musttail: Forward regparms of variadic functions on x86_64

[rnk]

Summary

If a variadic function body contains a musttail call, then we copy all
of the remaining register parameters into virtual registers in the
function prologue. We track the virtual registers through the function
body, and add them as additional registers to pass to the call. Because
this is all done in virtual registers, the register allocator usually
gives us good code. If the function does a call, however, it will have
to spill and reload all argument registers (ew).

Forwarding regparms on x86_32 is not implemented because most compilers
don't support varargs in 32-bit with regparms.

[majnemer]

lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp

151

if (const auto *CI = dyn_cast<CallInst>(I)) { would be a little nicer when considering that I is a const_iterator.

lib/Target/X86/X86ISelLowering.cpp

2291–2296

Do we want a TODO here for vectorcall?

[rnk]

• Add a const and munge some comments

[rnk]

Closed by commit rL216780 (authored by @rnk).

include/llvm/CodeGen/MachineFrameInfo.h

class MachineFrameInfo {
 
   /// True if the function includes inline assembly that adjusts the stack
   /// pointer.
   bool HasInlineAsmWithSPAdjust;
 
   /// True if the function contains a call to the llvm.vastart intrinsic.
   bool HasVAStart;
 
+  /// True if this is a varargs function that contains a musttail call.
+  bool HasMustTailInVarArgFunc;
+
   const TargetFrameLowering *getFrameLowering() const;
 public:
     explicit MachineFrameInfo(const TargetMachine &TM, bool RealignOpt)
     : TM(TM), RealignOption(RealignOpt) {
     StackSize = NumFixedObjects = OffsetAdjustment = MaxAlignment = 0;
     HasVarSizedObjects = false;
     FrameAddressTaken = false;
     ReturnAddressTaken = false;
     HasStackMap = false;
     HasPatchPoint = false;
     AdjustsStack = false;
     HasCalls = false;
     StackProtectorIdx = -1;
     FunctionContextIdx = -1;
     MaxCallFrameSize = 0;
     CSIValid = false;
     LocalFrameSize = 0;
     LocalFrameMaxAlign = 0;
     UseLocalStackAllocationBlock = false;
     HasInlineAsmWithSPAdjust = false;
     HasVAStart = false;
+    HasMustTailInVarArgFunc = false;
   }
 
   /// hasStackObjects - Return true if there are any stack objects in this
   /// function.
   ///
   bool hasStackObjects() const { return !Objects.empty(); }
 
   /// hasVarSizedObjects - This method may be called any time after instruction
public:
   /// Returns true if the function contains any stack-adjusting inline assembly.
   bool hasInlineAsmWithSPAdjust() const { return HasInlineAsmWithSPAdjust; }
   void setHasInlineAsmWithSPAdjust(bool B) { HasInlineAsmWithSPAdjust = B; }
 
   /// Returns true if the function calls the llvm.va_start intrinsic.
   bool hasVAStart() const { return HasVAStart; }
   void setHasVAStart(bool B) { HasVAStart = B; }
 
+  /// Returns true if the function is variadic and contains a musttail call.
+  bool hasMustTailInVarArgFunc() const { return HasMustTailInVarArgFunc; }
+  void setHasMustTailInVarArgFunc(bool B) { HasMustTailInVarArgFunc = B; }
+
   /// getMaxCallFrameSize - Return the maximum size of a call frame that must be
   /// allocated for an outgoing function call.  This is only available if
   /// CallFrameSetup/Destroy pseudo instructions are used by the target, and
   /// then only during or after prolog/epilog code insertion.
   ///
   unsigned getMaxCallFrameSize() const { return MaxCallFrameSize; }
   void setMaxCallFrameSize(unsigned S) { MaxCallFrameSize = S; }
 

lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp

for (BasicBlock::const_iterator I = BB->begin(), E = BB->end();
       // Look for calls to the @llvm.va_start intrinsic. We can omit some
       // prologue boilerplate for variadic functions that don't examine their
       // arguments.
       if (const auto *II = dyn_cast<IntrinsicInst>(I)) {
         if (II->getIntrinsicID() == Intrinsic::vastart)
           MF->getFrameInfo()->setHasVAStart(true);
       }
 
