Experimental support for aligned NEON spills.

ARM targets with NEON units have access to aligned vector loads and
stores that are potentially faster than unaligned operations.

Add support for spilling the callee-saved NEON registers to an aligned
stack area using 16-byte aligned NEON loads and store.

This feature is off by default, controlled by an -align-neon-spills
command line option.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147211 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 358 insertions, 11 deletions

diff --git a/lib/Target/ARM/ARMFrameLowering.cpp b/lib/Target/ARM/ARMFrameLowering.cpp
index 41b4b7ec12..420ede9957 100644
--- a/lib/Target/ARM/ARMFrameLowering.cpp
+++ b/lib/Target/ARM/ARMFrameLowering.cpp
@@ -16,15 +16,25 @@
 #include "ARMBaseRegisterInfo.h"
 #include "ARMMachineFunctionInfo.h"
 #include "MCTargetDesc/ARMAddressingModes.h"
+#include "llvm/Function.h"
 #include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/CodeGen/RegisterScavenging.h"
 #include "llvm/Target/TargetOptions.h"
+#include "llvm/Support/CommandLine.h"
 
 using namespace llvm;
 
+cl::opt<bool>
+SpillAlignedNEONRegs("align-neon-spills", cl::Hidden,
+                     cl::desc("Align ARM NEON spills in prolog and epilog"));
+
+static MachineBasicBlock::iterator
+skipAlignedDPRCS2Spills(MachineBasicBlock::iterator MI,
+                        unsigned NumAlignedDPRCS2Regs);
+
 /// hasFP - Return true if the specified function should have a dedicated frame
 /// pointer register.  This is true if the function has variable sized allocas
 /// or if frame pointer elimination is disabled.
@@ -139,6 +149,7 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF) const {
   // belongs to which callee-save spill areas.
   unsigned GPRCS1Size = 0, GPRCS2Size = 0, DPRCSSize = 0;
   int FramePtrSpillFI = 0;
+  int D8SpillFI = 0;
 
   // Allocate the vararg register save area. This is not counted in NumBytes.
   if (VARegSaveSize)
@@ -181,8 +192,13 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF) const {
       }
       break;
     default:
-      AFI->addDPRCalleeSavedAreaFrame(FI);
-      DPRCSSize += 8;
+      // This is a DPR. Exclude the aligned DPRCS2 spills.
+      if (Reg == ARM::D8)
+        D8SpillFI = FI;
+      if (Reg < ARM::D8 || Reg >= ARM::D8 + AFI->getNumAlignedDPRCS2Regs()) {
+        AFI->addDPRCalleeSavedAreaFrame(FI);
+        DPRCSSize += 8;
+      }
     }
   }
 
@@ -227,7 +243,17 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF) const {
       MBBI++;
   }
 
-  NumBytes = DPRCSOffset;
+  // Move past the aligned DPRCS2 area.
+  if (AFI->getNumAlignedDPRCS2Regs() > 0) {
+    MBBI = skipAlignedDPRCS2Spills(MBBI, AFI->getNumAlignedDPRCS2Regs());
+    // The code inserted by emitAlignedDPRCS2Spills realigns the stack, and
+    // leaves the stack pointer pointing to the DPRCS2 area.
+    //
+    // Adjust NumBytes to represent the stack slots below the DPRCS2 area.
+    NumBytes += MFI->getObjectOffset(D8SpillFI);
+  } else
+    NumBytes = DPRCSOffset;
+
   if (NumBytes) {
     // Adjust SP after all the callee-save spills.
     emitSPUpdate(isARM, MBB, MBBI, dl, TII, -NumBytes,
@@ -254,7 +280,9 @@ void ARMFrameLowering::emitPrologue(MachineFunction &MF) const {
 
   // If we need dynamic stack realignment, do it here. Be paranoid and make
   // sure if we also have VLAs, we have a base pointer for frame access.
-  if (RegInfo->needsStackRealignment(MF)) {
+  // If aligned NEON registers were sp[illed, the stack has already been
+  // realigned.
+  if (!AFI->getNumAlignedDPRCS2Regs() && RegInfo->needsStackRealignment(MF)) {
     unsigned MaxAlign = MFI->getMaxAlignment();
     assert (!AFI->isThumb1OnlyFunction());
     if (!AFI->isThumbFunction()) {
@@ -541,6 +569,7 @@ void ARMFrameLowering::emitPushInst(MachineBasicBlock &MBB,
                                     unsigned StmOpc, unsigned StrOpc,
                                     bool NoGap,
                                     bool(*Func)(unsigned, bool),
+                                    unsigned NumAlignedDPRCS2Regs,
                                     unsigned MIFlags) const {
   MachineFunction &MF = *MBB.getParent();
   const TargetInstrInfo &TII = *MF.getTarget().getInstrInfo();
@@ -556,6 +585,10 @@ void ARMFrameLowering::emitPushInst(MachineBasicBlock &MBB,
       unsigned Reg = CSI[i-1].getReg();
       if (!(Func)(Reg, STI.isTargetDarwin())) continue;
 
