[MergeFunctions] Remove alias support

[whitequark]

Summary

The alias support was dead code since 2011. It was last touched
in r124182, where it was reintroduced after being removed
in r110434, and since then it was gated behind a HasGlobalAliases
flag that was permanently stuck as false.

It is also broken. I'm not sure if it bitrotted or was just broken
in the first place because it appears to have never been tested,
but the following IR results in a crash:

define internal i32 @a(i32 %a, i32 %b) unnamed_addr {
  %c = add i32 %a, %b
  %d = xor i32 %a, %c
  ret i32 %c
}

define internal i32 @b(i32 %a, i32 %b) unnamed_addr {
  %c = add i32 %a, %b
  %d = xor i32 %a, %c
  ret i32 %c
}

It seems safe to remove buggy untested code that no one cared about
for seven years.

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

 #include <vector>
 
 using namespace llvm;
 
 #define DEBUG_TYPE "mergefunc"
 
 STATISTIC(NumFunctionsMerged, "Number of functions merged");
 STATISTIC(NumThunksWritten, "Number of thunks generated");
-STATISTIC(NumAliasesWritten, "Number of aliases generated");
 STATISTIC(NumDoubleWeak, "Number of new functions created");
 
 static cl::opt<unsigned> NumFunctionsForSanityCheck(
     "mergefunc-sanity",
     cl::desc("How many functions in module could be used for "
              "MergeFunctions pass sanity check. "
              "'0' disables this check. Works only with '-debug' key."),
     cl::init(0), cl::Hidden);

 /// by considering all pointer types to be equivalent. Once identified,
 /// MergeFunctions will fold them by replacing a call to one to a call to a
 /// bitcast of the other.
 ///
 class MergeFunctions : public ModulePass {
 public:
   static char ID;
   MergeFunctions()
-    : ModulePass(ID), FnTree(FunctionNodeCmp(&GlobalNumbers)), FNodesInTree(),
-      HasGlobalAliases(false) {
+    : ModulePass(ID), FnTree(FunctionNodeCmp(&GlobalNumbers)), FNodesInTree() {
     initializeMergeFunctionsPass(*PassRegistry::getPassRegistry());
   }
 
   bool runOnModule(Module &M) override;
 
 private:
   // The function comparison operator is provided here so that FunctionNodes do
   // not need to become larger with another pointer.
#endif
   /// necessary to make types match.
   void replaceDirectCallers(Function *Old, Function *New);
 
   /// Merge two equivalent functions. Upon completion, G may be deleted, or may
   /// be converted into a thunk. In either case, it should never be visited
   /// again.
   void mergeTwoFunctions(Function *F, Function *G);
 
-  /// Replace G with a thunk or an alias to F. Deletes G.
-  void writeThunkOrAlias(Function *F, Function *G);
-
   /// Fill PDIUnrelatedWL with instructions from the entry block that are
   /// unrelated to parameter related debug info.
   void filterInstsUnrelatedToPDI(BasicBlock *GEntryBlock,
                                  std::vector<Instruction *> &PDIUnrelatedWL);
 
   /// Erase the rest of the CFG (i.e. barring the entry block).
   void eraseTail(Function *G);
 
   /// Erase the instructions in PDIUnrelatedWL as they are unrelated to the
   /// parameter debug info, from the entry block.
   void eraseInstsUnrelatedToPDI(std::vector<Instruction *> &PDIUnrelatedWL);
 
   /// Replace G with a simple tail call to bitcast(F). Also (unless
   /// MergeFunctionsPDI holds) replace direct uses of G with bitcast(F),
   /// delete G.
   void writeThunk(Function *F, Function *G);
 
-  /// Replace G with an alias to F. Deletes G.
-  void writeAlias(Function *F, Function *G);
-
   /// Replace function F with function G in the function tree.
   void replaceFunctionInTree(const FunctionNode &FN, Function *G);
 
