[IR] Refactor GlobalIFunc to inherit from GlobalObject, Remove GlobalIndirectSymbol

As discussed in:
* https://reviews.llvm.org/D94166
* https://lists.llvm.org/pipermail/llvm-dev/2020-September/145031.html

The GlobalIndirectSymbol class lost most of its meaning in
https://reviews.llvm.org/D109792, which disambiguated getBaseObject
(now getAliaseeObject) between GlobalIFunc and everything else.
In addition, as long as GlobalIFunc is not a GlobalObject and
getAliaseeObject returns GlobalObjects, a GlobalAlias whose aliasee
is a GlobalIFunc cannot currently be modeled properly. Creating
aliases for GlobalIFuncs does happen in the wild (e.g. glibc). In addition,
calling getAliaseeObject on a GlobalIFunc will currently return nullptr,
which is undesirable because it should return the object itself for
non-aliases.

This patch refactors the GlobalIFunc class to inherit directly from
GlobalObject, and removes GlobalIndirectSymbol (while inlining the
relevant parts into GlobalAlias and GlobalIFunc). This allows for
calling getAliaseeObject() on a GlobalIFunc to return the GlobalIFunc
itself, making getAliaseeObject() more consistent and enabling
alias-to-ifunc to be properly modeled in the IR.

I exercised some judgement in the API clients of GlobalIndirectSymbol:
some were 'monomorphized' for GlobalAlias and GlobalIFunc, and
some remained shared (with the type adapted to become GlobalValue).

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D108872

Author: ibookstein

clang/lib/CodeGen/CodeGenModule.cpp

@@ -317,21 +317,21 @@ void CodeGenModule::applyGlobalValReplacements() {
 
 // This is only used in aliases that we created and we know they have a
 // linear structure.
-static const llvm::GlobalObject *getAliasedGlobal(
-    const llvm::GlobalIndirectSymbol &GIS) {
-  llvm::SmallPtrSet<const llvm::GlobalIndirectSymbol*, 4> Visited;
-  const llvm::Constant *C = &GIS;
+static const llvm::GlobalValue *getAliasedGlobal(const llvm::GlobalValue *GV) {
+  llvm::SmallPtrSet<const llvm::GlobalValue *, 4> Visited;
   for (;;) {
-    C = C->stripPointerCasts();
-    if (auto *GO = dyn_cast<llvm::GlobalObject>(C))
-      return GO;
-    // stripPointerCasts will not walk over weak aliases.
-    auto *GIS2 = dyn_cast<llvm::GlobalIndirectSymbol>(C);
-    if (!GIS2)
+    if (!GV || !Visited.insert(GV).second)
       return nullptr;
-    if (!Visited.insert(GIS2).second)
-      return nullptr;
-    C = GIS2->getIndirectSymbol();
+
+    const llvm::Constant *C;
+    if (auto *GA = dyn_cast<llvm::GlobalAlias>(GV))
+      C = GA->getAliasee();
+    else if (auto *GI = dyn_cast<llvm::GlobalIFunc>(GV))
+      C = GI->getResolver();
+    else
+      return GV;
+
+    GV = dyn_cast<llvm::GlobalValue>(C->stripPointerCasts());
   }
 }
 
@@ -350,9 +350,8 @@ void CodeGenModule::checkAliases() {
     else
       llvm_unreachable("Not an alias or ifunc?");
     StringRef MangledName = getMangledName(GD);
-    llvm::GlobalValue *Entry = GetGlobalValue(MangledName);
-    auto *Alias  = cast<llvm::GlobalIndirectSymbol>(Entry);
-    const llvm::GlobalValue *GV = getAliasedGlobal(*Alias);
+    llvm::GlobalValue *Alias = GetGlobalValue(MangledName);
+    const llvm::GlobalValue *GV = getAliasedGlobal(Alias);
     if (!GV) {
       Error = true;
       Diags.Report(Location, diag::err_cyclic_alias) << IsIFunc;
@@ -369,7 +368,10 @@ void CodeGenModule::checkAliases() {
         Diags.Report(Location, diag::err_ifunc_resolver_return);
     }
 
