Adding an A15 specific optimization pass for interactions between S/D/Q registers. The pass handles all the required transformations pre-regalloc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177169 91177308-0d34-0410-b5e6-96231b3b80d8

6 files changed, 777 insertions, 2 deletions

diff --git a/lib/Target/ARM/A15SDOptimizer.cpp b/lib/Target/ARM/A15SDOptimizer.cpp
new file mode 100644
index 0000000000..f0d4dbe2bf
--- /dev/null
+++ b/lib/Target/ARM/A15SDOptimizer.cpp
@@ -0,0 +1,704 @@
+//=== A15SDOptimizerPass.cpp - Optimize DPR and SPR register accesses on A15==//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// The Cortex-A15 processor employs a tracking scheme in its register renaming
+// in order to process each instruction's micro-ops speculatively and
+// out-of-order with appropriate forwarding. The ARM architecture allows VFP
+// instructions to read and write 32-bit S-registers.  Each S-register
+// corresponds to one half (upper or lower) of an overlaid 64-bit D-register.
+//
+// There are several instruction patterns which can be used to provide this
+// capability which can provide higher performance than other, potentially more
+// direct patterns, specifically around when one micro-op reads a D-register
+// operand that has recently been written as one or more S-register results.
+//
+// This file defines a pre-regalloc pass which looks for SPR producers which
+// are going to be used by a DPR (or QPR) consumers and creates the more
+// optimized access pattern.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "a15-sd-optimizer"
+#include "ARM.h"
+#include "ARMBaseInstrInfo.h"
+#include "ARMSubtarget.h"
+#include "ARMISelLowering.h"
+#include "ARMTargetMachine.h"
+
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstr.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Target/TargetRegisterInfo.h"
+
+#include <set>
+
+using namespace llvm;
+
+namespace {
+  struct A15SDOptimizer : public MachineFunctionPass {
+    static char ID;
+    A15SDOptimizer() : MachineFunctionPass(ID) {}
+
+    virtual bool runOnMachineFunction(MachineFunction &Fn);
+
+    virtual const char *getPassName() const {
+      return "ARM A15 S->D optimizer";
+    }
+
+  private:
+    const ARMBaseInstrInfo *TII;
+    const TargetRegisterInfo *TRI;
+    MachineRegisterInfo *MRI;
+
+    bool runOnInstruction(MachineInstr *MI);
+
+    //
+    // Instruction builder helpers
+    //
+    unsigned createDupLane(MachineBasicBlock &MBB,
+                           MachineBasicBlock::iterator InsertBefore,
+                           DebugLoc DL,
+                           unsigned Reg, unsigned Lane,
+                           bool QPR=false);
+
+    unsigned createExtractSubreg(MachineBasicBlock &MBB,
+                                 MachineBasicBlock::iterator InsertBefore,
+                                 DebugLoc DL,
+                                 unsigned DReg, unsigned Lane,
+                                 const TargetRegisterClass *TRC);
+
+    unsigned createVExt(MachineBasicBlock &MBB,
+                        MachineBasicBlock::iterator InsertBefore,
+                        DebugLoc DL,
+                        unsigned Ssub0, unsigned Ssub1);
+
+    unsigned createRegSequence(MachineBasicBlock &MBB,
+                               MachineBasicBlock::iterator InsertBefore,
+                               DebugLoc DL,
+                               unsigned Reg1, unsigned Reg2);
+
+    unsigned createInsertSubreg(MachineBasicBlock &MBB,
+                                MachineBasicBlock::iterator InsertBefore,
+                                DebugLoc DL, unsigned DReg, unsigned Lane,
+                                unsigned ToInsert);
+
+    unsigned createImplicitDef(MachineBasicBlock &MBB,
+                               MachineBasicBlock::iterator InsertBefore,
+                               DebugLoc DL);
+    
+    //
+    // Various property checkers
+    //
+    bool usesRegClass(MachineOperand &MO, const TargetRegisterClass *TRC);
+    bool hasPartialWrite(MachineInstr *MI);
+    SmallVector<unsigned, 8> getReadDPRs(MachineInstr *MI);
+    unsigned getDPRLaneFromSPR(unsigned SReg);
+
+    //
+    // Methods used for getting the definitions of partial registers
+    //
+
+    MachineInstr *elideCopies(MachineInstr *MI);
+    void elideCopiesAndPHIs(MachineInstr *MI,
+                            SmallVectorImpl<MachineInstr*> &Outs);
+
+    //
+    // Pattern optimization methods
+    //
+    unsigned optimizeAllLanesPattern(MachineInstr *MI, unsigned Reg);
+    unsigned optimizeSDPattern(MachineInstr *MI);
+    unsigned getPrefSPRLane(unsigned SReg);
+
+    //
+    // Sanitizing method - used to make sure if don't leave dead code around.
