clear "do not change" set.
+  KeepRegs.clear();
+
+  bool IsReturnBlock = (!BB->empty() && BB->back().getDesc().isReturn());
+
+  // Determine the live-out physregs for this block.
+  if (IsReturnBlock) {
+    // In a return block, examine the function live-out regs.
+    for (MachineRegisterInfo::liveout_iterator I = MRI.liveout_begin(),
+         E = MRI.liveout_end(); I != E; ++I) {
+      unsigned Reg = *I;
+      Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1);
+      KillIndices[Reg] = BB->size();
+      DefIndices[Reg] = ~0u;
+      // Repeat, for all aliases.
+      for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) {
+        unsigned AliasReg = *Alias;
+        Classes[AliasReg] = reinterpret_cast<TargetRegisterClass *>(-1);
+        KillIndices[AliasReg] = BB->size();
+        DefIndices[AliasReg] = ~0u;
+      }
+    }
+  } else {
+    // In a non-return block, examine the live-in regs of all successors.
+    for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
+         SE = BB->succ_end(); SI != SE; ++SI)
+      for (MachineBasicBlock::livein_iterator I = (*SI)->livein_begin(),
+           E = (*SI)->livein_end(); I != E; ++I) {
+        unsigned Reg = *I;
+        Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1);
+        KillIndices[Reg] = BB->size();
+        DefIndices[Reg] = ~0u;
+        // Repeat, for all aliases.
+        for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) {
+          unsigned AliasReg = *Alias;
+          Classes[AliasReg] = reinterpret_cast<TargetRegisterClass *>(-1);
+          KillIndices[AliasReg] = BB->size();
+          DefIndices[AliasReg] = ~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(BB);
+  for (const unsigned *I = TRI->getCalleeSavedRegs(); *I; ++I) {
+    unsigned Reg = *I;
+    if (!IsReturnBlock && !Pristine.test(Reg)) continue;
+    Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1);
+    KillIndices[Reg] = BB->size();
+    DefIndices[Reg] = ~0u;
+    // Repeat, for all aliases.
+    for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) {
+      unsigned AliasReg = *Alias;
+      Classes[AliasReg] = reinterpret_cast<TargetRegisterClass *>(-1);
+      KillIndices[AliasReg] = BB->size();
+      DefIndices[AliasReg] = ~0u;
+    }
+  }
+}
+
+void CriticalAntiDepBreaker::FinishBlock() {
+  RegRefs.clear();
+  KeepRegs.clear();
+}
+
+void CriticalAntiDepBreaker::Observe(MachineInstr *MI, unsigned Count,
+                                     unsigned InsertPosIndex) {
+  assert(Count < InsertPosIndex && "Instruction index out of expected range!");
+
+  // 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.
+  for (unsigned Reg = 0; Reg != TargetRegisterInfo::FirstVirtualRegister; ++Reg)
+    if (DefIndices[Reg] < InsertPosIndex && DefIndices[Reg] >= Count) {
+      assert(KillIndices[Reg] == ~0u && "Clobbered register is live!");
+      // Mark this register to be non-renamable.
+      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 SDep *CriticalPathStep(SUnit *SU) {
+  SDep *Next = 0;
+  unsigned NextDepth = 0;
+  // Find the predecessor edge with the greatest depth.
+  for (SUnit::pred_iterator P = SU->Preds.begin(), PE = SU->Preds.end();
+       P != PE; ++P) {
+    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) {
+  // 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;
+    unsigned Reg = MO.getReg();
+    if (Reg == 0) continue;
+    const TargetRegisterClass *NewRC = 0;
+    
+    if (i < MI->getDesc().getNumOperands())
+      NewRC = MI->getDesc().OpInfo[i].getRegClass(TRI);
+
+    // 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 (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) {
+      // 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 = *Alias;
+      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));
+
+    // It's not safe to change register allocation for source operands of
+    // that have special allocation requirements.
+    if (MO.isUse() && MI->getDesc().hasExtraSrcRegAllocReq()) {
+      if (KeepRegs.insert(Reg)) {
+        for (const unsigned *Subreg = TRI->getSubRegisters(Reg);
+             *Subreg; ++Subreg)
+          KeepRegs.insert(*Subreg);
+      }
+    }
+  }
+}
+
+void CriticalAntiDepBreaker::ScanInstruction(MachineInstr *MI,
+                                             unsigned Count) {
+  // Update liveness.
+  // Proceding upwards, registers that are defed but not used in this
+  // instruction are now dead.
+  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
+    MachineOperand &MO = MI->getOperand(i);
+    if (!MO.isReg()) continue;
+    unsigned Reg = MO.getReg();
+    if (Reg == 0) continue;
+    if (!MO.isDef()) continue;
+    // Ignore two-addr defs.
+    if (MI->isRegTiedToUseOperand(i)) continue;
+
+    DefIndices[Reg] = Count;
+    KillIndices[Reg] = ~0u;
+    assert(((KillIndices[Reg] == ~0u) !=
+            (DefIndices[Reg] == ~0u)) &&
+           "Kill and Def maps aren't consistent for Reg!");
+    KeepRegs.erase(Reg);
+    Classes[Reg] = 0;
+    RegRefs.erase(Reg);
