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 the are likely to be out of range of stack pointer and frame
  13 // pointer relative addressing.
  14 //
  15 //===----------------------------------------------------------------------===//
  16
  17 #define DEBUG_TYPE "localstackalloc"
  18 #include "llvm/Constants.h"
  19 #include "llvm/DerivedTypes.h"
  20 #include "llvm/Instructions.h"
  21 #include "llvm/Intrinsics.h"
  22 #include "llvm/LLVMContext.h"
  23 #include "llvm/Module.h"
  24 #include "llvm/Pass.h"
  25 #include "llvm/ADT/SmallSet.h"
  26 #include "llvm/ADT/Statistic.h"
  27 #include "llvm/ADT/STLExtras.h"
  28 #include "llvm/CodeGen/MachineFrameInfo.h"
  29 #include "llvm/CodeGen/MachineFunction.h"
  30 #include "llvm/CodeGen/MachineFunctionPass.h"
  31 #include "llvm/CodeGen/MachineRegisterInfo.h"
  32 #include "llvm/CodeGen/Passes.h"
  33 #include "llvm/Support/Debug.h"
  34 #include "llvm/Support/ErrorHandling.h"
  35 #include "llvm/Support/raw_ostream.h"
  36 #include "llvm/Target/TargetRegisterInfo.h"
  37 #include "llvm/Target/TargetFrameInfo.h"
  38
  39 using namespace llvm;
  40
  41 STATISTIC(NumAllocations, "Number of frame indices allocated into local block");
  42 STATISTIC(NumBaseRegisters, "Number of virtual frame base registers allocated");
  43 STATISTIC(NumReplacements, "Number of frame indices references replaced");
  44
  45 namespace {
  46   class FrameRef {
  47     MachineBasicBlock::iterator MI; // Instr referencing the frame
  48     int64_t LocalOffset;            // Local offset of the frame idx referenced
  49   public:
  50     FrameRef(MachineBasicBlock::iterator I, int64_t Offset) :
  51       MI(I), LocalOffset(Offset) {}
  52     bool operator<(const FrameRef &RHS) const {
  53       return LocalOffset < RHS.LocalOffset;
  54     }
  55     MachineBasicBlock::iterator getMachineInstr() { return MI; }
  56   };
  57
  58   class LocalStackSlotPass: public MachineFunctionPass {
  59     SmallVector<int64_t,16> LocalOffsets;
  60
  61     void AdjustStackOffset(MachineFrameInfo *MFI, int FrameIdx, int64_t &Offset,
  62                            bool StackGrowsDown, unsigned &MaxAlign);
  63     void calculateFrameObjectOffsets(MachineFunction &Fn);
  64     bool insertFrameReferenceRegisters(MachineFunction &Fn);
  65   public:
  66     static char ID; // Pass identification, replacement for typeid
  67     explicit LocalStackSlotPass() : MachineFunctionPass(ID) { }
  68     bool runOnMachineFunction(MachineFunction &MF);
  69
  70     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  71       AU.setPreservesCFG();
  72       MachineFunctionPass::getAnalysisUsage(AU);
  73     }
  74     const char *getPassName() const {
  75       return "Local Stack Slot Allocation";
  76     }
  77
  78   private:
  79   };
  80 } // end anonymous namespace
  81
  82 char LocalStackSlotPass::ID = 0;
  83
  84 FunctionPass *llvm::createLocalStackSlotAllocationPass() {
  85   return new LocalStackSlotPass();
  86 }
  87
  88 bool LocalStackSlotPass::runOnMachineFunction(MachineFunction &MF) {
  89   MachineFrameInfo *MFI = MF.getFrameInfo();
  90   const TargetRegisterInfo *TRI = MF.getTarget().getRegisterInfo();
  91   unsigned LocalObjectCount = MFI->getObjectIndexEnd();
  92
  93   // If the target doesn't want/need this pass, or if there are no locals
  94   // to consider, early exit.
  95   if (!TRI->requiresVirtualBaseRegisters(MF) || LocalObjectCount == 0)
  96     return true;
  97
  98   // Make sure we have enough space to store the local offsets.
