summaryrefslogtreecommitdiff
path: root/lib/Transforms/Utils/AddrModeMatcher.cpp
blob: 3a19b706eae83ce3f44f7e88847fc0b5262eb231 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
//===- AddrModeMatcher.cpp - Addressing mode matching facility --*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements target addressing mode matcher class.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Utils/AddrModeMatcher.h"
#include "llvm/Assembly/Writer.h"
#include "llvm/DataLayout.h"
#include "llvm/DerivedTypes.h"
#include "llvm/GlobalValue.h"
#include "llvm/Instruction.h"
#include "llvm/Support/CallSite.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
#include "llvm/Support/PatternMatch.h"
#include "llvm/Support/raw_ostream.h"

using namespace llvm;
using namespace llvm::PatternMatch;

void ExtAddrMode::print(raw_ostream &OS) const {
  bool NeedPlus = false;
  OS << "[";
  if (BaseGV) {
    OS << (NeedPlus ? " + " : "")
       << "GV:";
    WriteAsOperand(OS, BaseGV, /*PrintType=*/false);
    NeedPlus = true;
  }

  if (BaseOffs)
    OS << (NeedPlus ? " + " : "") << BaseOffs, NeedPlus = true;

  if (BaseReg) {
    OS << (NeedPlus ? " + " : "")
       << "Base:";
    WriteAsOperand(OS, BaseReg, /*PrintType=*/false);
    NeedPlus = true;
  }
  if (Scale) {
    OS << (NeedPlus ? " + " : "")
       << Scale << "*";
    WriteAsOperand(OS, ScaledReg, /*PrintType=*/false);
    NeedPlus = true;
  }

  OS << ']';
}

#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
void ExtAddrMode::dump() const {
  print(dbgs());
  dbgs() << '\n';
}
#endif


/// MatchScaledValue - Try adding ScaleReg*Scale to the current addressing mode.
/// Return true and update AddrMode if this addr mode is legal for the target,
/// false if not.
bool AddressingModeMatcher::MatchScaledValue(Value *ScaleReg, int64_t Scale,
                                             unsigned Depth) {
  // If Scale is 1, then this is the same as adding ScaleReg to the addressing
  // mode.  Just process that directly.
  if (Scale == 1)
    return MatchAddr(ScaleReg, Depth);
  
  // If the scale is 0, it takes nothing to add this.
  if (Scale == 0)
    return true;
  
  // If we already have a scale of this value, we can add to it, otherwise, we
  // need an available scale field.
  if (AddrMode.Scale != 0 && AddrMode.ScaledReg != ScaleReg)
    return false;

  ExtAddrMode TestAddrMode = AddrMode;

  // Add scale to turn X*4+X*3 -> X*7.  This could also do things like
  // [A+B + A*7] -> [B+A*8].
  TestAddrMode.Scale += Scale;
  TestAddrMode.ScaledReg = ScaleReg;

  // If the new address isn't legal, bail out.
  if (!TLI.isLegalAddressingMode(TestAddrMode, AccessTy))
    return false;

  // It was legal, so commit it.
  AddrMode = TestAddrMode;
  
  // Okay, we decided that we can add ScaleReg+Scale to AddrMode.  Check now
  // to see if ScaleReg is actually X+C.  If so, we can turn this into adding
  // X*Scale + C*Scale to addr mode.
  ConstantInt *CI = 0; Value *AddLHS = 0;
  if (isa<Instruction>(ScaleReg) &&  // not a constant expr.
      match(ScaleReg, m_Add(m_Value(AddLHS), m_ConstantInt(CI)))) {
    TestAddrMode.ScaledReg = AddLHS;
    TestAddrMode.BaseOffs += CI->getSExtValue()*TestAddrMode.Scale;
      
    // If this addressing mode is legal, commit it and remember that we folded
    // this instruction.
    if (TLI.isLegalAddressingMode(TestAddrMode, AccessTy)) {
      AddrModeInsts.push_back(cast<Instruction>(ScaleReg));
      AddrMode = TestAddrMode;
      return true;
    }
  }