+    // Repeat, for all subregs.
+    for (const unsigned *Subreg = TRI->getSubRegisters(Reg);
+         *Subreg; ++Subreg) {
+      unsigned SubregReg = *Subreg;
+      DefIndices[SubregReg] = Count;
+      KillIndices[SubregReg] = ~0u;
+      KeepRegs.erase(SubregReg);
+      Classes[SubregReg] = 0;
+      RegRefs.erase(SubregReg);
+    }
+    // Conservatively mark super-registers as unusable.
+    for (const unsigned *Super = TRI->getSuperRegisters(Reg);
+         *Super; ++Super) {
+      unsigned SuperReg = *Super;
+      Classes[SuperReg] = reinterpret_cast<TargetRegisterClass *>(-1);
+    }
+  }
+  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
+    MachineOperand &MO = MI->getOperand(i);
+    if (!MO.isReg()) continue;
+    unsigned Reg = MO.getReg();
+    if (Reg == 0) continue;
+    if (!MO.isUse()) continue;
+
+    const TargetRegisterClass *NewRC = 0;
+    if (i < MI->getDesc().getNumOperands())
+      NewRC = MI->getDesc().OpInfo[i].getRegClass(TRI);
+
+    // 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.
+    if (KillIndices[Reg] == ~0u) {
+      KillIndices[Reg] = Count;
+      DefIndices[Reg] = ~0u;
+          assert(((KillIndices[Reg] == ~0u) !=
+                  (DefIndices[Reg] == ~0u)) &&
+               "Kill and Def maps aren't consistent for Reg!");
+    }
+    // Repeat, for all aliases.
+    for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) {
+      unsigned AliasReg = *Alias;
+      if (KillIndices[AliasReg] == ~0u) {
+        KillIndices[AliasReg] = Count;
+        DefIndices[AliasReg] = ~0u;
+      }
+    }
+  }
+}
+
+unsigned
+CriticalAntiDepBreaker::findSuitableFreeRegister(unsigned AntiDepReg,
+                                                 unsigned LastNewReg,
+                                                 const TargetRegisterClass *RC) {
+  for (TargetRegisterClass::iterator R = RC->allocation_order_begin(MF),
+       RE = RC->allocation_order_end(MF); R != RE; ++R) {
+    unsigned NewReg = *R;
+    // 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 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;
+    return NewReg;
+  }
+
+  // No registers are free and available!
+  return 0;
+}
+
+unsigned CriticalAntiDepBreaker::
+BreakAntiDependencies(std::vector<SUnit>& SUnits,
+                      MachineBasicBlock::iterator& Begin,
+                      MachineBasicBlock::iterator& End,
+                      unsigned InsertPosIndex) {
+  // 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;
+
+  // Find the node at the bottom of the critical path.
+  SUnit *Max = 0;
+  for (unsigned i = 0, e = SUnits.size(); i != e; ++i) {
+    SUnit *SU = &SUnits[i];
+    if (!Max || SU->getDepth() + SU->Latency > Max->getDepth() + Max->Latency)
+      Max = SU;
+  }
+
+#ifndef NDEBUG
+  {
+    DEBUG(errs() << "Critical path has total latency "
+          << (Max->getDepth() + Max->Latency) << "\n");
+    DEBUG(errs() << "Available regs:");
+    for (unsigned Reg = 0; Reg < TRI->getNumRegs(); ++Reg) {
+      if (KillIndices[Reg] == ~0u)
+        DEBUG(errs() << " " << TRI->getName(Reg));
+    }
+    DEBUG(errs() << '\n');
+  }
+#endif
+
+  // Track progress along the critical path through the SUnit graph as we walk
+  // the instructions.
+  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.
+  unsigned LastNewReg[TargetRegisterInfo::FirstVirtualRegister] = {};
+
+  // 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;
+
+    // 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 (SDep *Edge = CriticalPathStep(CriticalPathSU)) {
+        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 (!AllocatableSet.test(AntiDepReg))
+            // Don't break anti-dependencies on non-allocatable registers.
+            AntiDepReg = 0;
+          else if (KeepRegs.count(AntiDepReg))
+            // Don't break anti-dependencies if an 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::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 = 0;
+        CriticalPathMI = 0;
+      }
+    }
+
+    PrescanInstruction(MI);
+
+    if (MI->getDesc().hasExtraDefRegAllocReq())
+      // 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.
+      for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
+        MachineOperand &MO = MI->getOperand(i);
+        if (!MO.isReg()) continue;
+        unsigned Reg = MO.getReg();
+        if (Reg == 0) continue;
+        if (MO.isUse() && AntiDepReg == Reg) {
+          AntiDepReg = 0;
+          break;
+        }
+      }
+    }
+
+    // Determine AntiDepReg's register class, if it is live and is
+    // consistently used within a single class.
+    const TargetRegisterClass *RC = AntiDepReg != 0 ? Classes[AntiDepReg] : 0;
+    assert((AntiDepReg == 0 || RC != NULL) &&
+           "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-depenence.
+    //
+    // TODO: Instead of picking the first free register, consider which might
+    // be the best.
+    if (AntiDepReg != 0) {
+      if (unsigned NewReg = findSuitableFreeRegister(AntiDepReg,
+                                                     LastNewReg[AntiDepReg],