+    //
+    void eraseInstrWithNoUses(MachineInstr *MI);
+
+    //
+    // A map used to track the changes done by this pass.
+    //
+    std::map<MachineInstr*, unsigned> Replacements;
+    std::set<MachineInstr *> DeadInstr;
+  };
+  char A15SDOptimizer::ID = 0;
+} // end anonymous namespace
+
+// Returns true if this is a use of a SPR register.
+bool A15SDOptimizer::usesRegClass(MachineOperand &MO,
+                                  const TargetRegisterClass *TRC) {
+  if (!MO.isReg())
+    return false;
+  unsigned Reg = MO.getReg();
+
+  if (TargetRegisterInfo::isVirtualRegister(Reg))
+    return MRI->getRegClass(Reg)->hasSuperClassEq(TRC);
+  else
+    return TRC->contains(Reg);
+}
+
+unsigned A15SDOptimizer::getDPRLaneFromSPR(unsigned SReg) {
+  unsigned DReg = TRI->getMatchingSuperReg(SReg, ARM::ssub_1,
+                                           &ARM::DPRRegClass);
+  if (DReg != ARM::NoRegister) return ARM::ssub_1;
+  return ARM::ssub_0;
+}
+
+// Get the subreg type that is most likely to be coalesced
+// for an SPR register that will be used in VDUP32d pseudo.
+unsigned A15SDOptimizer::getPrefSPRLane(unsigned SReg) {
+  if (!TRI->isVirtualRegister(SReg))
+    return getDPRLaneFromSPR(SReg);
+
+  MachineInstr *MI = MRI->getVRegDef(SReg);
+  if (!MI) return ARM::ssub_0;
+  MachineOperand *MO = MI->findRegisterDefOperand(SReg);
+
+  assert(MO->isReg() && "Non register operand found!");
+  if (!MO) return ARM::ssub_0;
+
+  if (MI->isCopy() && usesRegClass(MI->getOperand(1),
+                                    &ARM::SPRRegClass)) {
+    SReg = MI->getOperand(1).getReg();
+  }
+
+  if (TargetRegisterInfo::isVirtualRegister(SReg)) {
+    if (MO->getSubReg() == ARM::ssub_1) return ARM::ssub_1;
+    return ARM::ssub_0;
+  }
+  return getDPRLaneFromSPR(SReg);
+}
+
+// MI is known to be dead. Figure out what instructions
+// are also made dead by this and mark them for removal.
+void A15SDOptimizer::eraseInstrWithNoUses(MachineInstr *MI) {
+  SmallVector<MachineInstr *, 8> Front;
+  DeadInstr.insert(MI);
+
+  DEBUG(dbgs() << "Deleting base instruction " << *MI << "\n");
+  Front.push_back(MI);
+
+  while (Front.size() != 0) {
+    MI = Front.back();
+    Front.pop_back();
+
+    // MI is already known to be dead. We need to see
+    // if other instructions can also be removed.
+    for (unsigned int i = 0; i < MI->getNumOperands(); ++i) {
+      MachineOperand &MO = MI->getOperand(i);
+      if ((!MO.isReg()) || (!MO.isUse()))
+        continue;
+      unsigned Reg = MO.getReg();
+      if (!TRI->isVirtualRegister(Reg))
+        continue;
+      MachineOperand *Op = MI->findRegisterDefOperand(Reg);
+
+      if (!Op)
+        continue;
+
+      MachineInstr *Def = Op->getParent();
+
+      // We don't need to do anything if we have already marked
+      // this instruction as being dead.
+      if (DeadInstr.find(Def) != DeadInstr.end())
+        continue;
+
+      // Check if all the uses of this instruction are marked as
+      // dead. If so, we can also mark this instruction as being
+      // dead.
+      bool IsDead = true;
+      for (unsigned int j = 0; j < Def->getNumOperands(); ++j) {
+        MachineOperand &MODef = Def->getOperand(j);
+        if ((!MODef.isReg()) || (!MODef.isDef()))
+          continue;
+        unsigned DefReg = MODef.getReg();
+        if (!TRI->isVirtualRegister(DefReg)) {
+          IsDead = false;
+          break;
+        }
+        for (MachineRegisterInfo::use_iterator II = MRI->use_begin(Reg),
+                            EE = MRI->use_end();
+                            II != EE; ++II) {
+          // We don't care about self references.
