1//===- LocalStackSlotAllocation.cpp - Pre-allocate locals to stack slots --===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This pass assigns local frame indices to stack slots relative to one another
10// and allocates additional base registers to access them when the target
11// estimates they are likely to be out of range of stack pointer and frame
12// pointer relative addressing.
13//
14//===----------------------------------------------------------------------===//
15
16#include "llvm/ADT/SetVector.h"
17#include "llvm/ADT/SmallSet.h"
18#include "llvm/ADT/SmallVector.h"
19#include "llvm/ADT/Statistic.h"
20#include "llvm/CodeGen/MachineBasicBlock.h"
21#include "llvm/CodeGen/MachineFrameInfo.h"
22#include "llvm/CodeGen/MachineFunction.h"
23#include "llvm/CodeGen/MachineFunctionPass.h"
24#include "llvm/CodeGen/MachineInstr.h"
25#include "llvm/CodeGen/MachineOperand.h"
26#include "llvm/CodeGen/MachineRegisterInfo.h"
27#include "llvm/CodeGen/TargetFrameLowering.h"
28#include "llvm/CodeGen/TargetOpcodes.h"
29#include "llvm/CodeGen/TargetRegisterInfo.h"
30#include "llvm/CodeGen/TargetSubtargetInfo.h"
31#include "llvm/InitializePasses.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
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
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, Align &MaxAlign);
83    void AssignProtectedObjSet(const StackObjSet &UnassignedObjs,
84                               SmallSet<int, 16> &ProtectedObjs,
85                               MachineFrameInfo &MFI, bool StackGrowsDown,
86                               int64_t &Offset, Align &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
107char LocalStackSlotPass::ID = 0;
108
109char &llvm::LocalStackSlotAllocationID = LocalStackSlotPass::ID;
110INITIALIZE_PASS(LocalStackSlotPass, DEBUG_TYPE,
111                "Local Stack Slot Allocation", false, false)
112
113bool 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.
143void LocalStackSlotPass::AdjustStackOffset(MachineFrameInfo &MFI, int FrameIdx,
144                                           int64_t &Offset, bool StackGrowsDown,
145                                           Align &MaxAlign) {
146  // If the stack grows down, add the object size to find the lowest address.
147  if (StackGrowsDown)
148    Offset += MFI.getObjectSize(FrameIdx);
149
150  Align Alignment = MFI.getObjectAlign(FrameIdx);
151
152  // If the alignment of this object is greater than that of the stack, then
153  // increase the stack alignment to match.
154  MaxAlign = std::max(MaxAlign, Alignment);
155
156  // Adjust to alignment boundary.
157  Offset = alignTo(Offset, Alignment);
158
159  int64_t LocalOffset = StackGrowsDown ? -Offset : Offset;
160  LLVM_DEBUG(dbgs() << "Allocate FI(" << FrameIdx << ") to local offset "
161                    << LocalOffset << "\n");
162  // Keep the offset available for base register allocation
163  LocalOffsets[FrameIdx] = LocalOffset;
164  // And tell MFI about it for PEI to use later
165  MFI.mapLocalFrameObject(FrameIdx, LocalOffset);
166
167  if (!StackGrowsDown)
168    Offset += MFI.getObjectSize(FrameIdx);
169
170  ++NumAllocations;
171}
172
173/// AssignProtectedObjSet - Helper function to assign large stack objects (i.e.,
174/// those required to be close to the Stack Protector) to stack offsets.
175void LocalStackSlotPass::AssignProtectedObjSet(
176    const StackObjSet &UnassignedObjs, SmallSet<int, 16> &ProtectedObjs,
177    MachineFrameInfo &MFI, bool StackGrowsDown, int64_t &Offset,
178    Align &MaxAlign) {
179  for (StackObjSet::const_iterator I = UnassignedObjs.begin(),
180        E = UnassignedObjs.end(); I != E; ++I) {
181    int i = *I;
182    AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign);
183    ProtectedObjs.insert(i);
184  }
185}
186
187/// calculateFrameObjectOffsets - Calculate actual frame offsets for all of the
188/// abstract stack objects.
