[AMDGPU] Reserving VGPR for future SGPR Spill

[saiislam]

Summary

One VGPR register is allocated to handle a future spill of SGPR if "--amdgpu-reserve-vgpr-for-sgpr-spill" option is used

[saiislam]

Corrected formatting

[arsenm]

Can you add a test for this

[saiislam]

Removed reserved VGPR from RA list. Added a test case which
ensures spilling of SGPR into reserved VGPR.

[arsenm]

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

100

This should be the default, but the test disruption is probably significant without forcing the VGPR to be the high register, and adjust it down after RA

llvm/test/CodeGen/AMDGPU/reserve-vgpr-for-sgpr-spill.ll

13

I don't see how this test really stresses the need for SGPR spilling with no free VGPRs. I would expect this to look more like the tests in spill-wide-sgpr.ll, or spill-scavenge-offset.ll to force a high pressure

21

This is the removed form of the attribute. This should be something like frame-pointer=none, although I don't think it matters here

[saiislam]

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

100

Yes, you are right. 285 test cases in AMDGPU codegen are failing by making it default, in the current state.

llvm/test/CodeGen/AMDGPU/reserve-vgpr-for-sgpr-spill.ll

13

The arguments of the function are taking up all the 256 VGPRs, so FP (s34) and SGPRs (s30 and s31) are getting spilled into reserved VGPR (v32), and restored later.
Tests in spill-wide-sgpr.ll are also enforcing 2,4,8 way spilling of SGPR, which I can do here by modifying the body of parent_func while keeping the function signature of both functions. Will it be ok then?

[saiislam]

Now highest available VGPR is reserved in the beginning and it is
switched with lowest available VGPR after RA.

[arsenm]

llvm/lib/Target/AMDGPU/SILowerSGPRSpills.cpp

320–327

Should split into another function. It also isn't ensuring it's a CSR?

359

Commented out code

llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.h

488–489

This shouldn't be separate from the existing SGPR spill infrastructure. This is only pre-allocating one register

[saiislam]

Rebased and added code for proper lower shifting of reserved VGPR.

[saiislam]

Removed commented code.

[saiislam]

Handled few seg faults in the lit-tests.

[arsenm]

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

10867

Should add a note that this is a hack; if we split SGPR allocation from VGPR we shouldn't need to do this

llvm/lib/Target/AMDGPU/SILowerSGPRSpills.cpp

245

Don't need = NoRegister

254

Don't need != NoRegister. Also can invert and return early

258

doesn't need to be a reference

261–263

You can just unconditionally removeLiveIn. The isLiveIn is just a redundant map lookup

llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp

573–574

Should get a comment about what this is for. A better name might be removeVGPRForSGPRSpill?

575

++i

580–581

Can unconditionally removeLiveIn

582

You're going to end up calling sortUniqueLiveIns for every block, for every spill VGPR. The common case is only 1 spill VGPR, but you could collect all of the registers and do one walk through the function

llvm/test/CodeGen/AMDGPU/reserve-vgpr-for-sgpr-spill.ll

13

This won't use all 256 VGPRs. The arguments go to the stack after > 32. It happens to look like an external call looks like it uses a lot of registers, but this is likely to change in the future. A more reliable way would be to use an ugly blob of asm, like

