IR: Do not consider available_externally linkage to be linker-weak.

[pcc]

Summary

From the linker's perspective, an available_externally global is equivalent
to an external declaration (per isDeclarationForLinker()), so it is incorrect
to consider it to be a weak definition.

Also clean up some logic in the dead argument elimination pass and clarify
its comments to better explain how its behavior depends on linkage,
introduce GlobalValue::isStrongDefinitionForLinker() and start using
it throughout the optimizers and backend.

[• rafael]

LGTM with a nit

lib/Transforms/IPO/DeadArgumentElimination.cpp

340 ↗

(On Diff #29043)

! isStrongDefinitionForLinker

llvm/trunk/include/llvm/IR/GlobalValue.h

return Linkage == WeakAnyLinkage || Linkage == LinkOnceAnyLinkage ||
            Linkage == CommonLinkage || Linkage == ExternalWeakLinkage;
   }
 
   /// Whether the definition of this global may be replaced at link time.  NB:
   /// Using this method outside of the code generators is almost always a
   /// mistake: when working at the IR level use mayBeOverridden instead as it
   /// knows about ODR semantics.
   static bool isWeakForLinker(LinkageTypes Linkage)  {
-    return Linkage == AvailableExternallyLinkage || Linkage == WeakAnyLinkage ||
-           Linkage == WeakODRLinkage || Linkage == LinkOnceAnyLinkage ||
-           Linkage == LinkOnceODRLinkage || Linkage == CommonLinkage ||
-           Linkage == ExternalWeakLinkage;
+    return Linkage == WeakAnyLinkage || Linkage == WeakODRLinkage ||
+           Linkage == LinkOnceAnyLinkage || Linkage == LinkOnceODRLinkage ||
+           Linkage == CommonLinkage || Linkage == ExternalWeakLinkage;
   }
 
   bool hasExternalLinkage() const { return isExternalLinkage(Linkage); }
   bool hasAvailableExternallyLinkage() const {
     return isAvailableExternallyLinkage(Linkage);
   }
   bool hasLinkOnceLinkage() const {
     return isLinkOnceLinkage(Linkage);
/// @}
 
   bool isDeclarationForLinker() const {
     if (hasAvailableExternallyLinkage())
       return true;
 
     return isDeclaration();
   }
 
+  /// Returns true if this global's definition will be the one chosen by the
+  /// linker.
+  bool isStrongDefinitionForLinker() const {
+    return !(isDeclarationForLinker() || isWeakForLinker());
+  }
+
   /// This method unlinks 'this' from the containing module, but does not delete
   /// it.
   virtual void removeFromParent() = 0;
 
   /// This method unlinks 'this' from the containing module and deletes it.
   virtual void eraseFromParent() = 0;
 
   /// Get the module that this global value is contained inside of...

llvm/trunk/lib/Analysis/ValueTracking.cpp

if (auto *GO = dyn_cast<GlobalObject>(V)) {
     unsigned Align = GO->getAlignment();
     if (Align == 0) {
       if (auto *GVar = dyn_cast<GlobalVariable>(GO)) {
         Type *ObjectType = GVar->getType()->getElementType();
         if (ObjectType->isSized()) {
           // If the object is defined in the current Module, we'll be giving
           // it the preferred alignment. Otherwise, we have to assume that it
           // may only have the minimum ABI alignment.
-          if (!GVar->isDeclaration() && !GVar->isWeakForLinker())
+          if (GVar->isStrongDefinitionForLinker())
             Align = DL.getPreferredAlignment(GVar);
           else
             Align = DL.getABITypeAlignment(ObjectType);
         }
       }
     }
     if (Align > 0)
       KnownZero = APInt::getLowBitsSet(BitWidth,

llvm/trunk/lib/CodeGen/SelectionDAG/TargetLowering.cpp

 bool
 TargetLowering::isOffsetFoldingLegal(const GlobalAddressSDNode *GA) const {
   // Assume that everything is safe in static mode.
   if (getTargetMachine().getRelocationModel() == Reloc::Static)
     return true;
 
   // In dynamic-no-pic mode, assume that known defined values are safe.
   if (getTargetMachine().getRelocationModel() == Reloc::DynamicNoPIC &&
-      GA &&
-      !GA->getGlobal()->isDeclaration() &&
-      !GA->getGlobal()->isWeakForLinker())
+      GA && GA->getGlobal()->isStrongDefinitionForLinker())
     return true;
 