+      // D-registers in the aligned area DPRCS2 are NOT spilled here.
+      if (Reg >= ARM::D8 && Reg < ARM::D8 + NumAlignedDPRCS2Regs)
+        continue;
+
       // Add the callee-saved register as live-in unless it's LR and
       // @llvm.returnaddress is called. If LR is returned for
       // @llvm.returnaddress then it's already added to the function and
@@ -604,7 +637,8 @@ void ARMFrameLowering::emitPopInst(MachineBasicBlock &MBB,
                                    const std::vector<CalleeSavedInfo> &CSI,
                                    unsigned LdmOpc, unsigned LdrOpc,
                                    bool isVarArg, bool NoGap,
-                                   bool(*Func)(unsigned, bool)) const {
+                                   bool(*Func)(unsigned, bool),
+                                   unsigned NumAlignedDPRCS2Regs) const {
   MachineFunction &MF = *MBB.getParent();
   const TargetInstrInfo &TII = *MF.getTarget().getInstrInfo();
   ARMFunctionInfo *AFI = MF.getInfo<ARMFunctionInfo>();
@@ -624,6 +658,10 @@ void ARMFrameLowering::emitPopInst(MachineBasicBlock &MBB,
       unsigned Reg = CSI[i-1].getReg();
       if (!(Func)(Reg, STI.isTargetDarwin())) continue;
 
+      // The aligned reloads from area DPRCS2 are not inserted here.
+      if (Reg >= ARM::D8 && Reg < ARM::D8 + NumAlignedDPRCS2Regs)
+        continue;
+
       if (Reg == ARM::LR && !isTailCall && !isVarArg && STI.hasV5TOps()) {
         Reg = ARM::PC;
         LdmOpc = AFI->isThumbFunction() ? ARM::t2LDMIA_RET : ARM::LDMIA_RET;
@@ -676,6 +714,249 @@ void ARMFrameLowering::emitPopInst(MachineBasicBlock &MBB,
   }
 }
 
