LocalStackSlotAllocation.cpp revision 0bcf23f115608757b1d704e2f70a1148132551db 
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    void calculateFrameObjectOffsets(MachineFunction &Fn);
47
48    void insertFrameReferenceRegisters(MachineFunction &Fn);
49  public:
50    static char ID; // Pass identification, replacement for typeid
51    explicit LocalStackSlotPass() : MachineFunctionPass(ID) { }
52    bool runOnMachineFunction(MachineFunction &MF);
53
54    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
55      AU.setPreservesCFG();
56      MachineFunctionPass::getAnalysisUsage(AU);
57    }
58    const char *getPassName() const {
59      return "Local Stack Slot Allocation";
60    }
61
62  private:
63  };
64} // end anonymous namespace
65
66char LocalStackSlotPass::ID = 0;
67
68FunctionPass *llvm::createLocalStackSlotAllocationPass() {
69  return new LocalStackSlotPass();
70}
71
72bool LocalStackSlotPass::runOnMachineFunction(MachineFunction &MF) {
73  // Lay out the local blob.
74  calculateFrameObjectOffsets(MF);
75
76  // Insert virtual base registers to resolve frame index references.
77  insertFrameReferenceRegisters(MF);
78  return true;
79}
80
81/// AdjustStackOffset - Helper function used to adjust the stack frame offset.
82static inline void
83AdjustStackOffset(MachineFrameInfo *MFI, int FrameIdx, int64_t &Offset,
84                  unsigned &MaxAlign) {
85  unsigned Align = MFI->getObjectAlignment(FrameIdx);
86
87  // If the alignment of this object is greater than that of the stack, then
88  // increase the stack alignment to match.
89  MaxAlign = std::max(MaxAlign, Align);
90
91  // Adjust to alignment boundary.
92  Offset = (Offset + Align - 1) / Align * Align;
93
94  DEBUG(dbgs() << "Allocate FI(" << FrameIdx << ") to local offset "
95        << Offset << "\n");
96  MFI->mapLocalFrameObject(FrameIdx, Offset);
97  Offset += MFI->getObjectSize(FrameIdx);
98
99  ++NumAllocations;
100}
101
102/// calculateFrameObjectOffsets - Calculate actual frame offsets for all of the
103/// abstract stack objects.
104///
105void LocalStackSlotPass::calculateFrameObjectOffsets(MachineFunction &Fn) {
106  // Loop over all of the stack objects, assigning sequential addresses...
107  MachineFrameInfo *MFI = Fn.getFrameInfo();
108  int64_t Offset = 0;
109  unsigned MaxAlign = 0;
110
111  // Make sure that the stack protector comes before the local variables on the
112  // stack.
113  SmallSet<int, 16> LargeStackObjs;
114  if (MFI->getStackProtectorIndex() >= 0) {
115    AdjustStackOffset(MFI, MFI->getStackProtectorIndex(), Offset, MaxAlign);
116
117    // Assign large stack objects first.
118    for (unsigned i = 0, e = MFI->getObjectIndexEnd(); i != e; ++i) {
119      if (MFI->isDeadObjectIndex(i))
120        continue;
121      if (MFI->getStackProtectorIndex() == (int)i)
122        continue;
123      if (!MFI->MayNeedStackProtector(i))
124        continue;
125
126      AdjustStackOffset(MFI, i, Offset, MaxAlign);
127      LargeStackObjs.insert(i);
128    }
129  }
130
131  // Then assign frame offsets to stack objects that are not used to spill
132  // callee saved registers.