+                                                     RC)) {
+        DEBUG(errs() << "Breaking anti-dependence edge on "
+              << TRI->getName(AntiDepReg)
+              << " with " << RegRefs.count(AntiDepReg) << " references"
+              << " using " << TRI->getName(NewReg) << "!\n");
+
+        // Update the references to the old register to refer to the new
+        // register.
+        std::pair<std::multimap<unsigned, MachineOperand *>::iterator,
+                  std::multimap<unsigned, MachineOperand *>::iterator>
+           Range = RegRefs.equal_range(AntiDepReg);
+        for (std::multimap<unsigned, MachineOperand *>::iterator
+             Q = Range.first, QE = Range.second; Q != QE; ++Q)
+          Q->second->setReg(NewReg);
+
+        // We just went back in time and modified history; the
+        // liveness information for the anti-depenence 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] = 0;
+        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;
+}
diff --git a/lib/CodeGen/CriticalAntiDepBreaker.h b/lib/CodeGen/CriticalAntiDepBreaker.h
new file mode 100644
index 0000000000..830b29a060
--- /dev/null
+++ b/lib/CodeGen/CriticalAntiDepBreaker.h
@@ -0,0 +1,95 @@
+//=- llvm/CodeGen/CriticalAntiDepBreaker.h - Anti-Dep Support -*- C++ -*-=//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the CriticalAntiDepBreaker class, which
+// implements register anti-dependence breaking along a blocks
+// critical path during post-RA scheduler.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CODEGEN_CRITICALANTIDEPBREAKER_H
+#define LLVM_CODEGEN_CRITICALANTIDEPBREAKER_H
+
+#include "llvm/CodeGen/AntiDepBreaker.h"
+#include "llvm/CodeGen/MachineBasicBlock.h"
+#include "llvm/CodeGen/MachineFrameInfo.h"
+#include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/ScheduleDAG.h"
+#include "llvm/Target/TargetRegisterInfo.h"
+#include "llvm/ADT/BitVector.h"
+#include "llvm/ADT/SmallSet.h"
+
+namespace llvm {
+  class CriticalAntiDepBreaker : public AntiDepBreaker {
+    MachineFunction& MF;
+    MachineRegisterInfo &MRI;
+    const TargetRegisterInfo *TRI;
+
+    /// AllocatableSet - The set of allocatable registers.
+    /// We'll be ignoring anti-dependencies on non-allocatable registers,
+    /// because they may not be safe to break.
+    const BitVector AllocatableSet;
+
+    /// Classes - For live regs that are only used in one register class in a
+    /// live range, the register class. If the register is not live, the
+    /// corresponding value is null. If the register is live but used in
+    /// multiple register classes, the corresponding value is -1 casted to a
+    /// pointer.
+    const TargetRegisterClass *
+      Classes[TargetRegisterInfo::FirstVirtualRegister];
+
+    /// RegRegs - Map registers to all their references within a live range.
+    std::multimap<unsigned, MachineOperand *> RegRefs;
+
+    /// KillIndices - The index of the most recent kill (proceding bottom-up),
+    /// or ~0u if the register is not live.
+    unsigned KillIndices[TargetRegisterInfo::FirstVirtualRegister];
+
+    /// DefIndices - The index of the most recent complete def (proceding bottom
+    /// up), or ~0u if the register is live.
+    unsigned DefIndices[TargetRegisterInfo::FirstVirtualRegister];
+
+    /// KeepRegs - A set of registers which are live and cannot be changed to
+    /// break anti-dependencies.
+    SmallSet<unsigned, 4> KeepRegs;
+
+  public:
+    CriticalAntiDepBreaker(MachineFunction& MFi);
+    ~CriticalAntiDepBreaker();
+    
+    /// Start - Initialize anti-dep breaking for a new basic block.
+    void StartBlock(MachineBasicBlock *BB);
+
+    /// BreakAntiDependencies - Identifiy anti-dependencies along the critical path
+    /// of the ScheduleDAG and break them by renaming registers.
+    ///
+    unsigned BreakAntiDependencies(std::vector<SUnit>& SUnits,
+                                   MachineBasicBlock::iterator& Begin,
+                                   MachineBasicBlock::iterator& End,
+                                   unsigned InsertPosIndex);
+
+    /// Observe - Update liveness information to account for the current
+    /// instruction, which will not be scheduled.
+    ///
+    void Observe(MachineInstr *MI, unsigned Count, unsigned InsertPosIndex);
+
+    /// Finish - Finish anti-dep breaking for a basic block.
+    void FinishBlock();
+
+  private:
+    void PrescanInstruction(MachineInstr *MI);
+    void ScanInstruction(MachineInstr *MI, unsigned Count);
+    unsigned findSuitableFreeRegister(unsigned AntiDepReg,
+                                      unsigned LastNewReg,
+                                      const TargetRegisterClass *);
+  };
+}
+
+#endif
diff --git a/lib/CodeGen/PostRASchedulerList.cpp b/lib/CodeGen/PostRASchedulerList.cpp
index 106de5c064..ea66f02a4b 100644
--- a/lib/CodeGen/PostRASchedulerList.cpp
+++ b/lib/CodeGen/PostRASchedulerList.cpp
@@ -19,6 +19,7 @@
 //===----------------------------------------------------------------------===//
 