+/// Emit aligned spill instructions for NumAlignedDPRCS2Regs D-registers
+/// starting from d8.  It is assumed that the stack will be realigned before
+/// these instructions are executed.
+static void emitAlignedDPRCS2Spills(MachineBasicBlock &MBB,
+                                    MachineBasicBlock::iterator MI,
+                                    unsigned NumAlignedDPRCS2Regs,
+                                    const std::vector<CalleeSavedInfo> &CSI,
+                                    const TargetRegisterInfo *TRI) {
+  MachineFunction &MF = *MBB.getParent();
+  ARMFunctionInfo *AFI = MF.getInfo<ARMFunctionInfo>();
+  DebugLoc DL = MI->getDebugLoc();
+  const TargetInstrInfo &TII = *MF.getTarget().getInstrInfo();
+  MachineFrameInfo &MFI = *MF.getFrameInfo();
+
+  // Mark the D-register spill slots as properly aligned.  Since MFI computes
+  // stack slot layout backwards, this can actually mean that the d-reg stack
+  // slot offsets can be wrong. The offset for d8 will always be correct.
+  for (unsigned i = 0, e = CSI.size(); i != e; ++i) {
+    unsigned DNum = CSI[i].getReg() - ARM::D8;
+    if (DNum >= 8)
+      continue;
+    int FI = CSI[i].getFrameIdx();
+    // The even-numbered registers will be 16-byte aligned, the odd-numbered
+    // registers will be 8-byte aligned.
+    MFI.setObjectAlignment(FI, DNum % 2 ? 8 : 16);
+
+    // The stack slot for D8 needs to be maximally aligned because this is
+    // actually the point where we align the stack pointer.  MachineFrameInfo
+    // computes all offsets relative to the incoming stack pointer which is a
+    // bit weird when realigning the stack.  Any extra padding for this
+    // over-alignment is not realized because the code inserted below adjusts
+    // the stack pointer by numregs * 8 before aligning the stack pointer.
+    if (DNum == 0)
+      MFI.setObjectAlignment(FI, MFI.getMaxAlignment());
+  }
+
+  // Move the stack pointer to the d8 spill slot, and align it at the same
+  // time. Leave the stack slot address in the scratch register r4.
+  //
+  //   sub r4, sp, #numregs * 8
+  //   bic r4, r4, #align - 1
+  //   mov sp, r4
+  //
+  bool isThumb = AFI->isThumbFunction();
+  assert(!AFI->isThumb1OnlyFunction() && "Can't realign stack for thumb1");
+  AFI->setShouldRestoreSPFromFP(true);
+
+  // sub r4, sp, #numregs * 8
+  // The immediate is <= 64, so it doesn't need any special encoding.
+  unsigned Opc = isThumb ? ARM::t2SUBri : ARM::SUBri;
+  AddDefaultCC(AddDefaultPred(BuildMI(MBB, MI, DL, TII.get(Opc), ARM::R4)
+                              .addReg(ARM::SP)
+                              .addImm(8 * NumAlignedDPRCS2Regs)));
+
+  // bic r4, r4, #align-1
+  Opc = isThumb ? ARM::t2BICri : ARM::BICri;
+  unsigned MaxAlign = MF.getFrameInfo()->getMaxAlignment();
+  AddDefaultCC(AddDefaultPred(BuildMI(MBB, MI, DL, TII.get(Opc), ARM::R4)
+                              .addReg(ARM::R4, RegState::Kill)
+                              .addImm(MaxAlign - 1)));
+
+  // mov sp, r4
+  // The stack pointer must be adjusted before spilling anything, otherwise
+  // the stack slots could be clobbered by an interrupt handler.
+  // Leave r4 live, it is used below.
+  Opc = isThumb ? ARM::tMOVr : ARM::MOVr;
+  MachineInstrBuilder MIB = BuildMI(MBB, MI, DL, TII.get(Opc), ARM::SP)
+                            .addReg(ARM::R4);
+  MIB = AddDefaultPred(MIB);
+  if (!isThumb)
+    AddDefaultCC(MIB);
+
+  // Now spill NumAlignedDPRCS2Regs registers starting from d8.
+  // r4 holds the stack slot address.
+  unsigned NextReg = ARM::D8;
+
+  // 16-byte aligned vst1.64 with 4 d-regs and address writeback.
+  // The writeback is only needed when emitting two vst1.64 instructions.
+  if (NumAlignedDPRCS2Regs >= 6) {
+    unsigned SupReg = TRI->getMatchingSuperReg(NextReg, ARM::dsub_0,
+                                               ARM::QQPRRegisterClass);
+    MBB.addLiveIn(SupReg);
+    AddDefaultPred(BuildMI(MBB, MI, DL, TII.get(ARM::VST1d64Qwb_fixed),
+                           ARM::R4)
+                   .addReg(ARM::R4, RegState::Kill).addImm(16)
+                   .addReg(NextReg)
+                   .addReg(SupReg, RegState::ImplicitKill));
+    NextReg += 4;
+    NumAlignedDPRCS2Regs -= 4;
+  }
+
+  // We won't modify r4 beyond this point.  It currently points to the next
+  // register to be spilled.
+  unsigned R4BaseReg = NextReg;
+
+  // 16-byte aligned vst1.64 with 4 d-regs, no writeback.
+  if (NumAlignedDPRCS2Regs >= 4) {
+    unsigned SupReg = TRI->getMatchingSuperReg(NextReg, ARM::dsub_0,
+                                               ARM::QQPRRegisterClass);
+    MBB.addLiveIn(SupReg);
+    AddDefaultPred(BuildMI(MBB, MI, DL, TII.get(ARM::VST1d64Q))
+                   .addReg(ARM::R4).addImm(16).addReg(NextReg)
+                   .addReg(SupReg, RegState::ImplicitKill));
+    NextReg += 4;
+    NumAlignedDPRCS2Regs -= 4;
+  }
+
+  // 16-byte aligned vst1.64 with 2 d-regs.
+  if (NumAlignedDPRCS2Regs >= 2) {
+    unsigned SupReg = TRI->getMatchingSuperReg(NextReg, ARM::dsub_0,
+                                               ARM::QPRRegisterClass);
+    MBB.addLiveIn(SupReg);
+    AddDefaultPred(BuildMI(MBB, MI, DL, TII.get(ARM::VST1q64))
+                   .addReg(ARM::R4).addImm(16).addReg(NextReg)
+                   .addReg(SupReg, RegState::ImplicitKill));
+    NextReg += 2;
+    NumAlignedDPRCS2Regs -= 2;
+  }
+
+  // Finally, use a vanilla vstr.64 for the odd last register.
+  if (NumAlignedDPRCS2Regs) {
+    MBB.addLiveIn(NextReg);
+    // vstr.64 uses addrmode5 which has an offset scale of 4.
+    AddDefaultPred(BuildMI(MBB, MI, DL, TII.get(ARM::VSTRD))
+                   .addReg(NextReg)
+                   .addReg(ARM::R4).addImm((NextReg-R4BaseReg)*2));
+  }
+
+  // The last spill instruction inserted should kill the scratch register r4.
+  llvm::prior(MI)->addRegisterKilled(ARM::R4, TRI);
+}
+
