//===-- GlobalDCE.cpp - DCE unreachable internal functions ----------------===//
//
+// 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
//===----------------------------------------------------------------------===//
#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/Pass.h"
-#include "Support/Statistic.h"
-#include <set>
+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");
+ 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<Constant *, 32> AliveGlobals;
+ SmallPtrSet<Constant *, 8> SeenConstants;
+ SmallPtrSet<GlobalVariable *, 8> DiscardableGlobalInitializers;
- /// 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 RemoveUnusedConstantPointerRef(GlobalValue &GV);
- bool SafeToDestroyConstant(Constant *C);
+ /// \brief Checks if C is alive or is a ConstantExpr that refers to an alive
+ /// value.
+ bool ContainsUsedGlobal(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)
+
+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);
+
+ typedef std::multimap<const Comdat *, GlobalValue *> ComdatGVPairsTy;
+ ComdatGVPairsTy ComdatGVPairs;
+
// Loop over the module, adding globals which are obviously necessary.
for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
- Changed |= RemoveUnusedConstantPointerRef(*I);
+ Changed |= RemoveUnusedGlobalValue(*I);
// Functions with external linkage are needed if they have a body
- if (I->hasExternalLinkage() && !I->isExternal())
- GlobalIsNeeded(I);
+ if (!I->isDeclaration() && !I->hasAvailableExternallyLinkage()) {
+ if (!I->isDiscardableIfUnused())
+ GlobalIsNeeded(I);
+ else if (const Comdat *C = I->getComdat())
+ ComdatGVPairs.insert(std::make_pair(C, I));
+ }
}
- for (Module::giterator I = M.gbegin(), E = M.gend(); I != E; ++I) {
- Changed |= RemoveUnusedConstantPointerRef(*I);
- // Externally visible globals are needed, if they have an initializer.
- if (I->hasExternalLinkage() && !I->isExternal())
+ 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->isDeclaration() && !I->hasAvailableExternallyLinkage()) {
+ if (!I->isDiscardableIfUnused())
+ GlobalIsNeeded(I);
+ else if (const Comdat *C = I->getComdat())
+ ComdatGVPairs.insert(std::make_pair(C, I));
+ }
+ }
+
+ 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->isDiscardableIfUnused()) {
GlobalIsNeeded(I);
+ } else if (const Comdat *C = I->getComdat()) {
+ ComdatGVPairs.insert(std::make_pair(C, I));
+ }
}
+ for (ComdatGVPairsTy::iterator I = ComdatGVPairs.begin(),
+ E = ComdatGVPairs.end();
+ I != E;) {
+ ComdatGVPairsTy::iterator UB = ComdatGVPairs.upper_bound(I->first);
+ bool CanDiscard = std::all_of(I, UB, [](ComdatGVPairsTy::value_type Pair) {
+ return Pair.second->isDiscardableIfUnused();
+ });
+ if (!CanDiscard) {
+ std::for_each(I, UB, [this](ComdatGVPairsTy::value_type Pair) {
+ GlobalIsNeeded(Pair.second);
+ });
+ }
+ I = UB;
+ }
// 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)
+ 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);
+ I->setInitializer(nullptr);
}
-
// 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())
+ if (!I->isDeclaration())
I->deleteBody();
}
+ // 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(nullptr);
+ }
+
+ // Look for available externally constants that we can turn into normal
+ // externals by deleting their initalizers. This allows us to remove other
+ // globals that are referenced by the initializer.
+ if (!DiscardableGlobalInitializers.empty()) {
+ for (GlobalVariable *GV : DiscardableGlobalInitializers) {
+ if (!ContainsUsedGlobal(GV->getInitializer())) {
+ GV->setInitializer(nullptr);
+ GV->setLinkage(GlobalValue::ExternalLinkage);
+ Changed = true;
+ }
+ }
+ }
+
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) {
- RemoveUnusedConstantPointerRef(*DeadFunctions[i]);
+ RemoveUnusedGlobalValue(*DeadFunctions[i]);
M.getFunctionList().erase(DeadFunctions[i]);
}
NumFunctions += DeadFunctions.size();
if (!DeadGlobalVars.empty()) {
for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
- RemoveUnusedConstantPointerRef(*DeadGlobalVars[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();
+ SeenConstants.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;
-
- // Otherwise insert it now, so we do not infinitely recurse
- AliveGlobals.insert(I, G);
-
+ 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());
+ if (GV->hasInitializer()) {
+ if (GV->hasAvailableExternallyLinkage())
+ DiscardableGlobalInitializers.insert(GV);
+ else
+ 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...
+
+ if (F->hasPrefixData())
+ MarkUsedGlobalsAsNeeded(F->getPrefixData());
+
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))
GlobalIsNeeded(GV);
else if (Constant *C = dyn_cast<Constant>(*U))
- MarkUsedGlobalsAsNeeded(C);
+ MarkUsedGlobalsAsNeeded(C);
}
}
void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
- if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(C))
- GlobalIsNeeded(CPR->getValue());
- 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));
+ 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 we've already processed this constant there's no need to do it again.
+ Constant *Op = dyn_cast<Constant>(*I);
+ if (Op && SeenConstants.insert(Op))
+ MarkUsedGlobalsAsNeeded(Op);
}
}
-// RemoveUnusedConstantPointerRef - 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;
- }
+bool GlobalDCE::ContainsUsedGlobal(Constant *C) {
+ // C contains a used global If C is alive or we visited it while marking
+ // values alive.
+ if (AliveGlobals.count(C) || SeenConstants.count(C))
+ return true;
+ // Now check all operands of a ConstantExpr.
+ for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I) {
+ Constant *Op = dyn_cast<Constant>(*I);
+ if (Op && ContainsUsedGlobal(Op))
+ 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.
+// 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::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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