1 //===-- DeadArgumentElimination.cpp - Eliminate dead arguments ------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass deletes dead arguments from internal functions. Dead argument
11 // elimination removes arguments which are directly dead, as well as arguments
12 // only passed into function calls as dead arguments of other functions. This
13 // pass also deletes dead arguments in a similar way.
15 // This pass is often useful as a cleanup pass to run after aggressive
16 // interprocedural passes, which add possibly-dead arguments.
18 //===----------------------------------------------------------------------===//
20 #include "llvm/Transforms/IPO.h"
21 #include "llvm/Module.h"
22 #include "llvm/Pass.h"
23 #include "llvm/DerivedTypes.h"
24 #include "llvm/Constant.h"
25 #include "llvm/Instructions.h"
26 #include "llvm/Support/CallSite.h"
27 #include "llvm/Support/Debug.h"
28 #include "llvm/ADT/Statistic.h"
29 #include "llvm/ADT/iterator"
34 Statistic<> NumArgumentsEliminated("deadargelim",
35 "Number of unread args removed");
36 Statistic<> NumRetValsEliminated("deadargelim",
37 "Number of unused return values removed");
39 /// DAE - The dead argument elimination pass.
41 class DAE : public ModulePass {
42 /// Liveness enum - During our initial pass over the program, we determine
43 /// that things are either definately alive, definately dead, or in need of
44 /// interprocedural analysis (MaybeLive).
46 enum Liveness { Live, MaybeLive, Dead };
48 /// LiveArguments, MaybeLiveArguments, DeadArguments - These sets contain
49 /// all of the arguments in the program. The Dead set contains arguments
50 /// which are completely dead (never used in the function). The MaybeLive
51 /// set contains arguments which are only passed into other function calls,
52 /// thus may be live and may be dead. The Live set contains arguments which
53 /// are known to be alive.
55 std::set<Argument*> DeadArguments, MaybeLiveArguments, LiveArguments;
57 /// DeadRetVal, MaybeLiveRetVal, LifeRetVal - These sets contain all of the
58 /// functions in the program. The Dead set contains functions whose return
59 /// value is known to be dead. The MaybeLive set contains functions whose
60 /// return values are only used by return instructions, and the Live set
61 /// contains functions whose return values are used, functions that are
62 /// external, and functions that already return void.
64 std::set<Function*> DeadRetVal, MaybeLiveRetVal, LiveRetVal;
66 /// InstructionsToInspect - As we mark arguments and return values
67 /// MaybeLive, we keep track of which instructions could make the values
68 /// live here. Once the entire program has had the return value and
69 /// arguments analyzed, this set is scanned to promote the MaybeLive objects
70 /// to be Live if they really are used.
71 std::vector<Instruction*> InstructionsToInspect;
73 /// CallSites - Keep track of the call sites of functions that have
74 /// MaybeLive arguments or return values.
75 std::multimap<Function*, CallSite> CallSites;
78 bool runOnModule(Module &M);
80 virtual bool ShouldHackArguments() const { return false; }
83 Liveness getArgumentLiveness(const Argument &A);
84 bool isMaybeLiveArgumentNowLive(Argument *Arg);
86 void SurveyFunction(Function &Fn);
88 void MarkArgumentLive(Argument *Arg);
89 void MarkRetValLive(Function *F);
90 void MarkReturnInstArgumentLive(ReturnInst *RI);
92 void RemoveDeadArgumentsFromFunction(Function *F);
94 RegisterOpt<DAE> X("deadargelim", "Dead Argument Elimination");
96 /// DAH - DeadArgumentHacking pass - Same as dead argument elimination, but
97 /// deletes arguments to functions which are external. This is only for use
99 struct DAH : public DAE {
100 virtual bool ShouldHackArguments() const { return true; }
102 RegisterPass<DAH> Y("deadarghaX0r",
103 "Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)");
106 /// createDeadArgEliminationPass - This pass removes arguments from functions
107 /// which are not used by the body of the function.