+      // If we have a musttail call in a variadic funciton, we need to ensure we
+      // forward implicit register parameters.
+      if (auto *CI = dyn_cast<CallInst>(I)) {
+        if (CI->isMustTailCall() && Fn->isVarArg())
+          MF->getFrameInfo()->setHasMustTailInVarArgFunc(true);
+      }
+
       // Mark values used outside their block as exported, by allocating
       // a virtual register for them.
       if (isUsedOutsideOfDefiningBlock(I))
         if (!isa<AllocaInst>(I) ||
             !StaticAllocaMap.count(cast<AllocaInst>(I)))
           InitializeRegForValue(I);
 
       // Collect llvm.dbg.declare information. This is done now instead of

lib/Target/X86/X86ISelLowering.cpp

int FI = MFI->CreateFixedObject(ValVT.getSizeInBits()/8,
                                     VA.getLocMemOffset(), isImmutable);
     SDValue FIN = DAG.getFrameIndex(FI, getPointerTy());
     return DAG.getLoad(ValVT, dl, Chain, FIN,
                        MachinePointerInfo::getFixedStack(FI),
                        false, false, false, 0);
   }
 }
 
+static ArrayRef<MCPhysReg> get64BitArgumentGPRs(CallingConv::ID CallConv,
+                                                const X86Subtarget *Subtarget) {
+  assert(Subtarget->is64Bit());
+
+  if (Subtarget->isCallingConvWin64(CallConv)) {
+    static const MCPhysReg GPR64ArgRegsWin64[] = {
+      X86::RCX, X86::RDX, X86::R8,  X86::R9
+    };
+    return GPR64ArgRegsWin64;
+  }
+
+  static const MCPhysReg GPR64ArgRegs64Bit[] = {
+    X86::RDI, X86::RSI, X86::RDX, X86::RCX, X86::R8, X86::R9
+  };
+  return GPR64ArgRegs64Bit;
+}
+
+static ArrayRef<MCPhysReg> get64BitArgumentXMMs(MachineFunction &MF,
+                                                CallingConv::ID CallConv,
+                                                const X86Subtarget *Subtarget) {
+  assert(Subtarget->is64Bit());
+  if (Subtarget->isCallingConvWin64(CallConv)) {
+    // The XMM registers which might contain var arg parameters are shadowed
+    // in their paired GPR.  So we only need to save the GPR to their home
+    // slots.
+    return None;
+  }
+
+  const Function *Fn = MF.getFunction();
+  bool NoImplicitFloatOps = Fn->getAttributes().
+      hasAttribute(AttributeSet::FunctionIndex, Attribute::NoImplicitFloat);
+  assert(!(MF.getTarget().Options.UseSoftFloat && NoImplicitFloatOps) &&
+         "SSE register cannot be used when SSE is disabled!");
+  if (MF.getTarget().Options.UseSoftFloat || NoImplicitFloatOps ||
+      !Subtarget->hasSSE1())
+    // Kernel mode asks for SSE to be disabled, so there are no XMM argument
+    // registers.
+    return None;
+
+  static const MCPhysReg XMMArgRegs64Bit[] = {
+    X86::XMM0, X86::XMM1, X86::XMM2, X86::XMM3,
+    X86::XMM4, X86::XMM5, X86::XMM6, X86::XMM7
+  };
+  return XMMArgRegs64Bit;
+}
+
 SDValue
 X86TargetLowering::LowerFormalArguments(SDValue Chain,
                                         CallingConv::ID CallConv,
                                         bool isVarArg,
                                       const SmallVectorImpl<ISD::InputArg> &Ins,
                                         SDLoc dl,
                                         SelectionDAG &DAG,
                                         SmallVectorImpl<SDValue> &InVals)
X86TargetLowering::LowerFormalArguments(SDValue Chain,
   // Align stack specially for tail calls.
   if (FuncIsMadeTailCallSafe(CallConv,
                              MF.getTarget().Options.GuaranteedTailCallOpt))
     StackSize = GetAlignedArgumentStackSize(StackSize, DAG);
 