+          if (&*II == Def)
+            continue;
+          if (DeadInstr.find(&*II) == DeadInstr.end()) {
+            IsDead = false;
+            break;
+          }
+        }
+      }
+
+      if (!IsDead) continue;
+
+      DEBUG(dbgs() << "Deleting instruction " << *Def << "\n");
+      DeadInstr.insert(Def);
+    }
+  }
+}
+
+// Creates the more optimized patterns and generally does all the code
+// transformations in this pass.
+unsigned A15SDOptimizer::optimizeSDPattern(MachineInstr *MI) {
+  if (MI->isCopy()) {
+    return optimizeAllLanesPattern(MI, MI->getOperand(1).getReg());
+  }
+
+  if (MI->isInsertSubreg()) {
+    unsigned DPRReg = MI->getOperand(1).getReg();
+    unsigned SPRReg = MI->getOperand(2).getReg();
+
+    if (TRI->isVirtualRegister(DPRReg) && TRI->isVirtualRegister(SPRReg)) {
+      MachineInstr *DPRMI = MRI->getVRegDef(MI->getOperand(1).getReg());
+      MachineInstr *SPRMI = MRI->getVRegDef(MI->getOperand(2).getReg());
+
+      if (DPRMI && SPRMI) {
+        // See if the first operand of this insert_subreg is IMPLICIT_DEF
+        MachineInstr *ECDef = elideCopies(DPRMI);
+        if (ECDef != 0 && ECDef->isImplicitDef()) {
+          // Another corner case - if we're inserting something that is purely
+          // a subreg copy of a DPR, just use that DPR.
+
+          MachineInstr *EC = elideCopies(SPRMI);
+          // Is it a subreg copy of ssub_0?
+          if (EC && EC->isCopy() &&
+              EC->getOperand(1).getSubReg() == ARM::ssub_0) {
+            DEBUG(dbgs() << "Found a subreg copy: " << *SPRMI);
+
+            // Find the thing we're subreg copying out of - is it of the same
+            // regclass as DPRMI? (i.e. a DPR or QPR).
+            unsigned FullReg = SPRMI->getOperand(1).getReg();
+            const TargetRegisterClass *TRC =
+              MRI->getRegClass(MI->getOperand(1).getReg());
+            if (TRC->hasSuperClassEq(MRI->getRegClass(FullReg))) {
+              DEBUG(dbgs() << "Subreg copy is compatible - returning ");
+              DEBUG(dbgs() << PrintReg(FullReg) << "\n");
+              eraseInstrWithNoUses(MI);
+              return FullReg;
+            }
+          }
+
+          return optimizeAllLanesPattern(MI, MI->getOperand(2).getReg());
+        }
+      }
+    }
+    return optimizeAllLanesPattern(MI, MI->getOperand(0).getReg());
+  }
+
+  if (MI->isRegSequence() && usesRegClass(MI->getOperand(1),
+                                          &ARM::SPRRegClass)) {
+    // See if all bar one of the operands are IMPLICIT_DEF and insert the
+    // optimizer pattern accordingly.
+    unsigned NumImplicit = 0, NumTotal = 0;
+    unsigned NonImplicitReg = ~0U;
+
+    for (unsigned I = 1; I < MI->getNumExplicitOperands(); ++I) {
+      if (!MI->getOperand(I).isReg())
+        continue;
+      ++NumTotal;
+      unsigned OpReg = MI->getOperand(I).getReg();
+
+      if (!TRI->isVirtualRegister(OpReg))
+        break;
+
+      MachineInstr *Def = MRI->getVRegDef(OpReg);
+      if (!Def)
+        break;
+      if (Def->isImplicitDef())
+        ++NumImplicit;
+      else
+        NonImplicitReg = MI->getOperand(I).getReg();
+    }
+
+    if (NumImplicit == NumTotal - 1)
+      return optimizeAllLanesPattern(MI, NonImplicitReg);
+    else
+      return optimizeAllLanesPattern(MI, MI->getOperand(0).getReg());
+  }
+
+  assert(0 && "Unhandled update pattern!");
+  return 0;
+}
+
+// Return true if this MachineInstr inserts a scalar (SPR) value into
+// a D or Q register.
+bool A15SDOptimizer::hasPartialWrite(MachineInstr *MI) {
+  // The only way we can do a partial register update is through a COPY,
+  // INSERT_SUBREG or REG_SEQUENCE.
+  if (MI->isCopy() && usesRegClass(MI->getOperand(1), &ARM::SPRRegClass))
+    return true;
+
+  if (MI->isInsertSubreg() && usesRegClass(MI->getOperand(2),
+                                           &ARM::SPRRegClass))
+    return true;
+
+  if (MI->isRegSequence() && usesRegClass(MI->getOperand(1), &ARM::SPRRegClass))
+    return true;
+
+  return false;
+}
+
+// Looks through full copies to get the instruction that defines the input
+// operand for MI.