+/// Skip past the code inserted by emitAlignedDPRCS2Spills, and return an
+/// iterator to the following instruction.
+static MachineBasicBlock::iterator
+skipAlignedDPRCS2Spills(MachineBasicBlock::iterator MI,
+                        unsigned NumAlignedDPRCS2Regs) {
+  //   sub r4, sp, #numregs * 8
+  //   bic r4, r4, #align - 1
+  //   mov sp, r4
+  ++MI; ++MI; ++MI;
+  assert(MI->mayStore() && "Expecting spill instruction");
+
+  // These switches all fall through.
+  switch(NumAlignedDPRCS2Regs) {
+  case 7:
+    ++MI;
+    assert(MI->mayStore() && "Expecting spill instruction");
+  default:
+    ++MI;
+    assert(MI->mayStore() && "Expecting spill instruction");
+  case 1:
+  case 2:
+  case 4:
+    assert(MI->killsRegister(ARM::R4) && "Missed kill flag");
+    ++MI;
+  }
+  return MI;
+}
+
+/// Emit aligned reload instructions for NumAlignedDPRCS2Regs D-registers
+/// starting from d8.  These instructions are assumed to execute while the
+/// stack is still aligned, unlike the code inserted by emitPopInst.
+static void emitAlignedDPRCS2Restores(MachineBasicBlock &MBB,
+                                      MachineBasicBlock::iterator MI,
+                                      unsigned NumAlignedDPRCS2Regs,
+                                      const std::vector<CalleeSavedInfo> &CSI,
+                                      const TargetRegisterInfo *TRI) {
+  MachineFunction &MF = *MBB.getParent();
+  ARMFunctionInfo *AFI = MF.getInfo<ARMFunctionInfo>();
+  DebugLoc DL = MI->getDebugLoc();
+  const TargetInstrInfo &TII = *MF.getTarget().getInstrInfo();
+
+  // Find the frame index assigned to d8.
+  int D8SpillFI = 0;
+  for (unsigned i = 0, e = CSI.size(); i != e; ++i)
+    if (CSI[i].getReg() == ARM::D8) {
+      D8SpillFI = CSI[i].getFrameIdx();
+      break;
+    }
+
+  // Materialize the address of the d8 spill slot into the scratch register r4.
+  // This can be fairly complicated if the stack frame is large, so just use
+  // the normal frame index elimination mechanism to do it.  This code runs as
+  // the initial part of the epilog where the stack and base pointers haven't
+  // been changed yet.
+  bool isThumb = AFI->isThumbFunction();
+  assert(!AFI->isThumb1OnlyFunction() && "Can't realign stack for thumb1");
+
+  unsigned Opc = isThumb ? ARM::t2ADDri : ARM::ADDri;
+  AddDefaultCC(AddDefaultPred(BuildMI(MBB, MI, DL, TII.get(Opc), ARM::R4)
+                              .addFrameIndex(D8SpillFI).addImm(0)));
+
+  // Now restore NumAlignedDPRCS2Regs registers starting from d8.
+  unsigned NextReg = ARM::D8;
+
+  // 16-byte aligned vld1.64 with 4 d-regs and writeback.
+  if (NumAlignedDPRCS2Regs >= 6) {
+    unsigned SupReg = TRI->getMatchingSuperReg(NextReg, ARM::dsub_0,
+                                               ARM::QQPRRegisterClass);
+    AddDefaultPred(BuildMI(MBB, MI, DL, TII.get(ARM::VLD1d64Qwb_fixed), NextReg)
+                   .addReg(ARM::R4, RegState::Define)
+                   .addReg(ARM::R4, RegState::Kill).addImm(16)
+                   .addReg(SupReg, RegState::ImplicitDefine));
+    NextReg += 4;
+    NumAlignedDPRCS2Regs -= 4;
+  }
+
+  // We won't modify r4 beyond this point.  It currently points to the next
+  // register to be spilled.
+  unsigned R4BaseReg = NextReg;
+
+  // 16-byte aligned vld1.64 with 4 d-regs, no writeback.
+  if (NumAlignedDPRCS2Regs >= 4) {
+    unsigned SupReg = TRI->getMatchingSuperReg(NextReg, ARM::dsub_0,
+                                               ARM::QQPRRegisterClass);
+    AddDefaultPred(BuildMI(MBB, MI, DL, TII.get(ARM::VLD1d64Q), NextReg)
+                   .addReg(ARM::R4).addImm(16)
+                   .addReg(SupReg, RegState::ImplicitDefine));
+    NextReg += 4;
+    NumAlignedDPRCS2Regs -= 4;
+  }
+
+  // 16-byte aligned vld1.64 with 2 d-regs.
+  if (NumAlignedDPRCS2Regs >= 2) {
+    unsigned SupReg = TRI->getMatchingSuperReg(NextReg, ARM::dsub_0,
+                                               ARM::QPRRegisterClass);
+    AddDefaultPred(BuildMI(MBB, MI, DL, TII.get(ARM::VLD1q64), NextReg)
+                   .addReg(ARM::R4).addImm(16)
+                   .addReg(SupReg, RegState::ImplicitDefine));
+    NextReg += 2;
+    NumAlignedDPRCS2Regs -= 2;
+  }
+
+  // Finally, use a vanilla vldr.64 for the remaining odd register.
+  if (NumAlignedDPRCS2Regs)
+    AddDefaultPred(BuildMI(MBB, MI, DL, TII.get(ARM::VLDRD), NextReg)
+                   .addReg(ARM::R4).addImm(2*(NextReg-R4BaseReg)));
+
+  // Last store kills r4.
+  llvm::prior(MI)->addRegisterKilled(ARM::R4, TRI);
+}
+
 bool ARMFrameLowering::spillCalleeSavedRegisters(MachineBasicBlock &MBB,
                                         MachineBasicBlock::iterator MI,
                                         const std::vector<CalleeSavedInfo> &CSI,
@@ -690,12 +971,19 @@ bool ARMFrameLowering::spillCalleeSavedRegisters(MachineBasicBlock &MBB,
   unsigned PushOneOpc = AFI->isThumbFunction() ?
     ARM::t2STR_PRE : ARM::STR_PRE_IMM;
   unsigned FltOpc = ARM::VSTMDDB_UPD;
-  emitPushInst(MBB, MI, CSI, PushOpc, PushOneOpc, false, &isARMArea1Register,
+  unsigned NumAlignedDPRCS2Regs = AFI->getNumAlignedDPRCS2Regs();
+  emitPushInst(MBB, MI, CSI, PushOpc, PushOneOpc, false, &isARMArea1Register, 0,
                MachineInstr::FrameSetup);
-  emitPushInst(MBB, MI, CSI, PushOpc, PushOneOpc, false, &isARMArea2Register,
+  emitPushInst(MBB, MI, CSI, PushOpc, PushOneOpc, false, &isARMArea2Register, 0,
                MachineInstr::FrameSetup);
   emitPushInst(MBB, MI, CSI, FltOpc, 0, true, &isARMArea3Register,
-               MachineInstr::FrameSetup);
+               NumAlignedDPRCS2Regs, MachineInstr::FrameSetup);
+
+  // The code above does not insert spill code for the aligned DPRCS2 registers.
+  // The stack realignment code will be inserted between the push instructions
+  // and these spills.
+  if (NumAlignedDPRCS2Regs)
+    emitAlignedDPRCS2Spills(MBB, MI, NumAlignedDPRCS2Regs, CSI, TRI);
 