   // If the function takes variable number of arguments, make a frame index for
   // the start of the first vararg value... for expansion of llvm.va_start. We
   // can skip this if there are no va_start calls.
-  if (isVarArg && MFI->hasVAStart()) {
-    if (Is64Bit || (CallConv != CallingConv::X86_FastCall &&
-                    CallConv != CallingConv::X86_ThisCall)) {
-      FuncInfo->setVarArgsFrameIndex(MFI->CreateFixedObject(1, StackSize,true));
-    }
-    if (Is64Bit) {
-      unsigned TotalNumIntRegs = 0, TotalNumXMMRegs = 0;
-
-      // FIXME: We should really autogenerate these arrays
-      static const MCPhysReg GPR64ArgRegsWin64[] = {
-        X86::RCX, X86::RDX, X86::R8,  X86::R9
-      };
-      static const MCPhysReg GPR64ArgRegs64Bit[] = {
-        X86::RDI, X86::RSI, X86::RDX, X86::RCX, X86::R8, X86::R9
-      };
-      static const MCPhysReg XMMArgRegs64Bit[] = {
-        X86::XMM0, X86::XMM1, X86::XMM2, X86::XMM3,
-        X86::XMM4, X86::XMM5, X86::XMM6, X86::XMM7
-      };
-      const MCPhysReg *GPR64ArgRegs;
-      unsigned NumXMMRegs = 0;
-
-      if (IsWin64) {
-        // The XMM registers which might contain var arg parameters are shadowed
-        // in their paired GPR.  So we only need to save the GPR to their home
-        // slots.
-        TotalNumIntRegs = 4;
-        GPR64ArgRegs = GPR64ArgRegsWin64;
-      } else {
-        TotalNumIntRegs = 6; TotalNumXMMRegs = 8;
-        GPR64ArgRegs = GPR64ArgRegs64Bit;
-
-        NumXMMRegs = CCInfo.getFirstUnallocated(XMMArgRegs64Bit,
-                                                TotalNumXMMRegs);
-      }
-      unsigned NumIntRegs = CCInfo.getFirstUnallocated(GPR64ArgRegs,
-                                                       TotalNumIntRegs);
-
-      bool NoImplicitFloatOps = Fn->getAttributes().
-        hasAttribute(AttributeSet::FunctionIndex, Attribute::NoImplicitFloat);
-      assert(!(NumXMMRegs && !Subtarget->hasSSE1()) &&
-             "SSE register cannot be used when SSE is disabled!");
-      assert(!(NumXMMRegs && MF.getTarget().Options.UseSoftFloat &&
-               NoImplicitFloatOps) &&
-             "SSE register cannot be used when SSE is disabled!");
-      if (MF.getTarget().Options.UseSoftFloat || NoImplicitFloatOps ||
-          !Subtarget->hasSSE1())
-        // Kernel mode asks for SSE to be disabled, so don't push them
-        // on the stack.
-        TotalNumXMMRegs = 0;
+  if (MFI->hasVAStart() &&
+      (Is64Bit || (CallConv != CallingConv::X86_FastCall &&
+                   CallConv != CallingConv::X86_ThisCall))) {
+    FuncInfo->setVarArgsFrameIndex(
+        MFI->CreateFixedObject(1, StackSize, true));
+  }
+
+  // 64-bit calling conventions support varargs and register parameters, so we
+  // have to do extra work to spill them in the prologue or forward them to
+  // musttail calls.
+  if (Is64Bit && isVarArg &&
+      (MFI->hasVAStart() || MFI->hasMustTailInVarArgFunc())) {
+    // Find the first unallocated argument registers.
+    ArrayRef<MCPhysReg> ArgGPRs = get64BitArgumentGPRs(CallConv, Subtarget);
+    ArrayRef<MCPhysReg> ArgXMMs = get64BitArgumentXMMs(MF, CallConv, Subtarget);
+    unsigned NumIntRegs =
+        CCInfo.getFirstUnallocated(ArgGPRs.data(), ArgGPRs.size());
+    unsigned NumXMMRegs =
+        CCInfo.getFirstUnallocated(ArgXMMs.data(), ArgXMMs.size());
 
