1 files changed, 449 insertions, 0 deletions
diff --git a/llvm/lib/Transforms/IPO/GlobalDCE.cpp b/llvm/lib/Transforms/IPO/GlobalDCE.cpp
new file mode 100644
index 000000000000..f010f7b703a6
--- /dev/null
+++ b/llvm/lib/Transforms/IPO/GlobalDCE.cpp
@@ -0,0 +1,449 @@
+//===-- GlobalDCE.cpp - DCE unreachable internal functions ----------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This transform is designed to eliminate unreachable internal globals from the
+// program.  It uses an aggressive algorithm, searching out globals that are
+// known to be alive.  After it finds all of the globals which are needed, it
+// deletes whatever is left over.  This allows it to delete recursive chunks of
+// the program which are unreachable.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/IPO/GlobalDCE.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/TypeMetadataUtils.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/IntrinsicInst.h"
+#include "llvm/IR/Module.h"
+#include "llvm/IR/Operator.h"
+#include "llvm/Pass.h"
+#include "llvm/Transforms/IPO.h"
+#include "llvm/Transforms/Utils/CtorUtils.h"
+#include "llvm/Transforms/Utils/GlobalStatus.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "globaldce"
+
+static cl::opt<bool>
+    ClEnableVFE("enable-vfe", cl::Hidden, cl::init(true), cl::ZeroOrMore,
+                cl::desc("Enable virtual function elimination"));
+
+STATISTIC(NumAliases  , "Number of global aliases removed");
+STATISTIC(NumFunctions, "Number of functions removed");
+STATISTIC(NumIFuncs,    "Number of indirect functions removed");
+STATISTIC(NumVariables, "Number of global variables removed");
+STATISTIC(NumVFuncs,    "Number of virtual functions removed");
+
+namespace {
+  class GlobalDCELegacyPass : public ModulePass {
+  public:
+    static char ID; // Pass identification, replacement for typeid
+    GlobalDCELegacyPass() : ModulePass(ID) {
+      initializeGlobalDCELegacyPassPass(*PassRegistry::getPassRegistry());
+    }
+
+    // run - Do the GlobalDCE pass on the specified module, optionally updating
+    // the specified callgraph to reflect the changes.
+    //
+    bool runOnModule(Module &M) override {
+      if (skipModule(M))
+        return false;
+
+      // We need a minimally functional dummy module analysis manager. It needs
+      // to at least know about the possibility of proxying a function analysis
+      // manager.
+      FunctionAnalysisManager DummyFAM;
+      ModuleAnalysisManager DummyMAM;
+      DummyMAM.registerPass(
+          [&] { return FunctionAnalysisManagerModuleProxy(DummyFAM); });
+
+      auto PA = Impl.run(M, DummyMAM);
+      return !PA.areAllPreserved();
+    }
+
+  private:
+    GlobalDCEPass Impl;
+  };
+}
+
+char GlobalDCELegacyPass::ID = 0;
+INITIALIZE_PASS(GlobalDCELegacyPass, "globaldce",
+                "Dead Global Elimination", false, false)
+
+// Public interface to the GlobalDCEPass.
+ModulePass *llvm::createGlobalDCEPass() {
+  return new GlobalDCELegacyPass();
+}
+
+/// Returns true if F is effectively empty.
+static bool isEmptyFunction(Function *F) {
+  BasicBlock &Entry = F->getEntryBlock();
+  for (auto &I : Entry) {
+    if (isa<DbgInfoIntrinsic>(I))
+      continue;
+    if (auto *RI = dyn_cast<ReturnInst>(&I))
+      return !RI->getReturnValue();
+    break;
+  }
+  return false;
+}
+
+/// Compute the set of GlobalValue that depends from V.
+/// The recursion stops as soon as a GlobalValue is met.
+void GlobalDCEPass::ComputeDependencies(Value *V,
+                                        SmallPtrSetImpl<GlobalValue *> &Deps) {
+  if (auto *I = dyn_cast<Instruction>(V)) {
+    Function *Parent = I->getParent()->getParent();
+    Deps.insert(Parent);
+  } else if (auto *GV = dyn_cast<GlobalValue>(V)) {
+    Deps.insert(GV);
+  } else if (auto *CE = dyn_cast<Constant>(V)) {
+    // Avoid walking the whole tree of a big ConstantExprs multiple times.