109 ModulePass *llvm::createDeadArgEliminationPass() { return new DAE(); }
110 ModulePass *llvm::createDeadArgHackingPass() { return new DAH(); }
112 static inline bool CallPassesValueThoughVararg(Instruction *Call,
114 CallSite CS = CallSite::get(Call);
115 const Type *CalledValueTy = CS.getCalledValue()->getType();
116 const Type *FTy = cast<PointerType>(CalledValueTy)->getElementType();
117 unsigned NumFixedArgs = cast<FunctionType>(FTy)->getNumParams();
118 for (CallSite::arg_iterator AI = CS.arg_begin()+NumFixedArgs;
119 AI != CS.arg_end(); ++AI)
120 if (AI->get() == Arg)
125 // getArgumentLiveness - Inspect an argument, determining if is known Live
126 // (used in a computation), MaybeLive (only passed as an argument to a call), or
128 DAE::Liveness DAE::getArgumentLiveness(const Argument &A) {
129 if (A.use_empty()) return Dead; // First check, directly dead?
131 // Scan through all of the uses, looking for non-argument passing uses.
132 for (Value::use_const_iterator I = A.use_begin(), E = A.use_end(); I!=E;++I) {
133 // Return instructions do not immediately effect liveness.
134 if (isa<ReturnInst>(*I))
137 CallSite CS = CallSite::get(const_cast<User*>(*I));
138 if (!CS.getInstruction()) {
139 // If its used by something that is not a call or invoke, it's alive!
142 // If it's an indirect call, mark it alive...
143 Function *Callee = CS.getCalledFunction();
144 if (!Callee) return Live;
146 // Check to see if it's passed through a va_arg area: if so, we cannot
148 if (CallPassesValueThoughVararg(CS.getInstruction(), &A))
149 return Live; // If passed through va_arg area, we cannot remove it
152 return MaybeLive; // It must be used, but only as argument to a function
156 // SurveyFunction - This performs the initial survey of the specified function,
157 // checking out whether or not it uses any of its incoming arguments or whether
158 // any callers use the return value. This fills in the
159 // (Dead|MaybeLive|Live)(Arguments|RetVal) sets.
161 // We consider arguments of non-internal functions to be intrinsically alive as
162 // well as arguments to functions which have their "address taken".
164 void DAE::SurveyFunction(Function &F) {
165 bool FunctionIntrinsicallyLive = false;
166 Liveness RetValLiveness = F.getReturnType() == Type::VoidTy ? Live : Dead;
168 if (!F.hasInternalLinkage() &&
169 (!ShouldHackArguments() || F.getIntrinsicID()))
170 FunctionIntrinsicallyLive = true;
172 for (Value::use_iterator I = F.use_begin(), E = F.use_end(); I != E; ++I) {
173 // If this use is anything other than a call site, the function is alive.
174 CallSite CS = CallSite::get(*I);
175 Instruction *TheCall = CS.getInstruction();
176 if (!TheCall) { // Not a direct call site?
177 FunctionIntrinsicallyLive = true;
181 // Check to see if the return value is used...
182 if (RetValLiveness != Live)
183 for (Value::use_iterator I = TheCall->use_begin(),
184 E = TheCall->use_end(); I != E; ++I)
185 if (isa<ReturnInst>(cast<Instruction>(*I))) {
186 RetValLiveness = MaybeLive;
187 } else if (isa<CallInst>(cast<Instruction>(*I)) ||
188 isa<InvokeInst>(cast<Instruction>(*I))) {
189 if (CallPassesValueThoughVararg(cast<Instruction>(*I), TheCall) ||
190 !CallSite::get(cast<Instruction>(*I)).getCalledFunction()) {
191 RetValLiveness = Live;
194 RetValLiveness = MaybeLive;
197 RetValLiveness = Live;
201 // If the function is PASSED IN as an argument, its address has been taken
202 for (CallSite::arg_iterator AI = CS.arg_begin(), E = CS.arg_end();
204 if (AI->get() == &F) {
205 FunctionIntrinsicallyLive = true;
208 if (FunctionIntrinsicallyLive) break;
211 if (FunctionIntrinsicallyLive) {
212 DEBUG(std::cerr << " Intrinsically live fn: " << F.getName() << "\n");
213 for (Function::aiterator AI = F.abegin(), E = F.aend(); AI != E; ++AI)
214 LiveArguments.insert(AI);
215 LiveRetVal.insert(&F);
219 switch (RetValLiveness) {
220 case Live: LiveRetVal.insert(&F); break;
221 case MaybeLive: MaybeLiveRetVal.insert(&F); break;
222 case Dead: DeadRetVal.insert(&F); break;
225 DEBUG(std::cerr << " Inspecting args for fn: " << F.getName() << "\n");
227 // If it is not intrinsically alive, we know that all users of the
228 // function are call sites. Mark all of the arguments live which are
229 // directly used, and keep track of all of the call sites of this function
230 // if there are any arguments we assume that are dead.