189void LocalStackSlotPass::calculateFrameObjectOffsets(MachineFunction &Fn) {
190  // Loop over all of the stack objects, assigning sequential addresses...
191  MachineFrameInfo &MFI = Fn.getFrameInfo();
192  const TargetFrameLowering &TFI = *Fn.getSubtarget().getFrameLowering();
193  bool StackGrowsDown =
194    TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsDown;
195  int64_t Offset = 0;
196  Align MaxAlign;
197
198  // Make sure that the stack protector comes before the local variables on the
199  // stack.
200  SmallSet<int, 16> ProtectedObjs;
201  if (MFI.hasStackProtectorIndex()) {
202    int StackProtectorFI = MFI.getStackProtectorIndex();
203
204    // We need to make sure we didn't pre-allocate the stack protector when
205    // doing this.
206    // If we already have a stack protector, this will re-assign it to a slot
207    // that is **not** covering the protected objects.
208    assert(!MFI.isObjectPreAllocated(StackProtectorFI) &&
209           "Stack protector pre-allocated in LocalStackSlotAllocation");
210
211    StackObjSet LargeArrayObjs;
212    StackObjSet SmallArrayObjs;
213    StackObjSet AddrOfObjs;
214
215    AdjustStackOffset(MFI, StackProtectorFI, Offset, StackGrowsDown, MaxAlign);
216
217    // Assign large stack objects first.
218    for (unsigned i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) {
219      if (MFI.isDeadObjectIndex(i))
220        continue;
221      if (StackProtectorFI == (int)i)
222        continue;
223      if (!TFI.isStackIdSafeForLocalArea(MFI.getStackID(i)))
224        continue;
225
226      switch (MFI.getObjectSSPLayout(i)) {
227      case MachineFrameInfo::SSPLK_None:
228        continue;
229      case MachineFrameInfo::SSPLK_SmallArray:
230        SmallArrayObjs.insert(i);
231        continue;
232      case MachineFrameInfo::SSPLK_AddrOf:
233        AddrOfObjs.insert(i);
234        continue;
235      case MachineFrameInfo::SSPLK_LargeArray:
236        LargeArrayObjs.insert(i);
237        continue;
238      }
239      llvm_unreachable("Unexpected SSPLayoutKind.");
240    }
241
242    AssignProtectedObjSet(LargeArrayObjs, ProtectedObjs, MFI, StackGrowsDown,
243                          Offset, MaxAlign);
244    AssignProtectedObjSet(SmallArrayObjs, ProtectedObjs, MFI, StackGrowsDown,
245                          Offset, MaxAlign);
246    AssignProtectedObjSet(AddrOfObjs, ProtectedObjs, MFI, StackGrowsDown,
247                          Offset, MaxAlign);
248  }
249
250  // Then assign frame offsets to stack objects that are not used to spill
251  // callee saved registers.
252  for (unsigned i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) {
253    if (MFI.isDeadObjectIndex(i))
254      continue;
255    if (MFI.getStackProtectorIndex() == (int)i)
256      continue;
257    if (ProtectedObjs.count(i))
258      continue;
259    if (!TFI.isStackIdSafeForLocalArea(MFI.getStackID(i)))
260      continue;
261
262    AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign);
263  }
264
265  // Remember how big this blob of stack space is
266  MFI.setLocalFrameSize(Offset);
267  MFI.setLocalFrameMaxAlign(MaxAlign);
268}
269
270static inline bool
271lookupCandidateBaseReg(unsigned BaseReg,
272                       int64_t BaseOffset,
273                       int64_t FrameSizeAdjust,
274                       int64_t LocalFrameOffset,
275                       const MachineInstr &MI,
276                       const TargetRegisterInfo *TRI) {
277  // Check if the relative offset from the where the base register references
278  // to the target address is in range for the instruction.
