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