1 //===-- DeadArgumentElimination.cpp - Eliminate dead arguments ------------===//
3 // This pass deletes dead arguments from internal functions. Dead argument
4 // elimination removes arguments which are directly dead, as well as arguments
5 // only passed into function calls as dead arguments of other functions.
7 // This pass is often useful as a cleanup pass to run after aggressive
8 // interprocedural passes, which add possibly-dead arguments.
10 //===----------------------------------------------------------------------===//
12 #include "llvm/Transforms/IPO.h"
13 #include "llvm/Module.h"
14 #include "llvm/Pass.h"
15 #include "llvm/DerivedTypes.h"
16 #include "llvm/Constant.h"
17 #include "llvm/iOther.h"
18 #include "llvm/iTerminators.h"
19 #include "llvm/Support/CallSite.h"
20 #include "Support/Statistic.h"
21 #include "Support/iterator"
25 Statistic<> NumArgumentsEliminated("deadargelim", "Number of args removed");
27 struct DAE : public Pass {
28 DAE(bool DFEF = false) : DeleteFromExternalFunctions(DFEF) {}
32 bool DeleteFromExternalFunctions;
33 bool FunctionArgumentsIntrinsicallyAlive(const Function &F);
34 void RemoveDeadArgumentsFromFunction(Function *F,
35 std::set<Argument*> &DeadArguments);
37 RegisterOpt<DAE> X("deadargelim", "Dead Argument Elimination");
40 /// createDeadArgEliminationPass - This pass removes arguments from functions
41 /// which are not used by the body of the function. If
42 /// DeleteFromExternalFunctions is true, the pass will modify functions that
43 /// have external linkage, which is not usually safe (this is used by bugpoint
44 /// to reduce testcases).
46 Pass *createDeadArgEliminationPass(bool DeleteFromExternalFunctions) {
47 return new DAE(DeleteFromExternalFunctions);
51 // FunctionArgumentsIntrinsicallyAlive - Return true if the arguments of the
52 // specified function are intrinsically alive.
54 // We consider arguments of non-internal functions to be intrinsically alive as
55 // well as arguments to functions which have their "address taken".
57 bool DAE::FunctionArgumentsIntrinsicallyAlive(const Function &F) {
58 if (!F.hasInternalLinkage() && !DeleteFromExternalFunctions) return true;
60 for (Value::use_const_iterator I = F.use_begin(), E = F.use_end(); I!=E; ++I){
61 // If this use is anything other than a call site, the function is alive.
62 CallSite CS = CallSite::get(const_cast<User*>(*I));
63 if (!CS.getInstruction()) return true; // Not a valid call site?
65 // If the function is PASSED IN as an argument, its address has been taken
66 for (CallSite::arg_iterator AI = CS.arg_begin(), E = CS.arg_end(); AI != E;
68 if (AI->get() == &F) return true;
74 enum ArgumentLiveness { Alive, MaybeLive, Dead };
77 // getArgumentLiveness - Inspect an argument, determining if is known Alive
78 // (used in a computation), MaybeLive (only passed as an argument to a call), or
80 static ArgumentLiveness getArgumentLiveness(const Argument &A) {
81 if (A.use_empty()) return Dead; // First check, directly dead?
83 // Scan through all of the uses, looking for non-argument passing uses.
84 for (Value::use_const_iterator I = A.use_begin(), E = A.use_end(); I!=E;++I) {
85 CallSite CS = CallSite::get(const_cast<User*>(*I));
86 if (!CS.getInstruction()) {
87 // If its used by something that is not a call or invoke, it's alive!
90 // If it's an indirect call, mark it alive...
91 Function *Callee = CS.getCalledFunction();
92 if (!Callee) return Alive;
94 // Check to see if it's passed through a va_arg area: if so, we cannot
96 unsigned NumFixedArgs = Callee->getFunctionType()->getNumParams();
97 for (CallSite::arg_iterator AI = CS.arg_begin()+NumFixedArgs;
98 AI != CS.arg_end(); ++AI)
99 if (AI->get() == &A) // If passed through va_arg area, we cannot remove it
103 return MaybeLive; // It must be used, but only as argument to a function
106 // isMaybeLiveArgumentNowAlive - Check to see if Arg is alive. At this point,
107 // we know that the only uses of Arg are to be passed in as an argument to a
108 // function call. Check to see if the formal argument passed in is in the
109 // LiveArguments set. If so, return true.