   // Otherwise assume nothing is safe.
   return false;
 }
 
 //===----------------------------------------------------------------------===//
 //  Optimization Methods

llvm/trunk/lib/Target/AArch64/AArch64Subtarget.cpp

: AArch64GenSubtargetInfo(TT, CPU, FS), ARMProcFamily(Others),
       InstrInfo(initializeSubtargetDependencies(FS)),
       TSInfo(TM.getDataLayout()), TLInfo(TM, *this) {}
 
 /// ClassifyGlobalReference - Find the target operand flags that describe
 /// how a global value should be referenced for the current subtarget.
 unsigned char
 AArch64Subtarget::ClassifyGlobalReference(const GlobalValue *GV,
                                         const TargetMachine &TM) const {
-  bool isDecl = GV->isDeclarationForLinker();
+  bool isDef = GV->isStrongDefinitionForLinker();
 
   // MachO large model always goes via a GOT, simply to get a single 8-byte
   // absolute relocation on all global addresses.
   if (TM.getCodeModel() == CodeModel::Large && isTargetMachO())
     return AArch64II::MO_GOT;
 
   // The small code mode's direct accesses use ADRP, which cannot necessarily
   // produce the value 0 (if the code is above 4GB).
-  if (TM.getCodeModel() == CodeModel::Small &&
-      GV->isWeakForLinker() && isDecl) {
+  if (TM.getCodeModel() == CodeModel::Small && GV->hasExternalWeakLinkage()) {
     // In PIC mode use the GOT, but in absolute mode use a constant pool load.
     if (TM.getRelocationModel() == Reloc::Static)
         return AArch64II::MO_CONSTPOOL;
     else
         return AArch64II::MO_GOT;
   }
 
   // If symbol visibility is hidden, the extra load is not needed if
   // the symbol is definitely defined in the current translation unit.
 
   // The handling of non-hidden symbols in PIC mode is rather target-dependent:
   //   + On MachO, if the symbol is defined in this module the GOT can be
   //     skipped.
   //   + On ELF, the R_AARCH64_COPY relocation means that even symbols actually
   //     defined could end up in unexpected places. Use a GOT.
   if (TM.getRelocationModel() != Reloc::Static && GV->hasDefaultVisibility()) {
     if (isTargetMachO())
-      return (isDecl || GV->isWeakForLinker()) ? AArch64II::MO_GOT
-                                               : AArch64II::MO_NO_FLAG;
+      return isDef ? AArch64II::MO_NO_FLAG : AArch64II::MO_GOT;
     else
       // No need to go through the GOT for local symbols on ELF.
       return GV->hasLocalLinkage() ? AArch64II::MO_NO_FLAG : AArch64II::MO_GOT;
   }
 
   return AArch64II::MO_NO_FLAG;
 }
 

llvm/trunk/lib/Target/ARM/ARMISelLowering.cpp

if (GlobalAddressSDNode *G = dyn_cast<GlobalAddressSDNode>(Callee)) {
       Callee = DAG.getLoad(getPointerTy(), dl,
                            DAG.getEntryNode(), CPAddr,
                            MachinePointerInfo::getConstantPool(),
                            false, false, false, 0);
     }
   } else if (GlobalAddressSDNode *G = dyn_cast<GlobalAddressSDNode>(Callee)) {
     const GlobalValue *GV = G->getGlobal();
     isDirect = true;
-    bool isExt = GV->isDeclaration() || GV->isWeakForLinker();
-    bool isStub = (isExt && Subtarget->isTargetMachO()) &&
+    bool isDef = GV->isStrongDefinitionForLinker();
+    bool isStub = (!isDef && Subtarget->isTargetMachO()) &&
                    getTargetMachine().getRelocationModel() != Reloc::Static;
     isARMFunc = !Subtarget->isThumb() || (isStub && !Subtarget->isMClass());
     // ARM call to a local ARM function is predicable.
-    isLocalARMFunc = !Subtarget->isThumb() && (!isExt || !ARMInterworking);
+    isLocalARMFunc = !Subtarget->isThumb() && (isDef || !ARMInterworking);
     // tBX takes a register source operand.
     if (isStub && Subtarget->isThumb1Only() && !Subtarget->hasV5TOps()) {
       assert(Subtarget->isTargetMachO() && "WrapperPIC use on non-MachO?");
       Callee = DAG.getNode(ARMISD::WrapperPIC, dl, getPointerTy(),
                            DAG.getTargetGlobalAddress(GV, dl, getPointerTy(),
                                                       0, ARMII::MO_NONLAZY));
       Callee = DAG.getLoad(getPointerTy(), dl, DAG.getEntryNode(), Callee,
                            MachinePointerInfo::getGOT(), false, false, true, 0);

llvm/trunk/lib/Target/ARM/ARMSubtarget.cpp

 
 /// GVIsIndirectSymbol - true if the GV will be accessed via an indirect symbol.
 bool
 ARMSubtarget::GVIsIndirectSymbol(const GlobalValue *GV,
                                  Reloc::Model RelocM) const {
   if (RelocM == Reloc::Static)
     return false;
 
