LocalStackSlotAllocation.cpp revision a0fc005321ac163f10ebc5216a85068a496969df 
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/CodeGen/MachineFrameInfo.h"
28#include "llvm/CodeGen/MachineFunction.h"
29#include "llvm/CodeGen/MachineFunctionPass.h"
30#include "llvm/CodeGen/MachineRegisterInfo.h"
31#include "llvm/CodeGen/Passes.h"
32#include "llvm/Support/Debug.h"
33#include "llvm/Support/ErrorHandling.h"
34#include "llvm/Support/raw_ostream.h"
35#include "llvm/Target/TargetRegisterInfo.h"
36#include "llvm/Target/TargetFrameInfo.h"
37
38using namespace llvm;
39
40STATISTIC(NumAllocations, "Number of frame indices allocated into local block");
41STATISTIC(NumBaseRegisters, "Number of virtual frame base registers allocated");
42STATISTIC(NumReplacements, "Number of frame indices references replaced");
43
44namespace {
45  class LocalStackSlotPass: public MachineFunctionPass {
46    SmallVector<int64_t,16> LocalOffsets;
47
48    void AdjustStackOffset(MachineFrameInfo *MFI, int FrameIdx, int64_t &Offset,
49                           unsigned &MaxAlign);
50    void calculateFrameObjectOffsets(MachineFunction &Fn);
51    void insertFrameReferenceRegisters(MachineFunction &Fn);
52  public:
53    static char ID; // Pass identification, replacement for typeid
54    explicit LocalStackSlotPass() : MachineFunctionPass(ID) { }
55    bool runOnMachineFunction(MachineFunction &MF);
56
57    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
58      AU.setPreservesCFG();
59      MachineFunctionPass::getAnalysisUsage(AU);
60    }
61    const char *getPassName() const {
62      return "Local Stack Slot Allocation";
63    }
64
65  private:
66  };
67} // end anonymous namespace
68
69char LocalStackSlotPass::ID = 0;
70
71FunctionPass *llvm::createLocalStackSlotAllocationPass() {
72  return new LocalStackSlotPass();
73}
74
75bool LocalStackSlotPass::runOnMachineFunction(MachineFunction &MF) {
76  MachineFrameInfo *MFI = MF.getFrameInfo();
77  unsigned LocalObjectCount = MFI->getObjectIndexEnd();
78
79  // Early exit if there are no locals to consider
80  if (!LocalObjectCount)
81    return true;
82
83  // Make sure we have enough space to store the local offsets.
84  LocalOffsets.resize(MFI->getObjectIndexEnd());
85
86  // Lay out the local blob.
87  calculateFrameObjectOffsets(MF);
88
89  // Insert virtual base registers to resolve frame index references.
90  insertFrameReferenceRegisters(MF);
91
92  // Tell MFI whether any base registers were allocated. PEI will only
93  // want to use the local block allocations from this pass if there were any.
94  // Otherwise, PEI can do a bit better job of getting the alignment right
95  // without a hole at the start since it knows the alignment of the stack
96  // at the start of local allocation, and this pass doesn't.
97  MFI->setUseLocalStackAllocationBlock(NumBaseRegisters > 0);
98
99  return true;
100}
101
102/// AdjustStackOffset - Helper function used to adjust the stack frame offset.
103void LocalStackSlotPass::AdjustStackOffset(MachineFrameInfo *MFI,
104                                           int FrameIdx, int64_t &Offset,
105                                           unsigned &MaxAlign) {
106  unsigned Align = MFI->getObjectAlignment(FrameIdx);
107
108  // If the alignment of this object is greater than that of the stack, then
109  // increase the stack alignment to match.
110  MaxAlign = std::max(MaxAlign, Align);
111
112  // Adjust to alignment boundary.
113  Offset = (Offset + Align - 1) / Align * Align;
114
115  DEBUG(dbgs() << "Allocate FI(" << FrameIdx << ") to local offset "
116        << Offset << "\n");
117  // Keep the offset available for base register allocation
118  LocalOffsets[FrameIdx] = Offset;
119  // And tell MFI about it for PEI to use later
120  MFI->mapLocalFrameObject(FrameIdx, Offset);
121  Offset += MFI->getObjectSize(FrameIdx);
122
123  ++NumAllocations;
124}
125
126/// calculateFrameObjectOffsets - Calculate actual frame offsets for all of the
127/// abstract stack objects.
128///
129void LocalStackSlotPass::calculateFrameObjectOffsets(MachineFunction &Fn) {
130  // Loop over all of the stack objects, assigning sequential addresses...
131  MachineFrameInfo *MFI = Fn.getFrameInfo();
132  int64_t Offset = 0;
133  unsigned MaxAlign = 0;
134
135  // Make sure that the stack protector comes before the local variables on the
136  // stack.
137  SmallSet<int, 16> LargeStackObjs;
138  if (MFI->getStackProtectorIndex() >= 0) {
139    AdjustStackOffset(MFI, MFI->getStackProtectorIndex(), Offset, MaxAlign);
140
141    // Assign large stack objects first.
