lib/CodeGen/LocalStackSlotAllocation.cpp

   1 //===- LocalStackSlotAllocation.cpp - Pre-allocate locals to stack slots --===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This pass assigns local frame indices to stack slots relative to one another
  11 // and allocates additional base registers to access them when the target
  12 // estimates they are likely to be out of range of stack pointer and frame
  13 // pointer relative addressing.
  14 //
  15 //===----------------------------------------------------------------------===//
  16
  17 #include "llvm/ADT/SetVector.h"
  18 #include "llvm/ADT/SmallSet.h"
  19 #include "llvm/ADT/SmallVector.h"
  20 #include "llvm/ADT/Statistic.h"
  21 #include "llvm/CodeGen/MachineBasicBlock.h"
  22 #include "llvm/CodeGen/MachineFrameInfo.h"
  23 #include "llvm/CodeGen/MachineFunction.h"
  24 #include "llvm/CodeGen/MachineFunctionPass.h"
  25 #include "llvm/CodeGen/MachineInstr.h"
  26 #include "llvm/CodeGen/MachineOperand.h"
  27 #include "llvm/CodeGen/MachineRegisterInfo.h"
  28 #include "llvm/CodeGen/TargetFrameLowering.h"
  29 #include "llvm/CodeGen/TargetOpcodes.h"
  30 #include "llvm/CodeGen/TargetRegisterInfo.h"
  31 #include "llvm/CodeGen/TargetSubtargetInfo.h"
  32 #include "llvm/Pass.h"
  33 #include "llvm/Support/Debug.h"
  34 #include "llvm/Support/ErrorHandling.h"
  35 #include "llvm/Support/raw_ostream.h"
  36 #include <algorithm>
  37 #include <cassert>
  38 #include <cstdint>
  39 #include <tuple>
  40
  41 using namespace llvm;
  42
  43 #define DEBUG_TYPE "localstackalloc"
  44
  45 STATISTIC(NumAllocations, "Number of frame indices allocated into local block");
  46 STATISTIC(NumBaseRegisters, "Number of virtual frame base registers allocated");
  47 STATISTIC(NumReplacements, "Number of frame indices references replaced");
  48
  49 namespace {
  50
  51   class FrameRef {
  52     MachineBasicBlock::iterator MI; // Instr referencing the frame
  53     int64_t LocalOffset;            // Local offset of the frame idx referenced
  54     int FrameIdx;                   // The frame index
  55
  56     // Order reference instruction appears in program. Used to ensure
  57     // deterministic order when multiple instructions may reference the same
  58     // location.
  59     unsigned Order;
  60
  61   public:
  62     FrameRef(MachineInstr *I, int64_t Offset, int Idx, unsigned Ord) :
  63       MI(I), LocalOffset(Offset), FrameIdx(Idx), Order(Ord) {}
  64
  65     bool operator<(const FrameRef &RHS) const {
  66       return std::tie(LocalOffset, FrameIdx, Order) <
  67              std::tie(RHS.LocalOffset, RHS.FrameIdx, RHS.Order);
  68     }
  69
  70     MachineBasicBlock::iterator getMachineInstr() const { return MI; }
  71     int64_t getLocalOffset() const { return LocalOffset; }
  72     int getFrameIndex() const { return FrameIdx; }
  73   };
  74
  75   class LocalStackSlotPass: public MachineFunctionPass {
  76     SmallVector<int64_t, 16> LocalOffsets;
  77
  78     /// StackObjSet - A set of stack object indexes
  79     using StackObjSet = SmallSetVector<int, 8>;
  80
  81     void AdjustStackOffset(MachineFrameInfo &MFI, int FrameIdx, int64_t &Offset,
  82                            bool StackGrowsDown, unsigned &MaxAlign);
  83     void AssignProtectedObjSet(const StackObjSet &UnassignedObjs,
  84                                SmallSet<int, 16> &ProtectedObjs,
  85                                MachineFrameInfo &MFI, bool StackGrowsDown,
  86                                int64_t &Offset, unsigned &MaxAlign);
  87     void calculateFrameObjectOffsets(MachineFunction &Fn);
  88     bool insertFrameReferenceRegisters(MachineFunction &Fn);