call void asm sideeffect "",
  "~{v0},~{v1},~{v2},~{v3},~{v4},~{v5},~{v6},~{v7},~{v8},~{v9}
  ,~{v10},~{v11},~{v12},~{v13},~{v14},~{v15},~{v16},~{v17},~{v18},~{v19}
  ,~{v20},~{v21},~{v22},~{v23},~{v24},~{v25},~{v26},~{v27},~{v28},~{v29}
  ,~{v30},~{v31},~{v32},~{v33},~{v34},~{v35},~{v36},~{v37},~{v38},~{v39}
  ,~{v40},~{v41},~{v42},~{v43},~{v44},~{v45},~{v46},~{v47},~{v48},~{v49}
  ,~{v50},~{v51},~{v52},~{v53},~{v54},~{v55},~{v56},~{v57},~{v58},~{v59}
  ,~{v60},~{v61},~{v62},~{v63},~{v64},~{v65},~{v66},~{v67},~{v68},~{v69}
  ,~{v70},~{v71},~{v72},~{v73},~{v74},~{v75},~{v76},~{v77},~{v78},~{v79}
  ,~{v80},~{v81},~{v82},~{v83},~{v84},~{v85},~{v86},~{v87},~{v88},~{v89}
  ,~{v90},~{v91},~{v92},~{v93},~{v94},~{v95},~{v96},~{v97},~{v98},~{v99}
  ,~{v100},~{v101},~{v102},~{v103},~{v104},~{v105},~{v106},~{v107},~{v108},~{v109}
  ,~{v110},~{v111},~{v112},~{v113},~{v114},~{v115},~{v116},~{v117},~{v118},~{v119}
  ,~{v120},~{v121},~{v122},~{v123},~{v124},~{v125},~{v126},~{v127},~{v128},~{v129}
  ,~{v130},~{v131},~{v132},~{v133},~{v134},~{v135},~{v136},~{v137},~{v138},~{v139}
  ,~{v140},~{v141},~{v142},~{v143},~{v144},~{v145},~{v146},~{v147},~{v148},~{v149}
  ,~{v150},~{v151},~{v152},~{v153},~{v154},~{v155},~{v156},~{v157},~{v158},~{v159}
  ,~{v160},~{v161},~{v162},~{v163},~{v164},~{v165},~{v166},~{v167},~{v168},~{v169}
  ,~{v170},~{v171},~{v172},~{v173},~{v174},~{v175},~{v176},~{v177},~{v178},~{v179}
  ,~{v180},~{v181},~{v182},~{v183},~{v184},~{v185},~{v186},~{v187},~{v188},~{v189}
  ,~{v190},~{v191},~{v192},~{v193},~{v194},~{v195},~{v196},~{v197},~{v198},~{v199}
  ,~{v200},~{v201},~{v202},~{v203},~{v204},~{v205},~{v206},~{v207},~{v208},~{v209}
  ,~{v210},~{v211},~{v212},~{v213},~{v214},~{v215},~{v216},~{v217},~{v218},~{v219}
  ,~{v220},~{v221},~{v222},~{v223},~{v224},~{v225},~{v226},~{v227},~{v228},~{v229}
  ,~{v230},~{v231},~{v232},~{v233},~{v234},~{v235},~{v236},~{v237},~{v238},~{v239}
  ,~{v240},~{v241},~{v242},~{v243},~{v244},~{v245},~{v246},~{v247},~{v248},~{v249}
  ,~{v250},~{v251},~{v252},~{v253},~{v254},~{v255}"() #1

You can also use amdgpu_max_waves_per_eu to artifically limit the number of available VGPRs to reduce number of registers you have to list

[saiislam]

Fixed failing test cases. Now VGPR reservation happens in an
independent function and doesn't overload allocateSGPRSpillToVGPR.
Updated test case to hard code usage of all but one VGPRs and use
it for SGPR spill.

[arsenm]

llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp

347–348

Better name might be reserveVGPRforSGPRSpills?

355

Define and initialize LaneVGPR at the same time

358

Don't need this? It was never added?

361

Don't need DummyFI, can just pass None

[saiislam]

Cleaned code for reserving vgpr.

[cdevadas]

llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp

315

Doesn't it limit the total allowable SGPR spills to 64?
What happens if more than 64 callee-saved SGPRs used in the functions?

[saiislam]

llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp

315

Yes, this patch is limited to 64 SGPR spills only.
Does it make sense to handle that case in a separate patch?

[kerbowa]

llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp

315

It seems like there is a fallback to spilling to VMEM if there are no free lanes. Doesn't this also limit the number of SGPR to VGPR spills to 32 on Navi?

[arsenm]

llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp

315

That fallback is 90% broken. This is ultimately still a workaround for the fact that SGPRs and VGPRs are allocated at the same time, so we can run out of VGPRs before we see all the SGPR spills that need to be handled

[arsenm]

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

10871–10872

Don't need the dummy stack object anymore

llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp

349

The FI argument is unused

[saiislam]

Removed unsued frame index argument from vgpr reserving function. Added condition to reserve vgpr only if machine function has stack objects.

