//===-- GlobalDCE.cpp - DCE unreachable internal functions ----------------===//
//
-// This transform is designed to eliminate unreachable internal globals
-// FIXME: GlobalDCE should update the callgraph, not destroy it!
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// 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.h"
-#include "llvm/Module.h"
-#include "llvm/Constants.h"
-#include "llvm/DerivedTypes.h"
-#include "llvm/Analysis/CallGraph.h"
-#include "Support/DepthFirstIterator.h"
-#include "Support/Statistic.h"
-#include <algorithm>
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Transforms/Utils/CtorUtils.h"
+#include "llvm/Transforms/Utils/GlobalStatus.h"
+#include "llvm/Pass.h"
+#include <unordered_map>
+using namespace llvm;
+
+#define DEBUG_TYPE "globaldce"
+
+STATISTIC(NumAliases , "Number of global aliases removed");
+STATISTIC(NumFunctions, "Number of functions removed");
+STATISTIC(NumVariables, "Number of global variables removed");
namespace {
- Statistic<> NumFunctions("globaldce","Number of functions removed");
- Statistic<> NumVariables("globaldce","Number of global variables removed");
- Statistic<> NumCPRs("globaldce", "Number of const pointer refs removed");
- Statistic<> NumConsts("globaldce", "Number of init constants removed");
-
- bool RemoveUnreachableFunctions(Module &M, CallGraph &CallGraph) {
- // Calculate which functions are reachable from the external functions in
- // the call graph.
- //
- std::set<CallGraphNode*> ReachableNodes(df_begin(&CallGraph),
- df_end(&CallGraph));
-
- // Loop over the functions in the module twice. The first time is used to
- // drop references that functions have to each other before they are
- // deleted. The second pass removes the functions that need to be removed.
- //
- std::vector<CallGraphNode*> FunctionsToDelete; // Track unused functions
- for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
- CallGraphNode *N = CallGraph[I];
-
- if (!ReachableNodes.count(N)) { // Not reachable??
- I->dropAllReferences();
- N->removeAllCalledFunctions();
- FunctionsToDelete.push_back(N);
- ++NumFunctions;
- }
+ struct GlobalDCE : public ModulePass {
+ static char ID; // Pass identification, replacement for typeid
+ GlobalDCE() : ModulePass(ID) {
+ initializeGlobalDCEPass(*PassRegistry::getPassRegistry());
}
-
- // Nothing to do if no unreachable functions have been found...
- if (FunctionsToDelete.empty()) return false;
-
- // Unreachable functions have been found and should have no references to
- // them, delete them now.
- //
- for (std::vector<CallGraphNode*>::iterator I = FunctionsToDelete.begin(),
- E = FunctionsToDelete.end(); I != E; ++I)
- delete CallGraph.removeFunctionFromModule(*I);
-
- // Walk the function list, removing prototypes for functions which are not
- // used.
- for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
- if (I->use_size() == 0 && I->isExternal()) {
- CallGraph[I]->removeAllCalledFunctions();
- delete CallGraph.removeFunctionFromModule(I);
- }
- return true;
- }
-
- struct GlobalDCE : public Pass {
// run - Do the GlobalDCE pass on the specified module, optionally updating
// the specified callgraph to reflect the changes.
//
- bool run(Module &M) {
- return RemoveUnreachableFunctions(M, getAnalysis<CallGraph>()) |
- RemoveUnreachableGlobalVariables(M);
- }
-
- // getAnalysisUsage - This function works on the call graph of a module.
- // It is capable of updating the call graph to reflect the new state of the
- // module.
- //
- virtual void getAnalysisUsage(AnalysisUsage &AU) const {
- AU.addRequired<CallGraph>();
- }
+ bool runOnModule(Module &M) override;
private:
- std::vector<GlobalValue*> WorkList;
+ SmallPtrSet<GlobalValue*, 32> AliveGlobals;
+ SmallPtrSet<Constant *, 8> SeenConstants;
+ std::unordered_multimap<Comdat *, GlobalValue *> ComdatMembers;
- inline bool RemoveIfDead(GlobalValue *GV);
- void DestroyInitializer(Constant *C);
+ /// GlobalIsNeeded - mark the specific global value as needed, and
+ /// recursively mark anything that it uses as also needed.