 #define DEBUG_TYPE "post-RA-sched"
+#include "CriticalAntiDepBreaker.h"
 #include "ExactHazardRecognizer.h"
 #include "SimpleHazardRecognizer.h"
 #include "ScheduleDAGInstrs.h"
@@ -40,6 +41,7 @@
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/raw_ostream.h"
+#include "llvm/ADT/BitVector.h"
 #include "llvm/ADT/Statistic.h"
 #include <map>
 #include <set>
@@ -47,6 +49,7 @@ using namespace llvm;
 
 STATISTIC(NumNoops, "Number of noops inserted");
 STATISTIC(NumStalls, "Number of pipeline stalls");
+STATISTIC(NumFixedAnti, "Number of fixed anti-dependencies");
 
 // Post-RA scheduling is enabled with
 // TargetSubtarget.enablePostRAScheduler(). This flag can be used to
@@ -55,10 +58,11 @@ static cl::opt<bool>
 EnablePostRAScheduler("post-RA-scheduler",
                        cl::desc("Enable scheduling after register allocation"),
                        cl::init(false), cl::Hidden);
-static cl::opt<bool>
+static cl::opt<std::string>
 EnableAntiDepBreaking("break-anti-dependencies",
-                      cl::desc("Break post-RA scheduling anti-dependencies"),
-                      cl::init(true), cl::Hidden);
+                      cl::desc("Break post-RA scheduling anti-dependencies: "
+                               "\"critical\", \"all\", or \"none\""),
+                      cl::init("none"), cl::Hidden);
 static cl::opt<bool>
 EnablePostRAHazardAvoidance("avoid-hazards",
                       cl::desc("Enable exact hazard avoidance"),
@@ -116,56 +120,30 @@ namespace {
     /// Topo - A topological ordering for SUnits.
     ScheduleDAGTopologicalSort Topo;
 
-    /// AllocatableSet - The set of allocatable registers.
-    /// We'll be ignoring anti-dependencies on non-allocatable registers,
-    /// because they may not be safe to break.
-    const BitVector AllocatableSet;
-
     /// HazardRec - The hazard recognizer to use.
     ScheduleHazardRecognizer *HazardRec;
 
+    /// AntiDepBreak - Anti-dependence breaking object, or NULL if none
+    AntiDepBreaker *AntiDepBreak;
+
     /// AA - AliasAnalysis for making memory reference queries.
     AliasAnalysis *AA;
 
-    /// AntiDepMode - Anti-dependence breaking mode
-    TargetSubtarget::AntiDepBreakMode AntiDepMode;
-
-    /// Classes - For live regs that are only used in one register class in a
-    /// live range, the register class. If the register is not live, the
-    /// corresponding value is null. If the register is live but used in
-    /// multiple register classes, the corresponding value is -1 casted to a
-    /// pointer.
-    const TargetRegisterClass *
-      Classes[TargetRegisterInfo::FirstVirtualRegister];
-
-    /// RegRegs - Map registers to all their references within a live range.
-    std::multimap<unsigned, MachineOperand *> RegRefs;
-
     /// KillIndices - The index of the most recent kill (proceding bottom-up),
     /// or ~0u if the register is not live.
     unsigned KillIndices[TargetRegisterInfo::FirstVirtualRegister];
 
-    /// DefIndices - The index of the most recent complete def (proceding bottom
-    /// up), or ~0u if the register is live.
-    unsigned DefIndices[TargetRegisterInfo::FirstVirtualRegister];
-
-    /// KeepRegs - A set of registers which are live and cannot be changed to
-    /// break anti-dependencies.
-    SmallSet<unsigned, 4> KeepRegs;
-
   public:
     SchedulePostRATDList(MachineFunction &MF,
                          const MachineLoopInfo &MLI,
                          const MachineDominatorTree &MDT,
                          ScheduleHazardRecognizer *HR,
-                         AliasAnalysis *aa,
-                         TargetSubtarget::AntiDepBreakMode adm)
+                         AntiDepBreaker *ADB,
+                         AliasAnalysis *aa)
       : ScheduleDAGInstrs(MF, MLI, MDT), Topo(SUnits),
-        AllocatableSet(TRI->getAllocatableSet(MF)),
-      HazardRec(HR), AA(aa), AntiDepMode(adm) {}
+      HazardRec(HR), AntiDepBreak(ADB), AA(aa) {}
 