  99   LocalOffsets.resize(MFI->getObjectIndexEnd());
 100
 101   // Lay out the local blob.
 102   calculateFrameObjectOffsets(MF);
 103
 104   // Insert virtual base registers to resolve frame index references.
 105   bool UsedBaseRegs = insertFrameReferenceRegisters(MF);
 106
 107   // Tell MFI whether any base registers were allocated. PEI will only
 108   // want to use the local block allocations from this pass if there were any.
 109   // Otherwise, PEI can do a bit better job of getting the alignment right
 110   // without a hole at the start since it knows the alignment of the stack
 111   // at the start of local allocation, and this pass doesn't.
 112   MFI->setUseLocalStackAllocationBlock(UsedBaseRegs);
 113
 114   return true;
 115 }
 116
 117 /// AdjustStackOffset - Helper function used to adjust the stack frame offset.
 118 void LocalStackSlotPass::AdjustStackOffset(MachineFrameInfo *MFI,
 119                                            int FrameIdx, int64_t &Offset,
 120                                            bool StackGrowsDown,
 121                                            unsigned &MaxAlign) {
 122   // If the stack grows down, add the object size to find the lowest address.
 123   if (StackGrowsDown)
 124     Offset += MFI->getObjectSize(FrameIdx);
 125
 126   unsigned Align = MFI->getObjectAlignment(FrameIdx);
 127
 128   // If the alignment of this object is greater than that of the stack, then
 129   // increase the stack alignment to match.
 130   MaxAlign = std::max(MaxAlign, Align);
 131
 132   // Adjust to alignment boundary.
 133   Offset = (Offset + Align - 1) / Align * Align;
 134
 135   int64_t LocalOffset = StackGrowsDown ? -Offset : Offset;
 136   DEBUG(dbgs() << "Allocate FI(" << FrameIdx << ") to local offset "
 137         << LocalOffset << "\n");
 138   // Keep the offset available for base register allocation
 139   LocalOffsets[FrameIdx] = LocalOffset;
 140   // And tell MFI about it for PEI to use later
 141   MFI->mapLocalFrameObject(FrameIdx, LocalOffset);
 142
 143   if (!StackGrowsDown)
 144     Offset += MFI->getObjectSize(FrameIdx);
 145
 146   ++NumAllocations;
 147 }
 148
 149 /// calculateFrameObjectOffsets - Calculate actual frame offsets for all of the
 150 /// abstract stack objects.
 151 ///
 152 void LocalStackSlotPass::calculateFrameObjectOffsets(MachineFunction &Fn) {
 153   // Loop over all of the stack objects, assigning sequential addresses...
 154   MachineFrameInfo *MFI = Fn.getFrameInfo();
 155   const TargetFrameInfo &TFI = *Fn.getTarget().getFrameInfo();
 156   bool StackGrowsDown =
 157     TFI.getStackGrowthDirection() == TargetFrameInfo::StackGrowsDown;
 158   int64_t Offset = 0;
 159   unsigned MaxAlign = 0;
 160
 161   // Make sure that the stack protector comes before the local variables on the
 162   // stack.
 163   SmallSet<int, 16> LargeStackObjs;
 164   if (MFI->getStackProtectorIndex() >= 0) {
 165     AdjustStackOffset(MFI, MFI->getStackProtectorIndex(), Offset,
 166                       StackGrowsDown, MaxAlign);
 167
 168     // Assign large stack objects first.
 169     for (unsigned i = 0, e = MFI->getObjectIndexEnd(); i != e; ++i) {
 170       if (MFI->isDeadObjectIndex(i))
 171         continue;
 172       if (MFI->getStackProtectorIndex() == (int)i)
 173         continue;
 174       if (!MFI->MayNeedStackProtector(i))
 175         continue;
 176
 177       AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign);
 178       LargeStackObjs.insert(i);
 179     }
 180   }
 181
 182   // Then assign frame offsets to stack objects that are not used to spill
 183   // callee saved registers.