111 static bool isMaybeLiveArgumentNowAlive(Argument *Arg,
112 const std::set<Argument*> &LiveArguments) {
113 for (Value::use_iterator I = Arg->use_begin(), E = Arg->use_end(); I!=E; ++I){
114 CallSite CS = CallSite::get(*I);
116 // We know that this can only be used for direct calls...
117 Function *Callee = cast<Function>(CS.getCalledValue());
119 // Loop over all of the arguments (because Arg may be passed into the call
120 // multiple times) and check to see if any are now alive...
121 CallSite::arg_iterator CSAI = CS.arg_begin();
122 for (Function::aiterator AI = Callee->abegin(), E = Callee->aend();
123 AI != E; ++AI, ++CSAI)
124 // If this is the argument we are looking for, check to see if it's alive
125 if (*CSAI == Arg && LiveArguments.count(AI))
131 // MarkArgumentLive - The MaybeLive argument 'Arg' is now known to be alive.
132 // Mark it live in the specified sets and recursively mark arguments in callers
133 // live that are needed to pass in a value.
135 static void MarkArgumentLive(Argument *Arg,
136 std::set<Argument*> &MaybeLiveArguments,
137 std::set<Argument*> &LiveArguments,
138 const std::multimap<Function*, CallSite> &CallSites) {
139 DEBUG(std::cerr << " MaybeLive argument now live: " << Arg->getName()<<"\n");
140 assert(MaybeLiveArguments.count(Arg) && !LiveArguments.count(Arg) &&
141 "Arg not MaybeLive?");
142 MaybeLiveArguments.erase(Arg);
143 LiveArguments.insert(Arg);
145 // Loop over all of the call sites of the function, making any arguments
146 // passed in to provide a value for this argument live as necessary.
148 Function *Fn = Arg->getParent();
149 unsigned ArgNo = std::distance(Fn->abegin(), Function::aiterator(Arg));
151 std::multimap<Function*, CallSite>::const_iterator I =
152 CallSites.lower_bound(Fn);
153 for (; I != CallSites.end() && I->first == Fn; ++I) {
154 const CallSite &CS = I->second;
155 if (Argument *ActualArg = dyn_cast<Argument>(*(CS.arg_begin()+ArgNo)))
156 if (MaybeLiveArguments.count(ActualArg))
157 MarkArgumentLive(ActualArg, MaybeLiveArguments, LiveArguments,
162 // RemoveDeadArgumentsFromFunction - We know that F has dead arguments, as
163 // specified by the DeadArguments list. Transform the function and all of the
164 // callees of the function to not have these arguments.
166 void DAE::RemoveDeadArgumentsFromFunction(Function *F,
167 std::set<Argument*> &DeadArguments){
168 // Start by computing a new prototype for the function, which is the same as
169 // the old function, but has fewer arguments.
170 const FunctionType *FTy = F->getFunctionType();
171 std::vector<const Type*> Params;
173 for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I)
174 if (!DeadArguments.count(I))
175 Params.push_back(I->getType());
177 FunctionType *NFTy = FunctionType::get(FTy->getReturnType(), Params,
180 // Create the new function body and insert it into the module...
181 Function *NF = new Function(NFTy, F->getLinkage(), F->getName());
182 F->getParent()->getFunctionList().insert(F, NF);
184 // Loop over all of the callers of the function, transforming the call sites
185 // to pass in a smaller number of arguments into the new function.
187 while (!F->use_empty()) {
188 CallSite CS = CallSite::get(F->use_back());
189 Instruction *Call = CS.getInstruction();
190 CS.setCalledFunction(NF); // Reduce the uses count of F
192 // Loop over the operands, deleting dead ones...
193 CallSite::arg_iterator AI = CS.arg_begin();
194 for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I)
195 if (DeadArguments.count(I)) { // Remove operands for dead arguments
196 AI = Call->op_erase(AI);
198 ++AI; // Leave live operands alone...
202 // Since we have now created the new function, splice the body of the old
203 // function right into the new function, leaving the old rotting hulk of the
205 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
207 // Loop over the argument list, transfering uses of the old arguments over to
208 // the new arguments, also transfering over the names as well. While we're at
209 // it, remove the dead arguments from the DeadArguments list.