+ void GlobalIsNeeded(GlobalValue *GV);
+ void MarkUsedGlobalsAsNeeded(Constant *C);
- bool RemoveUnreachableGlobalVariables(Module &M);
- bool RemoveUnusedConstantPointerRef(GlobalValue &GV);
- bool SafeToDestroyConstant(Constant *C);
+ bool RemoveUnusedGlobalValue(GlobalValue &GV);
};
- RegisterOpt<GlobalDCE> X("globaldce", "Dead Global Elimination");
}
-Pass *createGlobalDCEPass() { return new GlobalDCE(); }
+/// Returns true if F contains only a single "ret" instruction.
+static bool isEmptyFunction(Function *F) {
+ BasicBlock &Entry = F->getEntryBlock();
+ if (Entry.size() != 1 || !isa<ReturnInst>(Entry.front()))
+ return false;
+ ReturnInst &RI = cast<ReturnInst>(Entry.front());
+ return RI.getReturnValue() == nullptr;
+}
+char GlobalDCE::ID = 0;
+INITIALIZE_PASS(GlobalDCE, "globaldce",
+ "Dead Global Elimination", false, false)
-// RemoveIfDead - If this global value is dead, remove it from the current
-// module and return true.
-//
-bool GlobalDCE::RemoveIfDead(GlobalValue *GV) {
- // If there is only one use of the global value, it might be a
- // ConstantPointerRef... which means that this global might actually be
- // dead.
- if (GV->use_size() == 1)
- RemoveUnusedConstantPointerRef(*GV);
-
- if (!GV->use_empty()) return false;
-
- if (GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV)) {
- // Eliminate all global variables that are unused, and that are internal, or
- // do not have an initializer.
- //
- if (GVar->hasInternalLinkage() || GVar->isExternal()) {
- Constant *Init = GVar->hasInitializer() ? GVar->getInitializer() : 0;
- GV->getParent()->getGlobalList().erase(GVar);
- ++NumVariables;
-
- // If there was an initializer for the global variable, try to destroy it
- // now.
- if (Init) DestroyInitializer(Init);
-
- // If the global variable is still on the worklist, remove it now.
- std::vector<GlobalValue*>::iterator I = std::find(WorkList.begin(),
- WorkList.end(), GV);
- while (I != WorkList.end()) {
- I = WorkList.erase(I);
- I = std::find(I, WorkList.end(), GV);
- }
+ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
- return true;
- }
- } else {
- Function *F = cast<Function>(GV);
- // FIXME: TODO
+bool GlobalDCE::runOnModule(Module &M) {
+ bool Changed = false;
+ // 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));
+
+ // Loop over the module, adding globals which are obviously necessary.
+ for (Function &F : M) {
+ Changed |= RemoveUnusedGlobalValue(F);
+ // Functions with external linkage are needed if they have a body
+ if (!F.isDeclaration() && !F.hasAvailableExternallyLinkage())
+ if (!F.isDiscardableIfUnused())
+ GlobalIsNeeded(&F);
}
- return false;
-}
-// DestroyInitializer - A global variable was just destroyed and C is its
-// initializer. If we can, destroy C and all of the constants it refers to.
-//
-void GlobalDCE::DestroyInitializer(Constant *C) {
- // Cannot destroy constants still being used, and cannot destroy primitive
- // types.
- if (!C->use_empty() || C->getType()->isPrimitiveType()) return;
+ for (GlobalVariable &GV : M.globals()) {
+ Changed |= RemoveUnusedGlobalValue(GV);
+ // Externally visible & appending globals are needed, if they have an
+ // initializer.