 184   for (unsigned i = 0, e = MFI->getObjectIndexEnd(); i != e; ++i) {
 185     if (MFI->isDeadObjectIndex(i))
 186       continue;
 187     if (MFI->getStackProtectorIndex() == (int)i)
 188       continue;
 189     if (LargeStackObjs.count(i))
 190       continue;
 191
 192     AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign);
 193   }
 194
 195   // Remember how big this blob of stack space is
 196   MFI->setLocalFrameSize(Offset);
 197   MFI->setLocalFrameMaxAlign(MaxAlign);
 198 }
 199
 200 static inline bool
 201 lookupCandidateBaseReg(const SmallVector<std::pair<unsigned, int64_t>, 8> &Regs,
 202                        std::pair<unsigned, int64_t> &RegOffset,
 203                        int64_t FrameSizeAdjust,
 204                        int64_t LocalFrameOffset,
 205                        const MachineInstr *MI,
 206                        const TargetRegisterInfo *TRI) {
 207   unsigned e = Regs.size();
 208   for (unsigned i = 0; i < e; ++i) {
 209     RegOffset = Regs[i];
 210     // Check if the relative offset from the where the base register references
 211     // to the target address is in range for the instruction.
 212     int64_t Offset = FrameSizeAdjust + LocalFrameOffset - RegOffset.second;
 213     if (TRI->isFrameOffsetLegal(MI, Offset))
 214       return true;
 215   }
 216   return false;
 217 }
 218
 219 bool LocalStackSlotPass::insertFrameReferenceRegisters(MachineFunction &Fn) {
 220   // Scan the function's instructions looking for frame index references.
 221   // For each, ask the target if it wants a virtual base register for it
 222   // based on what we can tell it about where the local will end up in the
 223   // stack frame. If it wants one, re-use a suitable one we've previously
 224   // allocated, or if there isn't one that fits the bill, allocate a new one
 225   // and ask the target to create a defining instruction for it.
 226   bool UsedBaseReg = false;
 227
 228   MachineFrameInfo *MFI = Fn.getFrameInfo();
 229   const TargetRegisterInfo *TRI = Fn.getTarget().getRegisterInfo();
 230   const TargetFrameInfo &TFI = *Fn.getTarget().getFrameInfo();
 231   bool StackGrowsDown =
 232     TFI.getStackGrowthDirection() == TargetFrameInfo::StackGrowsDown;
 233   MachineBasicBlock::iterator InsertionPt = Fn.begin()->begin();
 234
 235   // Collect all of the instructions in the block that reference
 236   // a frame index. Also store the frame index referenced to ease later
 237   // lookup. (For any insn that has more than one FI reference, we arbitrarily
 238   // choose the first one).
 239   SmallVector<FrameRef, 64> FrameReferenceInsns;
 240   // A base register definition is a register+offset pair.
 241   SmallVector<std::pair<unsigned, int64_t>, 8> BaseRegisters;
 242
 243
 244   for (MachineFunction::iterator BB = Fn.begin(), E = Fn.end(); BB != E; ++BB) {
 245     for (MachineBasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
 246       MachineInstr *MI = I;
 247       // Debug value instructions can't be out of range, so they don't need
 248       // any updates.
 249       if (MI->isDebugValue())
 250         continue;
 251       // For now, allocate the base register(s) within the basic block
 252       // where they're used, and don't try to keep them around outside
 253       // of that. It may be beneficial to try sharing them more broadly
 254       // than that, but the increased register pressure makes that a
 255       // tricky thing to balance. Investigate if re-materializing these
 256       // becomes an issue.
 257       for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
 258         // Consider replacing all frame index operands that reference
 259         // an object allocated in the local block.
 260         if (MI->getOperand(i).isFI()) {
 261           // Don't try this with values not in the local block.