     ~SchedulePostRATDList() {
-      delete HazardRec;
     }
 
     /// StartBlock - Initialize register live-range state for scheduling in
@@ -177,11 +155,6 @@ namespace {
     ///
     void Schedule();
     
-    /// FixupKills - Fix register kill flags that have been made
-    /// invalid due to scheduling
-    ///
-    void FixupKills(MachineBasicBlock *MBB);
-
     /// Observe - Update liveness information to account for the current
     /// instruction, which will not be scheduled.
     ///
@@ -191,17 +164,16 @@ namespace {
     ///
     void FinishBlock();
 
+    /// FixupKills - Fix register kill flags that have been made
+    /// invalid due to scheduling
+    ///
+    void FixupKills(MachineBasicBlock *MBB);
+
   private:
-    void PrescanInstruction(MachineInstr *MI);
-    void ScanInstruction(MachineInstr *MI, unsigned Count);
     void ReleaseSucc(SUnit *SU, SDep *SuccEdge);
     void ReleaseSuccessors(SUnit *SU);
     void ScheduleNodeTopDown(SUnit *SU, unsigned CurCycle);
     void ListScheduleTopDown();
-    bool BreakAntiDependencies();
-    unsigned findSuitableFreeRegister(unsigned AntiDepReg,
-                                      unsigned LastNewReg,
-                                      const TargetRegisterClass *);
     void StartBlockForKills(MachineBasicBlock *BB);
     
     // ToggleKillFlag - Toggle a register operand kill flag. Other
@@ -250,8 +222,9 @@ bool PostRAScheduler::runOnMachineFunction(MachineFunction &Fn) {
 
   // Check for antidep breaking override...
   if (EnableAntiDepBreaking.getPosition() > 0) {
-    AntiDepMode = (EnableAntiDepBreaking) ? 
-      TargetSubtarget::ANTIDEP_CRITICAL : TargetSubtarget::ANTIDEP_NONE;
+    AntiDepMode = (EnableAntiDepBreaking == "all") ? TargetSubtarget::ANTIDEP_ALL :
+      (EnableAntiDepBreaking == "critical") ? TargetSubtarget::ANTIDEP_CRITICAL :
+      TargetSubtarget::ANTIDEP_NONE;
   }
 
   DEBUG(errs() << "PostRAScheduler\n");
@@ -262,8 +235,12 @@ bool PostRAScheduler::runOnMachineFunction(MachineFunction &Fn) {
   ScheduleHazardRecognizer *HR = EnablePostRAHazardAvoidance ?
     (ScheduleHazardRecognizer *)new ExactHazardRecognizer(InstrItins) :
     (ScheduleHazardRecognizer *)new SimpleHazardRecognizer();
+  AntiDepBreaker *ADB = 
+    (AntiDepMode == TargetSubtarget::ANTIDEP_ALL) ? NULL /* FIXME */ :
+    (AntiDepMode == TargetSubtarget::ANTIDEP_CRITICAL) ? 
+    new CriticalAntiDepBreaker(Fn) : NULL;
 
-  SchedulePostRATDList Scheduler(Fn, MLI, MDT, HR, AA, AntiDepMode);
+  SchedulePostRATDList Scheduler(Fn, MLI, MDT, HR, ADB, AA);
 
   // Loop over all of the basic blocks
   for (MachineFunction::iterator MBB = Fn.begin(), MBBe = Fn.end();
@@ -311,6 +288,9 @@ bool PostRAScheduler::runOnMachineFunction(MachineFunction &Fn) {
     Scheduler.FixupKills(MBB);
   }
 
+  delete HR;
+  delete ADB;
+
   return true;
 }
   
@@ -321,78 +301,10 @@ void SchedulePostRATDList::StartBlock(MachineBasicBlock *BB) {
   // Call the superclass.
   ScheduleDAGInstrs::StartBlock(BB);
 