  89
  90   public:
  91     static char ID; // Pass identification, replacement for typeid
  92
  93     explicit LocalStackSlotPass() : MachineFunctionPass(ID) {
  94       initializeLocalStackSlotPassPass(*PassRegistry::getPassRegistry());
  95     }
  96
  97     bool runOnMachineFunction(MachineFunction &MF) override;
  98
  99     void getAnalysisUsage(AnalysisUsage &AU) const override {
 100       AU.setPreservesCFG();
 101       MachineFunctionPass::getAnalysisUsage(AU);
 102     }
 103   };
 104
 105 } // end anonymous namespace
 106
 107 char LocalStackSlotPass::ID = 0;
 108
 109 char &llvm::LocalStackSlotAllocationID = LocalStackSlotPass::ID;
 110 INITIALIZE_PASS(LocalStackSlotPass, DEBUG_TYPE,
 111                 "Local Stack Slot Allocation", false, false)
 112
 113 bool LocalStackSlotPass::runOnMachineFunction(MachineFunction &MF) {
 114   MachineFrameInfo &MFI = MF.getFrameInfo();
 115   const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
 116   unsigned LocalObjectCount = MFI.getObjectIndexEnd();
 117
 118   // If the target doesn't want/need this pass, or if there are no locals
 119   // to consider, early exit.
 120   if (!TRI->requiresVirtualBaseRegisters(MF) || LocalObjectCount == 0)
 121     return true;
 122
 123   // Make sure we have enough space to store the local offsets.
 124   LocalOffsets.resize(MFI.getObjectIndexEnd());
 125
 126   // Lay out the local blob.
 127   calculateFrameObjectOffsets(MF);
 128
 129   // Insert virtual base registers to resolve frame index references.
 130   bool UsedBaseRegs = insertFrameReferenceRegisters(MF);
 131
 132   // Tell MFI whether any base registers were allocated. PEI will only
 133   // want to use the local block allocations from this pass if there were any.
 134   // Otherwise, PEI can do a bit better job of getting the alignment right
 135   // without a hole at the start since it knows the alignment of the stack
 136   // at the start of local allocation, and this pass doesn't.
 137   MFI.setUseLocalStackAllocationBlock(UsedBaseRegs);
 138
 139   return true;
 140 }
 141
 142 /// AdjustStackOffset - Helper function used to adjust the stack frame offset.
 143 void LocalStackSlotPass::AdjustStackOffset(MachineFrameInfo &MFI,
 144                                            int FrameIdx, int64_t &Offset,
 145                                            bool StackGrowsDown,
 146                                            unsigned &MaxAlign) {
 147   // If the stack grows down, add the object size to find the lowest address.
 148   if (StackGrowsDown)
 149     Offset += MFI.getObjectSize(FrameIdx);
 150
 151   unsigned Align = MFI.getObjectAlignment(FrameIdx);
 152
 153   // If the alignment of this object is greater than that of the stack, then
 154   // increase the stack alignment to match.
 155   MaxAlign = std::max(MaxAlign, Align);
 156
 157   // Adjust to alignment boundary.
 158   Offset = (Offset + Align - 1) / Align * Align;
 159
 160   int64_t LocalOffset = StackGrowsDown ? -Offset : Offset;
 161   LLVM_DEBUG(dbgs() << "Allocate FI(" << FrameIdx << ") to local offset "
 162                     << LocalOffset << "\n");
 163   // Keep the offset available for base register allocation
 164   LocalOffsets[FrameIdx] = LocalOffset;
 165   // And tell MFI about it for PEI to use later
 166   MFI.mapLocalFrameObject(FrameIdx, LocalOffset);
 167
 168   if (!StackGrowsDown)
 169     Offset += MFI.getObjectSize(FrameIdx);
 170
 171   ++NumAllocations;
 172 }
 173
 174 /// AssignProtectedObjSet - Helper function to assign large stack objects (i.e.,
 175 /// those required to be close to the Stack Protector) to stack offsets.