+MachineInstr *A15SDOptimizer::elideCopies(MachineInstr *MI) {
+  if (!MI->isFullCopy())
+    return MI;
+  if (!TRI->isVirtualRegister(MI->getOperand(1).getReg()))
+    return NULL;
+  MachineInstr *Def = MRI->getVRegDef(MI->getOperand(1).getReg());
+  if (!Def)
+    return NULL;
+  return elideCopies(Def);
+}
+
+// Look through full copies and PHIs to get the set of non-copy MachineInstrs
+// that can produce MI.
+void A15SDOptimizer::elideCopiesAndPHIs(MachineInstr *MI,
+                                        SmallVectorImpl<MachineInstr*> &Outs) {
+   // Looking through PHIs may create loops so we need to track what
+   // instructions we have visited before.
+   std::set<MachineInstr *> Reached;
+   SmallVector<MachineInstr *, 8> Front;
+   Front.push_back(MI);
+   while (Front.size() != 0) {
+     MI = Front.back();
+     Front.pop_back();
+
+     // If we have already explored this MachineInstr, ignore it.
+     if (Reached.find(MI) != Reached.end())
+       continue;
+     Reached.insert(MI);
+     if (MI->isPHI()) {
+       for (unsigned I = 1, E = MI->getNumOperands(); I != E; I += 2) {
+         unsigned Reg = MI->getOperand(I).getReg();
+         if (!TRI->isVirtualRegister(Reg)) {
+           continue;
+         }
+         MachineInstr *NewMI = MRI->getVRegDef(Reg);
+         if (!NewMI)
+           continue;
+         Front.push_back(NewMI);
+       }
+     } else if (MI->isFullCopy()) {
+       if (!TRI->isVirtualRegister(MI->getOperand(1).getReg()))
+         continue;
+       MachineInstr *NewMI = MRI->getVRegDef(MI->getOperand(1).getReg());
+       if (!NewMI)
+         continue;
+       Front.push_back(NewMI);
+     } else {
+       DEBUG(dbgs() << "Found partial copy" << *MI <<"\n");
+       Outs.push_back(MI);
+     }
+   }
+}
+
+// Return the DPR virtual registers that are read by this machine instruction
+// (if any).
+SmallVector<unsigned, 8> A15SDOptimizer::getReadDPRs(MachineInstr *MI) {
+  if (MI->isCopyLike() || MI->isInsertSubreg() || MI->isRegSequence() ||
+      MI->isKill())
+    return SmallVector<unsigned, 8>();
+
+  SmallVector<unsigned, 8> Defs;
+  for (unsigned i = 0; i < MI->getNumOperands(); ++i) {
+    MachineOperand &MO = MI->getOperand(i);
+
+    if (!MO.isReg() || !MO.isUse())
+      continue;
+    if (!usesRegClass(MO, &ARM::DPRRegClass) &&
+        !usesRegClass(MO, &ARM::QPRRegClass))
+      continue;
+
+    Defs.push_back(MO.getReg());
+  }
+  return Defs;
+}
+
+// Creates a DPR register from an SPR one by using a VDUP.
+unsigned
+A15SDOptimizer::createDupLane(MachineBasicBlock &MBB,
+                              MachineBasicBlock::iterator InsertBefore,
+                              DebugLoc DL,
+                              unsigned Reg, unsigned Lane, bool QPR) {
+  unsigned Out = MRI->createVirtualRegister(QPR ? &ARM::QPRRegClass :
+                                                  &ARM::DPRRegClass);
+  AddDefaultPred(BuildMI(MBB,
+                         InsertBefore,
+                         DL,
+                         TII->get(QPR ? ARM::VDUPLN32q : ARM::VDUPLN32d),
+                         Out)
+                   .addReg(Reg)
+                   .addImm(Lane));
+ 
+  return Out;
+}
+
+// Creates a SPR register from a DPR by copying the value in lane 0.
+unsigned
+A15SDOptimizer::createExtractSubreg(MachineBasicBlock &MBB,
+                                    MachineBasicBlock::iterator InsertBefore,
+                                    DebugLoc DL,
+                                    unsigned DReg, unsigned Lane,
+                                    const TargetRegisterClass *TRC) {
+  unsigned Out = MRI->createVirtualRegister(TRC);
+  BuildMI(MBB,
+          InsertBefore,
+          DL,
+          TII->get(TargetOpcode::COPY), Out)
+    .addReg(DReg, 0, Lane);
+
+  return Out;
+}
+
+// Takes two SPR registers and creates a DPR by using a REG_SEQUENCE.