  // Otherwise, not (x+c)*scale, just return what we have.
  return true;
}

/// MightBeFoldableInst - This is a little filter, which returns true if an
/// addressing computation involving I might be folded into a load/store
/// accessing it.  This doesn't need to be perfect, but needs to accept at least
/// the set of instructions that MatchOperationAddr can.
static bool MightBeFoldableInst(Instruction *I) {
  switch (I->getOpcode()) {
  case Instruction::BitCast:
    // Don't touch identity bitcasts.
    if (I->getType() == I->getOperand(0)->getType())
      return false;
    return I->getType()->isPointerTy() || I->getType()->isIntegerTy();
  case Instruction::PtrToInt:
    // PtrToInt is always a noop, as we know that the int type is pointer sized.
    return true;
  case Instruction::IntToPtr:
    // We know the input is intptr_t, so this is foldable.
    return true;
  case Instruction::Add:
    return true;
  case Instruction::Mul:
  case Instruction::Shl:
    // Can only handle X*C and X << C.
    return isa<ConstantInt>(I->getOperand(1));
  case Instruction::GetElementPtr:
    return true;
  default:
    return false;
  }
}


/// MatchOperationAddr - Given an instruction or constant expr, see if we can
/// fold the operation into the addressing mode.  If so, update the addressing
/// mode and return true, otherwise return false without modifying AddrMode.
bool AddressingModeMatcher::MatchOperationAddr(User *AddrInst, unsigned Opcode,
                                               unsigned Depth) {
  // Avoid exponential behavior on extremely deep expression trees.
  if (Depth >= 5) return false;
  
  switch (Opcode) {
  case Instruction::PtrToInt:
    // PtrToInt is always a noop, as we know that the int type is pointer sized.
    return MatchAddr(AddrInst->getOperand(0), Depth);
  case Instruction::IntToPtr:
    // This inttoptr is a no-op if the integer type is pointer sized.
    if (TLI.getValueType(AddrInst->getOperand(0)->getType()) ==
        TLI.getPointerTy())
      return MatchAddr(AddrInst->getOperand(0), Depth);
    return false;
  case Instruction::BitCast:
    // BitCast is always a noop, and we can handle it as long as it is
    // int->int or pointer->pointer (we don't want int<->fp or something).
    if ((AddrInst->getOperand(0)->getType()->isPointerTy() ||
         AddrInst->getOperand(0)->getType()->isIntegerTy()) &&
        // Don't touch identity bitcasts.  These were probably put here by LSR,
        // and we don't want to mess around with them.  Assume it knows what it
        // is doing.
        AddrInst->getOperand(0)->getType() != AddrInst->getType())
      return MatchAddr(AddrInst->getOperand(0), Depth);
    return false;
  case Instruction::Add: {
    // Check to see if we can merge in the RHS then the LHS.  If so, we win.
    ExtAddrMode BackupAddrMode = AddrMode;
    unsigned OldSize = AddrModeInsts.size();
    if (MatchAddr(AddrInst->getOperand(1), Depth+1) &&
        MatchAddr(AddrInst->getOperand(0), Depth+1))
      return true;
    
    // Restore the old addr mode info.
    AddrMode = BackupAddrMode;
    AddrModeInsts.resize(OldSize);
    
    // Otherwise this was over-aggressive.  Try merging in the LHS then the RHS.
    if (MatchAddr(AddrInst->getOperand(0), Depth+1) &&
        MatchAddr(AddrInst->getOperand(1), Depth+1))
      return true;
    