 176 void LocalStackSlotPass::AssignProtectedObjSet(const StackObjSet &UnassignedObjs,
 177                                            SmallSet<int, 16> &ProtectedObjs,
 178                                            MachineFrameInfo &MFI,
 179                                            bool StackGrowsDown, int64_t &Offset,
 180                                            unsigned &MaxAlign) {
 181   for (StackObjSet::const_iterator I = UnassignedObjs.begin(),
 182         E = UnassignedObjs.end(); I != E; ++I) {
 183     int i = *I;
 184     AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign);
 185     ProtectedObjs.insert(i);
 186   }
 187 }
 188
 189 /// calculateFrameObjectOffsets - Calculate actual frame offsets for all of the
 190 /// abstract stack objects.
 191 void LocalStackSlotPass::calculateFrameObjectOffsets(MachineFunction &Fn) {
 192   // Loop over all of the stack objects, assigning sequential addresses...
 193   MachineFrameInfo &MFI = Fn.getFrameInfo();
 194   const TargetFrameLowering &TFI = *Fn.getSubtarget().getFrameLowering();
 195   bool StackGrowsDown =
 196     TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsDown;
 197   int64_t Offset = 0;
 198   unsigned MaxAlign = 0;
 199
 200   // Make sure that the stack protector comes before the local variables on the
 201   // stack.
 202   SmallSet<int, 16> ProtectedObjs;
 203   if (MFI.getStackProtectorIndex() >= 0) {
 204     StackObjSet LargeArrayObjs;
 205     StackObjSet SmallArrayObjs;
 206     StackObjSet AddrOfObjs;
 207
 208     AdjustStackOffset(MFI, MFI.getStackProtectorIndex(), Offset,
 209                       StackGrowsDown, MaxAlign);
 210
 211     // Assign large stack objects first.
 212     for (unsigned i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) {
 213       if (MFI.isDeadObjectIndex(i))
 214         continue;
 215       if (MFI.getStackProtectorIndex() == (int)i)
 216         continue;
 217
 218       switch (MFI.getObjectSSPLayout(i)) {
 219       case MachineFrameInfo::SSPLK_None:
 220         continue;
 221       case MachineFrameInfo::SSPLK_SmallArray:
 222         SmallArrayObjs.insert(i);
 223         continue;
 224       case MachineFrameInfo::SSPLK_AddrOf:
 225         AddrOfObjs.insert(i);
 226         continue;
 227       case MachineFrameInfo::SSPLK_LargeArray:
 228         LargeArrayObjs.insert(i);
 229         continue;
 230       }
 231       llvm_unreachable("Unexpected SSPLayoutKind.");
 232     }
 233
 234     AssignProtectedObjSet(LargeArrayObjs, ProtectedObjs, MFI, StackGrowsDown,
 235                           Offset, MaxAlign);
 236     AssignProtectedObjSet(SmallArrayObjs, ProtectedObjs, MFI, StackGrowsDown,
 237                           Offset, MaxAlign);
 238     AssignProtectedObjSet(AddrOfObjs, ProtectedObjs, MFI, StackGrowsDown,
 239                           Offset, MaxAlign);
 240   }
 241
 242   // Then assign frame offsets to stack objects that are not used to spill
 243   // callee saved registers.
 244   for (unsigned i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) {
 245     if (MFI.isDeadObjectIndex(i))
 246       continue;
 247     if (MFI.getStackProtectorIndex() == (int)i)
 248       continue;
 249     if (ProtectedObjs.count(i))
 250       continue;
 251
 252     AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign);
 253   }
 254
 255   // Remember how big this blob of stack space is
 256   MFI.setLocalFrameSize(Offset);
 257   MFI.setLocalFrameMaxAlign(MaxAlign);
 258 }
 259
 260 static inline bool
 261 lookupCandidateBaseReg(unsigned BaseReg,
 262                        int64_t BaseOffset,
 263                        int64_t FrameSizeAdjust,
 264                        int64_t LocalFrameOffset,
 265                        const MachineInstr &MI,
 266                        const TargetRegisterInfo *TRI) {
 267   // Check if the relative offset from the where the base register references
 268   // to the target address is in range for the instruction.