-  bool isDecl = GV->isDeclarationForLinker();
+  bool isDef = GV->isStrongDefinitionForLinker();
 
   if (!isTargetMachO()) {
     // Extra load is needed for all externally visible.
     if (GV->hasLocalLinkage() || GV->hasHiddenVisibility())
       return false;
     return true;
   } else {
-    if (RelocM == Reloc::PIC_) {
-      // If this is a strong reference to a definition, it is definitely not
-      // through a stub.
-      if (!isDecl && !GV->isWeakForLinker())
-        return false;
+    if (isDef)
 
-      // Unless we have a symbol with hidden visibility, we have to go through a
-      // normal $non_lazy_ptr stub because this symbol might be resolved late.
-      if (!GV->hasHiddenVisibility())  // Non-hidden $non_lazy_ptr reference.
-        return true;
 
+    if (RelocM == Reloc::PIC_) {
       // If symbol visibility is hidden, we have a stub for common symbol
       // references and external declarations.
-      if (isDecl || GV->hasCommonLinkage())
+      if (GV->isDeclarationForLinker() || GV->hasCommonLinkage())
         // Hidden $non_lazy_ptr reference.
         return true;
-
-      return false;
-    } else {
-      // If this is a strong reference to a definition, it is definitely not
-      // through a stub.
-      if (!isDecl && !GV->isWeakForLinker())
-        return false;
-
-      // Unless we have a symbol with hidden visibility, we have to go through a
-      // normal $non_lazy_ptr stub because this symbol might be resolved late.
-      if (!GV->hasHiddenVisibility())  // Non-hidden $non_lazy_ptr reference.
-        return true;
     }
   }
 
   return false;
 }
 
 unsigned ARMSubtarget::getMispredictionPenalty() const {
   return SchedModel.MispredictPenalty;

llvm/trunk/lib/Target/PowerPC/PPCAsmPrinter.cpp

case MachineOperand::MO_BlockAddress:
     return;
   case MachineOperand::MO_GlobalAddress: {
     // Computing the address of a global symbol, not calling it.
     const GlobalValue *GV = MO.getGlobal();
     MCSymbol *SymToPrint;
 
     // External or weakly linked global variables need non-lazily-resolved stubs
     if (TM.getRelocationModel() != Reloc::Static &&
-        (GV->isDeclaration() || GV->isWeakForLinker())) {
+        !GV->isStrongDefinitionForLinker()) {
       if (!GV->hasHiddenVisibility()) {
         SymToPrint = getSymbolWithGlobalValueBase(GV, "$non_lazy_ptr");
         MachineModuleInfoImpl::StubValueTy &StubSym = 
           MMI->getObjFileInfo<MachineModuleInfoMachO>()
             .getGVStubEntry(SymToPrint);
         if (!StubSym.getPointer())
           StubSym = MachineModuleInfoImpl::
             StubValueTy(getSymbol(GV), !GV->hasInternalLinkage());
case PPC::ADDIStocHA: {
     bool IsAvailExt = false;
 
     if (MO.isGlobal()) {
       const GlobalValue *GV = MO.getGlobal();
       MOSymbol = getSymbol(GV);
       IsExternal = GV->isDeclaration();
       IsCommon = GV->hasCommonLinkage();
       IsNonLocalFunction = GV->getType()->getElementType()->isFunctionTy() &&
-        (GV->isDeclaration() || GV->isWeakForLinker());
+        !GV->isStrongDefinitionForLinker();
       IsAvailExt = GV->hasAvailableExternallyLinkage();
     } else if (MO.isCPI())
       MOSymbol = GetCPISymbol(MO.getIndex());
     else if (MO.isJTI())
       MOSymbol = GetJTISymbol(MO.getIndex());
     else if (MO.isBlockAddress())
       MOSymbol = GetBlockAddressSymbol(MO.getBlockAddress());
 
case PPC::ADDItocL: {
     bool IsExternal = false;
     bool IsNonLocalFunction = false;
 