    // Otherwise we definitely can't merge the ADD in.
    AddrMode = BackupAddrMode;
    AddrModeInsts.resize(OldSize);
    break;
  }
  //case Instruction::Or:
  // TODO: We can handle "Or Val, Imm" iff this OR is equivalent to an ADD.
  //break;
  case Instruction::Mul:
  case Instruction::Shl: {
    // Can only handle X*C and X << C.
    ConstantInt *RHS = dyn_cast<ConstantInt>(AddrInst->getOperand(1));
    if (!RHS) return false;
    int64_t Scale = RHS->getSExtValue();
    if (Opcode == Instruction::Shl)
      Scale = 1LL << Scale;
    
    return MatchScaledValue(AddrInst->getOperand(0), Scale, Depth);
  }
  case Instruction::GetElementPtr: {
    // Scan the GEP.  We check it if it contains constant offsets and at most
    // one variable offset.
    int VariableOperand = -1;
    unsigned VariableScale = 0;
    
    int64_t ConstantOffset = 0;
    const DataLayout *TD = TLI.getDataLayout();
    gep_type_iterator GTI = gep_type_begin(AddrInst);
    for (unsigned i = 1, e = AddrInst->getNumOperands(); i != e; ++i, ++GTI) {
      if (StructType *STy = dyn_cast<StructType>(*GTI)) {
        const StructLayout *SL = TD->getStructLayout(STy);
        unsigned Idx =
          cast<ConstantInt>(AddrInst->getOperand(i))->getZExtValue();
        ConstantOffset += SL->getElementOffset(Idx);
      } else {
        uint64_t TypeSize = TD->getTypeAllocSize(GTI.getIndexedType());
        if (ConstantInt *CI = dyn_cast<ConstantInt>(AddrInst->getOperand(i))) {
          ConstantOffset += CI->getSExtValue()*TypeSize;
        } else if (TypeSize) {  // Scales of zero don't do anything.
          // We only allow one variable index at the moment.
          if (VariableOperand != -1)
            return false;
          
          // Remember the variable index.
          VariableOperand = i;
          VariableScale = TypeSize;
        }
      }
    }
    
    // A common case is for the GEP to only do a constant offset.  In this case,
    // just add it to the disp field and check validity.
    if (VariableOperand == -1) {
      AddrMode.BaseOffs += ConstantOffset;
      if (ConstantOffset == 0 || TLI.isLegalAddressingMode(AddrMode, AccessTy)){
        // Check to see if we can fold the base pointer in too.
        if (MatchAddr(AddrInst->getOperand(0), Depth+1))
          return true;
      }
      AddrMode.BaseOffs -= ConstantOffset;
      return false;
    }

    // Save the valid addressing mode in case we can't match.
    ExtAddrMode BackupAddrMode = AddrMode;
    unsigned OldSize = AddrModeInsts.size();

    // See if the scale and offset amount is valid for this target.
    AddrMode.BaseOffs += ConstantOffset;

    // Match the base operand of the GEP.
    if (!MatchAddr(AddrInst->getOperand(0), Depth+1)) {
      // If it couldn't be matched, just stuff the value in a register.
      if (AddrMode.HasBaseReg) {
        AddrMode = BackupAddrMode;
        AddrModeInsts.resize(OldSize);
        return false;
      }
      AddrMode.HasBaseReg = true;
      AddrMode.BaseReg = AddrInst->getOperand(0);
    }

    // Match the remaining variable portion of the GEP.
    if (!MatchScaledValue(AddrInst->getOperand(VariableOperand), VariableScale,
                          Depth)) {
      // If it couldn't be matched, try stuffing the base into a register
      // instead of matching it, and retrying the match of the scale.
      AddrMode = BackupAddrMode;
      AddrModeInsts.resize(OldSize);
      if (AddrMode.HasBaseReg)
        return false;
      AddrMode.HasBaseReg = true;
      AddrMode.BaseReg = AddrInst->getOperand(0);
      AddrMode.BaseOffs += ConstantOffset;
      if (!MatchScaledValue(AddrInst->getOperand(VariableOperand),
                            VariableScale, Depth)) {
        // If even that didn't work, bail.
        AddrMode = BackupAddrMode;
        AddrModeInsts.resize(OldSize);
        return false;
      }
    }