+    // Gather all the live in physical registers.
+    SmallVector<SDValue, 6> LiveGPRs;
+    SmallVector<SDValue, 8> LiveXMMRegs;
+    SDValue ALVal;
+    for (MCPhysReg Reg : ArgGPRs.slice(NumIntRegs)) {
+      unsigned GPR = MF.addLiveIn(Reg, &X86::GR64RegClass);
+      LiveGPRs.push_back(
+          DAG.getCopyFromReg(DAG.getEntryNode(), dl, GPR, MVT::i64));
+    }
+    if (!ArgXMMs.empty()) {
+      unsigned AL = MF.addLiveIn(X86::AL, &X86::GR8RegClass);
+      ALVal = DAG.getCopyFromReg(DAG.getEntryNode(), dl, AL, MVT::i8);
+      for (MCPhysReg Reg : ArgXMMs.slice(NumXMMRegs)) {
+        unsigned XMMReg = MF.addLiveIn(Reg, &X86::VR128RegClass);
+        LiveXMMRegs.push_back(
+            DAG.getCopyFromReg(Chain, dl, XMMReg, MVT::v4f32));
+      }
+    }
+
+    // Store them to the va_list returned by va_start.
+    if (MFI->hasVAStart()) {
       if (IsWin64) {
         const TargetFrameLowering &TFI = *MF.getSubtarget().getFrameLowering();
         // Get to the caller-allocated home save location.  Add 8 to account
         // for the return address.
         int HomeOffset = TFI.getOffsetOfLocalArea() + 8;
         FuncInfo->setRegSaveFrameIndex(
           MFI->CreateFixedObject(1, NumIntRegs * 8 + HomeOffset, false));
         // Fixup to set vararg frame on shadow area (4 x i64).
         if (NumIntRegs < 4)
           FuncInfo->setVarArgsFrameIndex(FuncInfo->getRegSaveFrameIndex());
       } else {
         // For X86-64, if there are vararg parameters that are passed via
         // registers, then we must store them to their spots on the stack so
         // they may be loaded by deferencing the result of va_next.
         FuncInfo->setVarArgsGPOffset(NumIntRegs * 8);
-        FuncInfo->setVarArgsFPOffset(TotalNumIntRegs * 8 + NumXMMRegs * 16);
-        FuncInfo->setRegSaveFrameIndex(
-          MFI->CreateStackObject(TotalNumIntRegs * 8 + TotalNumXMMRegs * 16, 16,
-                               false));
+        FuncInfo->setVarArgsFPOffset(ArgGPRs.size() * 8 + NumXMMRegs * 16);
+        FuncInfo->setRegSaveFrameIndex(MFI->CreateStackObject(
+            ArgGPRs.size() * 8 + ArgXMMs.size() * 16, 16, false));
       }
 
       // Store the integer parameter registers.
       SmallVector<SDValue, 8> MemOps;
       SDValue RSFIN = DAG.getFrameIndex(FuncInfo->getRegSaveFrameIndex(),
                                         getPointerTy());
       unsigned Offset = FuncInfo->getVarArgsGPOffset();
-      for (; NumIntRegs != TotalNumIntRegs; ++NumIntRegs) {
+      for (SDValue Val : LiveGPRs) {
         SDValue FIN = DAG.getNode(ISD::ADD, dl, getPointerTy(), RSFIN,
                                   DAG.getIntPtrConstant(Offset));
-        unsigned VReg = MF.addLiveIn(GPR64ArgRegs[NumIntRegs],
-                                     &X86::GR64RegClass);
-        SDValue Val = DAG.getCopyFromReg(Chain, dl, VReg, MVT::i64);
         SDValue Store =
           DAG.getStore(Val.getValue(1), dl, Val, FIN,
                        MachinePointerInfo::getFixedStack(
                          FuncInfo->getRegSaveFrameIndex(), Offset),
                        false, false, 0);
         MemOps.push_back(Store);
         Offset += 8;
       }
 