[saiislam]

Handled a test case which was failing in debug only.

[saiislam]

Switched the nested loops in lowerShiftReservedVGPR() to move
sorting of unique live-ins to the outer loop. Improved cleanup of
reserved vgpr after lowering it to a lower value, and when it needs
to be removed.

[saiislam]

Removed a break

[arsenm]

llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp

584

Register()

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

 
 STATISTIC(NumTailCalls, "Number of tail calls");
 
 static cl::opt<bool> DisableLoopAlignment(
   "amdgpu-disable-loop-alignment",
   cl::desc("Do not align and prefetch loops"),
   cl::init(false));
 
+static cl::opt<bool> VGPRReserveforSGPRSpill(
+    "amdgpu-reserve-vgpr-for-sgpr-spill",
+    cl::desc("Allocates one VGPR for future SGPR Spill"), cl::init(true));
+
 static bool hasFP32Denormals(const MachineFunction &MF) {
   const SIMachineFunctionInfo *Info = MF.getInfo<SIMachineFunctionInfo>();
   return Info->getMode().allFP32Denormals();
 }
 
 static bool hasFP64FP16Denormals(const MachineFunction &MF) {
   const SIMachineFunctionInfo *Info = MF.getInfo<SIMachineFunctionInfo>();
   return Info->getMode().allFP64FP16Denormals();
if (ST.isWave32() && !MF.empty()) {
     for (auto &MBB : MF) {
       for (auto &MI : MBB) {
         TII->fixImplicitOperands(MI);
       }
     }
   }
 
   TargetLoweringBase::finalizeLowering(MF);
+
+  // Allocate a VGPR for future SGPR Spill if
+  // "amdgpu-reserve-vgpr-for-sgpr-spill" option is used
+  // FIXME: We won't need this hack if we split SGPR allocation from VGPR
+  if (VGPRReserveforSGPRSpill && !Info->VGPRReservedForSGPRSpill &&
+      !Info->isEntryFunction() && MF.getFrameInfo().hasStackObjects())
+    Info->reserveVGPRforSGPRSpills(MF);
 }
 
 void SITargetLowering::computeKnownBitsForFrameIndex(const SDValue Op,
                                                      KnownBits &Known,
                                                      const APInt &DemandedElts,
                                                      const SelectionDAG &DAG,
                                                      unsigned Depth) const {
   TargetLowering::computeKnownBitsForFrameIndex(Op, Known, DemandedElts,
                                                 DAG, Depth);

llvm/lib/Target/AMDGPU/SILowerSGPRSpills.cpp

if (!CSI.empty()) {
         insertCSRRestores(*RestoreBlock, CSI, LIS);
       return true;
     }
   }
 
   return false;
 }
 
+static ArrayRef<MCPhysReg> getAllVGPR32(const GCNSubtarget &ST,
+                                        const MachineFunction &MF) {
+  return makeArrayRef(AMDGPU::VGPR_32RegClass.begin(), ST.getMaxNumVGPRs(MF));
+}
+
+// Find lowest available VGPR and use it as VGPR reserved for SGPR spills.
+static bool lowerShiftReservedVGPR(MachineFunction &MF,
+                                   const GCNSubtarget &ST) {
+  MachineRegisterInfo &MRI = MF.getRegInfo();
+  MachineFrameInfo &FrameInfo = MF.getFrameInfo();
+  SIMachineFunctionInfo *FuncInfo = MF.getInfo<SIMachineFunctionInfo>();
+  Register LowestAvailableVGPR, ReservedVGPR;
+  ArrayRef<MCPhysReg> AllVGPR32s = getAllVGPR32(ST, MF);
+  for (MCPhysReg Reg : AllVGPR32s) {
+    if (MRI.isAllocatable(Reg) && !MRI.isPhysRegUsed(Reg)) {
+      LowestAvailableVGPR = Reg;
+      break;
+    }
+  }
+
+  if (!LowestAvailableVGPR)
+    return false;
+
+  ReservedVGPR = FuncInfo->VGPRReservedForSGPRSpill;
+  const MCPhysReg *CSRegs = MF.getRegInfo().getCalleeSavedRegs();
+  int i = 0;
+
+  for (MachineBasicBlock &MBB : MF) {
+    for (auto Reg : FuncInfo->getSGPRSpillVGPRs()) {
+      if (Reg.VGPR == ReservedVGPR) {
+        MBB.removeLiveIn(ReservedVGPR);
+        MBB.addLiveIn(LowestAvailableVGPR);
+        Optional<int> FI;
+        if (FuncInfo->isCalleeSavedReg(CSRegs, LowestAvailableVGPR))
+          FI = FrameInfo.CreateSpillStackObject(4, Align(4));
+
+        FuncInfo->setSGPRSpillVGPRs(LowestAvailableVGPR, FI, i);
+      }
+      ++i;
+    }
+    MBB.sortUniqueLiveIns();
+  }
+
+  return true;
+}
+
 bool SILowerSGPRSpills::runOnMachineFunction(MachineFunction &MF) {
   const GCNSubtarget &ST = MF.getSubtarget<GCNSubtarget>();
   TII = ST.getInstrInfo();
   TRI = &TII->getRegisterInfo();
 