279  int64_t Offset = FrameSizeAdjust + LocalFrameOffset - BaseOffset;
280  return TRI->isFrameOffsetLegal(&MI, BaseReg, Offset);
281}
282
283bool LocalStackSlotPass::insertFrameReferenceRegisters(MachineFunction &Fn) {
284  // Scan the function's instructions looking for frame index references.
285  // For each, ask the target if it wants a virtual base register for it
286  // based on what we can tell it about where the local will end up in the
287  // stack frame. If it wants one, re-use a suitable one we've previously
288  // allocated, or if there isn't one that fits the bill, allocate a new one
289  // and ask the target to create a defining instruction for it.
290  bool UsedBaseReg = false;
291
292  MachineFrameInfo &MFI = Fn.getFrameInfo();
293  const TargetRegisterInfo *TRI = Fn.getSubtarget().getRegisterInfo();
294  const TargetFrameLowering &TFI = *Fn.getSubtarget().getFrameLowering();
295  bool StackGrowsDown =
296    TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsDown;
297
298  // Collect all of the instructions in the block that reference
299  // a frame index. Also store the frame index referenced to ease later
300  // lookup. (For any insn that has more than one FI reference, we arbitrarily
301  // choose the first one).
302  SmallVector<FrameRef, 64> FrameReferenceInsns;
303
304  unsigned Order = 0;
305
306  for (MachineBasicBlock &BB : Fn) {
307    for (MachineInstr &MI : BB) {
308      // Debug value, stackmap and patchpoint instructions can't be out of
309      // range, so they don't need any updates.
310      if (MI.isDebugInstr() || MI.getOpcode() == TargetOpcode::STATEPOINT ||
311          MI.getOpcode() == TargetOpcode::STACKMAP ||
312          MI.getOpcode() == TargetOpcode::PATCHPOINT)
313        continue;
314
315      // For now, allocate the base register(s) within the basic block
316      // where they're used, and don't try to keep them around outside
317      // of that. It may be beneficial to try sharing them more broadly
318      // than that, but the increased register pressure makes that a
319      // tricky thing to balance. Investigate if re-materializing these
320      // becomes an issue.
321      for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
322        // Consider replacing all frame index operands that reference
323        // an object allocated in the local block.
324        if (MI.getOperand(i).isFI()) {
325          // Don't try this with values not in the local block.
326          if (!MFI.isObjectPreAllocated(MI.getOperand(i).getIndex()))
327            break;
328          int Idx = MI.getOperand(i).getIndex();
329          int64_t LocalOffset = LocalOffsets[Idx];
330          if (!TRI->needsFrameBaseReg(&MI, LocalOffset))
331            break;
332          FrameReferenceInsns.push_back(FrameRef(&MI, LocalOffset, Idx, Order++));
333          break;
334        }
335      }
336    }
337  }
338
339  // Sort the frame references by local offset.
340  // Use frame index as a tie-breaker in case MI's have the same offset.
341  llvm::sort(FrameReferenceInsns);
342
343  MachineBasicBlock *Entry = &Fn.front();
344
345  unsigned BaseReg = 0;
346  int64_t BaseOffset = 0;
347
348  // Loop through the frame references and allocate for them as necessary.
349  for (int ref = 0, e = FrameReferenceInsns.size(); ref < e ; ++ref) {
350    FrameRef &FR = FrameReferenceInsns[ref];
351    MachineInstr &MI = *FR.getMachineInstr();
352    int64_t LocalOffset = FR.getLocalOffset();
353    int FrameIdx = FR.getFrameIndex();
354    assert(MFI.isObjectPreAllocated(FrameIdx) &&
355           "Only pre-allocated locals expected!");
356
357    // We need to keep the references to the stack protector slot through frame
358    // index operands so that it gets resolved by PEI rather than this pass.