     if (MO.isGlobal()) {
       const GlobalValue *GV = MO.getGlobal();
       MOSymbol = getSymbol(GV);
       IsExternal = GV->isDeclaration();
       IsNonLocalFunction = GV->getType()->getElementType()->isFunctionTy() &&
-        (GV->isDeclaration() || GV->isWeakForLinker());
+        !GV->isStrongDefinitionForLinker();
     } else if (MO.isCPI())
       MOSymbol = GetCPISymbol(MO.getIndex());
 
     if (IsNonLocalFunction || IsExternal ||
         TM.getCodeModel() == CodeModel::Large)
       MOSymbol = lookUpOrCreateTOCEntry(MOSymbol);
 
     const MCExpr *Exp =

llvm/trunk/lib/Target/PowerPC/PPCFastISel.cpp

else {
     unsigned HighPartReg = createResultReg(RC);
     BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DbgLoc, TII.get(PPC::ADDIStocHA),
             HighPartReg).addReg(PPC::X2).addGlobalAddress(GV);
 
     // If/when switches are implemented, jump tables should be handled
     // on the "if" path here.
     if (CModel == CodeModel::Large ||
         (GV->getType()->getElementType()->isFunctionTy() &&
-         (GV->isDeclaration() || GV->isWeakForLinker())) ||
+         !GV->isStrongDefinitionForLinker()) ||
         GV->isDeclaration() || GV->hasCommonLinkage() ||
         GV->hasAvailableExternallyLinkage())
       BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DbgLoc, TII.get(PPC::LDtocL),
               DestReg).addGlobalAddress(GV).addReg(HighPartReg);
     else
       // Otherwise generate the ADDItocL.
       BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DbgLoc, TII.get(PPC::ADDItocL),
               DestReg).addReg(HighPartReg).addGlobalAddress(GV);

llvm/trunk/lib/Target/PowerPC/PPCISelDAGToDAG.cpp

case PPCISD::TOC_ENTRY: {
     if (isa<JumpTableSDNode>(GA) || isa<BlockAddressSDNode>(GA) ||
         CModel == CodeModel::Large)
       return transferMemOperands(N, CurDAG->getMachineNode(PPC::LDtocL, dl,
                                       MVT::i64, GA, SDValue(Tmp, 0)));
 
     if (GlobalAddressSDNode *G = dyn_cast<GlobalAddressSDNode>(GA)) {
       const GlobalValue *GValue = G->getGlobal();
       if ((GValue->getType()->getElementType()->isFunctionTy() &&
-           (GValue->isDeclaration() || GValue->isWeakForLinker())) ||
+           !GValue->isStrongDefinitionForLinker()) ||
           GValue->isDeclaration() || GValue->hasCommonLinkage() ||
           GValue->hasAvailableExternallyLinkage())
         return transferMemOperands(N, CurDAG->getMachineNode(PPC::LDtocL, dl,
                                         MVT::i64, GA, SDValue(Tmp, 0)));
     }
 
     return CurDAG->getMachineNode(PPC::ADDItocL, dl, MVT::i64,
                                   SDValue(Tmp, 0), GA);

llvm/trunk/lib/Target/PowerPC/PPCISelLowering.cpp

unsigned PrepareCall(SelectionDAG &DAG, SDValue &Callee, SDValue &InFlag,
   if (isFunctionGlobalAddress(Callee)) {
     GlobalAddressSDNode *G = cast<GlobalAddressSDNode>(Callee);
     // A call to a TLS address is actually an indirect call to a
     // thread-specific pointer.
     unsigned OpFlags = 0;
     if ((DAG.getTarget().getRelocationModel() != Reloc::Static &&
          (Subtarget.getTargetTriple().isMacOSX() &&
           Subtarget.getTargetTriple().isMacOSXVersionLT(10, 5)) &&
-         (G->getGlobal()->isDeclaration() ||
-          G->getGlobal()->isWeakForLinker())) ||
+         !G->getGlobal()->isStrongDefinitionForLinker()) ||
         (Subtarget.isTargetELF() && !isPPC64 &&
          !G->getGlobal()->hasLocalLinkage() &&
          DAG.getTarget().getRelocationModel() == Reloc::PIC_)) {
       // PC-relative references to external symbols should go through $stub,
       // unless we're building with the leopard linker or later, which
       // automatically synthesizes these stubs.
       OpFlags = PPCII::MO_PLT_OR_STUB;
     }
unsigned PrepareCall(SelectionDAG &DAG, SDValue &Callee, SDValue &InFlag,
 
   return CallOpc;
 }
 
 static
 bool isLocalCall(const SDValue &Callee)
 {
   if (GlobalAddressSDNode *G = dyn_cast<GlobalAddressSDNode>(Callee))
-    return !G->getGlobal()->isDeclaration() &&
-           !G->getGlobal()->isWeakForLinker();
+    return G->getGlobal()->isStrongDefinitionForLinker();
   return false;
 }
 