-    llvm::Constant *Aliasee = Alias->getIndirectSymbol();
+    llvm::Constant *Aliasee =
+        IsIFunc ? cast<llvm::GlobalIFunc>(Alias)->getResolver()
+                : cast<llvm::GlobalAlias>(Alias)->getAliasee();
+
     llvm::GlobalValue *AliaseeGV;
     if (auto CE = dyn_cast<llvm::ConstantExpr>(Aliasee))
       AliaseeGV = cast<llvm::GlobalValue>(CE->getOperand(0));
@@ -388,13 +390,17 @@ void CodeGenModule::checkAliases() {
     // compatibility with gcc we implement it by just pointing the alias
     // to its aliasee's aliasee. We also warn, since the user is probably
     // expecting the link to be weak.
-    if (auto GA = dyn_cast<llvm::GlobalIndirectSymbol>(AliaseeGV)) {
+    if (auto *GA = dyn_cast<llvm::GlobalAlias>(AliaseeGV)) {
       if (GA->isInterposable()) {
         Diags.Report(Location, diag::warn_alias_to_weak_alias)
             << GV->getName() << GA->getName() << IsIFunc;
         Aliasee = llvm::ConstantExpr::getPointerBitCastOrAddrSpaceCast(
-            GA->getIndirectSymbol(), Alias->getType());
-        Alias->setIndirectSymbol(Aliasee);
+            GA->getAliasee(), Alias->getType());
+
+        if (IsIFunc)
+          cast<llvm::GlobalIFunc>(Alias)->setResolver(Aliasee);
+        else
+          cast<llvm::GlobalAlias>(Alias)->setAliasee(Aliasee);
       }
     }
   }
@@ -403,8 +409,7 @@ void CodeGenModule::checkAliases() {
 
   for (const GlobalDecl &GD : Aliases) {
     StringRef MangledName = getMangledName(GD);
-    llvm::GlobalValue *Entry = GetGlobalValue(MangledName);
-    auto *Alias = cast<llvm::GlobalIndirectSymbol>(Entry);
+    llvm::GlobalValue *Alias = GetGlobalValue(MangledName);
     Alias->replaceAllUsesWith(llvm::UndefValue::get(Alias->getType()));
     Alias->eraseFromParent();
   }

llvm/include/llvm-c/Core.h

@@ -1580,10 +1580,10 @@ LLVMTypeRef LLVMX86AMXType(void);
       macro(ConstantVector)                 \
       macro(GlobalValue)                    \
         macro(GlobalAlias)                  \
-        macro(GlobalIFunc)                  \
         macro(GlobalObject)                 \
           macro(Function)                   \
           macro(GlobalVariable)             \
+          macro(GlobalIFunc)                \
       macro(UndefValue)                     \
       macro(PoisonValue)                    \
     macro(Instruction)                      \

llvm/include/llvm/AsmParser/LLParser.h

@@ -304,11 +304,10 @@ namespace llvm {
                      unsigned DLLStorageClass, bool DSOLocal,
                      GlobalVariable::ThreadLocalMode TLM,
                      GlobalVariable::UnnamedAddr UnnamedAddr);
-    bool parseIndirectSymbol(const std::string &Name, LocTy NameLoc,
-                             unsigned L, unsigned Visibility,
-                             unsigned DLLStorageClass, bool DSOLocal,
-                             GlobalVariable::ThreadLocalMode TLM,
-                             GlobalVariable::UnnamedAddr UnnamedAddr);
+    bool parseAliasOrIFunc(const std::string &Name, LocTy NameLoc, unsigned L,
+                           unsigned Visibility, unsigned DLLStorageClass,
+                           bool DSOLocal, GlobalVariable::ThreadLocalMode TLM,
+                           GlobalVariable::UnnamedAddr UnnamedAddr);
     bool parseComdat();
     bool parseStandaloneMetadata();
     bool parseNamedMetadata();

llvm/include/llvm/CodeGen/AsmPrinter.h

@@ -41,7 +41,6 @@ class DIEAbbrev;
 class DwarfDebug;
 class GCMetadataPrinter;
 class GCStrategy;
-class GlobalIndirectSymbol;
 class GlobalObject;
 class GlobalValue;
 class GlobalVariable;
@@ -795,8 +794,8 @@ class AsmPrinter : public MachineFunctionPass {
   void emitModuleCommandLines(Module &M);
 