+    auto Where = ConstantDependenciesCache.find(CE);
+    if (Where != ConstantDependenciesCache.end()) {
+      auto const &K = Where->second;
+      Deps.insert(K.begin(), K.end());
+    } else {
+      SmallPtrSetImpl<GlobalValue *> &LocalDeps = ConstantDependenciesCache[CE];
+      for (User *CEUser : CE->users())
+        ComputeDependencies(CEUser, LocalDeps);
+      Deps.insert(LocalDeps.begin(), LocalDeps.end());
+    }
+  }
+}
+
+void GlobalDCEPass::UpdateGVDependencies(GlobalValue &GV) {
+  SmallPtrSet<GlobalValue *, 8> Deps;
+  for (User *User : GV.users())
+    ComputeDependencies(User, Deps);
+  Deps.erase(&GV); // Remove self-reference.
+  for (GlobalValue *GVU : Deps) {
+    // If this is a dep from a vtable to a virtual function, and we have
+    // complete information about all virtual call sites which could call
+    // though this vtable, then skip it, because the call site information will
+    // be more precise.
+    if (VFESafeVTables.count(GVU) && isa<Function>(&GV)) {
+      LLVM_DEBUG(dbgs() << "Ignoring dep " << GVU->getName() << " -> "
+                        << GV.getName() << "\n");
+      continue;
+    }
+    GVDependencies[GVU].insert(&GV);
+  }
+}
+
+/// Mark Global value as Live
+void GlobalDCEPass::MarkLive(GlobalValue &GV,
+                             SmallVectorImpl<GlobalValue *> *Updates) {
+  auto const Ret = AliveGlobals.insert(&GV);
+  if (!Ret.second)
+    return;
+
+  if (Updates)
+    Updates->push_back(&GV);
+  if (Comdat *C = GV.getComdat()) {
+    for (auto &&CM : make_range(ComdatMembers.equal_range(C))) {
+      MarkLive(*CM.second, Updates); // Recursion depth is only two because only
+                                     // globals in the same comdat are visited.
+    }
+  }
+}
+
+void GlobalDCEPass::ScanVTables(Module &M) {
+  SmallVector<MDNode *, 2> Types;
+  LLVM_DEBUG(dbgs() << "Building type info -> vtable map\n");
+
+  auto *LTOPostLinkMD =
+      cast_or_null<ConstantAsMetadata>(M.getModuleFlag("LTOPostLink"));
+  bool LTOPostLink =
+      LTOPostLinkMD &&
+      (cast<ConstantInt>(LTOPostLinkMD->getValue())->getZExtValue() != 0);
+
+  for (GlobalVariable &GV : M.globals()) {
+    Types.clear();
+    GV.getMetadata(LLVMContext::MD_type, Types);
+    if (GV.isDeclaration() || Types.empty())
+      continue;
+
+    // Use the typeid metadata on the vtable to build a mapping from typeids to
+    // the list of (GV, offset) pairs which are the possible vtables for that
+    // typeid.
+    for (MDNode *Type : Types) {
+      Metadata *TypeID = Type->getOperand(1).get();
+
+      uint64_t Offset =
+          cast<ConstantInt>(
+              cast<ConstantAsMetadata>(Type->getOperand(0))->getValue())
+              ->getZExtValue();
+
+      TypeIdMap[TypeID].insert(std::make_pair(&GV, Offset));
+    }
+
+    // If the type corresponding to the vtable is private to this translation
+    // unit, we know that we can see all virtual functions which might use it,
+    // so VFE is safe.
+    if (auto GO = dyn_cast<GlobalObject>(&GV)) {
+      GlobalObject::VCallVisibility TypeVis = GO->getVCallVisibility();
+      if (TypeVis == GlobalObject::VCallVisibilityTranslationUnit ||
+          (LTOPostLink &&
+           TypeVis == GlobalObject::VCallVisibilityLinkageUnit)) {
+        LLVM_DEBUG(dbgs() << GV.getName() << " is safe for VFE\n");
+        VFESafeVTables.insert(&GV);
+      }
+    }
+  }
+}
+
+void GlobalDCEPass::ScanVTableLoad(Function *Caller, Metadata *TypeId,
+                                   uint64_t CallOffset) {
+  for (auto &VTableInfo : TypeIdMap[TypeId]) {
+    GlobalVariable *VTable = VTableInfo.first;
+    uint64_t VTableOffset = VTableInfo.second;
+
+    Constant *Ptr =
+        getPointerAtOffset(VTable->getInitializer(), VTableOffset + CallOffset,
+                           *Caller->getParent());
+    if (!Ptr) {
+      LLVM_DEBUG(dbgs() << "can't find pointer in vtable!\n");
+      VFESafeVTables.erase(VTable);