    return true;
  }
  }
  return false;
}

/// MatchAddr - If we can, try to add the value of 'Addr' into the current
/// addressing mode.  If Addr can't be added to AddrMode this returns false and
/// leaves AddrMode unmodified.  This assumes that Addr is either a pointer type
/// or intptr_t for the target.
///
bool AddressingModeMatcher::MatchAddr(Value *Addr, unsigned Depth) {
  if (ConstantInt *CI = dyn_cast<ConstantInt>(Addr)) {
    // Fold in immediates if legal for the target.
    AddrMode.BaseOffs += CI->getSExtValue();
    if (TLI.isLegalAddressingMode(AddrMode, AccessTy))
      return true;
    AddrMode.BaseOffs -= CI->getSExtValue();
  } else if (GlobalValue *GV = dyn_cast<GlobalValue>(Addr)) {
    // If this is a global variable, try to fold it into the addressing mode.
    if (AddrMode.BaseGV == 0) {
      AddrMode.BaseGV = GV;
      if (TLI.isLegalAddressingMode(AddrMode, AccessTy))
        return true;
      AddrMode.BaseGV = 0;
    }
  } else if (Instruction *I = dyn_cast<Instruction>(Addr)) {
    ExtAddrMode BackupAddrMode = AddrMode;
    unsigned OldSize = AddrModeInsts.size();

    // Check to see if it is possible to fold this operation.
    if (MatchOperationAddr(I, I->getOpcode(), Depth)) {
      // Okay, it's possible to fold this.  Check to see if it is actually
      // *profitable* to do so.  We use a simple cost model to avoid increasing
      // register pressure too much.
      if (I->hasOneUse() ||
          IsProfitableToFoldIntoAddressingMode(I, BackupAddrMode, AddrMode)) {
        AddrModeInsts.push_back(I);
        return true;
      }
      
      // It isn't profitable to do this, roll back.
      //cerr << "NOT FOLDING: " << *I;
      AddrMode = BackupAddrMode;
      AddrModeInsts.resize(OldSize);
    }
  } else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Addr)) {
    if (MatchOperationAddr(CE, CE->getOpcode(), Depth))
      return true;
  } else if (isa<ConstantPointerNull>(Addr)) {
    // Null pointer gets folded without affecting the addressing mode.
    return true;
  }

  // Worse case, the target should support [reg] addressing modes. :)
  if (!AddrMode.HasBaseReg) {
    AddrMode.HasBaseReg = true;
    AddrMode.BaseReg = Addr;
    // Still check for legality in case the target supports [imm] but not [i+r].
    if (TLI.isLegalAddressingMode(AddrMode, AccessTy))
      return true;
    AddrMode.HasBaseReg = false;
    AddrMode.BaseReg = 0;
  }

  // If the base register is already taken, see if we can do [r+r].
  if (AddrMode.Scale == 0) {
    AddrMode.Scale = 1;
    AddrMode.ScaledReg = Addr;
    if (TLI.isLegalAddressingMode(AddrMode, AccessTy))
      return true;
    AddrMode.Scale = 0;
    AddrMode.ScaledReg = 0;
  }
  // Couldn't match.
  return false;
}


/// IsOperandAMemoryOperand - Check to see if all uses of OpVal by the specified
/// inline asm call are due to memory operands.  If so, return true, otherwise
/// return false.
static bool IsOperandAMemoryOperand(CallInst *CI, InlineAsm *IA, Value *OpVal,
                                    const TargetLowering &TLI) {
  TargetLowering::AsmOperandInfoVector TargetConstraints = TLI.ParseConstraints(ImmutableCallSite(CI));
  for (unsigned i = 0, e = TargetConstraints.size(); i != e; ++i) {
    TargetLowering::AsmOperandInfo &OpInfo = TargetConstraints[i];
    
    // Compute the constraint code and ConstraintType to use.
    TLI.ComputeConstraintToUse(OpInfo, SDValue());