   VRM = getAnalysisIfAvailable<VirtRegMap>();
 
   assert(SaveBlocks.empty() && RestoreBlocks.empty());
bool SILowerSGPRSpills::runOnMachineFunction(MachineFunction &MF) {
   // handled as SpilledToReg in regular PrologEpilogInserter.
   if ((TRI->spillSGPRToVGPR() && (HasCSRs || FuncInfo->hasSpilledSGPRs())) ||
       SpillVGPRToAGPR) {
     // Process all SGPR spills before frame offsets are finalized. Ideally SGPRs
     // are spilled to VGPRs, in which case we can eliminate the stack usage.
     //
     // This operates under the assumption that only other SGPR spills are users
     // of the frame index.
+
+    lowerShiftReservedVGPR(MF, ST);
+
     for (MachineBasicBlock &MBB : MF) {
       MachineBasicBlock::iterator Next;
       for (auto I = MBB.begin(), E = MBB.end(); I != E; I = Next) {
         MachineInstr &MI = *I;
         Next = std::next(I);
 
         if (SpillToAGPR && TII->isVGPRSpill(MI)) {
           // Try to eliminate stack used by VGPR spills before frame
           // finalization.
           unsigned FIOp = AMDGPU::getNamedOperandIdx(MI.getOpcode(),
                                                      AMDGPU::OpName::vaddr);
           int FI = MI.getOperand(FIOp).getIndex();
           Register VReg =
               TII->getNamedOperand(MI, AMDGPU::OpName::vdata)->getReg();
for (MachineBasicBlock &MBB : MF) {
           assert(Spilled && "failed to spill SGPR to VGPR when allocated");
         }
       }
     }
 
     for (MachineBasicBlock &MBB : MF) {
       for (auto SSpill : FuncInfo->getSGPRSpillVGPRs())
         MBB.addLiveIn(SSpill.VGPR);
 
       for (MCPhysReg Reg : FuncInfo->getVGPRSpillAGPRs())
         MBB.addLiveIn(Reg);
 
       for (MCPhysReg Reg : FuncInfo->getAGPRSpillVGPRs())
         MBB.addLiveIn(Reg);
 
       MBB.sortUniqueLiveIns();
     }
 
     MadeChange = true;
+  } else if (FuncInfo->VGPRReservedForSGPRSpill) {
+    FuncInfo->removeVGPRForSGPRSpill(FuncInfo->VGPRReservedForSGPRSpill, MF);
   }
 
   SaveBlocks.clear();
   RestoreBlocks.clear();
 
   return MadeChange;
 }

llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.h

private:
   SmallVector<MCPhysReg, 32> SpillVGPR;
 
 public: // FIXME
   /// If this is set, an SGPR used for save/restore of the register used for the
   /// frame pointer.
   Register SGPRForFPSaveRestoreCopy;
   Optional<int> FramePointerSaveIndex;
 
+  Register VGPRReservedForSGPRSpill;
+  bool isCalleeSavedReg(const MCPhysReg *CSRegs, MCPhysReg Reg);
+
 public:
   SIMachineFunctionInfo(const MachineFunction &MF);
 
   bool initializeBaseYamlFields(const yaml::SIMachineFunctionInfo &YamlMFI);
 