 SDValue
 PPCTargetLowering::LowerCallResult(SDValue Chain, SDValue InFlag,
                                    CallingConv::ID CallConv, bool isVarArg,
                                    const SmallVectorImpl<ISD::InputArg> &Ins,
                                    SDLoc dl, SelectionDAG &DAG,

llvm/trunk/lib/Target/X86/X86FastISel.cpp

if (CalleeOp) {
     // external symbols most go through the PLT in PIC mode.  If the symbol
     // has hidden or protected visibility, or if it is static or local, then
     // we don't need to use the PLT - we can directly call it.
     if (Subtarget->isTargetELF() &&
         TM.getRelocationModel() == Reloc::PIC_ &&
         GV->hasDefaultVisibility() && !GV->hasLocalLinkage()) {
       OpFlags = X86II::MO_PLT;
     } else if (Subtarget->isPICStyleStubAny() &&
-               (GV->isDeclaration() || GV->isWeakForLinker()) &&
+               !GV->isStrongDefinitionForLinker() &&
                (!Subtarget->getTargetTriple().isMacOSX() ||
                 Subtarget->getTargetTriple().isMacOSXVersionLT(10, 5))) {
       // PC-relative references to external symbols should go through $stub,
       // unless we're building with the leopard linker or later, which
       // automatically synthesizes these stubs.
       OpFlags = X86II::MO_DARWIN_STUB;
     }
 

llvm/trunk/lib/Target/X86/X86ISelLowering.cpp

if (!GV->hasDLLImportStorageClass()) {
       // external symbols most go through the PLT in PIC mode.  If the symbol
       // has hidden or protected visibility, or if it is static or local, then
       // we don't need to use the PLT - we can directly call it.
       if (Subtarget->isTargetELF() &&
           DAG.getTarget().getRelocationModel() == Reloc::PIC_ &&
           GV->hasDefaultVisibility() && !GV->hasLocalLinkage()) {
         OpFlags = X86II::MO_PLT;
       } else if (Subtarget->isPICStyleStubAny() &&
-                 (GV->isDeclaration() || GV->isWeakForLinker()) &&
+                 !GV->isStrongDefinitionForLinker() &&
                  (!Subtarget->getTargetTriple().isMacOSX() ||
                   Subtarget->getTargetTriple().isMacOSXVersionLT(10, 5))) {
         // PC-relative references to external symbols should go through $stub,
         // unless we're building with the leopard linker or later, which
         // automatically synthesizes these stubs.
         OpFlags = X86II::MO_DARWIN_STUB;
       } else if (Subtarget->isPICStyleRIPRel() && isa<Function>(GV) &&
                  cast<Function>(GV)->hasFnAttribute(Attribute::NonLazyBind)) {

llvm/trunk/lib/Target/X86/X86Subtarget.cpp

 /// context.
 unsigned char X86Subtarget::
 ClassifyGlobalReference(const GlobalValue *GV, const TargetMachine &TM) const {
   // DLLImport only exists on windows, it is implemented as a load from a
   // DLLIMPORT stub.
   if (GV->hasDLLImportStorageClass())
     return X86II::MO_DLLIMPORT;
 
-  bool isDecl = GV->isDeclarationForLinker();
+  bool isDef = GV->isStrongDefinitionForLinker();
 
   // X86-64 in PIC mode.
   if (isPICStyleRIPRel()) {
     // Large model never uses stubs.
     if (TM.getCodeModel() == CodeModel::Large)
       return X86II::MO_NO_FLAG;
 
     if (isTargetDarwin()) {
       // If symbol visibility is hidden, the extra load is not needed if
       // target is x86-64 or the symbol is definitely defined in the current
       // translation unit.
-      if (GV->hasDefaultVisibility() &&
-          (isDecl || GV->isWeakForLinker()))
+      if (GV->hasDefaultVisibility() && !isDef)
         return X86II::MO_GOTPCREL;
     } else if (!isTargetWin64()) {
       assert(isTargetELF() && "Unknown rip-relative target");
 
       // Extra load is needed for all externally visible.
       if (!GV->hasLocalLinkage() && GV->hasDefaultVisibility())
         return X86II::MO_GOTPCREL;
     }
ClassifyGlobalReference(const GlobalValue *GV, const TargetMachine &TM) const {
   }
 
   if (isPICStyleStubPIC()) {  // Darwin/32 in PIC mode.
     // Determine whether we have a stub reference and/or whether the reference
     // is relative to the PIC base or not.
 