    // If this asm operand is our Value*, and if it isn't an indirect memory
    // operand, we can't fold it!
    if (OpInfo.CallOperandVal == OpVal &&
        (OpInfo.ConstraintType != TargetLowering::C_Memory ||
         !OpInfo.isIndirect))
      return false;
  }

  return true;
}


/// FindAllMemoryUses - Recursively walk all the uses of I until we find a
/// memory use.  If we find an obviously non-foldable instruction, return true.
/// Add the ultimately found memory instructions to MemoryUses.
static bool FindAllMemoryUses(Instruction *I,
                SmallVectorImpl<std::pair<Instruction*,unsigned> > &MemoryUses,
                              SmallPtrSet<Instruction*, 16> &ConsideredInsts,
                              const TargetLowering &TLI) {
  // If we already considered this instruction, we're done.
  if (!ConsideredInsts.insert(I))
    return false;
  
  // If this is an obviously unfoldable instruction, bail out.
  if (!MightBeFoldableInst(I))
    return true;

  // Loop over all the uses, recursively processing them.
  for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
       UI != E; ++UI) {
    User *U = *UI;

    if (LoadInst *LI = dyn_cast<LoadInst>(U)) {
      MemoryUses.push_back(std::make_pair(LI, UI.getOperandNo()));
      continue;
    }
    
    if (StoreInst *SI = dyn_cast<StoreInst>(U)) {
      unsigned opNo = UI.getOperandNo();
      if (opNo == 0) return true; // Storing addr, not into addr.
      MemoryUses.push_back(std::make_pair(SI, opNo));
      continue;
    }
    
    if (CallInst *CI = dyn_cast<CallInst>(U)) {
      InlineAsm *IA = dyn_cast<InlineAsm>(CI->getCalledValue());
      if (!IA) return true;
      
      // If this is a memory operand, we're cool, otherwise bail out.
      if (!IsOperandAMemoryOperand(CI, IA, I, TLI))
        return true;
      continue;
    }
    
    if (FindAllMemoryUses(cast<Instruction>(U), MemoryUses, ConsideredInsts,
                          TLI))
      return true;
  }

  return false;
}


/// ValueAlreadyLiveAtInst - Retrn true if Val is already known to be live at
/// the use site that we're folding it into.  If so, there is no cost to
/// include it in the addressing mode.  KnownLive1 and KnownLive2 are two values
/// that we know are live at the instruction already.
bool AddressingModeMatcher::ValueAlreadyLiveAtInst(Value *Val,Value *KnownLive1,
                                                   Value *KnownLive2) {
  // If Val is either of the known-live values, we know it is live!
  if (Val == 0 || Val == KnownLive1 || Val == KnownLive2)
    return true;
  
  // All values other than instructions and arguments (e.g. constants) are live.
  if (!isa<Instruction>(Val) && !isa<Argument>(Val)) return true;
  
  // If Val is a constant sized alloca in the entry block, it is live, this is
  // true because it is just a reference to the stack/frame pointer, which is
  // live for the whole function.
  if (AllocaInst *AI = dyn_cast<AllocaInst>(Val))
    if (AI->isStaticAlloca())
      return true;
  
  // Check to see if this value is already used in the memory instruction's
  // block.  If so, it's already live into the block at the very least, so we
  // can reasonably fold it.
  return Val->isUsedInBasicBlock(MemoryInst->getParent());
}