 262           if (!MFI->isObjectPreAllocated(MI->getOperand(i).getIndex()))
 263             break;
 264           FrameReferenceInsns.
 265             push_back(FrameRef(MI, LocalOffsets[MI->getOperand(i).getIndex()]));
 266           break;
 267         }
 268       }
 269     }
 270   }
 271   // Sort the frame references by local offset
 272   array_pod_sort(FrameReferenceInsns.begin(), FrameReferenceInsns.end());
 273
 274
 275   // Loop throught the frame references and allocate for them as necessary
 276   for (int ref = 0, e = FrameReferenceInsns.size(); ref < e ; ++ref) {
 277     MachineBasicBlock::iterator I =
 278       FrameReferenceInsns[ref].getMachineInstr();
 279     MachineInstr *MI = I;
 280     for (unsigned idx = 0, e = MI->getNumOperands(); idx != e; ++idx) {
 281       // Consider replacing all frame index operands that reference
 282       // an object allocated in the local block.
 283       if (MI->getOperand(idx).isFI()) {
 284         int FrameIdx = MI->getOperand(idx).getIndex();
 285
 286         assert(MFI->isObjectPreAllocated(FrameIdx) &&
 287                "Only pre-allocated locals expected!");
 288
 289         DEBUG(dbgs() << "Considering: " << *MI);
 290         if (TRI->needsFrameBaseReg(MI, LocalOffsets[FrameIdx])) {
 291           unsigned BaseReg = 0;
 292           int64_t Offset = 0;
 293           int64_t FrameSizeAdjust =
 294             StackGrowsDown ? MFI->getLocalFrameSize() : 0;
 295
 296           DEBUG(dbgs() << "  Replacing FI in: " << *MI);
 297
 298           // If we have a suitable base register available, use it; otherwise
 299           // create a new one. Note that any offset encoded in the
 300           // instruction itself will be taken into account by the target,
 301           // so we don't have to adjust for it here when reusing a base
 302           // register.
 303           std::pair<unsigned, int64_t> RegOffset;
 304           if (lookupCandidateBaseReg(BaseRegisters, RegOffset,
 305                                      FrameSizeAdjust,
 306                                      LocalOffsets[FrameIdx],
 307                                      MI, TRI)) {
 308             DEBUG(dbgs() << "  Reusing base register " <<
 309                   RegOffset.first << "\n");
 310             // We found a register to reuse.
 311             BaseReg = RegOffset.first;
 312             Offset = FrameSizeAdjust + LocalOffsets[FrameIdx] -
 313               RegOffset.second;
 314           } else {
 315             // No previously defined register was in range, so create a
 316             // new one.
 317             int64_t InstrOffset = TRI->getFrameIndexInstrOffset(MI, idx);
 318             const TargetRegisterClass *RC = TRI->getPointerRegClass();
 319             BaseReg = Fn.getRegInfo().createVirtualRegister(RC);
 320
 321             DEBUG(dbgs() << "  Materializing base register " << BaseReg <<
 322                   " at frame local offset " <<
 323                   LocalOffsets[FrameIdx] + InstrOffset << "\n");
 324             // Tell the target to insert the instruction to initialize
 325             // the base register.
 326             TRI->materializeFrameBaseRegister(InsertionPt, BaseReg,
 327                                               FrameIdx, InstrOffset);
 328
 329             // The base register already includes any offset specified
 330             // by the instruction, so account for that so it doesn't get
 331             // applied twice.
 332             Offset = -InstrOffset;
 333
 334             int64_t BaseOffset = FrameSizeAdjust + LocalOffsets[FrameIdx] +
 335               InstrOffset;
 336             BaseRegisters.push_back(
 337               std::pair<unsigned, int64_t>(BaseReg, BaseOffset));
 338             ++NumBaseRegisters;
 339             UsedBaseReg = true;
 340           }
 341           assert(BaseReg != 0 && "Unable to allocate virtual base register!");
 342
 343           // Modify the instruction to use the new base register rather
 344           // than the frame index operand.
 345           TRI->resolveFrameIndex(I, BaseReg, Offset);
 346           DEBUG(dbgs() << "Resolved: " << *MI);
 347
 348           ++NumReplacements;
 349         }
 350       }
 351     }
 352   }
 353   return UsedBaseReg;
 354 }