+ if (!GV.isDeclaration() && !GV.hasAvailableExternallyLinkage())
+ if (!GV.isDiscardableIfUnused())
+ GlobalIsNeeded(&GV);
+ }
+
+ for (GlobalAlias &GA : M.aliases()) {
+ Changed |= RemoveUnusedGlobalValue(GA);
+ // Externally visible aliases are needed.
+ if (!GA.isDiscardableIfUnused())
+ GlobalIsNeeded(&GA);
+ }
- // If this is a CPR, the global value referred to may be dead now! Add it to
- // the worklist.
+ // Now that all globals which are needed are in the AliveGlobals set, we loop
+ // through the program, deleting those which are not alive.
//
- if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(C)) {
- WorkList.push_back(CPR->getValue());
- C->destroyConstant();
- ++NumCPRs;
- } else {
- bool DestroyContents = true;
- // As an optimization to the GlobalDCE algorithm, do attempt to destroy the
- // contents of an array of primitive types, because we know that this will
- // never succeed, and there could be a lot of them.
- //
- if (ConstantArray *CA = dyn_cast<ConstantArray>(C))
- if (CA->getType()->getElementType()->isPrimitiveType())
- DestroyContents = false; // Nothing we can do with the subcontents
+ // 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();
+ }
+ }
- // All other constants refer to other constants. Destroy them if possible
- // as well.
- //
- std::vector<Value*> SubConstants;
- if (DestroyContents) SubConstants.insert(SubConstants.end(),
- C->op_begin(), C->op_end());
-
- // Destroy the actual constant...
- C->destroyConstant();
- ++NumConsts;
-
- if (DestroyContents) {
- // Remove duplicates from SubConstants, so that we do not call
- // DestroyInitializer on the same constant twice (the first call might
- // delete it, so this would be bad)
- //
- std::sort(SubConstants.begin(), SubConstants.end());
- SubConstants.erase(std::unique(SubConstants.begin(), SubConstants.end()),
- SubConstants.end());
-
- // Loop over the subconstants, destroying them as well.
- for (unsigned i = 0, e = SubConstants.size(); i != e; ++i)
- DestroyInitializer(cast<Constant>(SubConstants[i]));
+ // 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();
}
- }
-}
-bool GlobalDCE::RemoveUnreachableGlobalVariables(Module &M) {
- bool Changed = false;
- WorkList.reserve(M.gsize());
+ // 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);
+ }
- // Insert all of the globals into the WorkList, making sure to run
- // RemoveUnusedConstantPointerRef at least once on all globals...
- //
- for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
- Changed |= RemoveUnusedConstantPointerRef(*I);
- WorkList.push_back(I);
- }
- for (Module::giterator I = M.gbegin(), E = M.gend(); I != E; ++I) {
- Changed |= RemoveUnusedConstantPointerRef(*I);
- WorkList.push_back(I);
+ if (!DeadFunctions.empty()) {
+ // Now that all interferences have been dropped, delete the actual objects
+ // themselves.
+ for (Function *F : DeadFunctions) {
+ RemoveUnusedGlobalValue(*F);
+ M.getFunctionList().erase(F);
+ }
+ NumFunctions += DeadFunctions.size();
+ Changed = true;
}
- // Loop over the worklist, deleting global objects that we can. Whenever we
- // delete something that might make something else dead, it gets added to the
- // worklist.
- //
- while (!WorkList.empty()) {
- GlobalValue *GV = WorkList.back();
- WorkList.pop_back();
+ if (!DeadGlobalVars.empty()) {
+ for (GlobalVariable *GV : DeadGlobalVars) {
+ RemoveUnusedGlobalValue(*GV);
+ M.getGlobalList().erase(GV);
+ }
+ NumVariables += DeadGlobalVars.size();
+ Changed = true;
+ }
- Changed |= RemoveIfDead(GV);
+ // Now delete any dead aliases.
