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, 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
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,
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.
176void 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.
191void 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.hasStackProtectorIndex()) {
204    int StackProtectorFI = MFI.getStackProtectorIndex();
205
206    // We need to make sure we didn't pre-allocate the stack protector when
207    // doing this.
208    // If we already have a stack protector, this will re-assign it to a slot
209    // that is **not** covering the protected objects.
210    assert(!MFI.isObjectPreAllocated(StackProtectorFI) &&
211           "Stack protector pre-allocated in LocalStackSlotAllocation");
212
213    StackObjSet LargeArrayObjs;
214    StackObjSet SmallArrayObjs;
215    StackObjSet AddrOfObjs;
216
217    AdjustStackOffset(MFI, StackProtectorFI, Offset, StackGrowsDown, MaxAlign);
218
219    // Assign large stack objects first.
220    for (unsigned i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) {
221      if (MFI.isDeadObjectIndex(i))
222        continue;
223      if (StackProtectorFI == (int)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
260    AdjustStackOffset(MFI, i, Offset, StackGrowsDown, MaxAlign);
261  }
262
263  // Remember how big this blob of stack space is
264  MFI.setLocalFrameSize(Offset);
265  MFI.setLocalFrameMaxAlign(assumeAligned(MaxAlign));
266}
267
268static inline bool
269lookupCandidateBaseReg(unsigned BaseReg,
270                       int64_t BaseOffset,
271                       int64_t FrameSizeAdjust,
272                       int64_t LocalFrameOffset,
273                       const MachineInstr &MI,
274                       const TargetRegisterInfo *TRI) {
275  // Check if the relative offset from the where the base register references
276  // to the target address is in range for the instruction.
277  int64_t Offset = FrameSizeAdjust + LocalFrameOffset - BaseOffset;
278  return TRI->isFrameOffsetLegal(&MI, BaseReg, Offset);
279}
280
281bool LocalStackSlotPass::insertFrameReferenceRegisters(MachineFunction &Fn) {
282  // Scan the function's instructions looking for frame index references.
283  // For each, ask the target if it wants a virtual base register for it
284  // based on what we can tell it about where the local will end up in the
285  // stack frame. If it wants one, re-use a suitable one we've previously
286  // allocated, or if there isn't one that fits the bill, allocate a new one
287  // and ask the target to create a defining instruction for it.
288  bool UsedBaseReg = false;
289
290  MachineFrameInfo &MFI = Fn.getFrameInfo();
291  const TargetRegisterInfo *TRI = Fn.getSubtarget().getRegisterInfo();
292  const TargetFrameLowering &TFI = *Fn.getSubtarget().getFrameLowering();
293  bool StackGrowsDown =
294    TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsDown;
295
296  // Collect all of the instructions in the block that reference
297  // a frame index. Also store the frame index referenced to ease later
298  // lookup. (For any insn that has more than one FI reference, we arbitrarily
299  // choose the first one).
300  SmallVector<FrameRef, 64> FrameReferenceInsns;
301
302  unsigned Order = 0;
303
304  for (MachineBasicBlock &BB : Fn) {
305    for (MachineInstr &MI : BB) {
306      // Debug value, stackmap and patchpoint instructions can't be out of
307      // range, so they don't need any updates.
308      if (MI.isDebugInstr() || MI.getOpcode() == TargetOpcode::STATEPOINT ||
309          MI.getOpcode() == TargetOpcode::STACKMAP ||
310          MI.getOpcode() == TargetOpcode::PATCHPOINT)
311        continue;
312
313      // For now, allocate the base register(s) within the basic block
314      // where they're used, and don't try to keep them around outside
315      // of that. It may be beneficial to try sharing them more broadly
316      // than that, but the increased register pressure makes that a
317      // tricky thing to balance. Investigate if re-materializing these
318      // becomes an issue.
319      for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
320        // Consider replacing all frame index operands that reference
321        // an object allocated in the local block.
322        if (MI.getOperand(i).isFI()) {
323          // Don't try this with values not in the local block.
324          if (!MFI.isObjectPreAllocated(MI.getOperand(i).getIndex()))
325            break;
326          int Idx = MI.getOperand(i).getIndex();
327          int64_t LocalOffset = LocalOffsets[Idx];
328          if (!TRI->needsFrameBaseReg(&MI, LocalOffset))
329            break;
330          FrameReferenceInsns.push_back(FrameRef(&MI, LocalOffset, Idx, Order++));
331          break;
332        }
333      }
334    }
335  }
336
337  // Sort the frame references by local offset.
338  // Use frame index as a tie-breaker in case MI's have the same offset.