/// IsProfitableToFoldIntoAddressingMode - It is possible for the addressing
/// mode of the machine to fold the specified instruction into a load or store
/// that ultimately uses it.  However, the specified instruction has multiple
/// uses.  Given this, it may actually increase register pressure to fold it
/// into the load.  For example, consider this code:
///
///     X = ...
///     Y = X+1
///     use(Y)   -> nonload/store
///     Z = Y+1
///     load Z
///
/// In this case, Y has multiple uses, and can be folded into the load of Z
/// (yielding load [X+2]).  However, doing this will cause both "X" and "X+1" to
/// be live at the use(Y) line.  If we don't fold Y into load Z, we use one
/// fewer register.  Since Y can't be folded into "use(Y)" we don't increase the
/// number of computations either.
///
/// Note that this (like most of CodeGenPrepare) is just a rough heuristic.  If
/// X was live across 'load Z' for other reasons, we actually *would* want to
/// fold the addressing mode in the Z case.  This would make Y die earlier.
bool AddressingModeMatcher::
IsProfitableToFoldIntoAddressingMode(Instruction *I, ExtAddrMode &AMBefore,
                                     ExtAddrMode &AMAfter) {
  if (IgnoreProfitability) return true;
  
  // AMBefore is the addressing mode before this instruction was folded into it,
  // and AMAfter is the addressing mode after the instruction was folded.  Get
  // the set of registers referenced by AMAfter and subtract out those
  // referenced by AMBefore: this is the set of values which folding in this
  // address extends the lifetime of.
  //
  // Note that there are only two potential values being referenced here,
  // BaseReg and ScaleReg (global addresses are always available, as are any
  // folded immediates).
  Value *BaseReg = AMAfter.BaseReg, *ScaledReg = AMAfter.ScaledReg;
  
  // If the BaseReg or ScaledReg was referenced by the previous addrmode, their
  // lifetime wasn't extended by adding this instruction.
  if (ValueAlreadyLiveAtInst(BaseReg, AMBefore.BaseReg, AMBefore.ScaledReg))
    BaseReg = 0;
  if (ValueAlreadyLiveAtInst(ScaledReg, AMBefore.BaseReg, AMBefore.ScaledReg))
    ScaledReg = 0;

  // If folding this instruction (and it's subexprs) didn't extend any live
  // ranges, we're ok with it.
  if (BaseReg == 0 && ScaledReg == 0)
    return true;

  // If all uses of this instruction are ultimately load/store/inlineasm's,
  // check to see if their addressing modes will include this instruction.  If
  // so, we can fold it into all uses, so it doesn't matter if it has multiple
  // uses.
  SmallVector<std::pair<Instruction*,unsigned>, 16> MemoryUses;
  SmallPtrSet<Instruction*, 16> ConsideredInsts;
  if (FindAllMemoryUses(I, MemoryUses, ConsideredInsts, TLI))
    return false;  // Has a non-memory, non-foldable use!
  
  // Now that we know that all uses of this instruction are part of a chain of
  // computation involving only operations that could theoretically be folded
  // into a memory use, loop over each of these uses and see if they could
  // *actually* fold the instruction.
  SmallVector<Instruction*, 32> MatchedAddrModeInsts;
  for (unsigned i = 0, e = MemoryUses.size(); i != e; ++i) {
    Instruction *User = MemoryUses[i].first;
    unsigned OpNo = MemoryUses[i].second;
    
    // Get the access type of this use.  If the use isn't a pointer, we don't
    // know what it accesses.
    Value *Address = User->getOperand(OpNo);
    if (!Address->getType()->isPointerTy())
      return false;
    Type *AddressAccessTy =
      cast<PointerType>(Address->getType())->getElementType();
    
    // Do a match against the root of this address, ignoring profitability. This
    // will tell us if the addressing mode for the memory operation will
    // *actually* cover the shared instruction.
    ExtAddrMode Result;
    AddressingModeMatcher Matcher(MatchedAddrModeInsts, TLI, AddressAccessTy,
                                  MemoryInst, Result);
    Matcher.IgnoreProfitability = true;
    bool Success = Matcher.MatchAddr(Address, 0);
    (void)Success; assert(Success && "Couldn't select *anything*?");

    // If the match didn't cover I, then it won't be shared by it.
    if (std::find(MatchedAddrModeInsts.begin(), MatchedAddrModeInsts.end(),
                  I) == MatchedAddrModeInsts.end())
      return false;
    
    MatchedAddrModeInsts.clear();
  }
  
  return true;
}