232 bool AnyMaybeLiveArgs = false;
233 for (Function::aiterator AI = F.abegin(), E = F.aend(); AI != E; ++AI)
234 switch (getArgumentLiveness(*AI)) {
236 DEBUG(std::cerr << " Arg live by use: " << AI->getName() << "\n");
237 LiveArguments.insert(AI);
240 DEBUG(std::cerr << " Arg definitely dead: " <<AI->getName()<<"\n");
241 DeadArguments.insert(AI);
244 DEBUG(std::cerr << " Arg only passed to calls: "
245 << AI->getName() << "\n");
246 AnyMaybeLiveArgs = true;
247 MaybeLiveArguments.insert(AI);
251 // If there are any "MaybeLive" arguments, we need to check callees of
252 // this function when/if they become alive. Record which functions are
254 if (AnyMaybeLiveArgs || RetValLiveness == MaybeLive)
255 for (Value::use_iterator I = F.use_begin(), E = F.use_end();
257 if (AnyMaybeLiveArgs)
258 CallSites.insert(std::make_pair(&F, CallSite::get(*I)));
260 if (RetValLiveness == MaybeLive)
261 for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
263 InstructionsToInspect.push_back(cast<Instruction>(*UI));
267 // isMaybeLiveArgumentNowLive - Check to see if Arg is alive. At this point, we
268 // know that the only uses of Arg are to be passed in as an argument to a
269 // function call or return. Check to see if the formal argument passed in is in
270 // the LiveArguments set. If so, return true.
272 bool DAE::isMaybeLiveArgumentNowLive(Argument *Arg) {
273 for (Value::use_iterator I = Arg->use_begin(), E = Arg->use_end(); I!=E; ++I){
274 if (isa<ReturnInst>(*I)) {
275 if (LiveRetVal.count(Arg->getParent())) return true;
279 CallSite CS = CallSite::get(*I);
281 // We know that this can only be used for direct calls...
282 Function *Callee = CS.getCalledFunction();
284 // Loop over all of the arguments (because Arg may be passed into the call
285 // multiple times) and check to see if any are now alive...
286 CallSite::arg_iterator CSAI = CS.arg_begin();
287 for (Function::aiterator AI = Callee->abegin(), E = Callee->aend();
288 AI != E; ++AI, ++CSAI)
289 // If this is the argument we are looking for, check to see if it's alive
290 if (*CSAI == Arg && LiveArguments.count(AI))
296 /// MarkArgumentLive - The MaybeLive argument 'Arg' is now known to be alive.
297 /// Mark it live in the specified sets and recursively mark arguments in callers
298 /// live that are needed to pass in a value.
300 void DAE::MarkArgumentLive(Argument *Arg) {
301 std::set<Argument*>::iterator It = MaybeLiveArguments.lower_bound(Arg);
302 if (It == MaybeLiveArguments.end() || *It != Arg) return;
304 DEBUG(std::cerr << " MaybeLive argument now live: " << Arg->getName()<<"\n");
305 MaybeLiveArguments.erase(It);
306 LiveArguments.insert(Arg);
308 // Loop over all of the call sites of the function, making any arguments
309 // passed in to provide a value for this argument live as necessary.
311 Function *Fn = Arg->getParent();
312 unsigned ArgNo = std::distance(Fn->abegin(), Function::aiterator(Arg));
314 std::multimap<Function*, CallSite>::iterator I = CallSites.lower_bound(Fn);
315 for (; I != CallSites.end() && I->first == Fn; ++I) {
316 CallSite CS = I->second;
317 Value *ArgVal = *(CS.arg_begin()+ArgNo);
318 if (Argument *ActualArg = dyn_cast<Argument>(ArgVal)) {
319 MarkArgumentLive(ActualArg);
321 // If the value passed in at this call site is a return value computed by
322 // some other call site, make sure to mark the return value at the other
323 // call site as being needed.