-  // Reset the hazard recognizer.
+  // Reset the hazard recognizer and anti-dep breaker.
   HazardRec->Reset();
-
-  // Clear out the register class data.
-  std::fill(Classes, array_endof(Classes),
-            static_cast<const TargetRegisterClass *>(0));
-
-  // Initialize the indices to indicate that no registers are live.
-  std::fill(KillIndices, array_endof(KillIndices), ~0u);
-  std::fill(DefIndices, array_endof(DefIndices), BB->size());
-
-  // Clear "do not change" set.
-  KeepRegs.clear();
-
-  bool IsReturnBlock = (!BB->empty() && BB->back().getDesc().isReturn());
-
-  // Determine the live-out physregs for this block.
-  if (IsReturnBlock) {
-    // In a return block, examine the function live-out regs.
-    for (MachineRegisterInfo::liveout_iterator I = MRI.liveout_begin(),
-         E = MRI.liveout_end(); I != E; ++I) {
-      unsigned Reg = *I;
-      Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1);
-      KillIndices[Reg] = BB->size();
-      DefIndices[Reg] = ~0u;
-      // Repeat, for all aliases.
-      for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) {
-        unsigned AliasReg = *Alias;
-        Classes[AliasReg] = reinterpret_cast<TargetRegisterClass *>(-1);
-        KillIndices[AliasReg] = BB->size();
-        DefIndices[AliasReg] = ~0u;
-      }
-    }
-  } else {
-    // In a non-return block, examine the live-in regs of all successors.
-    for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
-         SE = BB->succ_end(); SI != SE; ++SI)
-      for (MachineBasicBlock::livein_iterator I = (*SI)->livein_begin(),
-           E = (*SI)->livein_end(); I != E; ++I) {
-        unsigned Reg = *I;
-        Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1);
-        KillIndices[Reg] = BB->size();
-        DefIndices[Reg] = ~0u;
-        // Repeat, for all aliases.
-        for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) {
-          unsigned AliasReg = *Alias;
-          Classes[AliasReg] = reinterpret_cast<TargetRegisterClass *>(-1);
-          KillIndices[AliasReg] = BB->size();
-          DefIndices[AliasReg] = ~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(BB);
-  for (const unsigned *I = TRI->getCalleeSavedRegs(); *I; ++I) {
-    unsigned Reg = *I;
-    if (!IsReturnBlock && !Pristine.test(Reg)) continue;
-    Classes[Reg] = reinterpret_cast<TargetRegisterClass *>(-1);
-    KillIndices[Reg] = BB->size();
-    DefIndices[Reg] = ~0u;
-    // Repeat, for all aliases.
-    for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) {
-      unsigned AliasReg = *Alias;
-      Classes[AliasReg] = reinterpret_cast<TargetRegisterClass *>(-1);
-      KillIndices[AliasReg] = BB->size();
-      DefIndices[AliasReg] = ~0u;
-    }
-  }
+  if (AntiDepBreak != NULL)
+    AntiDepBreak->StartBlock(BB);
 }
 
 /// Schedule - Schedule the instruction range using list scheduling.
@@ -403,8 +315,11 @@ void SchedulePostRATDList::Schedule() {
   // Build the scheduling graph.
   BuildSchedGraph(AA);
 
-  if (AntiDepMode != TargetSubtarget::ANTIDEP_NONE) {
-    if (BreakAntiDependencies()) {
+  if (AntiDepBreak != NULL) {
+    unsigned Broken = 
+      AntiDepBreak->BreakAntiDependencies(SUnits, Begin, InsertPos,
+                                          InsertPosIndex);
+    if (Broken > 0) {
       // We made changes. Update the dependency graph.
       // Theoretically we could update the graph in place:
       // When a live range is changed to use a different register, remove
@@ -415,6 +330,8 @@ void SchedulePostRATDList::Schedule() {
       EntrySU = SUnit();
       ExitSU = SUnit();
       BuildSchedGraph(AA);
+
+      NumFixedAnti += Broken;
     }
   }
 
@@ -432,436 +349,20 @@ void SchedulePostRATDList::Schedule() {
 /// instruction, which will not be scheduled.
 ///
 void SchedulePostRATDList::Observe(MachineInstr *MI, unsigned Count) {
-  assert(Count < InsertPosIndex && "Instruction index out of expected range!");
-
-  // 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.
-  for (unsigned Reg = 0; Reg != TargetRegisterInfo::FirstVirtualRegister; ++Reg)
-    if (DefIndices[Reg] < InsertPosIndex && DefIndices[Reg] >= Count) {
-      assert(KillIndices[Reg] == ~0u && "Clobbered register is live!");
-      // Mark this register to be non-renamable.
-      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);
+  if (AntiDepBreak != NULL)
+    AntiDepBreak->Observe(MI, Count, InsertPosIndex);
 }
 
 /// FinishBlock - Clean up register live-range state.
 ///
 void SchedulePostRATDList::FinishBlock() {
-  RegRefs.clear();
+  if (AntiDepBreak != NULL)
+    AntiDepBreak->FinishBlock();
 