+unsigned
+A15SDOptimizer::createRegSequence(MachineBasicBlock &MBB,
+                                  MachineBasicBlock::iterator InsertBefore,
+                                  DebugLoc DL,
+                                  unsigned Reg1, unsigned Reg2) {
+  unsigned Out = MRI->createVirtualRegister(&ARM::QPRRegClass);
+  BuildMI(MBB,
+          InsertBefore,
+          DL,
+          TII->get(TargetOpcode::REG_SEQUENCE), Out)
+    .addReg(Reg1)
+    .addImm(ARM::dsub_0)
+    .addReg(Reg2)
+    .addImm(ARM::dsub_1);
+  return Out;
+}
+
+// Takes two DPR registers that have previously been VDUPed (Ssub0 and Ssub1)
+// and merges them into one DPR register.
+unsigned
+A15SDOptimizer::createVExt(MachineBasicBlock &MBB,
+                           MachineBasicBlock::iterator InsertBefore,
+                           DebugLoc DL,
+                           unsigned Ssub0, unsigned Ssub1) {
+  unsigned Out = MRI->createVirtualRegister(&ARM::DPRRegClass);
+  AddDefaultPred(BuildMI(MBB,
+                         InsertBefore,
+                         DL,
+                         TII->get(ARM::VEXTd32), Out)
+                   .addReg(Ssub0)
+                   .addReg(Ssub1)
+                   .addImm(1));
+  return Out;
+}
+
+unsigned
+A15SDOptimizer::createInsertSubreg(MachineBasicBlock &MBB,
+                                   MachineBasicBlock::iterator InsertBefore,
+                                   DebugLoc DL, unsigned DReg, unsigned Lane,
+                                   unsigned ToInsert) {
+  unsigned Out = MRI->createVirtualRegister(&ARM::DPR_VFP2RegClass);
+  BuildMI(MBB,
+          InsertBefore,
+          DL,
+          TII->get(TargetOpcode::INSERT_SUBREG), Out)
+    .addReg(DReg)
+    .addReg(ToInsert)
+    .addImm(Lane);
+
+  return Out;
+}
+
+unsigned
+A15SDOptimizer::createImplicitDef(MachineBasicBlock &MBB,
+                                  MachineBasicBlock::iterator InsertBefore,
+                                  DebugLoc DL) {
+  unsigned Out = MRI->createVirtualRegister(&ARM::DPRRegClass);
+  BuildMI(MBB,
+          InsertBefore,
+          DL,
+          TII->get(TargetOpcode::IMPLICIT_DEF), Out);
+  return Out;
+}
+
+// This function inserts instructions in order to optimize interactions between
+// SPR registers and DPR/QPR registers. It does so by performing VDUPs on all
+// lanes, and the using VEXT instructions to recompose the result.
+unsigned
+A15SDOptimizer::optimizeAllLanesPattern(MachineInstr *MI, unsigned Reg) {
+  MachineBasicBlock::iterator InsertPt(MI);
+  DebugLoc DL = MI->getDebugLoc();
+  MachineBasicBlock &MBB = *MI->getParent();
+  InsertPt++;
+  unsigned Out;
+
+  if (MRI->getRegClass(Reg)->hasSuperClassEq(&ARM::QPRRegClass)) {
+    unsigned DSub0 = createExtractSubreg(MBB, InsertPt, DL, Reg,
+                                         ARM::dsub_0, &ARM::DPRRegClass);
+    unsigned DSub1 = createExtractSubreg(MBB, InsertPt, DL, Reg,
+                                         ARM::dsub_1, &ARM::DPRRegClass);
+
+    unsigned Out1 = createDupLane(MBB, InsertPt, DL, DSub0, 0);
+    unsigned Out2 = createDupLane(MBB, InsertPt, DL, DSub0, 1);
+    Out = createVExt(MBB, InsertPt, DL, Out1, Out2);
+
+    unsigned Out3 = createDupLane(MBB, InsertPt, DL, DSub1, 0);
+    unsigned Out4 = createDupLane(MBB, InsertPt, DL, DSub1, 1);
+    Out2 = createVExt(MBB, InsertPt, DL, Out3, Out4);
+
+    Out = createRegSequence(MBB, InsertPt, DL, Out, Out2);
+
+  } else if (MRI->getRegClass(Reg)->hasSuperClassEq(&ARM::DPRRegClass)) {
+    unsigned Out1 = createDupLane(MBB, InsertPt, DL, Reg, 0);
+    unsigned Out2 = createDupLane(MBB, InsertPt, DL, Reg, 1);
+    Out = createVExt(MBB, InsertPt, DL, Out1, Out2);
+
+  } else {
