//===-- 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/GlobalDCE.h"
-#include "llvm/Analysis/CallGraph.h"
-#include "llvm/Module.h"
-#include "llvm/Function.h"
+#include "llvm/Transforms/IPO.h"
+#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 "Support/DepthFirstIterator.h"
-#include <set>
+#include <unordered_map>
+using namespace llvm;
-static 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));
+#define DEBUG_TYPE "globaldce"
- // 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->removeAllCalledMethods();
- FunctionsToDelete.push_back(N);
+STATISTIC(NumAliases , "Number of global aliases removed");
+STATISTIC(NumFunctions, "Number of functions removed");
+STATISTIC(NumVariables, "Number of global variables removed");
+
+namespace {
+ struct GlobalDCE : public ModulePass {
+ static char ID; // Pass identification, replacement for typeid
+ GlobalDCE() : ModulePass(ID) {
+ initializeGlobalDCEPass(*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;
+
+ private:
+ SmallPtrSet<GlobalValue*, 32> AliveGlobals;
+ SmallPtrSet<Constant *, 8> SeenConstants;
+ std::unordered_multimap<Comdat *, GlobalValue *> ComdatMembers;
+
+ /// 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 RemoveUnusedGlobalValue(GlobalValue &GV);
+ };
+}
+
+/// 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)
+
+ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
+
+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);
+ }
+
+ 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);
}
- // Nothing to do if no unreachable functions have been found...
- if (FunctionsToDelete.empty()) return false;
+ for (GlobalAlias &GA : M.aliases()) {
+ Changed |= RemoveUnusedGlobalValue(GA);
+ // Externally visible aliases are needed.
+ if (!GA.isDiscardableIfUnused())
+ GlobalIsNeeded(&GA);
+ }
- // Unreachables functions have been found and should have no references to
- // them, delete them now.
+ // Now that all globals which are needed are in the AliveGlobals set, we loop
+ // through the program, deleting those which are not alive.
//
- for (std::vector<CallGraphNode*>::iterator I = FunctionsToDelete.begin(),
- E = FunctionsToDelete.end(); I != E; ++I)
- delete CallGraph.removeMethodFromModule(*I);
- return true;
-}
+ // 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();
+ }
+ }
-namespace {
- struct GlobalDCE : public Pass {
- const char *getPassName() const { return "Dead Global Elimination"; }
+ // 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();
+ }
- // 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>());
+ // 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);
}
- // 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::ID);
+ 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;
+ }
+
+ if (!DeadGlobalVars.empty()) {
+ for (GlobalVariable *GV : DeadGlobalVars) {
+ RemoveUnusedGlobalValue(*GV);
+ M.getGlobalList().erase(GV);
+ }
+ NumVariables += DeadGlobalVars.size();
+ Changed = true;
+ }
+
+ // 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 released
+ AliveGlobals.clear();
+ SeenConstants.clear();
+ ComdatMembers.clear();
+
+ return Changed;
}
-Pass *createGlobalDCEPass() { return new GlobalDCE(); }
+/// 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);
+ }
+}
+
+// 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::RemoveUnusedGlobalValue(GlobalValue &GV) {
+ if (GV.use_empty())
+ return false;
+ GV.removeDeadConstantUsers();
+ return GV.use_empty();
+}
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