-      if (TotalNumXMMRegs != 0 && NumXMMRegs != TotalNumXMMRegs) {
+      if (!ArgXMMs.empty() && NumXMMRegs != ArgXMMs.size()) {
         // Now store the XMM (fp + vector) parameter registers.
         SmallVector<SDValue, 12> SaveXMMOps;
         SaveXMMOps.push_back(Chain);
-
-        unsigned AL = MF.addLiveIn(X86::AL, &X86::GR8RegClass);
-        SDValue ALVal = DAG.getCopyFromReg(DAG.getEntryNode(), dl, AL, MVT::i8);
         SaveXMMOps.push_back(ALVal);
-
         SaveXMMOps.push_back(DAG.getIntPtrConstant(
                                FuncInfo->getRegSaveFrameIndex()));
         SaveXMMOps.push_back(DAG.getIntPtrConstant(
                                FuncInfo->getVarArgsFPOffset()));
-
-        for (; NumXMMRegs != TotalNumXMMRegs; ++NumXMMRegs) {
-          unsigned VReg = MF.addLiveIn(XMMArgRegs64Bit[NumXMMRegs],
-                                       &X86::VR128RegClass);
-          SDValue Val = DAG.getCopyFromReg(Chain, dl, VReg, MVT::v4f32);
-          SaveXMMOps.push_back(Val);
-        }
+        SaveXMMOps.insert(SaveXMMOps.end(), LiveXMMRegs.begin(),
+                          LiveXMMRegs.end());
         MemOps.push_back(DAG.getNode(X86ISD::VASTART_SAVE_XMM_REGS, dl,
                                      MVT::Other, SaveXMMOps));
       }
 
       if (!MemOps.empty())
         Chain = DAG.getNode(ISD::TokenFactor, dl, MVT::Other, MemOps);
+    } else {
+      // TODO: Save virtual registers away some where so we can do
+      // getCopyFromReg in the musttail call lowering bb.
+      assert(MFI->hasMustTailInVarArgFunc());
+      auto &Forwards = FuncInfo->getForwardedMustTailRegParms();
+      typedef X86MachineFunctionInfo::Forward Forward;
+
+      // Add all GPRs, al, and XMMs to the list of forwards.
+      for (unsigned I = 0, E = LiveGPRs.size(); I != E; ++I) {
+        unsigned VReg =
+            MF.getRegInfo().createVirtualRegister(&X86::GR64RegClass);
+        Chain = DAG.getCopyToReg(Chain, dl, VReg, LiveGPRs[I]);
+        Forwards.push_back(Forward(VReg, ArgGPRs[NumIntRegs + I], MVT::i64));
+      }
+
+      if (!ArgXMMs.empty()) {
+        unsigned ALVReg =
+            MF.getRegInfo().createVirtualRegister(&X86::GR8RegClass);
+        Chain = DAG.getCopyToReg(Chain, dl, ALVReg, ALVal);
+        Forwards.push_back(Forward(ALVReg, X86::AL, MVT::i8));
+
+        for (unsigned I = 0, E = LiveXMMRegs.size(); I != E; ++I) {
+          unsigned VReg =
+              MF.getRegInfo().createVirtualRegister(&X86::VR128RegClass);
+          Chain = DAG.getCopyToReg(Chain, dl, VReg, LiveXMMRegs[I]);
+          Forwards.push_back(
+              Forward(VReg, ArgXMMs[NumXMMRegs + I], MVT::v4f32));
+        }
+      }
     }
   }
 
   // Some CCs need callee pop.
   if (X86::isCalleePop(CallConv, Is64Bit, isVarArg,
                        MF.getTarget().Options.GuaranteedTailCallOpt)) {
     FuncInfo->setBytesToPopOnReturn(StackSize); // Callee pops everything.
   } else {
X86TargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
   bool &isTailCall                      = CLI.IsTailCall;
   bool isVarArg                         = CLI.IsVarArg;
 
   MachineFunction &MF = DAG.getMachineFunction();
   bool Is64Bit        = Subtarget->is64Bit();
   bool IsWin64        = Subtarget->isCallingConvWin64(CallConv);
   StructReturnType SR = callIsStructReturn(Outs);
   bool IsSibcall      = false;
+  X86MachineFunctionInfo *X86Info = MF.getInfo<X86MachineFunctionInfo>();
 
   if (MF.getTarget().Options.DisableTailCalls)
     isTailCall = false;
 
   bool IsMustTail = CLI.CS && CLI.CS->isMustTailCall();
   if (IsMustTail) {
     // Force this to be a tail call.  The verifier rules are enough to ensure
     // that we can lower this successfully without moving the return address
if (IsSibcall)
     NumBytes = 0;
   else if (MF.getTarget().Options.GuaranteedTailCallOpt &&
            IsTailCallConvention(CallConv))
     NumBytes = GetAlignedArgumentStackSize(NumBytes, DAG);
 
   int FPDiff = 0;
   if (isTailCall && !IsSibcall && !IsMustTail) {
     // Lower arguments at fp - stackoffset + fpdiff.
-    X86MachineFunctionInfo *X86Info = MF.getInfo<X86MachineFunctionInfo>();
     unsigned NumBytesCallerPushed = X86Info->getBytesToPopOnReturn();
 