   return true;
 }
@@ -710,15 +998,22 @@ bool ARMFrameLowering::restoreCalleeSavedRegisters(MachineBasicBlock &MBB,
   MachineFunction &MF = *MBB.getParent();
   ARMFunctionInfo *AFI = MF.getInfo<ARMFunctionInfo>();
   bool isVarArg = AFI->getVarArgsRegSaveSize() > 0;
+  unsigned NumAlignedDPRCS2Regs = AFI->getNumAlignedDPRCS2Regs();
+
+  // The emitPopInst calls below do not insert reloads for the aligned DPRCS2
+  // registers. Do that here instead.
+  if (NumAlignedDPRCS2Regs)
+    emitAlignedDPRCS2Restores(MBB, MI, NumAlignedDPRCS2Regs, CSI, TRI);
 
   unsigned PopOpc = AFI->isThumbFunction() ? ARM::t2LDMIA_UPD : ARM::LDMIA_UPD;
   unsigned LdrOpc = AFI->isThumbFunction() ? ARM::t2LDR_POST :ARM::LDR_POST_IMM;
   unsigned FltOpc = ARM::VLDMDIA_UPD;
-  emitPopInst(MBB, MI, CSI, FltOpc, 0, isVarArg, true, &isARMArea3Register);
+  emitPopInst(MBB, MI, CSI, FltOpc, 0, isVarArg, true, &isARMArea3Register,
+              NumAlignedDPRCS2Regs);
   emitPopInst(MBB, MI, CSI, PopOpc, LdrOpc, isVarArg, false,
-              &isARMArea2Register);
+              &isARMArea2Register, 0);
   emitPopInst(MBB, MI, CSI, PopOpc, LdrOpc, isVarArg, false,
-              &isARMArea1Register);
+              &isARMArea1Register, 0);
 