133  for (unsigned i = 0, e = MFI->getObjectIndexEnd(); i != e; ++i) {
134    if (MFI->isDeadObjectIndex(i))
135      continue;
136    if (MFI->getStackProtectorIndex() == (int)i)
137      continue;
138    if (LargeStackObjs.count(i))
139      continue;
140
141    AdjustStackOffset(MFI, i, Offset, MaxAlign);
142  }
143
144  // Remember how big this blob of stack space is
145  MFI->setLocalFrameSize(Offset);
146  MFI->setLocalFrameMaxAlign(MaxAlign);
147}
148
149static inline bool
150lookupCandidateBaseReg(const SmallVector<std::pair<unsigned, int64_t>, 8> &Regs,
151                       std::pair<unsigned, int64_t> &RegOffset,
152                       const MachineInstr *MI,
153                       const TargetRegisterInfo *TRI) {
154  unsigned e = Regs.size();
155  for (unsigned i = 0; i < e; ++i) {
156    RegOffset = Regs[i];
157    if (TRI->isBaseRegInRange(MI, RegOffset.first, RegOffset.second))
158      return true;
159  }
160  return false;
161}
162
163void LocalStackSlotPass::insertFrameReferenceRegisters(MachineFunction &Fn) {
164  // Scan the function's instructions looking for frame index references.
165  // For each, ask the target if it wants a virtual base register for it
166  // based on what we can tell it about where the local will end up in the
167  // stack frame. If it wants one, re-use a suitable one we've previously
168  // allocated, or if there isn't one that fits the bill, allocate a new one
169  // and ask the target to create a defining instruction for it.
170
171  MachineFrameInfo *MFI = Fn.getFrameInfo();
172  const TargetRegisterInfo *TRI = Fn.getTarget().getRegisterInfo();
173
174  for (MachineFunction::iterator BB = Fn.begin(),
175         E = Fn.end(); BB != E; ++BB) {
176    for (MachineBasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
177      MachineInstr *MI = I;
178      // Debug value instructions can't be out of range, so they don't need
179      // any updates.
180      // FIXME: When we extend this stuff to handle functions with both
181      // VLAs and dynamic realignment, we should update the debug values
182      // to reference the new base pointer when possible.
183      if (MI->isDebugValue())
184        continue;
185
186      // A base register definition is a register+offset pair.
187      SmallVector<std::pair<unsigned, int64_t>, 8> BaseRegisters;
188
189      // For now, allocate the base register(s) within the basic block
190      // where they're used, and don't try to keep them around outside
191      // of that. It may be beneficial to try sharing them more broadly
192      // than that, but the increased register pressure makes that a
193      // tricky thing to balance. Investigate if re-materializing these
194      // becomes an issue.
195      for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
196        // Consider replacing all frame index operands that reference
197        // an object allocated in the local block.
198        if (MI->getOperand(i).isFI()) {
199          int FrameIdx = MI->getOperand(i).getIndex();
200          // Don't try this with values not in the local block.
201          if (!MFI->isObjectPreAllocated(FrameIdx))
202            continue;
203
204          DEBUG(dbgs() << "Considering: " << *MI);
205          if (TRI->needsFrameBaseReg(MI, i)) {
206            unsigned BaseReg = 0;
207            unsigned Offset = 0;
208
209            DEBUG(dbgs() << "  Replacing FI in: " << *MI);
210
211            // If we have a suitable base register available, use it; otherwise
212            // create a new one.
213
214            std::pair<unsigned, int64_t> RegOffset;
215            if (lookupCandidateBaseReg(BaseRegisters, RegOffset, MI, TRI)) {
216              // We found a register to reuse.
217              BaseReg = RegOffset.first;
218              Offset = RegOffset.second;
219            } else {
220              // No previously defined register was in range, so create a
221              // new one.
222              const TargetRegisterClass *RC = TRI->getPointerRegClass();
223              BaseReg = Fn.getRegInfo().createVirtualRegister(RC);
224
225              // Tell the target to insert the instruction to initialize
226              // the base register.
227              TRI->materializeFrameBaseRegister(I, BaseReg, FrameIdx);
228
229              BaseRegisters.push_back(std::pair<unsigned, int64_t>(BaseReg,
230                                                                   Offset));
231              ++NumBaseRegisters;
232            }
233            assert(BaseReg != 0 && "Unable to allocate virtual base register!");
234
235            // Modify the instruction to use the new base register rather
236            // than the frame index operand.
237            TRI->resolveFrameIndex(I, BaseReg, Offset);
238
239            ++NumReplacements;
240          }
241
242        }
243      }
244    }
245  }
246}
247