     FPDiff = NumBytesCallerPushed - NumBytes;
 
     // Set the delta of movement of the returnaddr stackslot.
     // But only set if delta is greater than previous delta.
     if (FPDiff < X86Info->getTCReturnAddrDelta())
       X86Info->setTCReturnAddrDelta(FPDiff);
if (!isTailCall) {
       if (G && !G->getGlobal()->hasHiddenVisibility() &&
           !G->getGlobal()->hasProtectedVisibility())
         Callee = LowerGlobalAddress(Callee, DAG);
       else if (isa<ExternalSymbolSDNode>(Callee))
         Callee = LowerExternalSymbol(Callee, DAG);
     }
   }
 
-  if (Is64Bit && isVarArg && !IsWin64) {
+  if (Is64Bit && isVarArg && !IsWin64 && !IsMustTail) {
     // From AMD64 ABI document:
     // For calls that may call functions that use varargs or stdargs
     // (prototype-less calls or calls to functions containing ellipsis (...) in
     // the declaration) %al is used as hidden argument to specify the number
     // of SSE registers used. The contents of %al do not need to match exactly
     // the number of registers, but must be an ubound on the number of SSE
     // registers used and is in the range 0 - 8 inclusive.
 
     // Count the number of XMM registers allocated.
     static const MCPhysReg XMMArgRegs[] = {
       X86::XMM0, X86::XMM1, X86::XMM2, X86::XMM3,
       X86::XMM4, X86::XMM5, X86::XMM6, X86::XMM7
     };
     unsigned NumXMMRegs = CCInfo.getFirstUnallocated(XMMArgRegs, 8);
     assert((Subtarget->hasSSE1() || !NumXMMRegs)
            && "SSE registers cannot be used when SSE is disabled");
 
     RegsToPass.push_back(std::make_pair(unsigned(X86::AL),
                                         DAG.getConstant(NumXMMRegs, MVT::i8)));
   }
 
+  if (Is64Bit && isVarArg && IsMustTail) {
+    const auto &Forwards = X86Info->getForwardedMustTailRegParms();
+    for (const auto &F : Forwards) {
+      SDValue Val = DAG.getCopyFromReg(Chain, dl, F.VReg, F.VT);
+      RegsToPass.push_back(std::make_pair(unsigned(F.PReg), Val));
+    }
+  }
+
   // For tail calls lower the arguments to the 'real' stack slots.  Sibcalls
   // don't need this because the eligibility check rejects calls that require
   // shuffling arguments passed in memory.
   if (!IsSibcall && isTailCall) {
     // Force all the incoming stack arguments to be loaded from the stack
     // before any new outgoing arguments are stored to the stack, because the
     // outgoing stack slots may alias the incoming argument stack slots, and
     // the alias isn't otherwise explicit. This is slightly more conservative

lib/Target/X86/X86MachineFunctionInfo.h

 // This file declares X86-specific per-machine-function information.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_LIB_TARGET_X86_X86MACHINEFUNCTIONINFO_H
 #define LLVM_LIB_TARGET_X86_X86MACHINEFUNCTIONINFO_H
 