   GCMetadataPrinter *GetOrCreateGCPrinter(GCStrategy &S);
-  /// Emit GlobalAlias or GlobalIFunc.
-  void emitGlobalIndirectSymbol(Module &M, const GlobalIndirectSymbol &GIS);
+  void emitGlobalAlias(Module &M, const GlobalAlias &GA);
+  void emitGlobalIFunc(Module &M, const GlobalIFunc &GI);
 
   /// This method decides whether the specified basic block requires a label.
   bool shouldEmitLabelForBasicBlock(const MachineBasicBlock &MBB) const;

llvm/include/llvm/IR/GlobalAlias.h

@@ -15,7 +15,8 @@
 #define LLVM_IR_GLOBALALIAS_H
 
 #include "llvm/ADT/ilist_node.h"
-#include "llvm/IR/GlobalIndirectSymbol.h"
+#include "llvm/IR/GlobalValue.h"
+#include "llvm/IR/OperandTraits.h"
 #include "llvm/IR/Value.h"
 
 namespace llvm {
@@ -24,8 +25,7 @@ class Twine;
 class Module;
 template <typename ValueSubClass> class SymbolTableListTraits;
 
-class GlobalAlias : public GlobalIndirectSymbol,
-                    public ilist_node<GlobalAlias> {
+class GlobalAlias : public GlobalValue, public ilist_node<GlobalAlias> {
   friend class SymbolTableListTraits<GlobalAlias>;
 
   GlobalAlias(Type *Ty, unsigned AddressSpace, LinkageTypes Linkage,
@@ -58,6 +58,17 @@ class GlobalAlias : public GlobalIndirectSymbol,
   // Linkage, Type, Parent and AddressSpace taken from the Aliasee.
   static GlobalAlias *create(const Twine &Name, GlobalValue *Aliasee);
 
+  // allocate space for exactly one operand
+  void *operator new(size_t S) { return User::operator new(S, 1); }
+  void operator delete(void *Ptr) { User::operator delete(Ptr); }
+
+  /// Provide fast operand accessors
+  DECLARE_TRANSPARENT_OPERAND_ACCESSORS(Constant);
+
+  void copyAttributesFrom(const GlobalAlias *Src) {
+    GlobalValue::copyAttributesFrom(Src);
+  }
+
   /// removeFromParent - This method unlinks 'this' from the containing module,
   /// but does not delete it.
   ///
@@ -71,11 +82,9 @@ class GlobalAlias : public GlobalIndirectSymbol,
   /// These methods retrieve and set alias target.
   void setAliasee(Constant *Aliasee);
   const Constant *getAliasee() const {
-    return getIndirectSymbol();
-  }
-  Constant *getAliasee() {
-    return getIndirectSymbol();
+    return static_cast<Constant *>(Op<0>().get());
   }
+  Constant *getAliasee() { return static_cast<Constant *>(Op<0>().get()); }
 
   const GlobalObject *getAliaseeObject() const;
   GlobalObject *getAliaseeObject() {
@@ -94,6 +103,12 @@ class GlobalAlias : public GlobalIndirectSymbol,
   }
 };
 
+template <>
+struct OperandTraits<GlobalAlias>
+    : public FixedNumOperandTraits<GlobalAlias, 1> {};
+
+DEFINE_TRANSPARENT_OPERAND_ACCESSORS(GlobalAlias, Constant)
+
 } // end namespace llvm
 
 #endif // LLVM_IR_GLOBALALIAS_H

llvm/include/llvm/IR/GlobalIFunc.h

@@ -18,7 +18,9 @@
 #define LLVM_IR_GLOBALIFUNC_H
 
 #include "llvm/ADT/ilist_node.h"
-#include "llvm/IR/GlobalIndirectSymbol.h"
+#include "llvm/IR/Constant.h"
+#include "llvm/IR/GlobalObject.h"
+#include "llvm/IR/OperandTraits.h"
 #include "llvm/IR/Value.h"
 
 namespace llvm {
@@ -29,8 +31,7 @@ class Module;
 // Traits class for using GlobalIFunc in symbol table in Module.
 template <typename ValueSubClass> class SymbolTableListTraits;
 