   ArrayRef<SpilledReg> getSGPRToVGPRSpills(int FrameIndex) const {
     auto I = SGPRToVGPRSpills.find(FrameIndex);
     return (I == SGPRToVGPRSpills.end()) ?
       ArrayRef<SpilledReg>() : makeArrayRef(I->second);
   }
 
   ArrayRef<SGPRSpillVGPRCSR> getSGPRSpillVGPRs() const {
     return SpillVGPRs;
   }
 
+  void setSGPRSpillVGPRs(Register NewVGPR, Optional<int> newFI, int Index) {
+    SpillVGPRs[Index].VGPR = NewVGPR;
+    SpillVGPRs[Index].FI = newFI;
+    VGPRReservedForSGPRSpill = NewVGPR;
+  }
+
+  bool removeVGPRForSGPRSpill(Register ReservedVGPR, MachineFunction &MF);
+
   ArrayRef<MCPhysReg> getAGPRSpillVGPRs() const {
     return SpillAGPR;
   }
 
   ArrayRef<MCPhysReg> getVGPRSpillAGPRs() const {
     return SpillVGPR;
   }
 
   MCPhysReg getVGPRToAGPRSpill(int FrameIndex, unsigned Lane) const {
     auto I = VGPRToAGPRSpills.find(FrameIndex);
     return (I == VGPRToAGPRSpills.end()) ? (MCPhysReg)AMDGPU::NoRegister
                                          : I->second.Lanes[Lane];
   }
 
   bool haveFreeLanesForSGPRSpill(const MachineFunction &MF,
                                  unsigned NumLane) const;
   bool allocateSGPRSpillToVGPR(MachineFunction &MF, int FI);
+  bool reserveVGPRforSGPRSpills(MachineFunction &MF);
   bool allocateVGPRSpillToAGPR(MachineFunction &MF, int FI, bool isAGPRtoVGPR);
   void removeDeadFrameIndices(MachineFrameInfo &MFI);
 
   bool hasCalculatedTID() const { return TIDReg != 0; };
   Register getTIDReg() const { return TIDReg; };
   void setTIDReg(Register Reg) { TIDReg = Reg; }
 
   unsigned getBytesInStackArgArea() const {

llvm/lib/Target/AMDGPU/SIMachineFunctionInfo.cpp

 
 Register SIMachineFunctionInfo::addImplicitBufferPtr(const SIRegisterInfo &TRI) {
   ArgInfo.ImplicitBufferPtr = ArgDescriptor::createRegister(TRI.getMatchingSuperReg(
     getNextUserSGPR(), AMDGPU::sub0, &AMDGPU::SReg_64RegClass));
   NumUserSGPRs += 2;
   return ArgInfo.ImplicitBufferPtr.getRegister();
 }
 
-static bool isCalleeSavedReg(const MCPhysReg *CSRegs, MCPhysReg Reg) {
+bool SIMachineFunctionInfo::isCalleeSavedReg(const MCPhysReg *CSRegs,
+                                             MCPhysReg Reg) {
   for (unsigned I = 0; CSRegs[I]; ++I) {
     if (CSRegs[I] == Reg)
       return true;
   }
 
   return false;
 }
 
bool SIMachineFunctionInfo::allocateSGPRSpillToVGPR(MachineFunction &MF,
   if (!SpillLanes.empty())
     return true;
 
   const GCNSubtarget &ST = MF.getSubtarget<GCNSubtarget>();
   const SIRegisterInfo *TRI = ST.getRegisterInfo();
   MachineFrameInfo &FrameInfo = MF.getFrameInfo();
   MachineRegisterInfo &MRI = MF.getRegInfo();
   unsigned WaveSize = ST.getWavefrontSize();
+  SIMachineFunctionInfo *FuncInfo = MF.getInfo<SIMachineFunctionInfo>();
 
   unsigned Size = FrameInfo.getObjectSize(FI);
   assert(Size >= 4 && Size <= 64 && "invalid sgpr spill size");
   assert(TRI->spillSGPRToVGPR() && "not spilling SGPRs to VGPRs");
 
   int NumLanes = Size / 4;
 
   const MCPhysReg *CSRegs = MRI.getCalleeSavedRegs();
 