+    assert(MRI->getRegClass(Reg)->hasSuperClassEq(&ARM::SPRRegClass) &&
+           "Found unexpected regclass!");
+
+    unsigned PrefLane = getPrefSPRLane(Reg);
+    unsigned Lane;
+    switch (PrefLane) {
+      case ARM::ssub_0: Lane = 0; break;
+      case ARM::ssub_1: Lane = 1; break;
+      default: llvm_unreachable("Unknown preferred lane!");
+    }
+
+    bool UsesQPR = usesRegClass(MI->getOperand(0), &ARM::QPRRegClass);
+
+    Out = createImplicitDef(MBB, InsertPt, DL);
+    Out = createInsertSubreg(MBB, InsertPt, DL, Out, PrefLane, Reg);
+    Out = createDupLane(MBB, InsertPt, DL, Out, Lane, UsesQPR);
+    eraseInstrWithNoUses(MI);
+  }
+  return Out;
+}
+
+bool A15SDOptimizer::runOnInstruction(MachineInstr *MI) {
+  // We look for instructions that write S registers that are then read as
+  // D/Q registers. These can only be caused by COPY, INSERT_SUBREG and
+  // REG_SEQUENCE pseudos that insert an SPR value into a DPR register or
+  // merge two SPR values to form a DPR register.  In order avoid false
+  // positives we make sure that there is an SPR producer so we look past
+  // COPY and PHI nodes to find it.
+  //
+  // The best code pattern for when an SPR producer is going to be used by a
+  // DPR or QPR consumer depends on whether the other lanes of the
+  // corresponding DPR/QPR are currently defined.
+  //
+  // We can handle these efficiently, depending on the type of
+  // pseudo-instruction that is producing the pattern
+  //
+  //   * COPY:          * VDUP all lanes and merge the results together
+  //                      using VEXTs.
+  //
+  //   * INSERT_SUBREG: * If the SPR value was originally in another DPR/QPR
+  //                      lane, and the other lane(s) of the DPR/QPR register
+  //                      that we are inserting in are undefined, use the
+  //                      original DPR/QPR value. 
+  //                    * Otherwise, fall back on the same stategy as COPY.
+  //
+  //   * REG_SEQUENCE:  * If all except one of the input operands are
+  //                      IMPLICIT_DEFs, insert the VDUP pattern for just the
+  //                      defined input operand
+  //                    * Otherwise, fall back on the same stategy as COPY.
+  //
+
+  // First, get all the reads of D-registers done by this instruction.
+  SmallVector<unsigned, 8> Defs = getReadDPRs(MI);
+  bool Modified = false;
+
+  for (SmallVector<unsigned, 8>::iterator I = Defs.begin(), E = Defs.end();
+     I != E; ++I) {
+    // Follow the def-use chain for this DPR through COPYs, and also through
+    // PHIs (which are essentially multi-way COPYs). It is because of PHIs that
+    // we can end up with multiple defs of this DPR.
+
+    SmallVector<MachineInstr *, 8> DefSrcs;
+    if (!TRI->isVirtualRegister(*I))
+      continue;
+    MachineInstr *Def = MRI->getVRegDef(*I);
+    if (!Def)
+      continue;
+
+    elideCopiesAndPHIs(Def, DefSrcs);
+
+    for (SmallVector<MachineInstr*, 8>::iterator II = DefSrcs.begin(),
+      EE = DefSrcs.end(); II != EE; ++II) {
+      MachineInstr *MI = *II;
+
+      // If we've already analyzed and replaced this operand, don't do
+      // anything.
+      if (Replacements.find(MI) != Replacements.end())
+        continue;
+
+      // Now, work out if the instruction causes a SPR->DPR dependency.
+      if (!hasPartialWrite(MI))
+        continue;
+
+      // Collect all the uses of this MI's DPR def for updating later.
+      SmallVector<MachineOperand*, 8> Uses;
+      unsigned DPRDefReg = MI->getOperand(0).getReg();
+      for (MachineRegisterInfo::use_iterator I = MRI->use_begin(DPRDefReg),
+             E = MRI->use_end(); I != E; ++I)
+        Uses.push_back(&I.getOperand());
+
+      // We can optimize this.