324 CallSite ArgCS = CallSite::get(ArgVal);
325 if (ArgCS.getInstruction())
326 if (Function *Fn = ArgCS.getCalledFunction())
332 /// MarkArgumentLive - The MaybeLive return value for the specified function is
333 /// now known to be alive. Propagate this fact to the return instructions which
335 void DAE::MarkRetValLive(Function *F) {
336 assert(F && "Shame shame, we can't have null pointers here!");
338 // Check to see if we already knew it was live
339 std::set<Function*>::iterator I = MaybeLiveRetVal.lower_bound(F);
340 if (I == MaybeLiveRetVal.end() || *I != F) return; // It's already alive!
342 DEBUG(std::cerr << " MaybeLive retval now live: " << F->getName() << "\n");
344 MaybeLiveRetVal.erase(I);
345 LiveRetVal.insert(F); // It is now known to be live!
347 // Loop over all of the functions, noticing that the return value is now live.
348 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
349 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()))
350 MarkReturnInstArgumentLive(RI);
353 void DAE::MarkReturnInstArgumentLive(ReturnInst *RI) {
354 Value *Op = RI->getOperand(0);
355 if (Argument *A = dyn_cast<Argument>(Op)) {
357 } else if (CallInst *CI = dyn_cast<CallInst>(Op)) {
358 if (Function *F = CI->getCalledFunction())
360 } else if (InvokeInst *II = dyn_cast<InvokeInst>(Op)) {
361 if (Function *F = II->getCalledFunction())
366 // RemoveDeadArgumentsFromFunction - We know that F has dead arguments, as
367 // specified by the DeadArguments list. Transform the function and all of the
368 // callees of the function to not have these arguments.
370 void DAE::RemoveDeadArgumentsFromFunction(Function *F) {
371 // Start by computing a new prototype for the function, which is the same as
372 // the old function, but has fewer arguments.
373 const FunctionType *FTy = F->getFunctionType();
374 std::vector<const Type*> Params;
376 for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I)
377 if (!DeadArguments.count(I))
378 Params.push_back(I->getType());
380 const Type *RetTy = FTy->getReturnType();
381 if (DeadRetVal.count(F)) {
382 RetTy = Type::VoidTy;
386 // Work around LLVM bug PR56: the CWriter cannot emit varargs functions which
387 // have zero fixed arguments.
389 // FIXME: once this bug is fixed in the CWriter, this hack should be removed.
391 bool ExtraArgHack = false;
392 if (Params.empty() && FTy->isVarArg()) {
394 Params.push_back(Type::IntTy);
397 FunctionType *NFTy = FunctionType::get(RetTy, Params, FTy->isVarArg());
399 // Create the new function body and insert it into the module...
400 Function *NF = new Function(NFTy, F->getLinkage(), F->getName());
401 F->getParent()->getFunctionList().insert(F, NF);
403 // Loop over all of the callers of the function, transforming the call sites
404 // to pass in a smaller number of arguments into the new function.
406 std::vector<Value*> Args;
407 while (!F->use_empty()) {
408 CallSite CS = CallSite::get(F->use_back());
409 Instruction *Call = CS.getInstruction();
411 // Loop over the operands, deleting dead ones...
412 CallSite::arg_iterator AI = CS.arg_begin();
413 for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I, ++AI)
414 if (!DeadArguments.count(I)) // Remove operands for dead arguments
418 Args.push_back(Constant::getNullValue(Type::IntTy));
420 // Push any varargs arguments on the list
421 for (; AI != CS.arg_end(); ++AI)
425 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
426 New = new InvokeInst(NF, II->getNormalDest(), II->getUnwindDest(),
429 New = new CallInst(NF, Args, "", Call);
433 if (!Call->use_empty()) {
434 if (New->getType() == Type::VoidTy)
435 Call->replaceAllUsesWith(Constant::getNullValue(Call->getType()));
437 Call->replaceAllUsesWith(New);
438 std::string Name = Call->getName();
444 // Finally, remove the old call from the program, reducing the use-count of
446 Call->getParent()->getInstList().erase(Call);
449 // Since we have now created the new function, splice the body of the old
450 // function right into the new function, leaving the old rotting hulk of the
452 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
454 // Loop over the argument list, transfering uses of the old arguments over to
455 // the new arguments, also transfering over the names as well. While we're at
456 // it, remove the dead arguments from the DeadArguments list.
458 for (Function::aiterator I = F->abegin(), E = F->aend(), I2 = NF->abegin();
460 if (!DeadArguments.count(I)) {
461 // If this is a live argument, move the name and users over to the new
463 I->replaceAllUsesWith(I2);
464 I2->setName(I->getName());
467 // If this argument is dead, replace any uses of it with null constants
468 // (these are guaranteed to only be operands to call instructions which
469 // will later be simplified).
