//===-- GlobalDCE.cpp - DCE unreachable internal functions ----------------===//
-//
+//
// The LLVM Compiler Infrastructure
//
-// This file was developed by the LLVM research group and is distributed under
-// the University of Illinois Open Source License. See LICENSE.TXT for details.
-//
+// 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
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/IPO.h"
-#include "llvm/Constants.h"
-#include "llvm/Module.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/Statistic.h"
-#include <set>
+#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<> NumGVs("globaldce", "Number of global values removed");
+ struct GlobalDCE : public ModulePass {
+ static char ID; // Pass identification, replacement for typeid
+ GlobalDCE() : ModulePass(ID) {
+ initializeGlobalDCEPass(*PassRegistry::getPassRegistry());
+ }
- 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);
+ bool runOnModule(Module &M) override;
private:
- std::set<GlobalValue*> AliveGlobals;
+ SmallPtrSet<GlobalValue*, 32> AliveGlobals;
+ SmallPtrSet<Constant *, 8> SeenConstants;
+ std::unordered_multimap<Comdat *, GlobalValue *> ComdatMembers;
- /// MarkGlobalIsNeeded - the specific global value as needed, and
+ /// 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 SafeToDestroyConstant(Constant* C);
bool RemoveUnusedGlobalValue(GlobalValue &GV);
};
- RegisterOpt<GlobalDCE> X("globaldce", "Dead Global Elimination");
}
-Pass *llvm::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)
+
+ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
-bool GlobalDCE::run(Module &M) {
+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 (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
- Changed |= RemoveUnusedGlobalValue(*I);
+ for (Function &F : M) {
+ Changed |= RemoveUnusedGlobalValue(F);
// Functions with external linkage are needed if they have a body
- if ((!I->hasInternalLinkage() && !I->hasLinkOnceLinkage()) &&
- !I->isExternal())
- GlobalIsNeeded(I);
+ if (!F.isDeclaration() && !F.hasAvailableExternallyLinkage())
+ if (!F.isDiscardableIfUnused())
+ GlobalIsNeeded(&F);
}
- for (Module::giterator I = M.gbegin(), E = M.gend(); I != E; ++I) {
- Changed |= RemoveUnusedGlobalValue(*I);
+ for (GlobalVariable &GV : M.globals()) {
+ Changed |= RemoveUnusedGlobalValue(GV);
// Externally visible & appending globals are needed, if they have an
// initializer.
- if ((!I->hasInternalLinkage() && !I->hasLinkOnceLinkage()) &&
- !I->isExternal())
- GlobalIsNeeded(I);
+ 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);
+ }
// 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 (Module::giterator I = M.gbegin(), E = M.gend(); I != E; ++I)
- if (!AliveGlobals.count(I)) {
- DeadGlobalVars.push_back(I); // Keep track of dead globals
- I->setInitializer(0);
+ 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 (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->isExternal())
- I->deleteBody();
+ 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);
}
if (!DeadFunctions.empty()) {
- // Now that all interreferences have been dropped, delete the actual objects
+ // 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]);
+ for (Function *F : DeadFunctions) {
+ RemoveUnusedGlobalValue(*F);
+ M.getFunctionList().erase(F);
}
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]);
+ 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;
}
-/// MarkGlobalIsNeeded - the specific global value as needed, and
+/// GlobalIsNeeded - the specific global value as needed, and
/// recursively mark anything that it uses as also needed.
void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
- std::set<GlobalValue*>::iterator I = AliveGlobals.lower_bound(G);
-
// If the global is already in the set, no need to reprocess it.
- if (I != AliveGlobals.end() && *I == G) return;
+ if (!AliveGlobals.insert(G).second)
+ return;
- // Otherwise insert it now, so we do not infinitely recurse
- AliveGlobals.insert(I, G);
+ 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);
- // For all basic blocks...
- for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
- // For all instructions...
- for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
- // For all operands...
- for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
- if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
+
+ 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);
+ else if (Constant *C = dyn_cast<Constant>(U))
+ MarkUsedGlobalsAsNeeded(C);
}
}
void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
- GlobalIsNeeded(GV);
- else {
- // 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)
- MarkUsedGlobalsAsNeeded(cast<Constant>(*I));
+ 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);
}
}
// might make it deader.
//
bool GlobalDCE::RemoveUnusedGlobalValue(GlobalValue &GV) {
- for (Value::use_iterator I = GV.use_begin(), E = GV.use_end(); I != E; ++I)
- if (GlobalValue* User = dyn_cast<GlobalValue>(*I))
- if (User->removeDeadConstantUsers()) { // Only if unreferenced...
- ++NumGVs;
- }
- 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;
+ if (GV.use_empty())
+ return false;
+ GV.removeDeadConstantUsers();
+ return GV.use_empty();
}
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