+      unsigned NewReg = optimizeSDPattern(MI);
+
+      if (NewReg != 0) {
+        Modified = true;
+        for (SmallVector<MachineOperand*, 8>::const_iterator I = Uses.begin(),
+               E = Uses.end(); I != E; ++I) {
+          DEBUG(dbgs() << "Replacing operand "
+                       << **I << " with "
+                       << PrintReg(NewReg) << "\n");
+          (*I)->substVirtReg(NewReg, 0, *TRI);
+        }
+      }
+      Replacements[MI] = NewReg;
+    }
+  }
+  return Modified;
+}
+
+bool A15SDOptimizer::runOnMachineFunction(MachineFunction &Fn) {
+  TII = static_cast<const ARMBaseInstrInfo*>(Fn.getTarget().getInstrInfo());
+  TRI = Fn.getTarget().getRegisterInfo();
+  MRI = &Fn.getRegInfo();
+  bool Modified = false;
+
+  DEBUG(dbgs() << "Running on function " << Fn.getName()<< "\n");
+
+  DeadInstr.clear();
+  Replacements.clear();
+
+  for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
+       ++MFI) {
+
+    for (MachineBasicBlock::iterator MI = MFI->begin(), ME = MFI->end();
+      MI != ME;) {
+      Modified |= runOnInstruction(MI++);
+    }
+ 
+  }
+
+  for (std::set<MachineInstr *>::iterator I = DeadInstr.begin(),
+                                            E = DeadInstr.end();
+                                            I != E; ++I) {
+    (*I)->eraseFromParent();
+  }
+
+  return Modified;
+}
+
+FunctionPass *llvm::createA15SDOptimizerPass() {
+  return new A15SDOptimizer();
+}
diff --git a/lib/Target/ARM/ARM.h b/lib/Target/ARM/ARM.h
index 5faf8c320c..80e5f37eb0 100644
--- a/lib/Target/ARM/ARM.h
+++ b/lib/Target/ARM/ARM.h
@@ -35,6 +35,7 @@ FunctionPass *createARMISelDag(ARMBaseTargetMachine &TM,
 FunctionPass *createARMJITCodeEmitterPass(ARMBaseTargetMachine &TM,
                                           JITCodeEmitter &JCE);
 
+FunctionPass *createA15SDOptimizerPass();
 FunctionPass *createARMLoadStoreOptimizationPass(bool PreAlloc = false);
 FunctionPass *createARMExpandPseudoPass();
 FunctionPass *createARMGlobalBaseRegPass();
diff --git a/lib/Target/ARM/ARMBaseInstrInfo.cpp b/lib/Target/ARM/ARMBaseInstrInfo.cpp
index ed001ea24a..ed8b9cd9a1 100644
--- a/lib/Target/ARM/ARMBaseInstrInfo.cpp
+++ b/lib/Target/ARM/ARMBaseInstrInfo.cpp
@@ -1125,7 +1125,7 @@ bool ARMBaseInstrInfo::expandPostRAPseudo(MachineBasicBlock::iterator MI) const{
   // copyPhysReg() calls.  Look for VMOVS instructions that can legally be
   // widened to VMOVD.  We prefer the VMOVD when possible because it may be
   // changed into a VORR that can go down the NEON pipeline.
-  if (!WidenVMOVS || !MI->isCopy())
+  if (!WidenVMOVS || !MI->isCopy() || Subtarget.isCortexA15())
     return false;
 
   // Look for a copy between even S-registers.  That is where we keep floats
diff --git a/lib/Target/ARM/ARMTargetMachine.cpp b/lib/Target/ARM/ARMTargetMachine.cpp
index 774521852a..3003760df7 100644
--- a/lib/Target/ARM/ARMTargetMachine.cpp
+++ b/lib/Target/ARM/ARMTargetMachine.cpp
@@ -28,6 +28,11 @@ EnableGlobalMerge("global-merge", cl::Hidden,
                   cl::desc("Enable global merge pass"),
                   cl::init(true));
 
+static cl::opt<bool>
+DisableA15SDOptimization("disable-a15-sd-optimization", cl::Hidden,
+                   cl::desc("Inhibit optimization of S->D register accesses on A15"),
+                   cl::init(false));
+
 extern "C" void LLVMInitializeARMTarget() {
   // Register the target.
   RegisterTargetMachine<ARMTargetMachine> X(TheARMTarget);
@@ -164,6 +169,12 @@ bool ARMPassConfig::addPreRegAlloc() {
     addPass(createARMLoadStoreOptimizationPass(true));
   if (getOptLevel() != CodeGenOpt::None && getARMSubtarget().isLikeA9())
     addPass(createMLxExpansionPass());
+  // Since the A15SDOptimizer pass can insert VDUP instructions, it can only be
+  // enabled when NEON is available.