-class GlobalIFunc final : public GlobalIndirectSymbol,
-                          public ilist_node<GlobalIFunc> {
+class GlobalIFunc final : public GlobalObject, public ilist_node<GlobalIFunc> {
   friend class SymbolTableListTraits<GlobalIFunc>;
 
   GlobalIFunc(Type *Ty, unsigned AddressSpace, LinkageTypes Linkage,
@@ -46,6 +47,17 @@ class GlobalIFunc final : public GlobalIndirectSymbol,
                              LinkageTypes Linkage, const Twine &Name,
                              Constant *Resolver, Module *Parent);
 
+  // allocate space for exactly one operand
+  void *operator new(size_t S) { return User::operator new(S, 1); }
+  void operator delete(void *Ptr) { User::operator delete(Ptr); }
+
+  /// Provide fast operand accessors
+  DECLARE_TRANSPARENT_OPERAND_ACCESSORS(Constant);
+
+  void copyAttributesFrom(const GlobalIFunc *Src) {
+    GlobalObject::copyAttributesFrom(Src);
+  }
+
   /// This method unlinks 'this' from the containing module, but does not
   /// delete it.
   void removeFromParent();
@@ -54,11 +66,11 @@ class GlobalIFunc final : public GlobalIndirectSymbol,
   void eraseFromParent();
 
   /// These methods retrieve and set ifunc resolver function.
-  void setResolver(Constant *Resolver) {
-    setIndirectSymbol(Resolver);
+  void setResolver(Constant *Resolver) { Op<0>().set(Resolver); }
+  const Constant *getResolver() const {
+    return static_cast<Constant *>(Op<0>().get());
   }
-  const Constant *getResolver() const { return getIndirectSymbol(); }
-  Constant *getResolver() { return getIndirectSymbol(); }
+  Constant *getResolver() { return static_cast<Constant *>(Op<0>().get()); }
 
   // Return the resolver function after peeling off potential ConstantExpr
   // indirection.
@@ -74,6 +86,12 @@ class GlobalIFunc final : public GlobalIndirectSymbol,
   }
 };
 
+template <>
+struct OperandTraits<GlobalIFunc>
+    : public FixedNumOperandTraits<GlobalIFunc, 1> {};
+
+DEFINE_TRANSPARENT_OPERAND_ACCESSORS(GlobalIFunc, Constant)
+
 } // end namespace llvm
 
 #endif // LLVM_IR_GLOBALIFUNC_H

llvm/include/llvm/IR/GlobalIndirectSymbol.h

@@ -153,7 +153,8 @@ class GlobalObject : public GlobalValue {
   // Methods for support type inquiry through isa, cast, and dyn_cast:
   static bool classof(const Value *V) {
     return V->getValueID() == Value::FunctionVal ||
-           V->getValueID() == Value::GlobalVariableVal;
+           V->getValueID() == Value::GlobalVariableVal ||
+           V->getValueID() == Value::GlobalIFuncVal;
   }
 
 private:

llvm/include/llvm/IR/GlobalObject.h

@@ -37,7 +37,6 @@ class DataLayout;
 class Function;
 class GlobalAlias;
 class GlobalIFunc;
-class GlobalIndirectSymbol;
 class GlobalObject;
 class GlobalValue;
 class GlobalVariable;
@@ -1016,21 +1015,16 @@ template <> struct isa_impl<GlobalIFunc, Value> {
   }
 };
 
-template <> struct isa_impl<GlobalIndirectSymbol, Value> {
-  static inline bool doit(const Value &Val) {
-    return isa<GlobalAlias>(Val) || isa<GlobalIFunc>(Val);
-  }
-};
-
 template <> struct isa_impl<GlobalValue, Value> {
   static inline bool doit(const Value &Val) {
-    return isa<GlobalObject>(Val) || isa<GlobalIndirectSymbol>(Val);
+    return isa<GlobalObject>(Val) || isa<GlobalAlias>(Val);
   }
 };
 
 template <> struct isa_impl<GlobalObject, Value> {
   static inline bool doit(const Value &Val) {
-    return isa<GlobalVariable>(Val) || isa<Function>(Val);
+    return isa<GlobalVariable>(Val) || isa<Function>(Val) ||
+           isa<GlobalIFunc>(Val);
   }
 };