339  llvm::sort(FrameReferenceInsns);
340
341  MachineBasicBlock *Entry = &Fn.front();
342
343  unsigned BaseReg = 0;
344  int64_t BaseOffset = 0;
345
346  // Loop through the frame references and allocate for them as necessary.
347  for (int ref = 0, e = FrameReferenceInsns.size(); ref < e ; ++ref) {
348    FrameRef &FR = FrameReferenceInsns[ref];
349    MachineInstr &MI = *FR.getMachineInstr();
350    int64_t LocalOffset = FR.getLocalOffset();
351    int FrameIdx = FR.getFrameIndex();
352    assert(MFI.isObjectPreAllocated(FrameIdx) &&
353           "Only pre-allocated locals expected!");
354
355    // We need to keep the references to the stack protector slot through frame
356    // index operands so that it gets resolved by PEI rather than this pass.
357    // This avoids accesses to the stack protector though virtual base
358    // registers, and forces PEI to address it using fp/sp/bp.
359    if (MFI.hasStackProtectorIndex() &&
360        FrameIdx == MFI.getStackProtectorIndex())
361      continue;
362
363    LLVM_DEBUG(dbgs() << "Considering: " << MI);
364
365    unsigned idx = 0;
366    for (unsigned f = MI.getNumOperands(); idx != f; ++idx) {
367      if (!MI.getOperand(idx).isFI())
368        continue;
369
370      if (FrameIdx == MI.getOperand(idx).getIndex())
371        break;
372    }
373
374    assert(idx < MI.getNumOperands() && "Cannot find FI operand");
375
376    int64_t Offset = 0;
377    int64_t FrameSizeAdjust = StackGrowsDown ? MFI.getLocalFrameSize() : 0;
378
379    LLVM_DEBUG(dbgs() << "  Replacing FI in: " << MI);
380
381    // If we have a suitable base register available, use it; otherwise
382    // create a new one. Note that any offset encoded in the
383    // instruction itself will be taken into account by the target,
384    // so we don't have to adjust for it here when reusing a base
385    // register.
386    if (UsedBaseReg &&
387        lookupCandidateBaseReg(BaseReg, BaseOffset, FrameSizeAdjust,
388                               LocalOffset, MI, TRI)) {
389      LLVM_DEBUG(dbgs() << "  Reusing base register " << BaseReg << "\n");
390      // We found a register to reuse.
391      Offset = FrameSizeAdjust + LocalOffset - BaseOffset;
392    } else {
393      // No previously defined register was in range, so create a new one.
394      int64_t InstrOffset = TRI->getFrameIndexInstrOffset(&MI, idx);
395
396      int64_t PrevBaseOffset = BaseOffset;
397      BaseOffset = FrameSizeAdjust + LocalOffset + InstrOffset;
398
399      // We'd like to avoid creating single-use virtual base registers.
400      // Because the FrameRefs are in sorted order, and we've already
401      // processed all FrameRefs before this one, just check whether or not
402      // the next FrameRef will be able to reuse this new register. If not,
403      // then don't bother creating it.
404      if (ref + 1 >= e ||
405          !lookupCandidateBaseReg(
406              BaseReg, BaseOffset, FrameSizeAdjust,
407              FrameReferenceInsns[ref + 1].getLocalOffset(),
408              *FrameReferenceInsns[ref + 1].getMachineInstr(), TRI)) {
409        BaseOffset = PrevBaseOffset;
410        continue;
411      }
412
413      const MachineFunction *MF = MI.getMF();
414      const TargetRegisterClass *RC = TRI->getPointerRegClass(*MF);
415      BaseReg = Fn.getRegInfo().createVirtualRegister(RC);
416
417      LLVM_DEBUG(dbgs() << "  Materializing base register " << BaseReg
418                        << " at frame local offset "
419                        << LocalOffset + InstrOffset << "\n");
420
421      // Tell the target to insert the instruction to initialize
422      // the base register.
423      //            MachineBasicBlock::iterator InsertionPt = Entry->begin();
424      TRI->materializeFrameBaseRegister(Entry, BaseReg, FrameIdx,
425                                        InstrOffset);
426
427      // The base register already includes any offset specified
428      // by the instruction, so account for that so it doesn't get
429      // applied twice.
430      Offset = -InstrOffset;
431
432      ++NumBaseRegisters;
433      UsedBaseReg = true;
434    }
435    assert(BaseReg != 0 && "Unable to allocate virtual base register!");
436
437    // Modify the instruction to use the new base register rather
438    // than the frame index operand.
439    TRI->resolveFrameIndex(MI, BaseReg, Offset);
440    LLVM_DEBUG(dbgs() << "Resolved: " << MI);
441
442    ++NumReplacements;
443  }
444
445  return UsedBaseReg;
446}
447