   // Make sure to handle the case where a wide SGPR spill may span between two
   // VGPRs.
   for (int I = 0; I < NumLanes; ++I, ++NumVGPRSpillLanes) {
     Register LaneVGPR;
     unsigned VGPRIndex = (NumVGPRSpillLanes % WaveSize);
 
-    if (VGPRIndex == 0) {
+    if (VGPRIndex == 0 && !FuncInfo->VGPRReservedForSGPRSpill) {
       LaneVGPR = TRI->findUnusedRegister(MRI, &AMDGPU::VGPR_32RegClass, MF);
       if (LaneVGPR == AMDGPU::NoRegister) {
         // We have no VGPRs left for spilling SGPRs. Reset because we will not
         // partially spill the SGPR to VGPRs.
         SGPRToVGPRSpills.erase(FI);
         NumVGPRSpillLanes -= I;
         return false;
       }
for (int I = 0; I < NumLanes; ++I, ++NumVGPRSpillLanes) {
     }
 
     SpillLanes.push_back(SpilledReg(LaneVGPR, VGPRIndex));
   }
 
   return true;
 }
 
+/// Reserve a VGPR for spilling of SGPRs
+bool SIMachineFunctionInfo::reserveVGPRforSGPRSpills(MachineFunction &MF) {
+  const GCNSubtarget &ST = MF.getSubtarget<GCNSubtarget>();
+  const SIRegisterInfo *TRI = ST.getRegisterInfo();
+  SIMachineFunctionInfo *FuncInfo = MF.getInfo<SIMachineFunctionInfo>();
+
+  Register LaneVGPR = TRI->findUnusedRegister(
+      MF.getRegInfo(), &AMDGPU::VGPR_32RegClass, MF, true);
+  SpillVGPRs.push_back(SGPRSpillVGPRCSR(LaneVGPR, None));
+  FuncInfo->VGPRReservedForSGPRSpill = LaneVGPR;
+  return true;
+}
+
 /// Reserve AGPRs or VGPRs to support spilling for FrameIndex \p FI.
 /// Either AGPR is spilled to VGPR to vice versa.
 /// Returns true if a \p FI can be eliminated completely.
 bool SIMachineFunctionInfo::allocateVGPRSpillToAGPR(MachineFunction &MF,
                                                     int FI,
                                                     bool isAGPRtoVGPR) {
   MachineRegisterInfo &MRI = MF.getRegInfo();
   MachineFrameInfo &FrameInfo = MF.getFrameInfo();
   const GCNSubtarget &ST =  MF.getSubtarget<GCNSubtarget>();
 
bool SIMachineFunctionInfo::initializeBaseYamlFields(
   LDSSize = YamlMFI.LDSSize;
   HighBitsOf32BitAddress = YamlMFI.HighBitsOf32BitAddress;
   IsEntryFunction = YamlMFI.IsEntryFunction;
   NoSignedZerosFPMath = YamlMFI.NoSignedZerosFPMath;
   MemoryBound = YamlMFI.MemoryBound;
   WaveLimiter = YamlMFI.WaveLimiter;
   return false;
 }
+
+// Remove VGPR which was reserved for SGPR spills if there are no spilled SGPRs
+bool SIMachineFunctionInfo::removeVGPRForSGPRSpill(Register ReservedVGPR,
+                                                   MachineFunction &MF) {
+  for (auto *i = SpillVGPRs.begin(); i < SpillVGPRs.end(); i++) {
+    if (i->VGPR == ReservedVGPR) {
+      SpillVGPRs.erase(i);
+
+      for (MachineBasicBlock &MBB : MF) {
+        MBB.removeLiveIn(ReservedVGPR);
+        MBB.sortUniqueLiveIns();
+      }
+      this->VGPRReservedForSGPRSpill = AMDGPU::NoRegister;
+      return true;
+    }
+  }
+  return false;
+}

llvm/lib/Target/AMDGPU/SIRegisterInfo.h

public:
 