 269   int64_t Offset = FrameSizeAdjust + LocalFrameOffset - BaseOffset;
 270   return TRI->isFrameOffsetLegal(&MI, BaseReg, Offset);
 271 }
 272
 273 bool LocalStackSlotPass::insertFrameReferenceRegisters(MachineFunction &Fn) {
 274   // Scan the function's instructions looking for frame index references.
 275   // For each, ask the target if it wants a virtual base register for it
 276   // based on what we can tell it about where the local will end up in the
 277   // stack frame. If it wants one, re-use a suitable one we've previously
 278   // allocated, or if there isn't one that fits the bill, allocate a new one
 279   // and ask the target to create a defining instruction for it.
 280   bool UsedBaseReg = false;
 281
 282   MachineFrameInfo &MFI = Fn.getFrameInfo();
 283   const TargetRegisterInfo *TRI = Fn.getSubtarget().getRegisterInfo();
 284   const TargetFrameLowering &TFI = *Fn.getSubtarget().getFrameLowering();
 285   bool StackGrowsDown =
 286     TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsDown;
 287
 288   // Collect all of the instructions in the block that reference
 289   // a frame index. Also store the frame index referenced to ease later
 290   // lookup. (For any insn that has more than one FI reference, we arbitrarily
 291   // choose the first one).
 292   SmallVector<FrameRef, 64> FrameReferenceInsns;
 293
 294   unsigned Order = 0;
 295
 296   for (MachineBasicBlock &BB : Fn) {
 297     for (MachineInstr &MI : BB) {
 298       // Debug value, stackmap and patchpoint instructions can't be out of
 299       // range, so they don't need any updates.
 300       if (MI.isDebugInstr() || MI.getOpcode() == TargetOpcode::STATEPOINT ||
 301           MI.getOpcode() == TargetOpcode::STACKMAP ||
 302           MI.getOpcode() == TargetOpcode::PATCHPOINT)
 303         continue;
 304
 305       // For now, allocate the base register(s) within the basic block
 306       // where they're used, and don't try to keep them around outside
 307       // of that. It may be beneficial to try sharing them more broadly
 308       // than that, but the increased register pressure makes that a
 309       // tricky thing to balance. Investigate if re-materializing these
 310       // becomes an issue.
 311       for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
 312         // Consider replacing all frame index operands that reference
 313         // an object allocated in the local block.
 314         if (MI.getOperand(i).isFI()) {
 315           // Don't try this with values not in the local block.
 316           if (!MFI.isObjectPreAllocated(MI.getOperand(i).getIndex()))
 317             break;
 318           int Idx = MI.getOperand(i).getIndex();
 319           int64_t LocalOffset = LocalOffsets[Idx];
 320           if (!TRI->needsFrameBaseReg(&MI, LocalOffset))
 321             break;
 322           FrameReferenceInsns.push_back(FrameRef(&MI, LocalOffset, Idx, Order++));
 323           break;
 324         }
 325       }
 326     }
 327   }
 328
 329   // Sort the frame references by local offset.
 330   // Use frame index as a tie-breaker in case MI's have the same offset.
 331   llvm::sort(FrameReferenceInsns);
 332
 333   MachineBasicBlock *Entry = &Fn.front();
 334
 335   unsigned BaseReg = 0;
 336   int64_t BaseOffset = 0;
 337
 338   // Loop through the frame references and allocate for them as necessary.