470 I->replaceAllUsesWith(Constant::getNullValue(I->getType()));
471 DeadArguments.erase(I);
474 // If we change the return value of the function we must rewrite any return
475 // instructions. Check this now.
476 if (F->getReturnType() != NF->getReturnType())
477 for (Function::iterator BB = NF->begin(), E = NF->end(); BB != E; ++BB)
478 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
479 new ReturnInst(0, RI);
480 BB->getInstList().erase(RI);
483 // Now that the old function is dead, delete it.
484 F->getParent()->getFunctionList().erase(F);
487 bool DAE::runOnModule(Module &M) {
488 // First phase: loop through the module, determining which arguments are live.
489 // We assume all arguments are dead unless proven otherwise (allowing us to
490 // determine that dead arguments passed into recursive functions are dead).
492 DEBUG(std::cerr << "DAE - Determining liveness\n");
493 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
496 // Loop over the instructions to inspect, propagating liveness among arguments
497 // and return values which are MaybeLive.
499 while (!InstructionsToInspect.empty()) {
500 Instruction *I = InstructionsToInspect.back();
501 InstructionsToInspect.pop_back();
503 if (ReturnInst *RI = dyn_cast<ReturnInst>(I)) {
504 // For return instructions, we just have to check to see if the return
505 // value for the current function is known now to be alive. If so, any
506 // arguments used by it are now alive, and any call instruction return
507 // value is alive as well.
508 if (LiveRetVal.count(RI->getParent()->getParent()))
509 MarkReturnInstArgumentLive(RI);
512 CallSite CS = CallSite::get(I);
513 assert(CS.getInstruction() && "Unknown instruction for the I2I list!");
515 Function *Callee = CS.getCalledFunction();
517 // If we found a call or invoke instruction on this list, that means that
518 // an argument of the function is a call instruction. If the argument is
519 // live, then the return value of the called instruction is now live.
521 CallSite::arg_iterator AI = CS.arg_begin(); // ActualIterator
522 for (Function::aiterator FI = Callee->abegin(), E = Callee->aend();
523 FI != E; ++AI, ++FI) {
524 // If this argument is another call...
525 CallSite ArgCS = CallSite::get(*AI);
526 if (ArgCS.getInstruction() && LiveArguments.count(FI))
527 if (Function *Callee = ArgCS.getCalledFunction())
528 MarkRetValLive(Callee);
533 // Now we loop over all of the MaybeLive arguments, promoting them to be live
534 // arguments if one of the calls that uses the arguments to the calls they are
535 // passed into requires them to be live. Of course this could make other
536 // arguments live, so process callers recursively.
538 // Because elements can be removed from the MaybeLiveArguments set, copy it to
539 // a temporary vector.
541 std::vector<Argument*> TmpArgList(MaybeLiveArguments.begin(),
542 MaybeLiveArguments.end());
543 for (unsigned i = 0, e = TmpArgList.size(); i != e; ++i) {
544 Argument *MLA = TmpArgList[i];
545 if (MaybeLiveArguments.count(MLA) &&
546 isMaybeLiveArgumentNowLive(MLA))
547 MarkArgumentLive(MLA);
550 // Recover memory early...
553 // At this point, we know that all arguments in DeadArguments and
554 // MaybeLiveArguments are dead. If the two sets are empty, there is nothing
556 if (MaybeLiveArguments.empty() && DeadArguments.empty() &&
557 MaybeLiveRetVal.empty() && DeadRetVal.empty())
560 // Otherwise, compact into one set, and start eliminating the arguments from
562 DeadArguments.insert(MaybeLiveArguments.begin(), MaybeLiveArguments.end());
563 MaybeLiveArguments.clear();
564 DeadRetVal.insert(MaybeLiveRetVal.begin(), MaybeLiveRetVal.end());
565 MaybeLiveRetVal.clear();
567 LiveArguments.clear();
570 NumArgumentsEliminated += DeadArguments.size();
571 NumRetValsEliminated += DeadRetVal.size();
572 while (!DeadArguments.empty())
573 RemoveDeadArgumentsFromFunction((*DeadArguments.begin())->getParent());
575 while (!DeadRetVal.empty())
576 RemoveDeadArgumentsFromFunction(*DeadRetVal.begin());