+      return;
+    }
+
+    auto Callee = dyn_cast<Function>(Ptr->stripPointerCasts());
+    if (!Callee) {
+      LLVM_DEBUG(dbgs() << "vtable entry is not function pointer!\n");
+      VFESafeVTables.erase(VTable);
+      return;
+    }
+
+    LLVM_DEBUG(dbgs() << "vfunc dep " << Caller->getName() << " -> "
+                      << Callee->getName() << "\n");
+    GVDependencies[Caller].insert(Callee);
+  }
+}
+
+void GlobalDCEPass::ScanTypeCheckedLoadIntrinsics(Module &M) {
+  LLVM_DEBUG(dbgs() << "Scanning type.checked.load intrinsics\n");
+  Function *TypeCheckedLoadFunc =
+      M.getFunction(Intrinsic::getName(Intrinsic::type_checked_load));
+
+  if (!TypeCheckedLoadFunc)
+    return;
+
+  for (auto U : TypeCheckedLoadFunc->users()) {
+    auto CI = dyn_cast<CallInst>(U);
+    if (!CI)
+      continue;
+
+    auto *Offset = dyn_cast<ConstantInt>(CI->getArgOperand(1));
+    Value *TypeIdValue = CI->getArgOperand(2);
+    auto *TypeId = cast<MetadataAsValue>(TypeIdValue)->getMetadata();
+
+    if (Offset) {
+      ScanVTableLoad(CI->getFunction(), TypeId, Offset->getZExtValue());
+    } else {
+      // type.checked.load with a non-constant offset, so assume every entry in
+      // every matching vtable is used.
+      for (auto &VTableInfo : TypeIdMap[TypeId]) {
+        VFESafeVTables.erase(VTableInfo.first);
+      }
+    }
+  }
+}
+
+void GlobalDCEPass::AddVirtualFunctionDependencies(Module &M) {
+  if (!ClEnableVFE)
+    return;
+
+  ScanVTables(M);
+
+  if (VFESafeVTables.empty())
+    return;
+
+  ScanTypeCheckedLoadIntrinsics(M);
+
+  LLVM_DEBUG(
+    dbgs() << "VFE safe vtables:\n";
+    for (auto *VTable : VFESafeVTables)
+      dbgs() << "  " << VTable->getName() << "\n";
+  );
+}
+
+PreservedAnalyses GlobalDCEPass::run(Module &M, ModuleAnalysisManager &MAM) {
+  bool Changed = false;
+
+  // The algorithm first computes the set L of global variables that are
+  // trivially live.  Then it walks the initialization of these variables to
+  // compute the globals used to initialize them, which effectively builds a
+  // directed graph where nodes are global variables, and an edge from A to B
+  // means B is used to initialize A.  Finally, it propagates the liveness
+  // information through the graph starting from the nodes in L. Nodes note
+  // marked as alive are discarded.
+
+  // Remove empty functions from the global ctors list.
+  Changed |= optimizeGlobalCtorsList(M, isEmptyFunction);
+
+  // Collect the set of members for each comdat.
+  for (Function &F : M)
+    if (Comdat *C = F.getComdat())
+      ComdatMembers.insert(std::make_pair(C, &F));
+  for (GlobalVariable &GV : M.globals())
+    if (Comdat *C = GV.getComdat())
+      ComdatMembers.insert(std::make_pair(C, &GV));
+  for (GlobalAlias &GA : M.aliases())
+    if (Comdat *C = GA.getComdat())
+      ComdatMembers.insert(std::make_pair(C, &GA));
+
+  // Add dependencies between virtual call sites and the virtual functions they
+  // might call, if we have that information.
+  AddVirtualFunctionDependencies(M);
+
+  // Loop over the module, adding globals which are obviously necessary.
+  for (GlobalObject &GO : M.global_objects()) {
+    Changed |= RemoveUnusedGlobalValue(GO);
+    // Functions with external linkage are needed if they have a body.
+    // Externally visible & appending globals are needed, if they have an
+    // initializer.
+    if (!GO.isDeclaration())
+      if (!GO.isDiscardableIfUnused())
+        MarkLive(GO);
+
+    UpdateGVDependencies(GO);
+  }
+
+  // Compute direct dependencies of aliases.
+  for (GlobalAlias &GA : M.aliases()) {
+    Changed |= RemoveUnusedGlobalValue(GA);
+    // Externally visible aliases are needed.
+    if (!GA.isDiscardableIfUnused())
+      MarkLive(GA);
+
+    UpdateGVDependencies(GA);
+  }
+
+  // Compute direct dependencies of ifuncs.
+  for (GlobalIFunc &GIF : M.ifuncs()) {
+    Changed |= RemoveUnusedGlobalValue(GIF);
+    // Externally visible ifuncs are needed.
