Factor address mode matcher out of codegen prepare to make it available to other passes, e.g. loop strength reduction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@65134 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 593 insertions, 0 deletions

diff --git a/lib/Transforms/Utils/AddrModeMatcher.cpp b/lib/Transforms/Utils/AddrModeMatcher.cpp
new file mode 100644
index 0000000..766f1ed
--- /dev/null
+++ b/lib/Transforms/Utils/AddrModeMatcher.cpp
@@ -0,0 +1,593 @@
+//===- 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/DerivedTypes.h"
+#include "llvm/GlobalValue.h"
+#include "llvm/Instruction.h"
+#include "llvm/Assembly/Writer.h"
+#include "llvm/Target/TargetData.h"
+#include "llvm/Support/GetElementPtrTypeIterator.h"
+#include "llvm/Support/PatternMatch.h"
+
+using namespace llvm;
+using namespace llvm::PatternMatch;
+
+void ExtAddrMode::print(OStream &OS) const {
+  bool NeedPlus = false;
+  OS << "[";
+  if (BaseGV) {
+    OS << (NeedPlus ? " + " : "")
+       << "GV:";
+    WriteAsOperand(*OS.stream(), BaseGV, /*PrintType=*/false);
+    NeedPlus = true;
+  }
+
+  if (BaseOffs)
+    OS << (NeedPlus ? " + " : "") << BaseOffs, NeedPlus = true;
+
+  if (BaseReg) {
+    OS << (NeedPlus ? " + " : "")
+       << "Base:";
+    WriteAsOperand(*OS.stream(), BaseReg, /*PrintType=*/false);
+    NeedPlus = true;
+  }
+  if (Scale) {
+    OS << (NeedPlus ? " + " : "")
+       << Scale << "*";
+    WriteAsOperand(*OS.stream(), ScaledReg, /*PrintType=*/false);
+    NeedPlus = true;
+  }
+
+  OS << ']';
+}
+
+void ExtAddrMode::dump() const {
+  print(cerr);
+  cerr << '\n';
+}
+
+
+/// 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; Value *AddLHS;
+  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 isa<PointerType>(I->getType()) || isa<IntegerType>(I->getType());
+  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 ((isa<PointerType>(AddrInst->getOperand(0)->getType()) ||
+         isa<IntegerType>(AddrInst->getOperand(0)->getType())) &&
+        // 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 = 1 << 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 TargetData *TD = TLI.getTargetData();
+    gep_type_iterator GTI = gep_type_begin(AddrInst);
+    for (unsigned i = 1, e = AddrInst->getNumOperands(); i != e; ++i, ++GTI) {
+      if (const 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->getTypePaddedSize(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;
+    
+    // Check that this has no base reg yet.  If so, we won't have a place to
+    // put the base of the GEP (assuming it is not a null ptr).
+    bool SetBaseReg = true;
+    if (isa<ConstantPointerNull>(AddrInst->getOperand(0)))
+      SetBaseReg = false;   // null pointer base doesn't need representation.
+    else if (AddrMode.HasBaseReg)
+      return false;  // Base register already specified, can't match GEP.
+    else {
+      // Otherwise, we'll use the GEP base as the BaseReg.
+      AddrMode.HasBaseReg = true;
+      AddrMode.BaseReg = AddrInst->getOperand(0);
+    }
+    
+    // See if the scale and offset amount is valid for this target.
+    AddrMode.BaseOffs += ConstantOffset;
+    
+    if (!MatchScaledValue(AddrInst->getOperand(VariableOperand), VariableScale,
+                          Depth)) {
+      AddrMode = BackupAddrMode;
+      return false;
+    }
+    
+    // If we have a null as the base of the GEP, folding in the constant offset
+    // plus variable scale is all we can do.
+    if (!SetBaseReg) return true;
+      
+    // If this match succeeded, we know that we can form an address with the
+    // GepBase as the basereg.  Match the base pointer of the GEP more
+    // aggressively by zeroing out BaseReg and rematching.  If the base is
+    // (for example) another GEP, this allows merging in that other GEP into
+    // the addressing mode we're forming.
+    AddrMode.HasBaseReg = false;
+    AddrMode.BaseReg = 0;
+    bool Success = MatchAddr(AddrInst->getOperand(0), Depth+1);
+    assert(Success && "MatchAddr should be able to fill in BaseReg!");
+    Success=Success;
+    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) {
+  std::vector<InlineAsm::ConstraintInfo>
+  Constraints = IA->ParseConstraints();
+  
+  unsigned ArgNo = 1;   // ArgNo - The operand of the CallInst.
+  for (unsigned i = 0, e = Constraints.size(); i != e; ++i) {
+    TargetLowering::AsmOperandInfo OpInfo(Constraints[i]);
+    
+    // Compute the value type for each operand.
+    switch (OpInfo.Type) {
+      case InlineAsm::isOutput:
+        if (OpInfo.isIndirect)
+          OpInfo.CallOperandVal = CI->getOperand(ArgNo++);
+        break;
+      case InlineAsm::isInput:
+        OpInfo.CallOperandVal = CI->getOperand(ArgNo++);
+        break;
+      case InlineAsm::isClobber:
+        // Nothing to do.
+        break;
+    }
+    
+    // Compute the constraint code and ConstraintType to use.
+    TLI.ComputeConstraintToUse(OpInfo, SDValue(),
+                             OpInfo.ConstraintType == TargetLowering::C_Memory);
+    
+    // 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) {
+    if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
+      MemoryUses.push_back(std::make_pair(LI, UI.getOperandNo()));
+      continue;
+    }
+    
+    if (StoreInst *SI = dyn_cast<StoreInst>(*UI)) {
+      if (UI.getOperandNo() == 0) return true; // Storing addr, not into addr.
+      MemoryUses.push_back(std::make_pair(SI, UI.getOperandNo()));
+      continue;
+    }
+    
+    if (CallInst *CI = dyn_cast<CallInst>(*UI)) {
+      InlineAsm *IA = dyn_cast<InlineAsm>(CI->getCalledValue());
+      if (IA == 0) 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>(*UI), 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.
+  BasicBlock *MemBB = MemoryInst->getParent();
+  for (Value::use_iterator UI = Val->use_begin(), E = Val->use_end();
+       UI != E; ++UI)
+    // We know that uses of arguments and instructions have to be instructions.
+    if (cast<Instruction>(*UI)->getParent() == MemBB)
+      return true;
+  
+  return false;
+}
+
+
+
+/// 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 (!isa<PointerType>(Address->getType()))
+      return false;
+    const 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);
+    Success = 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;
+}