     // If this is a strong reference to a definition, it is definitely not
     // through a stub.
-    if (!isDecl && !GV->isWeakForLinker())
+    if (isDef)
       return X86II::MO_PIC_BASE_OFFSET;
 
     // Unless we have a symbol with hidden visibility, we have to go through a
     // normal $non_lazy_ptr stub because this symbol might be resolved late.
     if (!GV->hasHiddenVisibility())  // Non-hidden $non_lazy_ptr reference.
       return X86II::MO_DARWIN_NONLAZY_PIC_BASE;
 
     // If symbol visibility is hidden, we have a stub for common symbol
     // references and external declarations.
-    if (isDecl || GV->hasCommonLinkage()) {
+    if (GV->isDeclarationForLinker() || GV->hasCommonLinkage()) {
       // Hidden $non_lazy_ptr reference.
       return X86II::MO_DARWIN_HIDDEN_NONLAZY_PIC_BASE;
     }
 
     // Otherwise, no stub.
     return X86II::MO_PIC_BASE_OFFSET;
   }
 
   if (isPICStyleStubNoDynamic()) {  // Darwin/32 in -mdynamic-no-pic mode.
     // Determine whether we have a stub reference.
 
     // If this is a strong reference to a definition, it is definitely not
     // through a stub.
-    if (!isDecl && !GV->isWeakForLinker())
+    if (isDef)
       return X86II::MO_NO_FLAG;
 
     // Unless we have a symbol with hidden visibility, we have to go through a
     // normal $non_lazy_ptr stub because this symbol might be resolved late.
     if (!GV->hasHiddenVisibility())  // Non-hidden $non_lazy_ptr reference.
       return X86II::MO_DARWIN_NONLAZY;
 
     // Otherwise, no stub.

llvm/trunk/lib/Transforms/IPO/DeadArgumentElimination.cpp

bool DAE::DeleteDeadVarargs(Function &Fn) {
   return true;
 }
 
 /// RemoveDeadArgumentsFromCallers - Checks if the given function has any 
 /// arguments that are unused, and changes the caller parameters to be undefined
 /// instead.
 bool DAE::RemoveDeadArgumentsFromCallers(Function &Fn)
 {
-  if (Fn.isDeclaration() || Fn.mayBeOverridden())
+  // We cannot change the arguments if this TU does not define the function or
+  // if the linker may choose a function body from another TU, even if the
+  // nominal linkage indicates that other copies of the function have the same
+  // semantics. In the below example, the dead load from %p may not have been
+  // eliminated from the linker-chosen copy of f, so replacing %p with undef
+  // in callers may introduce undefined behavior.
+  //
+  // define linkonce_odr void @f(i32* %p) {
+  //   %v = load i32 %p
+  //   ret void
+  // }
+  if (!Fn.isStrongDefinitionForLinker())
     return false;
 
   // Functions with local linkage should already have been handled, except the
   // fragile (variadic) ones which we can improve here.
   if (Fn.hasLocalLinkage() && !Fn.getFunctionType()->isVarArg())
     return false;
 
-  // If a function seen at compile time is not necessarily the one linked to
-  // the binary being built, it is illegal to change the actual arguments
-  // passed to it. These functions can be captured by isWeakForLinker().
-  // *NOTE* that mayBeOverridden() is insufficient for this purpose as it
-  // doesn't include linkage types like AvailableExternallyLinkage and
-  // LinkOnceODRLinkage. Take link_odr* as an example, it indicates a set of
-  // *EQUIVALENT* globals that can be merged at link-time. However, the
-  // semantic of *EQUIVALENT*-functions includes parameters. Changing
-  // parameters breaks this assumption.
-  //
-  if (Fn.isWeakForLinker())
-    return false;
-
   if (Fn.use_empty())
     return false;
 
   SmallVector<unsigned, 8> UnusedArgs;
   for (Function::arg_iterator I = Fn.arg_begin(), E = Fn.arg_end(); 
        I != E; ++I) {
     Argument *Arg = I;