   /// \returns True if operands defined with this operand type can accept
   /// an inline constant. i.e. An integer value in the range (-16, 64) or
   /// -4.0f, -2.0f, -1.0f, -0.5f, 0.0f, 0.5f, 1.0f, 2.0f, 4.0f.
   bool opCanUseInlineConstant(unsigned OpType) const;
 
   MCRegister findUnusedRegister(const MachineRegisterInfo &MRI,
                                 const TargetRegisterClass *RC,
-                                const MachineFunction &MF) const;
+                                const MachineFunction &MF,
+                                bool ReserveHighestVGPR = false) const;
 
   const TargetRegisterClass *getRegClassForReg(const MachineRegisterInfo &MRI,
                                                Register Reg) const;
   bool isVGPR(const MachineRegisterInfo &MRI, Register Reg) const;
   bool isAGPR(const MachineRegisterInfo &MRI, Register Reg) const;
   bool isVectorRegister(const MachineRegisterInfo &MRI, Register Reg) const {
     return isVGPR(MRI, Reg) || isAGPR(MRI, Reg);
   }

llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp

BitVector SIRegisterInfo::getReservedRegs(const MachineFunction &MF) const {
 
   // FIXME: Stop using reserved registers for this.
   for (MCPhysReg Reg : MFI->getAGPRSpillVGPRs())
     reserveRegisterTuples(Reserved, Reg);
 
   for (MCPhysReg Reg : MFI->getVGPRSpillAGPRs())
     reserveRegisterTuples(Reserved, Reg);
 
+  if (MFI->VGPRReservedForSGPRSpill)
+    for (auto SSpill : MFI->getSGPRSpillVGPRs())
+      reserveRegisterTuples(Reserved, SSpill.VGPR);
+
   return Reserved;
 }
 
 bool SIRegisterInfo::canRealignStack(const MachineFunction &MF) const {
   const SIMachineFunctionInfo *Info = MF.getInfo<SIMachineFunctionInfo>();
   // On entry, the base address is 0, so it can't possibly need any more
   // alignment.
 
bool SIRegisterInfo::shouldRewriteCopySrc(
   //
   // We want to look through the COPY to find:
   //  => %3 = COPY %0
 
   // Plain copy.
   return getCommonSubClass(DefRC, SrcRC) != nullptr;
 }
 
-/// Returns a register that is not used at any point in the function.
+/// Returns a lowest register that is not used at any point in the function.
 ///        If all registers are used, then this function will return
-//         AMDGPU::NoRegister.
-MCRegister
-SIRegisterInfo::findUnusedRegister(const MachineRegisterInfo &MRI,
+///         AMDGPU::NoRegister. If \p ReserveHighestVGPR = true, then return
+///         highest unused register.
+MCRegister SIRegisterInfo::findUnusedRegister(const MachineRegisterInfo &MRI,
                                               const TargetRegisterClass *RC,
-                                   const MachineFunction &MF) const {
-
+                                              const MachineFunction &MF,
+                                              bool ReserveHighestVGPR) const {
+  if (ReserveHighestVGPR) {
+    for (MCRegister Reg : reverse(*RC))
+      if (MRI.isAllocatable(Reg) && !MRI.isPhysRegUsed(Reg))
+        return Reg;
+  } else {
     for (MCRegister Reg : *RC)
       if (MRI.isAllocatable(Reg) && !MRI.isPhysRegUsed(Reg))
         return Reg;
+  }
   return MCRegister();
 }
 
 ArrayRef<int16_t> SIRegisterInfo::getRegSplitParts(const TargetRegisterClass *RC,
                                                    unsigned EltSize) const {
   const unsigned RegBitWidth = AMDGPU::getRegBitWidth(*RC->MC);
   assert(RegBitWidth >= 32 && RegBitWidth <= 1024);
 