 #include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineValueType.h"
+#include <vector>
 
 namespace llvm {
 
 /// X86MachineFunctionInfo - This class is derived from MachineFunction and
 /// contains private X86 target-specific information for each MachineFunction.
 class X86MachineFunctionInfo : public MachineFunctionInfo {
   virtual void anchor();
 
class X86MachineFunctionInfo : public MachineFunctionInfo {
   unsigned VarArgsFPOffset;
   /// ArgumentStackSize - The number of bytes on stack consumed by the arguments
   /// being passed on the stack.
   unsigned ArgumentStackSize;
   /// NumLocalDynamics - Number of local-dynamic TLS accesses.
   unsigned NumLocalDynamics;
 
 public:
+  /// Describes a register that needs to be forwarded from the prologue to a
+  /// musttail call.
+  struct Forward {
+    Forward(unsigned VReg, MCPhysReg PReg, MVT VT)
+        : VReg(VReg), PReg(PReg), VT(VT) {}
+    unsigned VReg;
+    MCPhysReg PReg;
+    MVT VT;
+  };
+
+private:
+  /// ForwardedMustTailRegParms - A list of virtual and physical registers
+  /// that must be forwarded to every musttail call.
+  std::vector<Forward> ForwardedMustTailRegParms;
+
+public:
   X86MachineFunctionInfo() : ForceFramePointer(false),
                              CalleeSavedFrameSize(0),
                              BytesToPopOnReturn(0),
                              ReturnAddrIndex(0),
                              TailCallReturnAddrDelta(0),
                              SRetReturnReg(0),
                              GlobalBaseReg(0),
                              VarArgsFrameIndex(0),
public:
   void setVarArgsFPOffset(unsigned Offset) { VarArgsFPOffset = Offset; }
 
   unsigned getArgumentStackSize() const { return ArgumentStackSize; }
   void setArgumentStackSize(unsigned size) { ArgumentStackSize = size; }
 
   unsigned getNumLocalDynamicTLSAccesses() const { return NumLocalDynamics; }
   void incNumLocalDynamicTLSAccesses() { ++NumLocalDynamics; }
 
+  std::vector<Forward> &getForwardedMustTailRegParms() {
+    return ForwardedMustTailRegParms;
+  }
 };
 