211 for (Function::aiterator I = F->abegin(), E = F->aend(), I2 = NF->abegin();
213 if (!DeadArguments.count(I)) {
214 // If this is a live argument, move the name and users over to the new
216 I->replaceAllUsesWith(I2);
217 I2->setName(I->getName());
220 // If this argument is dead, replace any uses of it with null constants
221 // (these are guaranteed to only be operands to call instructions which
222 // will later be simplified).
223 I->replaceAllUsesWith(Constant::getNullValue(I->getType()));
224 DeadArguments.erase(I);
227 // Now that the old function is dead, delete it.
228 F->getParent()->getFunctionList().erase(F);
231 bool DAE::run(Module &M) {
232 // First phase: loop through the module, determining which arguments are live.
233 // We assume all arguments are dead unless proven otherwise (allowing us to
234 // determing that dead arguments passed into recursive functions are dead).
236 std::set<Argument*> LiveArguments, MaybeLiveArguments, DeadArguments;
237 std::multimap<Function*, CallSite> CallSites;
239 DEBUG(std::cerr << "DAE - Determining liveness\n");
240 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
242 // If the function is intrinsically alive, just mark the arguments alive.
243 if (FunctionArgumentsIntrinsicallyAlive(Fn)) {
244 for (Function::aiterator AI = Fn.abegin(), E = Fn.aend(); AI != E; ++AI)
245 LiveArguments.insert(AI);
246 DEBUG(std::cerr << " Args intrinsically live for fn: " << Fn.getName()
249 DEBUG(std::cerr << " Inspecting args for fn: " << Fn.getName() << "\n");
251 // If it is not intrinsically alive, we know that all users of the
252 // function are call sites. Mark all of the arguments live which are
253 // directly used, and keep track of all of the call sites of this function
254 // if there are any arguments we assume that are dead.
256 bool AnyMaybeLiveArgs = false;
257 for (Function::aiterator AI = Fn.abegin(), E = Fn.aend(); AI != E; ++AI)
258 switch (getArgumentLiveness(*AI)) {
260 DEBUG(std::cerr << " Arg live by use: " << AI->getName() << "\n");
261 LiveArguments.insert(AI);
264 DEBUG(std::cerr << " Arg definately dead: " <<AI->getName()<<"\n");
265 DeadArguments.insert(AI);
268 DEBUG(std::cerr << " Arg only passed to calls: "
269 << AI->getName() << "\n");
270 AnyMaybeLiveArgs = true;
271 MaybeLiveArguments.insert(AI);
275 // If there are any "MaybeLive" arguments, we need to check callees of
276 // this function when/if they become alive. Record which functions are
278 if (AnyMaybeLiveArgs)
279 for (Value::use_iterator I = Fn.use_begin(), E = Fn.use_end();
281 CallSites.insert(std::make_pair(&Fn, CallSite::get(*I)));
285 // Now we loop over all of the MaybeLive arguments, promoting them to be live
286 // arguments if one of the calls that uses the arguments to the calls they are
287 // passed into requires them to be live. Of course this could make other
288 // arguments live, so process callers recursively.
290 // Because elements can be removed from the MaybeLiveArguments list, copy it
291 // to a temporary vector.
293 std::vector<Argument*> TmpArgList(MaybeLiveArguments.begin(),
294 MaybeLiveArguments.end());
295 for (unsigned i = 0, e = TmpArgList.size(); i != e; ++i) {
296 Argument *MLA = TmpArgList[i];
297 if (MaybeLiveArguments.count(MLA) &&
298 isMaybeLiveArgumentNowAlive(MLA, LiveArguments)) {
299 MarkArgumentLive(MLA, MaybeLiveArguments, LiveArguments, CallSites);
303 // Recover memory early...
306 // At this point, we know that all arguments in DeadArguments and
307 // MaybeLiveArguments are dead. If the two sets are empty, there is nothing
309 if (MaybeLiveArguments.empty() && DeadArguments.empty())
312 // Otherwise, compact into one set, and start eliminating the arguments from
314 DeadArguments.insert(MaybeLiveArguments.begin(), MaybeLiveArguments.end());
315 MaybeLiveArguments.clear();
317 NumArgumentsEliminated += DeadArguments.size();
318 while (!DeadArguments.empty())
319 RemoveDeadArgumentsFromFunction((*DeadArguments.begin())->getParent(),