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