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llvm-mirror/lib/Transforms/IPO/GlobalDCE.cpp
Chris Lattner 721a5fac62 Use methods that are more explanatory
llvm-svn: 4142
2002-10-13 17:12:47 +00:00

251 lines
8.8 KiB
C++

//===-- GlobalDCE.cpp - DCE unreachable internal functions ----------------===//
//
// This transform is designed to eliminate unreachable internal globals
// FIXME: GlobalDCE should update the callgraph, not destroy it!
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/IPO.h"
#include "llvm/Module.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Analysis/CallGraph.h"
#include "Support/DepthFirstIterator.h"
#include "Support/Statistic.h"
#include <algorithm>
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");
Statistic<> NumConsts("globaldce", "Number of init constants removed");
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));
// 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->removeAllCalledFunctions();
FunctionsToDelete.push_back(N);
++NumFunctions;
}
}
// Nothing to do if no unreachable functions have been found...
if (FunctionsToDelete.empty()) return false;
// Unreachables functions have been found and should have no references to
// them, delete them now.
//
for (std::vector<CallGraphNode*>::iterator I = FunctionsToDelete.begin(),
E = FunctionsToDelete.end(); I != E; ++I)
delete CallGraph.removeFunctionFromModule(*I);
return true;
}
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 RemoveUnreachableFunctions(M, getAnalysis<CallGraph>()) |
RemoveUnreachableGlobalVariables(M);
}
// 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>();
}
private:
std::vector<GlobalValue*> WorkList;
inline bool RemoveIfDead(GlobalValue *GV);
void DestroyInitializer(Constant *C);
bool RemoveUnreachableGlobalVariables(Module &M);
bool RemoveUnusedConstantPointerRef(GlobalValue &GV);
bool SafeToDestroyConstant(Constant *C);
};
RegisterOpt<GlobalDCE> X("globaldce", "Dead Global Elimination");
}
Pass *createGlobalDCEPass() { return new GlobalDCE(); }
// RemoveIfDead - If this global value is dead, remove it from the current
// module and return true.
//
bool GlobalDCE::RemoveIfDead(GlobalValue *GV) {
// If there is only one use of the global value, it might be a
// ConstantPointerRef... which means that this global might actually be
// dead.
if (GV->use_size() == 1)
RemoveUnusedConstantPointerRef(*GV);
if (!GV->use_empty()) return false;
if (GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV)) {
// Eliminate all global variables that are unused, and that are internal, or
// do not have an initializer.
//
if (GVar->hasInternalLinkage() || GVar->isExternal()) {
Constant *Init = GVar->hasInitializer() ? GVar->getInitializer() : 0;
GV->getParent()->getGlobalList().erase(GVar);
++NumVariables;
// If there was an initializer for the global variable, try to destroy it
// now.
if (Init) DestroyInitializer(Init);
// If the global variable is still on the worklist, remove it now.
std::vector<GlobalValue*>::iterator I = std::find(WorkList.begin(),
WorkList.end(), GV);
while (I != WorkList.end())
I = std::find(WorkList.erase(I), WorkList.end(), GV);
return true;
}
} else {
Function *F = cast<Function>(GV);
// FIXME: TODO
}
return false;
}
// DestroyInitializer - A global variable was just destroyed and C is its
// initializer. If we can, destroy C and all of the constants it refers to.
//
void GlobalDCE::DestroyInitializer(Constant *C) {
// Cannot destroy constants still being used, and cannot destroy primitive
// types.
if (!C->use_empty() || C->getType()->isPrimitiveType()) return;
// If this is a CPR, the global value referred to may be dead now! Add it to
// the worklist.
//
if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(C)) {
WorkList.push_back(CPR->getValue());
C->destroyConstant();
++NumCPRs;
} else {
bool DestroyContents = true;
// As an optimization to the GlobalDCE algorithm, do attempt to destroy the
// contents of an array of primitive types, because we know that this will
// never succeed, and there could be a lot of them.
//
if (ConstantArray *CA = dyn_cast<ConstantArray>(C))
if (CA->getType()->getElementType()->isPrimitiveType())
DestroyContents = false; // Nothing we can do with the subcontents
// All other constants refer to other constants. Destroy them if possible
// as well.
//
std::vector<Value*> SubConstants;
if (DestroyContents) SubConstants.insert(SubConstants.end(),
C->op_begin(), C->op_end());
// Destroy the actual constant...
C->destroyConstant();
++NumConsts;
if (DestroyContents) {
// Remove duplicates from SubConstants, so that we do not call
// DestroyInitializer on the same constant twice (the first call might
// delete it, so this would be bad)
//
std::sort(SubConstants.begin(), SubConstants.end());
SubConstants.erase(std::unique(SubConstants.begin(), SubConstants.end()),
SubConstants.end());
// Loop over the subconstants, destroying them as well.
for (unsigned i = 0, e = SubConstants.size(); i != e; ++i)
DestroyInitializer(cast<Constant>(SubConstants[i]));
}
}
}
bool GlobalDCE::RemoveUnreachableGlobalVariables(Module &M) {
bool Changed = false;
WorkList.reserve(M.gsize());
// Insert all of the globals into the WorkList, making sure to run
// RemoveUnusedConstantPointerRef at least once on all globals...
//
for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
Changed |= RemoveUnusedConstantPointerRef(*I);
WorkList.push_back(I);
}
for (Module::giterator I = M.gbegin(), E = M.gend(); I != E; ++I) {
Changed |= RemoveUnusedConstantPointerRef(*I);
WorkList.push_back(I);
}
// Loop over the worklist, deleting global objects that we can. Whenever we
// delete something that might make something else dead, it gets added to the
// worklist.
//
while (!WorkList.empty()) {
GlobalValue *GV = WorkList.back();
WorkList.pop_back();
Changed |= RemoveIfDead(GV);
}
// Make sure that all memory is free'd from the worklist...
std::vector<GlobalValue*>().swap(WorkList);
return Changed;
}
// 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;
}
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;
}