 339   for (int ref = 0, e = FrameReferenceInsns.size(); ref < e ; ++ref) {
 340     FrameRef &FR = FrameReferenceInsns[ref];
 341     MachineInstr &MI = *FR.getMachineInstr();
 342     int64_t LocalOffset = FR.getLocalOffset();
 343     int FrameIdx = FR.getFrameIndex();
 344     assert(MFI.isObjectPreAllocated(FrameIdx) &&
 345            "Only pre-allocated locals expected!");
 346
 347     LLVM_DEBUG(dbgs() << "Considering: " << MI);
 348
 349     unsigned idx = 0;
 350     for (unsigned f = MI.getNumOperands(); idx != f; ++idx) {
 351       if (!MI.getOperand(idx).isFI())
 352         continue;
 353
 354       if (FrameIdx == MI.getOperand(idx).getIndex())
 355         break;
 356     }
 357
 358     assert(idx < MI.getNumOperands() && "Cannot find FI operand");
 359
 360     int64_t Offset = 0;
 361     int64_t FrameSizeAdjust = StackGrowsDown ? MFI.getLocalFrameSize() : 0;
 362
 363     LLVM_DEBUG(dbgs() << "  Replacing FI in: " << MI);
 364
 365     // If we have a suitable base register available, use it; otherwise
 366     // create a new one. Note that any offset encoded in the
 367     // instruction itself will be taken into account by the target,
 368     // so we don't have to adjust for it here when reusing a base
 369     // register.
 370     if (UsedBaseReg &&
 371         lookupCandidateBaseReg(BaseReg, BaseOffset, FrameSizeAdjust,
 372                                LocalOffset, MI, TRI)) {
 373       LLVM_DEBUG(dbgs() << "  Reusing base register " << BaseReg << "\n");
 374       // We found a register to reuse.
 375       Offset = FrameSizeAdjust + LocalOffset - BaseOffset;
 376     } else {
 377       // No previously defined register was in range, so create a new one.
 378       int64_t InstrOffset = TRI->getFrameIndexInstrOffset(&MI, idx);
 379
 380       int64_t PrevBaseOffset = BaseOffset;
 381       BaseOffset = FrameSizeAdjust + LocalOffset + InstrOffset;
 382
 383       // We'd like to avoid creating single-use virtual base registers.
 384       // Because the FrameRefs are in sorted order, and we've already
 385       // processed all FrameRefs before this one, just check whether or not
 386       // the next FrameRef will be able to reuse this new register. If not,
 387       // then don't bother creating it.
 388       if (ref + 1 >= e ||
 389           !lookupCandidateBaseReg(
 390               BaseReg, BaseOffset, FrameSizeAdjust,
 391               FrameReferenceInsns[ref + 1].getLocalOffset(),
 392               *FrameReferenceInsns[ref + 1].getMachineInstr(), TRI)) {
 393         BaseOffset = PrevBaseOffset;
 394         continue;
 395       }
 396
 397       const MachineFunction *MF = MI.getMF();
 398       const TargetRegisterClass *RC = TRI->getPointerRegClass(*MF);
 399       BaseReg = Fn.getRegInfo().createVirtualRegister(RC);
 400
 401       LLVM_DEBUG(dbgs() << "  Materializing base register " << BaseReg
 402                         << " at frame local offset "
 403                         << LocalOffset + InstrOffset << "\n");
 404
 405       // Tell the target to insert the instruction to initialize
 406       // the base register.
 407       //            MachineBasicBlock::iterator InsertionPt = Entry->begin();
 408       TRI->materializeFrameBaseRegister(Entry, BaseReg, FrameIdx,
 409                                         InstrOffset);
 410
 411       // The base register already includes any offset specified
 412       // by the instruction, so account for that so it doesn't get
 413       // applied twice.
 414       Offset = -InstrOffset;
 415
 416       ++NumBaseRegisters;
 417       UsedBaseReg = true;
 418     }
 419     assert(BaseReg != 0 && "Unable to allocate virtual base register!");
 420
 421     // Modify the instruction to use the new base register rather
 422     // than the frame index operand.
 423     TRI->resolveFrameIndex(MI, BaseReg, Offset);
 424     LLVM_DEBUG(dbgs() << "Resolved: " << MI);
 425
 426     ++NumReplacements;
 427   }
 428
 429   return UsedBaseReg;
 430 }