+    if (!GIF.isDiscardableIfUnused())
+      MarkLive(GIF);
+
+    UpdateGVDependencies(GIF);
+  }
+
+  // Propagate liveness from collected Global Values through the computed
+  // dependencies.
+  SmallVector<GlobalValue *, 8> NewLiveGVs{AliveGlobals.begin(),
+                                           AliveGlobals.end()};
+  while (!NewLiveGVs.empty()) {
+    GlobalValue *LGV = NewLiveGVs.pop_back_val();
+    for (auto *GVD : GVDependencies[LGV])
+      MarkLive(*GVD, &NewLiveGVs);
+  }
+
+  // Now that all globals which are needed are in the AliveGlobals set, we loop
+  // through the program, deleting those which are not alive.
+  //
+
+  // The first pass is to drop initializers of global variables which are dead.
+  std::vector<GlobalVariable *> DeadGlobalVars; // Keep track of dead globals
+  for (GlobalVariable &GV : M.globals())
+    if (!AliveGlobals.count(&GV)) {
+      DeadGlobalVars.push_back(&GV);         // Keep track of dead globals
+      if (GV.hasInitializer()) {
+        Constant *Init = GV.getInitializer();
+        GV.setInitializer(nullptr);
+        if (isSafeToDestroyConstant(Init))
+          Init->destroyConstant();
+      }
+    }
+
+  // The second pass drops the bodies of functions which are dead...
+  std::vector<Function *> DeadFunctions;
+  for (Function &F : M)
+    if (!AliveGlobals.count(&F)) {
+      DeadFunctions.push_back(&F);         // Keep track of dead globals
+      if (!F.isDeclaration())
+        F.deleteBody();
+    }
+
+  // The third pass drops targets of aliases which are dead...
+  std::vector<GlobalAlias*> DeadAliases;
+  for (GlobalAlias &GA : M.aliases())
+    if (!AliveGlobals.count(&GA)) {
+      DeadAliases.push_back(&GA);
+      GA.setAliasee(nullptr);
+    }
+
+  // The fourth pass drops targets of ifuncs which are dead...
+  std::vector<GlobalIFunc*> DeadIFuncs;
+  for (GlobalIFunc &GIF : M.ifuncs())
+    if (!AliveGlobals.count(&GIF)) {
+      DeadIFuncs.push_back(&GIF);
+      GIF.setResolver(nullptr);
+    }
+
+  // Now that all interferences have been dropped, delete the actual objects
+  // themselves.
+  auto EraseUnusedGlobalValue = [&](GlobalValue *GV) {
+    RemoveUnusedGlobalValue(*GV);
+    GV->eraseFromParent();
+    Changed = true;
+  };
+
+  NumFunctions += DeadFunctions.size();
+  for (Function *F : DeadFunctions) {
+    if (!F->use_empty()) {
+      // Virtual functions might still be referenced by one or more vtables,
+      // but if we've proven them to be unused then it's safe to replace the
+      // virtual function pointers with null, allowing us to remove the
+      // function itself.
+      ++NumVFuncs;
+      F->replaceNonMetadataUsesWith(ConstantPointerNull::get(F->getType()));
+    }
+    EraseUnusedGlobalValue(F);
+  }
+
+  NumVariables += DeadGlobalVars.size();
+  for (GlobalVariable *GV : DeadGlobalVars)
+    EraseUnusedGlobalValue(GV);
+
+  NumAliases += DeadAliases.size();
+  for (GlobalAlias *GA : DeadAliases)
+    EraseUnusedGlobalValue(GA);
+
+  NumIFuncs += DeadIFuncs.size();
+  for (GlobalIFunc *GIF : DeadIFuncs)
+    EraseUnusedGlobalValue(GIF);
+
+  // Make sure that all memory is released
+  AliveGlobals.clear();
+  ConstantDependenciesCache.clear();
+  GVDependencies.clear();
+  ComdatMembers.clear();
+  TypeIdMap.clear();
+  VFESafeVTables.clear();
+
+  if (Changed)
+    return PreservedAnalyses::none();
+  return PreservedAnalyses::all();
+}
+
+// RemoveUnusedGlobalValue - Loop over all of the uses of the specified
+// GlobalValue, looking for the constant pointer ref that may be pointing to it.
+// If found, check to see if the constant pointer ref is safe to destroy, and if
+// so, nuke it.  This will reduce the reference count on the global value, which
+// might make it deader.
+//
+bool GlobalDCEPass::RemoveUnusedGlobalValue(GlobalValue &GV) {
+  if (GV.use_empty())
+    return false;
+  GV.removeDeadConstantUsers();
+  return GV.use_empty();
+}