359    // This avoids accesses to the stack protector though virtual base
360    // registers, and forces PEI to address it using fp/sp/bp.
361    if (MFI.hasStackProtectorIndex() &&
362        FrameIdx == MFI.getStackProtectorIndex())
363      continue;
364
365    LLVM_DEBUG(dbgs() << "Considering: " << MI);
366
367    unsigned idx = 0;
368    for (unsigned f = MI.getNumOperands(); idx != f; ++idx) {
369      if (!MI.getOperand(idx).isFI())
370        continue;
371
372      if (FrameIdx == MI.getOperand(idx).getIndex())
373        break;
374    }
375
376    assert(idx < MI.getNumOperands() && "Cannot find FI operand");
377
378    int64_t Offset = 0;
379    int64_t FrameSizeAdjust = StackGrowsDown ? MFI.getLocalFrameSize() : 0;
380
381    LLVM_DEBUG(dbgs() << "  Replacing FI in: " << MI);
382
383    // If we have a suitable base register available, use it; otherwise
384    // create a new one. Note that any offset encoded in the
385    // instruction itself will be taken into account by the target,
386    // so we don't have to adjust for it here when reusing a base
387    // register.
388    if (UsedBaseReg &&
389        lookupCandidateBaseReg(BaseReg, BaseOffset, FrameSizeAdjust,
390                               LocalOffset, MI, TRI)) {
391      LLVM_DEBUG(dbgs() << "  Reusing base register " << BaseReg << "\n");
392      // We found a register to reuse.
393      Offset = FrameSizeAdjust + LocalOffset - BaseOffset;
394    } else {
395      // No previously defined register was in range, so create a new one.
396      int64_t InstrOffset = TRI->getFrameIndexInstrOffset(&MI, idx);
397
398      int64_t PrevBaseOffset = BaseOffset;
399      BaseOffset = FrameSizeAdjust + LocalOffset + InstrOffset;
400
401      // We'd like to avoid creating single-use virtual base registers.
402      // Because the FrameRefs are in sorted order, and we've already
403      // processed all FrameRefs before this one, just check whether or not
404      // the next FrameRef will be able to reuse this new register. If not,
405      // then don't bother creating it.
406      if (ref + 1 >= e ||
407          !lookupCandidateBaseReg(
408              BaseReg, BaseOffset, FrameSizeAdjust,
409              FrameReferenceInsns[ref + 1].getLocalOffset(),
410              *FrameReferenceInsns[ref + 1].getMachineInstr(), TRI)) {
411        BaseOffset = PrevBaseOffset;
412        continue;
413      }
414
415      const MachineFunction *MF = MI.getMF();
416      const TargetRegisterClass *RC = TRI->getPointerRegClass(*MF);
417      BaseReg = Fn.getRegInfo().createVirtualRegister(RC);
418
419      LLVM_DEBUG(dbgs() << "  Materializing base register " << BaseReg
420                        << " at frame local offset "
421                        << LocalOffset + InstrOffset << "\n");
422
423      // Tell the target to insert the instruction to initialize
424      // the base register.
425      //            MachineBasicBlock::iterator InsertionPt = Entry->begin();
426      TRI->materializeFrameBaseRegister(Entry, BaseReg, FrameIdx,
427                                        InstrOffset);
428
429      // The base register already includes any offset specified
430      // by the instruction, so account for that so it doesn't get
431      // applied twice.
432      Offset = -InstrOffset;
433
434      ++NumBaseRegisters;
435      UsedBaseReg = true;
436    }
437    assert(BaseReg != 0 && "Unable to allocate virtual base register!");
438
439    // Modify the instruction to use the new base register rather
440    // than the frame index operand.
441    TRI->resolveFrameIndex(MI, BaseReg, Offset);
442    LLVM_DEBUG(dbgs() << "Resolved: " << MI);
443
444    ++NumReplacements;
445  }
446
447  return UsedBaseReg;
448}
449