   return true;
 }
@@ -842,6 +1137,55 @@ static unsigned estimateRSStackSizeLimit(MachineFunction &MF,
   return Limit;
 }
 
+// In functions that realign the stack, it can be an advantage to spill the
+// callee-saved vector registers after realigning the stack. The vst1 and vld1
+// instructions take alignment hints that can improve performance.
+//
+static void checkNumAlignedDPRCS2Regs(MachineFunction &MF) {
+  MF.getInfo<ARMFunctionInfo>()->setNumAlignedDPRCS2Regs(0);
+  if (!SpillAlignedNEONRegs)
+    return;
+
+  // Naked functions don't spill callee-saved registers.
+  if (MF.getFunction()->hasFnAttr(Attribute::Naked))
+    return;
+
+  // We are planning to use NEON instructions vst1 / vld1.
+  if (!MF.getTarget().getSubtarget<ARMSubtarget>().hasNEON())
+    return;
+
+  // Don't bother if the default stack alignment is sufficiently high.
+  if (MF.getTarget().getFrameLowering()->getStackAlignment() >= 8)
+    return;
+
+  // Aligned spills require stack realignment.
+  const ARMBaseRegisterInfo *RegInfo =
+    static_cast<const ARMBaseRegisterInfo*>(MF.getTarget().getRegisterInfo());
+  if (!RegInfo->canRealignStack(MF))
+    return;
+
+  // We always spill contiguous d-registers starting from d8. Count how many
+  // needs spilling.  The register allocator will almost always use the
+  // callee-saved registers in order, but it can happen that there are holes in
+  // the range.  Registers above the hole will be spilled to the standard DPRCS
+  // area.
+  MachineRegisterInfo &MRI = MF.getRegInfo();
+  unsigned NumSpills = 0;
+  for (; NumSpills < 8; ++NumSpills)
+    if (!MRI.isPhysRegOrOverlapUsed(ARM::D8 + NumSpills))
+      break;
+
+  // Don't do this for just one d-register. It's not worth it.
+  if (NumSpills < 2)
+    return;
+
+  // Spill the first NumSpills D-registers after realigning the stack.
+  MF.getInfo<ARMFunctionInfo>()->setNumAlignedDPRCS2Regs(NumSpills);
+
+  // A scratch register is required for the vst1 / vld1 instructions.
+  MF.getRegInfo().setPhysRegUsed(ARM::R4);
+}
+
 void
 ARMFrameLowering::processFunctionBeforeCalleeSavedScan(MachineFunction &MF,
                                                        RegScavenger *RS) const {
@@ -888,6 +1232,9 @@ ARMFrameLowering::processFunctionBeforeCalleeSavedScan(MachineFunction &MF,
       MF.getRegInfo().setPhysRegUsed(ARM::R4);
   }
 
+  // See if we can spill vector registers to aligned stack.
+  checkNumAlignedDPRCS2Regs(MF);
+
   // Spill the BasePtr if it's used.
   if (RegInfo->hasBasePointer(MF))
     MF.getRegInfo().setPhysRegUsed(RegInfo->getBaseRegister());