llvm/test/CodeGen/AMDGPU/reserve-vgpr-for-sgpr-spill.ll

+; RUN: llc -mtriple amdgcn-amd-amdhsa -mcpu=gfx803 -O0 -verify-machineinstrs < %s | FileCheck -enable-var-scope -check-prefix=GCN %s
+
+define void @child_function() #0 {
+  call void asm sideeffect "", "~{vcc}" () #0
+  ret void
+}
+
+; GCN-LABEL: {{^}}parent_func:
+; CHECK:  v_writelane_b32 v255, s33, 2
+; CHECK:  v_writelane_b32 v255, s30, 0
+; CHECK:  v_writelane_b32 v255, s31, 1
+; CHECK:  s_swappc_b64 s[30:31], s[4:5]
+; CHECK:  v_readlane_b32 s4, v255, 0
+; CHECK:  v_readlane_b32 s5, v255, 1
+; CHECK:  v_readlane_b32 s33, v255, 2
+; GCN: ; NumVgprs: 256
+
+define void @parent_func() #0 {
+  call void asm sideeffect "",
+  "~{v0},~{v1},~{v2},~{v3},~{v4},~{v5},~{v6},~{v7},~{v8},~{v9}
+  ,~{v10},~{v11},~{v12},~{v13},~{v14},~{v15},~{v16},~{v17},~{v18},~{v19}
+  ,~{v20},~{v21},~{v22},~{v23},~{v24},~{v25},~{v26},~{v27},~{v28},~{v29}
+  ,~{v30},~{v31},~{v32},~{v33},~{v34},~{v35},~{v36},~{v37},~{v38},~{v39}
+  ,~{v40},~{v41},~{v42},~{v43},~{v44},~{v45},~{v46},~{v47},~{v48},~{v49}
+  ,~{v50},~{v51},~{v52},~{v53},~{v54},~{v55},~{v56},~{v57},~{v58},~{v59}
+  ,~{v60},~{v61},~{v62},~{v63},~{v64},~{v65},~{v66},~{v67},~{v68},~{v69}
+  ,~{v70},~{v71},~{v72},~{v73},~{v74},~{v75},~{v76},~{v77},~{v78},~{v79}
+  ,~{v80},~{v81},~{v82},~{v83},~{v84},~{v85},~{v86},~{v87},~{v88},~{v89}
+  ,~{v90},~{v91},~{v92},~{v93},~{v94},~{v95},~{v96},~{v97},~{v98},~{v99}
+  ,~{v100},~{v101},~{v102},~{v103},~{v104},~{v105},~{v106},~{v107},~{v108},~{v109}
+  ,~{v110},~{v111},~{v112},~{v113},~{v114},~{v115},~{v116},~{v117},~{v118},~{v119}
+  ,~{v120},~{v121},~{v122},~{v123},~{v124},~{v125},~{v126},~{v127},~{v128},~{v129}
+  ,~{v130},~{v131},~{v132},~{v133},~{v134},~{v135},~{v136},~{v137},~{v138},~{v139}
+  ,~{v140},~{v141},~{v142},~{v143},~{v144},~{v145},~{v146},~{v147},~{v148},~{v149}
+  ,~{v150},~{v151},~{v152},~{v153},~{v154},~{v155},~{v156},~{v157},~{v158},~{v159}
+  ,~{v160},~{v161},~{v162},~{v163},~{v164},~{v165},~{v166},~{v167},~{v168},~{v169}
+  ,~{v170},~{v171},~{v172},~{v173},~{v174},~{v175},~{v176},~{v177},~{v178},~{v179}
+  ,~{v180},~{v181},~{v182},~{v183},~{v184},~{v185},~{v186},~{v187},~{v188},~{v189}
+  ,~{v190},~{v191},~{v192},~{v193},~{v194},~{v195},~{v196},~{v197},~{v198},~{v199}
+  ,~{v200},~{v201},~{v202},~{v203},~{v204},~{v205},~{v206},~{v207},~{v208},~{v209}
+  ,~{v210},~{v211},~{v212},~{v213},~{v214},~{v215},~{v216},~{v217},~{v218},~{v219}
+  ,~{v220},~{v221},~{v222},~{v223},~{v224},~{v225},~{v226},~{v227},~{v228},~{v229}
+  ,~{v230},~{v231},~{v232},~{v233},~{v234},~{v235},~{v236},~{v237},~{v238},~{v239}
+  ,~{v240},~{v241},~{v242},~{v243},~{v244},~{v245},~{v246},~{v247},~{v248},~{v249}
+  ,~{v250},~{v251},~{v252},~{v253},~{v254}" () #0
+  call void @child_function()
+  ret void
+}
+
+attributes #0 = { nounwind noinline norecurse }