   // Call the superclass.
   ScheduleDAGInstrs::FinishBlock();
 }
 
-/// CriticalPathStep - Return the next SUnit after SU on the bottom-up
-/// critical path.
-static SDep *CriticalPathStep(SUnit *SU) {
-  SDep *Next = 0;
-  unsigned NextDepth = 0;
-  // Find the predecessor edge with the greatest depth.
-  for (SUnit::pred_iterator P = SU->Preds.begin(), PE = SU->Preds.end();
-       P != PE; ++P) {
-    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 SchedulePostRATDList::PrescanInstruction(MachineInstr *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;
-    unsigned Reg = MO.getReg();
-    if (Reg == 0) continue;
-    const TargetRegisterClass *NewRC = 0;
-    
-    if (i < MI->getDesc().getNumOperands())
-      NewRC = MI->getDesc().OpInfo[i].getRegClass(TRI);
-
-    // 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 (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) {
-      // 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 = *Alias;
-      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));
-
-    // It's not safe to change register allocation for source operands of
-    // that have special allocation requirements.
-    if (MO.isUse() && MI->getDesc().hasExtraSrcRegAllocReq()) {
-      if (KeepRegs.insert(Reg)) {
-        for (const unsigned *Subreg = TRI->getSubRegisters(Reg);
-             *Subreg; ++Subreg)
-          KeepRegs.insert(*Subreg);
-      }
-    }
-  }
-}
-
-void SchedulePostRATDList::ScanInstruction(MachineInstr *MI,
-                                           unsigned Count) {
-  // Update liveness.
-  // Proceding upwards, registers that are defed but not used in this
-  // instruction are now dead.
-  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
-    MachineOperand &MO = MI->getOperand(i);
-    if (!MO.isReg()) continue;
-    unsigned Reg = MO.getReg();
-    if (Reg == 0) continue;
-    if (!MO.isDef()) continue;
-    // Ignore two-addr defs.
-    if (MI->isRegTiedToUseOperand(i)) continue;
-
-    DefIndices[Reg] = Count;
-    KillIndices[Reg] = ~0u;
-    assert(((KillIndices[Reg] == ~0u) !=
-            (DefIndices[Reg] == ~0u)) &&
-           "Kill and Def maps aren't consistent for Reg!");
-    KeepRegs.erase(Reg);
-    Classes[Reg] = 0;
-    RegRefs.erase(Reg);
-    // Repeat, for all subregs.
-    for (const unsigned *Subreg = TRI->getSubRegisters(Reg);
-         *Subreg; ++Subreg) {
-      unsigned SubregReg = *Subreg;
-      DefIndices[SubregReg] = Count;
-      KillIndices[SubregReg] = ~0u;
-      KeepRegs.erase(SubregReg);
-      Classes[SubregReg] = 0;
-      RegRefs.erase(SubregReg);
-    }
-    // Conservatively mark super-registers as unusable.
-    for (const unsigned *Super = TRI->getSuperRegisters(Reg);
-         *Super; ++Super) {
-      unsigned SuperReg = *Super;
-      Classes[SuperReg] = reinterpret_cast<TargetRegisterClass *>(-1);
-    }
-  }
-  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
-    MachineOperand &MO = MI->getOperand(i);
-    if (!MO.isReg()) continue;
-    unsigned Reg = MO.getReg();
-    if (Reg == 0) continue;
-    if (!MO.isUse()) continue;
-
-    const TargetRegisterClass *NewRC = 0;
-    if (i < MI->getDesc().getNumOperands())
-      NewRC = MI->getDesc().OpInfo[i].getRegClass(TRI);
-
-    // 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.
-    if (KillIndices[Reg] == ~0u) {
-      KillIndices[Reg] = Count;
-      DefIndices[Reg] = ~0u;
-          assert(((KillIndices[Reg] == ~0u) !=
-                  (DefIndices[Reg] == ~0u)) &&
-               "Kill and Def maps aren't consistent for Reg!");
-    }
-    // Repeat, for all aliases.
-    for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) {
-      unsigned AliasReg = *Alias;
-      if (KillIndices[AliasReg] == ~0u) {
-        KillIndices[AliasReg] = Count;
-        DefIndices[AliasReg] = ~0u;
-      }
-    }
-  }
-}
-
-unsigned
-SchedulePostRATDList::findSuitableFreeRegister(unsigned AntiDepReg,
-                                               unsigned LastNewReg,
-                                               const TargetRegisterClass *RC) {
-  for (TargetRegisterClass::iterator R = RC->allocation_order_begin(MF),
-       RE = RC->allocation_order_end(MF); R != RE; ++R) {
-    unsigned NewReg = *R;
-    // 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 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;
-    return NewReg;
-  }
-
-  // No registers are free and available!
-  return 0;
-}
-
-/// BreakAntiDependencies - Identifiy anti-dependencies along the critical path
-/// of the ScheduleDAG and break them by renaming registers.
-///
-bool SchedulePostRATDList::BreakAntiDependencies() {
-  // 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 false;
-
-  // Find the node at the bottom of the critical path.
-  SUnit *Max = 0;
-  for (unsigned i = 0, e = SUnits.size(); i != e; ++i) {
-    SUnit *SU = &SUnits[i];
-    if (!Max || SU->getDepth() + SU->Latency > Max->getDepth() + Max->Latency)
-      Max = SU;
-  }
-
-#ifndef NDEBUG
-  {
-    DEBUG(errs() << "Critical path has total latency "