   /// The set of all distinct functions. Use the insert() and remove() methods
   /// to modify it. The map allows efficient lookup and deferring of Functions.
   FnTreeType FnTree;
   // Map functions to the iterators of the FunctionNode which contains them
   // in the FnTree. This must be updated carefully whenever the FnTree is
   // modified, i.e. in insert(), remove(), and replaceFunctionInTree(), to avoid
   // dangling iterators into FnTree. The invariant that preserves this is that
   // there is exactly one mapping F -> FN for each FunctionNode FN in FnTree.
   ValueMap<Function*, FnTreeType::iterator> FNodesInTree;
-
-  /// Whether or not the target supports global aliases.
-  bool HasGlobalAliases;
 };
 
 } // end anonymous namespace
 
 char MergeFunctions::ID = 0;
 INITIALIZE_PASS(MergeFunctions, "mergefunc", "Merge Functions", false, false)
 
 ModulePass *llvm::createMergeFunctionsPass() {
if (CS && CS.isCallee(U)) {
                                           NewArgAttrs));
 
       remove(CS.getInstruction()->getParent()->getParent());
       U->set(BitcastNew);
     }
   }
 }
 
-// Replace G with an alias to F if possible, or else a thunk to F. Deletes G.
-void MergeFunctions::writeThunkOrAlias(Function *F, Function *G) {
-  if (HasGlobalAliases && G->hasGlobalUnnamedAddr()) {
-    if (G->hasExternalLinkage() || G->hasLocalLinkage() ||
-        G->hasWeakLinkage()) {
-      writeAlias(F, G);
-      return;
-    }
-  }
-
-  writeThunk(F, G);
-}
-
 // Helper for writeThunk,
 // Selects proper bitcast operation,
 // but a bit simpler then CastInst::getCastOpcode.
 static Value *createCast(IRBuilder<> &Builder, Value *V, Type *DestTy) {
   Type *SrcTy = V->getType();
   if (SrcTy->isStructTy()) {
     assert(DestTy->isStructTy());
     assert(SrcTy->getStructNumElements() == DestTy->getStructNumElements());
if (MergeFunctionsPDI) {
     G->replaceAllUsesWith(NewG);
     G->eraseFromParent();
   }
 
   DEBUG(dbgs() << "writeThunk: " << H->getName() << '\n');
   ++NumThunksWritten;
 }
 
-// Replace G with an alias to F and delete G.
-void MergeFunctions::writeAlias(Function *F, Function *G) {
-  auto *GA = GlobalAlias::create(G->getLinkage(), "", F);
-  F->setAlignment(std::max(F->getAlignment(), G->getAlignment()));
-  GA->takeName(G);
-  GA->setVisibility(G->getVisibility());
-  removeUsers(G);
-  G->replaceAllUsesWith(GA);
-  G->eraseFromParent();
-
-  DEBUG(dbgs() << "writeAlias: " << GA->getName() << '\n');
-  ++NumAliasesWritten;
-}
-
 // Merge two equivalent functions. Upon completion, Function G is deleted.
 void MergeFunctions::mergeTwoFunctions(Function *F, Function *G) {
   if (F->isInterposable()) {
     assert(G->isInterposable());
 
     // Make them both thunks to the same internal function.
     Function *H = Function::Create(F->getFunctionType(), F->getLinkage(), "",
                                    F->getParent());
     H->copyAttributesFrom(F);
     H->takeName(F);
     removeUsers(F);
     F->replaceAllUsesWith(H);
 
     unsigned MaxAlignment = std::max(G->getAlignment(), H->getAlignment());
 
-    if (HasGlobalAliases) {
-      writeAlias(F, G);
-      writeAlias(F, H);
-    } else {
-      writeThunk(F, G);
-      writeThunk(F, H);
-    }
 
     F->setAlignment(MaxAlignment);
     F->setLinkage(GlobalValue::PrivateLinkage);
     ++NumDoubleWeak;
   } else {
-    writeThunkOrAlias(F, G);
+    writeThunk(F, G);
   }
 
   ++NumFunctionsMerged;
 }
 
 /// Replace function F by function G.
 void MergeFunctions::replaceFunctionInTree(const FunctionNode &FN,
                                            Function *G) {