 } // End llvm namespace
 
 #endif

test/CodeGen/X86/musttail-varargs.ll

+; RUN: llc < %s -enable-tail-merge=0 -mtriple=x86_64-linux | FileCheck %s --check-prefix=LINUX
+; RUN: llc < %s -enable-tail-merge=0 -mtriple=x86_64-windows | FileCheck %s --check-prefix=WINDOWS
+
+; Test that we actually spill and reload all arguments in the variadic argument
+; pack. Doing a normal call will clobber all argument registers, and we will
+; spill around it. A simple adjustment should not require any XMM spills.
+
+declare void(i8*, ...)* @get_f(i8* %this)
+
+define void @f_thunk(i8* %this, ...) {
+  %fptr = call void(i8*, ...)*(i8*)* @get_f(i8* %this)
+  musttail call void (i8*, ...)* %fptr(i8* %this, ...)
+  ret void
+}
+
+; Save and restore 6 GPRs, 8 XMMs, and AL around the call.
+
+; LINUX-LABEL: f_thunk:
+; LINUX-DAG: movq %rdi, {{.*}}
+; LINUX-DAG: movq %rsi, {{.*}}
+; LINUX-DAG: movq %rdx, {{.*}}
+; LINUX-DAG: movq %rcx, {{.*}}
+; LINUX-DAG: movq %r8, {{.*}}
+; LINUX-DAG: movq %r9, {{.*}}
+; LINUX-DAG: movb %al, {{.*}}
+; LINUX-DAG: movaps %xmm0, {{[0-9]*}}(%rsp)
+; LINUX-DAG: movaps %xmm1, {{[0-9]*}}(%rsp)
+; LINUX-DAG: movaps %xmm2, {{[0-9]*}}(%rsp)
+; LINUX-DAG: movaps %xmm3, {{[0-9]*}}(%rsp)
+; LINUX-DAG: movaps %xmm4, {{[0-9]*}}(%rsp)
+; LINUX-DAG: movaps %xmm5, {{[0-9]*}}(%rsp)
+; LINUX-DAG: movaps %xmm6, {{[0-9]*}}(%rsp)
+; LINUX-DAG: movaps %xmm7, {{[0-9]*}}(%rsp)
+; LINUX: callq get_f
+; LINUX-DAG: movaps {{[0-9]*}}(%rsp), %xmm0
+; LINUX-DAG: movaps {{[0-9]*}}(%rsp), %xmm1
+; LINUX-DAG: movaps {{[0-9]*}}(%rsp), %xmm2
+; LINUX-DAG: movaps {{[0-9]*}}(%rsp), %xmm3
+; LINUX-DAG: movaps {{[0-9]*}}(%rsp), %xmm4
+; LINUX-DAG: movaps {{[0-9]*}}(%rsp), %xmm5
+; LINUX-DAG: movaps {{[0-9]*}}(%rsp), %xmm6
+; LINUX-DAG: movaps {{[0-9]*}}(%rsp), %xmm7
+; LINUX-DAG: movq {{.*}}, %rdi
+; LINUX-DAG: movq {{.*}}, %rsi
+; LINUX-DAG: movq {{.*}}, %rdx
+; LINUX-DAG: movq {{.*}}, %rcx
+; LINUX-DAG: movq {{.*}}, %r8
+; LINUX-DAG: movq {{.*}}, %r9
+; LINUX-DAG: movb {{.*}}, %al
+; LINUX: jmpq *{{.*}}  # TAILCALL
+
+; WINDOWS-LABEL: f_thunk:
+; WINDOWS-NOT: mov{{.}}ps
+; WINDOWS-DAG: movq %rdx, {{.*}}
+; WINDOWS-DAG: movq %rcx, {{.*}}
+; WINDOWS-DAG: movq %r8, {{.*}}
+; WINDOWS-DAG: movq %r9, {{.*}}
+; WINDOWS-NOT: mov{{.}}ps
+; WINDOWS: callq get_f
+; WINDOWS-NOT: mov{{.}}ps
+; WINDOWS-DAG: movq {{.*}}, %rdx
+; WINDOWS-DAG: movq {{.*}}, %rcx
+; WINDOWS-DAG: movq {{.*}}, %r8
+; WINDOWS-DAG: movq {{.*}}, %r9
+; WINDOWS-NOT: mov{{.}}ps
+; WINDOWS: jmpq *{{.*}} # TAILCALL
+
+; This thunk shouldn't require any spills and reloads, assuming the register
+; allocator knows what it's doing.
+
+define void @g_thunk(i8* %fptr_i8, ...) {
+  %fptr = bitcast i8* %fptr_i8 to void (i8*, ...)*
+  musttail call void (i8*, ...)* %fptr(i8* %fptr_i8, ...)
+  ret void
+}
+
+; LINUX-LABEL: g_thunk:
+; LINUX-NOT: movq
+; LINUX: jmpq *%rdi  # TAILCALL
+
+; WINDOWS-LABEL: g_thunk:
+; WINDOWS-NOT: movq
+; WINDOWS: jmpq *%rcx # TAILCALL
+
+; Do a simple multi-exit multi-bb test.
+
+%struct.Foo = type { i1, i8*, i8* }
+
+@g = external global i32
+
+define void @h_thunk(%struct.Foo* %this, ...) {
+  %cond_p = getelementptr %struct.Foo* %this, i32 0, i32 0
+  %cond = load i1* %cond_p
+  br i1 %cond, label %then, label %else
+
+then:
+  %a_p = getelementptr %struct.Foo* %this, i32 0, i32 1
+  %a_i8 = load i8** %a_p
+  %a = bitcast i8* %a_i8 to void (%struct.Foo*, ...)*
+  musttail call void (%struct.Foo*, ...)* %a(%struct.Foo* %this, ...)
+  ret void
+
+else:
+  %b_p = getelementptr %struct.Foo* %this, i32 0, i32 2
+  %b_i8 = load i8** %b_p
+  %b = bitcast i8* %b_i8 to void (%struct.Foo*, ...)*
+  store i32 42, i32* @g
+  musttail call void (%struct.Foo*, ...)* %b(%struct.Foo* %this, ...)
+  ret void
+}
+
+; LINUX-LABEL: h_thunk:
+; LINUX: jne
+; LINUX: jmpq *{{.*}} # TAILCALL
+; LINUX: jmpq *{{.*}} # TAILCALL
+; WINDOWS-LABEL: h_thunk:
+; WINDOWS: jne
+; WINDOWS: jmpq *{{.*}} # TAILCALL
+; WINDOWS: jmpq *{{.*}} # TAILCALL