-          << (Max->getDepth() + Max->Latency) << "\n");
-    DEBUG(errs() << "Available regs:");
-    for (unsigned Reg = 0; Reg < TRI->getNumRegs(); ++Reg) {
-      if (KillIndices[Reg] == ~0u)
-        DEBUG(errs() << " " << TRI->getName(Reg));
-    }
-    DEBUG(errs() << '\n');
-  }
-#endif
-
-  // Track progress along the critical path through the SUnit graph as we walk
-  // the instructions.
-  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.
-  unsigned LastNewReg[TargetRegisterInfo::FirstVirtualRegister] = {};
-
-  // 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.
-  bool Changed = false;
-  unsigned Count = InsertPosIndex - 1;
-  for (MachineBasicBlock::iterator I = InsertPos, E = Begin;
-       I != E; --Count) {
-    MachineInstr *MI = --I;
-
-    // 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 (SDep *Edge = CriticalPathStep(CriticalPathSU)) {
-        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 (!AllocatableSet.test(AntiDepReg))
-            // Don't break anti-dependencies on non-allocatable registers.
-            AntiDepReg = 0;
-          else if (KeepRegs.count(AntiDepReg))
-            // Don't break anti-dependencies if an 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::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 = 0;
-        CriticalPathMI = 0;
-      }
-    }
-
-    PrescanInstruction(MI);
-
-    if (MI->getDesc().hasExtraDefRegAllocReq())
-      // 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.
-      for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
-        MachineOperand &MO = MI->getOperand(i);
-        if (!MO.isReg()) continue;
-        unsigned Reg = MO.getReg();
-        if (Reg == 0) continue;
-        if (MO.isUse() && AntiDepReg == Reg) {
-          AntiDepReg = 0;
-          break;
-        }
-      }
-    }
-
-    // Determine AntiDepReg's register class, if it is live and is
-    // consistently used within a single class.
-    const TargetRegisterClass *RC = AntiDepReg != 0 ? Classes[AntiDepReg] : 0;
-    assert((AntiDepReg == 0 || RC != NULL) &&
-           "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-depenence.
-    //
-    // TODO: Instead of picking the first free register, consider which might
-    // be the best.
-    if (AntiDepReg != 0) {
-      if (unsigned NewReg = findSuitableFreeRegister(AntiDepReg,
-                                                     LastNewReg[AntiDepReg],
-                                                     RC)) {
-        DEBUG(errs() << "Breaking anti-dependence edge on "
-              << TRI->getName(AntiDepReg)
-              << " with " << RegRefs.count(AntiDepReg) << " references"
-              << " using " << TRI->getName(NewReg) << "!\n");
-
-        // Update the references to the old register to refer to the new
-        // register.
-        std::pair<std::multimap<unsigned, MachineOperand *>::iterator,
-                  std::multimap<unsigned, MachineOperand *>::iterator>
-           Range = RegRefs.equal_range(AntiDepReg);
-        for (std::multimap<unsigned, MachineOperand *>::iterator
-             Q = Range.first, QE = Range.second; Q != QE; ++Q)
-          Q->second->setReg(NewReg);
-
-        // We just went back in time and modified history; the
-        // liveness information for the anti-depenence 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] = 0;
-        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);
-        Changed = true;
-        LastNewReg[AntiDepReg] = NewReg;
-      }
-    }
-
-    ScanInstruction(MI, Count);
-  }
-
-  return Changed;
-}
-
 /// StartBlockForKills - Initialize register live-range state for updating kills
 ///
 void SchedulePostRATDList::StartBlockForKills(MachineBasicBlock *BB) {
diff --git a/lib/Target/ARM/ARMSubtarget.h b/lib/Target/ARM/ARMSubtarget.h
index 74781593a0..e721a7fd68 100644
--- a/lib/Target/ARM/ARMSubtarget.h
+++ b/lib/Target/ARM/ARMSubtarget.h
@@ -130,7 +130,7 @@ protected:
   /// for Thumb1.
   bool enablePostRAScheduler(CodeGenOpt::Level OptLevel,
                              TargetSubtarget::AntiDepBreakMode& mode) const {
-    mode = TargetSubtarget::ANTIDEP_NONE;
+    mode = TargetSubtarget::ANTIDEP_CRITICAL;
     return PostRAScheduler && OptLevel >= CodeGenOpt::Default;
   }
 
diff --git a/test/CodeGen/X86/break-anti-dependencies.ll b/test/CodeGen/X86/break-anti-dependencies.ll
index 6b245c103e..972b3cd43c 100644
--- a/test/CodeGen/X86/break-anti-dependencies.ll
+++ b/test/CodeGen/X86/break-anti-dependencies.ll
@@ -1,7 +1,7 @@
-; RUN: llc < %s -march=x86-64 -post-RA-scheduler -break-anti-dependencies=false > %t
+; RUN: llc < %s -march=x86-64 -post-RA-scheduler -break-anti-dependencies=none > %t
 ; RUN:   grep {%xmm0} %t | count 14
 ; RUN:   not grep {%xmm1} %t
-; RUN: llc < %s -march=x86-64 -post-RA-scheduler -break-anti-dependencies > %t
+; RUN: llc < %s -march=x86-64 -post-RA-scheduler -break-anti-dependencies=critical > %t
 ; RUN:   grep {%xmm0} %t | count 7
 ; RUN:   grep {%xmm1} %t | count 7