+ if (!DeadAliases.empty()) {
+ for (GlobalAlias *GA : DeadAliases) {
+ RemoveUnusedGlobalValue(*GA);
+ M.getAliasList().erase(GA);
+ }
+ NumAliases += DeadAliases.size();
+ Changed = true;
}
- // Make sure that all memory is free'd from the worklist...
- std::vector<GlobalValue*>().swap(WorkList);
+ // Make sure that all memory is released
+ AliveGlobals.clear();
+ SeenConstants.clear();
+ ComdatMembers.clear();
+
return Changed;
}
+/// GlobalIsNeeded - the specific global value as needed, and
+/// recursively mark anything that it uses as also needed.
+void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
+ // If the global is already in the set, no need to reprocess it.
+ if (!AliveGlobals.insert(G).second)
+ return;
+
+ if (Comdat *C = G->getComdat()) {
+ for (auto &&CM : make_range(ComdatMembers.equal_range(C)))
+ GlobalIsNeeded(CM.second);
+ }
+
+ if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
+ // If this is a global variable, we must make sure to add any global values
+ // referenced by the initializer to the alive set.
+ if (GV->hasInitializer())
+ MarkUsedGlobalsAsNeeded(GV->getInitializer());
+ } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
+ // The target of a global alias is needed.
+ MarkUsedGlobalsAsNeeded(GA->getAliasee());
+ } else {
+ // Otherwise this must be a function object. We have to scan the body of
+ // the function looking for constants and global values which are used as
+ // operands. Any operands of these types must be processed to ensure that
+ // any globals used will be marked as needed.
+ Function *F = cast<Function>(G);
+
+ if (F->hasPrefixData())
+ MarkUsedGlobalsAsNeeded(F->getPrefixData());
+
+ if (F->hasPrologueData())
+ MarkUsedGlobalsAsNeeded(F->getPrologueData());
+
+ if (F->hasPersonalityFn())
+ MarkUsedGlobalsAsNeeded(F->getPersonalityFn());
+
+ for (BasicBlock &BB : *F)
+ for (Instruction &I : BB)
+ for (Use &U : I.operands())
+ if (GlobalValue *GV = dyn_cast<GlobalValue>(U))
+ GlobalIsNeeded(GV);
+ else if (Constant *C = dyn_cast<Constant>(U))
+ MarkUsedGlobalsAsNeeded(C);
+ }
+}
+
+void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
+ if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
+ return GlobalIsNeeded(GV);
+
+ // Loop over all of the operands of the constant, adding any globals they
+ // use to the list of needed globals.
+ for (Use &U : C->operands()) {
+ // If we've already processed this constant there's no need to do it again.
+ Constant *Op = dyn_cast<Constant>(U);
+ if (Op && SeenConstants.insert(Op).second)
+ MarkUsedGlobalsAsNeeded(Op);
+ }
+}
-// RemoveUnusedConstantPointerRef - Loop over all of the uses of the specified
+// 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 GlobalDCE::RemoveUnusedConstantPointerRef(GlobalValue &GV) {
- for (Value::use_iterator I = GV.use_begin(), E = GV.use_end(); I != E; ++I)
- if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(*I))
- if (SafeToDestroyConstant(CPR)) { // Only if unreferenced...
- CPR->destroyConstant();
- ++NumCPRs;
- return true;
- }
-
- return false;
-}
-
-// SafeToDestroyConstant - It is safe to destroy a constant iff it is only used
-// by constants itself. Note that constants cannot be cyclic, so this test is
-// pretty easy to implement recursively.
-//
-bool GlobalDCE::SafeToDestroyConstant(Constant *C) {
- for (Value::use_iterator I = C->use_begin(), E = C->use_end(); I != E; ++I)
- if (Constant *User = dyn_cast<Constant>(*I)) {
- if (!SafeToDestroyConstant(User)) return false;
- } else {
- return false;
- }
-
- return true;
+bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
+ if (GV.use_empty())
+ return false;
+ GV.removeDeadConstantUsers();
+ return GV.use_empty();
}
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