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//===-- GlobalDCE.cpp - DCE unreachable internal functions ----------------===//
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// The LLVM Compiler Infrastructure
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//===----------------------------------------------------------------------===//
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// This transform is designed to eliminate unreachable internal globals from the
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// program. It uses an aggressive algorithm, searching out globals that are
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// known to be alive. After it finds all of the globals which are needed, it
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// deletes whatever is left over. This allows it to delete recursive chunks of
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// the program which are unreachable.
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "globaldce"
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#include "llvm/Transforms/IPO.h"
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#include "llvm/Constants.h"
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#include "llvm/Module.h"
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#include "llvm/Pass.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/Statistic.h"
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STATISTIC(NumAliases , "Number of global aliases removed");
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STATISTIC(NumFunctions, "Number of functions removed");
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STATISTIC(NumVariables, "Number of global variables removed");
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struct GlobalDCE : public ModulePass {
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static char ID; // Pass identification, replacement for typeid
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GlobalDCE() : ModulePass(&ID) {}
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// run - Do the GlobalDCE pass on the specified module, optionally updating
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// the specified callgraph to reflect the changes.
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bool runOnModule(Module &M);
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SmallPtrSet<GlobalValue*, 32> AliveGlobals;
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/// GlobalIsNeeded - mark the specific global value as needed, and
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/// recursively mark anything that it uses as also needed.
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void GlobalIsNeeded(GlobalValue *GV);
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void MarkUsedGlobalsAsNeeded(Constant *C);
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bool RemoveUnusedGlobalValue(GlobalValue &GV);
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char GlobalDCE::ID = 0;
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static RegisterPass<GlobalDCE> X("globaldce", "Dead Global Elimination");
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ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
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bool GlobalDCE::runOnModule(Module &M) {
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// Loop over the module, adding globals which are obviously necessary.
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for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
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Changed |= RemoveUnusedGlobalValue(*I);
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// Functions with external linkage are needed if they have a body
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if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage() &&
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!I->isDeclaration() && !I->hasAvailableExternallyLinkage())
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for (Module::global_iterator I = M.global_begin(), E = M.global_end();
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Changed |= RemoveUnusedGlobalValue(*I);
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// Externally visible & appending globals are needed, if they have an
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if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage() &&
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!I->isDeclaration() && !I->hasAvailableExternallyLinkage())
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for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
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Changed |= RemoveUnusedGlobalValue(*I);
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// Externally visible aliases are needed.
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if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage())
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// Now that all globals which are needed are in the AliveGlobals set, we loop
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// through the program, deleting those which are not alive.
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// The first pass is to drop initializers of global variables which are dead.
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std::vector<GlobalVariable*> DeadGlobalVars; // Keep track of dead globals
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for (Module::global_iterator I = M.global_begin(), E = M.global_end();
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if (!AliveGlobals.count(I)) {
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DeadGlobalVars.push_back(I); // Keep track of dead globals
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// The second pass drops the bodies of functions which are dead...
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std::vector<Function*> DeadFunctions;
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for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
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if (!AliveGlobals.count(I)) {
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DeadFunctions.push_back(I); // Keep track of dead globals
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if (!I->isDeclaration())
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// The third pass drops targets of aliases which are dead...
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std::vector<GlobalAlias*> DeadAliases;
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for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;
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if (!AliveGlobals.count(I)) {
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DeadAliases.push_back(I);
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if (!DeadFunctions.empty()) {
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// Now that all interferences have been dropped, delete the actual objects
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for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
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RemoveUnusedGlobalValue(*DeadFunctions[i]);
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M.getFunctionList().erase(DeadFunctions[i]);
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NumFunctions += DeadFunctions.size();
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if (!DeadGlobalVars.empty()) {
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for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
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RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
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M.getGlobalList().erase(DeadGlobalVars[i]);
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NumVariables += DeadGlobalVars.size();
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// Now delete any dead aliases.
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if (!DeadAliases.empty()) {
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for (unsigned i = 0, e = DeadAliases.size(); i != e; ++i) {
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RemoveUnusedGlobalValue(*DeadAliases[i]);
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M.getAliasList().erase(DeadAliases[i]);
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NumAliases += DeadAliases.size();
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// Make sure that all memory is released
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AliveGlobals.clear();
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/// GlobalIsNeeded - the specific global value as needed, and
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/// recursively mark anything that it uses as also needed.
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void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
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// If the global is already in the set, no need to reprocess it.
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if (!AliveGlobals.insert(G))
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if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
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// If this is a global variable, we must make sure to add any global values
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// referenced by the initializer to the alive set.
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if (GV->hasInitializer())
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MarkUsedGlobalsAsNeeded(GV->getInitializer());
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} else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
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// The target of a global alias is needed.
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MarkUsedGlobalsAsNeeded(GA->getAliasee());
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// Otherwise this must be a function object. We have to scan the body of
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// the function looking for constants and global values which are used as
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// operands. Any operands of these types must be processed to ensure that
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// any globals used will be marked as needed.
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Function *F = cast<Function>(G);
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for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
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for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
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for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
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if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
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else if (Constant *C = dyn_cast<Constant>(*U))
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MarkUsedGlobalsAsNeeded(C);
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void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
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if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
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return GlobalIsNeeded(GV);
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// Loop over all of the operands of the constant, adding any globals they
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// use to the list of needed globals.
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for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I)
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if (Constant *OpC = dyn_cast<Constant>(*I))
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MarkUsedGlobalsAsNeeded(OpC);
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// RemoveUnusedGlobalValue - Loop over all of the uses of the specified
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// GlobalValue, looking for the constant pointer ref that may be pointing to it.
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// If found, check to see if the constant pointer ref is safe to destroy, and if
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// so, nuke it. This will reduce the reference count on the global value, which
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// might make it deader.
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bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
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if (GV.use_empty()) return false;
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GV.removeDeadConstantUsers();
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return GV.use_empty();