+  if (getOptLevel() != CodeGenOpt::None && getARMSubtarget().isCortexA15() &&
+    getARMSubtarget().hasNEON() && !DisableA15SDOptimization) {
+    addPass(createA15SDOptimizerPass());
+  }
   return true;
 }
 
@@ -174,7 +185,8 @@ bool ARMPassConfig::addPreSched2() {
       addPass(createARMLoadStoreOptimizationPass());
       printAndVerify("After ARM load / store optimizer");
     }
-    if (getARMSubtarget().hasNEON())
+    if ((DisableA15SDOptimization || !getARMSubtarget().isCortexA15()) &&
+      getARMSubtarget().hasNEON())
       addPass(createExecutionDependencyFixPass(&ARM::DPRRegClass));
   }
 
diff --git a/lib/Target/ARM/CMakeLists.txt b/lib/Target/ARM/CMakeLists.txt
index 586834cf73..b832508a08 100644
--- a/lib/Target/ARM/CMakeLists.txt
+++ b/lib/Target/ARM/CMakeLists.txt
@@ -15,6 +15,7 @@ tablegen(LLVM ARMGenDisassemblerTables.inc -gen-disassembler)
 add_public_tablegen_target(ARMCommonTableGen)
 
 add_llvm_target(ARMCodeGen
+  A15SDOptimizer.cpp
   ARMAsmPrinter.cpp
   ARMBaseInstrInfo.cpp
   ARMBaseRegisterInfo.cpp
diff --git a/test/CodeGen/ARM/a15-SD-dep.ll b/test/CodeGen/ARM/a15-SD-dep.ll
new file mode 100644
index 0000000000..17e3eba272
--- /dev/null
+++ b/test/CodeGen/ARM/a15-SD-dep.ll
@@ -0,0 +1,57 @@
+; RUN: llc -O1 -mcpu=cortex-a15 -mtriple=armv7-linux-gnueabi -disable-a15-sd-optimization -verify-machineinstrs < %s  | FileCheck -check-prefix=DISABLED %s
+; RUN: llc -O1 -mcpu=cortex-a15 -mtriple=armv7-linux-gnueabi -verify-machineinstrs < %s | FileCheck -check-prefix=ENABLED %s
+
+; CHECK-ENABLED: t1:
+; CHECK-DISABLED: t1:
+define <2 x float> @t1(float %f) {
+  ; CHECK-ENABLED: vdup.32 d{{[0-9]*}}, d0[0]
+  ; CHECK-DISABLED: vmov.32 d0[1], r{{.}}
+  %i1 = insertelement <2 x float> undef, float %f, i32 1
+  %i2 = fadd <2 x float> %i1, %i1
+  ret <2 x float> %i2
+}
+
+; CHECK-ENABLED: t2:
+; CHECK-DISABLED: t2:
+define <4 x float> @t2(float %g, float %f) {
+  ; CHECK-ENABLED: vdup.32 q{{[0-9]*}}, d0[0]
+  ; CHECK-DISABLED: vmov.32 d0[1], r{{.}}
+  %i1 = insertelement <4 x float> undef, float %f, i32 1
+  %i2 = fadd <4 x float> %i1, %i1
+  ret <4 x float> %i2
+}
+
+; CHECK-ENABLED: t3:
+; CHECK-DISABLED: t3:
+define arm_aapcs_vfpcc <2 x float> @t3(float %f) {
+  ; CHECK-ENABLED: vdup.32 d{{[0-9]*}}, d0[0] 
+  %i1 = insertelement <2 x float> undef, float %f, i32 1
+  %i2 = fadd <2 x float> %i1, %i1
+  ret <2 x float> %i2
+}
+
+; CHECK-ENABLED: t4:
+; CHECK-DISABLED: t4:
+define <2 x float> @t4(float %f) {
+  ; CHECK-ENABLED: vdup.32 d{{[0-9]*}}, d0[0]
+  ; CHECK-DISABLED-NOT: vdup
+  %i1 = insertelement <2 x float> undef, float %f, i32 1
+  br label %b
+
+  ; Block %b has an S-reg as live-in.
+b:
+  %i2 = fadd <2 x float> %i1, %i1
+  ret <2 x float> %i2
+}
+
+; CHECK-ENABLED: t5:
+; CHECK-DISABLED: t5:
+define arm_aapcs_vfpcc <4 x float> @t5(<4 x float> %q, float %f) {
+  ; CHECK-ENABLED: vdup.32 d{{[0-9]*}}, d{{[0-9]*}}[0]
+  ; CHECK-ENABLED: vadd.f32
+  ; CHECK-ENABLED-NEXT: bx lr
+  ; CHECK-DISABLED-NOT: vdup
+  %i1 = insertelement <4 x float> %q, float %f, i32 1
+  %i2 = fadd <4 x float> %i1, %i1
+  ret <4 x float> %i2
+}