-//===-- GlobalDCE.cpp - DCE unreachable internal methods ---------*- C++ -*--=//
+//===-- GlobalDCE.cpp - DCE unreachable internal functions ----------------===//
//
-// This transform is designed to eliminate unreachable internal globals
+// 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"
+#define DEBUG_TYPE "globaldce"
+#include "llvm/Transforms/IPO.h"
+#include "llvm/Constants.h"
#include "llvm/Module.h"
-#include "llvm/Method.h"
#include "llvm/Pass.h"
-#include "Support/DepthFirstIterator.h"
-#include <set>
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/Statistic.h"
+using namespace llvm;
-static bool RemoveUnreachableMethods(Module *M, cfg::CallGraph &CallGraph) {
- // Calculate which methods are reachable from the external methods in the call
- // graph.
- //
- std::set<cfg::CallGraphNode*> ReachableNodes(df_begin(&CallGraph),
- df_end(&CallGraph));
+STATISTIC(NumAliases , "Number of global aliases removed");
+STATISTIC(NumFunctions, "Number of functions removed");
+STATISTIC(NumVariables, "Number of global variables removed");
- // Loop over the methods in the module twice. The first time is used to drop
- // references that methods have to each other before they are deleted. The
- // second pass removes the methods that need to be removed.
- //
- std::vector<cfg::CallGraphNode*> MethodsToDelete; // Track unused methods
- for (Module::iterator I = M->begin(), E = M->end(); I != E; ++I) {
- cfg::CallGraphNode *N = CallGraph[*I];
- if (!ReachableNodes.count(N)) { // Not reachable??
- (*I)->dropAllReferences();
- N->removeAllCalledMethods();
- MethodsToDelete.push_back(N);
- }
+namespace {
+ struct GlobalDCE : public ModulePass {
+ static char ID; // Pass identification, replacement for typeid
+ GlobalDCE() : ModulePass(ID) {}
+
+ // run - Do the GlobalDCE pass on the specified module, optionally updating
+ // the specified callgraph to reflect the changes.
+ //
+ bool runOnModule(Module &M);
+
+ private:
+ SmallPtrSet<GlobalValue*, 32> AliveGlobals;
+
+ /// 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);
+ };
+}
+
+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;
+
+ // Loop over the module, adding globals which are obviously necessary.
+ for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
+ Changed |= RemoveUnusedGlobalValue(*I);
+ // Functions with external linkage are needed if they have a body
+ if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage() &&
+ !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
+ GlobalIsNeeded(I);
+ }
+
+ for (Module::global_iterator I = M.global_begin(), E = M.global_end();
+ I != E; ++I) {
+ Changed |= RemoveUnusedGlobalValue(*I);
+ // Externally visible & appending globals are needed, if they have an
+ // initializer.
+ if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage() &&
+ !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
+ GlobalIsNeeded(I);
}
- // Nothing to do if no unreachable methods have been found...
- if (MethodsToDelete.empty()) return false;
+ for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
+ I != E; ++I) {
+ Changed |= RemoveUnusedGlobalValue(*I);
+ // Externally visible aliases are needed.
+ if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage())
+ GlobalIsNeeded(I);
+ }
- // Unreachables methods 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<cfg::CallGraphNode*>::iterator I = MethodsToDelete.begin(),
- E = MethodsToDelete.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 (Module::global_iterator I = M.global_begin(), E = M.global_end();
+ I != E; ++I)
+ if (!AliveGlobals.count(I)) {
+ DeadGlobalVars.push_back(I); // Keep track of dead globals
+ I->setInitializer(0);
+ }
-namespace {
- 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 RemoveUnreachableMethods(M, getAnalysis<cfg::CallGraph>());
+ // The second pass drops the bodies of functions which are dead...
+ std::vector<Function*> DeadFunctions;
+ for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
+ if (!AliveGlobals.count(I)) {
+ DeadFunctions.push_back(I); // Keep track of dead globals
+ if (!I->isDeclaration())
+ I->deleteBody();
}
- // getAnalysisUsageInfo - 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 getAnalysisUsageInfo(Pass::AnalysisSet &Required,
- Pass::AnalysisSet &Destroyed,
- Pass::AnalysisSet &Provided) {
- Required.push_back(cfg::CallGraph::ID);
- // FIXME: This should update the callgraph, not destroy it!
- Destroyed.push_back(cfg::CallGraph::ID);
+ // The third pass drops targets of aliases which are dead...
+ std::vector<GlobalAlias*> DeadAliases;
+ for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;
+ ++I)
+ if (!AliveGlobals.count(I)) {
+ DeadAliases.push_back(I);
+ I->setAliasee(0);
}
- };
+
+ if (!DeadFunctions.empty()) {
+ // Now that all interferences have been dropped, delete the actual objects
+ // themselves.
+ for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
+ RemoveUnusedGlobalValue(*DeadFunctions[i]);
+ M.getFunctionList().erase(DeadFunctions[i]);
+ }
+ NumFunctions += DeadFunctions.size();
+ Changed = true;
+ }
+
+ if (!DeadGlobalVars.empty()) {
+ for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
+ RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
+ M.getGlobalList().erase(DeadGlobalVars[i]);
+ }
+ NumVariables += DeadGlobalVars.size();
+ Changed = true;
+ }
+
+ // Now delete any dead aliases.
+ if (!DeadAliases.empty()) {
+ for (unsigned i = 0, e = DeadAliases.size(); i != e; ++i) {
+ RemoveUnusedGlobalValue(*DeadAliases[i]);
+ M.getAliasList().erase(DeadAliases[i]);
+ }
+ NumAliases += DeadAliases.size();
+ Changed = true;
+ }
+
+ // Make sure that all memory is released
+ AliveGlobals.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))
+ return;
+
+ 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);
+
+ for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
+ for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
+ for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
+ 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 (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I)
+ if (Constant *OpC = dyn_cast<Constant>(*I))
+ MarkUsedGlobalsAsNeeded(OpC);
}
-Pass *createGlobalDCEPass() { return new GlobalDCE(); }
+// 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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