142    for (unsigned i = 0, e = MFI->getObjectIndexEnd(); i != e; ++i) {
143      if (MFI->isDeadObjectIndex(i))
144        continue;
145      if (MFI->getStackProtectorIndex() == (int)i)
146        continue;
147      if (!MFI->MayNeedStackProtector(i))
148        continue;
149
150      AdjustStackOffset(MFI, i, Offset, MaxAlign);
151      LargeStackObjs.insert(i);
152    }
153  }
154
155  // Then assign frame offsets to stack objects that are not used to spill
156  // callee saved registers.
157  for (unsigned i = 0, e = MFI->getObjectIndexEnd(); i != e; ++i) {
158    if (MFI->isDeadObjectIndex(i))
159      continue;
160    if (MFI->getStackProtectorIndex() == (int)i)
161      continue;
162    if (LargeStackObjs.count(i))
163      continue;
164
165    AdjustStackOffset(MFI, i, Offset, MaxAlign);
166  }
167
168  // Remember how big this blob of stack space is
169  MFI->setLocalFrameSize(Offset);
170  MFI->setLocalFrameMaxAlign(MaxAlign);
171}
172
173static inline bool
174lookupCandidateBaseReg(const SmallVector<std::pair<unsigned, int64_t>, 8> &Regs,
175                       std::pair<unsigned, int64_t> &RegOffset,
176                       int64_t LocalFrameOffset,
177                       const MachineInstr *MI,
178                       const TargetRegisterInfo *TRI) {
179  unsigned e = Regs.size();
180  for (unsigned i = 0; i < e; ++i) {
181    RegOffset = Regs[i];
182    // Check if the relative offset from the where the base register references
183    // to the target address is in range for the instruction.
184    int64_t Offset = LocalFrameOffset - RegOffset.second;
185    if (TRI->isBaseRegInRange(MI, RegOffset.first, Offset))
186      return true;
187  }
188  return false;
189}
190
191void LocalStackSlotPass::insertFrameReferenceRegisters(MachineFunction &Fn) {
192  // Scan the function's instructions looking for frame index references.
193  // For each, ask the target if it wants a virtual base register for it
194  // based on what we can tell it about where the local will end up in the
195  // stack frame. If it wants one, re-use a suitable one we've previously
196  // allocated, or if there isn't one that fits the bill, allocate a new one
197  // and ask the target to create a defining instruction for it.
198
199  MachineFrameInfo *MFI = Fn.getFrameInfo();
200  const TargetRegisterInfo *TRI = Fn.getTarget().getRegisterInfo();
201
202  for (MachineFunction::iterator BB = Fn.begin(),
203         E = Fn.end(); BB != E; ++BB) {
204    // A base register definition is a register+offset pair.
205    SmallVector<std::pair<unsigned, int64_t>, 8> BaseRegisters;
206
207    for (MachineBasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
208      MachineInstr *MI = I;
209      // Debug value instructions can't be out of range, so they don't need
210      // any updates.
211      // FIXME: When we extend this stuff to handle functions with both
212      // VLAs and dynamic realignment, we should update the debug values
213      // to reference the new base pointer when possible.
214      if (MI->isDebugValue())
215        continue;
216
217      // For now, allocate the base register(s) within the basic block
218      // where they're used, and don't try to keep them around outside
219      // of that. It may be beneficial to try sharing them more broadly
220      // than that, but the increased register pressure makes that a
221      // tricky thing to balance. Investigate if re-materializing these
222      // becomes an issue.
223      for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
224        // Consider replacing all frame index operands that reference
225        // an object allocated in the local block.
226        if (MI->getOperand(i).isFI()) {
227          int FrameIdx = MI->getOperand(i).getIndex();
228          // Don't try this with values not in the local block.
229          if (!MFI->isObjectPreAllocated(FrameIdx))
230            continue;
231
232          DEBUG(dbgs() << "Considering: " << *MI);
233          if (TRI->needsFrameBaseReg(MI, i)) {
234            unsigned BaseReg = 0;
235            unsigned Offset = 0;
236
237            DEBUG(dbgs() << "  Replacing FI in: " << *MI);
238
239            // If we have a suitable base register available, use it; otherwise
240            // create a new one.
241
242            std::pair<unsigned, int64_t> RegOffset;
243            if (lookupCandidateBaseReg(BaseRegisters, RegOffset,
244                                       LocalOffsets[FrameIdx], MI, TRI)) {
245              DEBUG(dbgs() << "  Reusing base register " <<
246                    RegOffset.first << "\n");
247              // We found a register to reuse.
248              BaseReg = RegOffset.first;
249              Offset = LocalOffsets[FrameIdx] - RegOffset.second;
250            } else {
251              // No previously defined register was in range, so create a
252              // new one.
253              const TargetRegisterClass *RC = TRI->getPointerRegClass();
254              BaseReg = Fn.getRegInfo().createVirtualRegister(RC);
255
256              // Tell the target to insert the instruction to initialize
257              // the base register.
258              TRI->materializeFrameBaseRegister(I, BaseReg, FrameIdx);
259
260              BaseRegisters.push_back(std::pair<unsigned, int64_t>(BaseReg,
261                                                                   Offset));
262              ++NumBaseRegisters;
263            }
264            assert(BaseReg != 0 && "Unable to allocate virtual base register!");
265
266            // Modify the instruction to use the new base register rather
267            // than the frame index operand.
268            TRI->resolveFrameIndex(I, BaseReg, Offset);
269
270            ++NumReplacements;
271          }
272
273        }
274      }
275    }
276  }
277}
278