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::arg_iterator AI = F.arg_begin(), E = F.arg_end();
215 LiveArguments.insert(AI);
216 LiveRetVal.insert(&F);
220 switch (RetValLiveness) {
221 case Live: LiveRetVal.insert(&F); break;
222 case MaybeLive: MaybeLiveRetVal.insert(&F); break;
223 case Dead: DeadRetVal.insert(&F); break;
226 DEBUG(std::cerr << " Inspecting args for fn: " << F.getName() << "\n");
228 // If it is not intrinsically alive, we know that all users of the
229 // function are call sites. Mark all of the arguments live which are
230 // directly used, and keep track of all of the call sites of this function
231 // if there are any arguments we assume that are dead.
233 bool AnyMaybeLiveArgs = false;
234 for (Function::arg_iterator AI = F.arg_begin(), E = F.arg_end();
236 switch (getArgumentLiveness(*AI)) {
238 DEBUG(std::cerr << " Arg live by use: " << AI->getName() << "\n");
239 LiveArguments.insert(AI);
242 DEBUG(std::cerr << " Arg definitely dead: " <<AI->getName()<<"\n");
243 DeadArguments.insert(AI);
246 DEBUG(std::cerr << " Arg only passed to calls: "
247 << AI->getName() << "\n");
248 AnyMaybeLiveArgs = true;
249 MaybeLiveArguments.insert(AI);
253 // If there are any "MaybeLive" arguments, we need to check callees of
254 // this function when/if they become alive. Record which functions are
256 if (AnyMaybeLiveArgs || RetValLiveness == MaybeLive)
257 for (Value::use_iterator I = F.use_begin(), E = F.use_end();
259 if (AnyMaybeLiveArgs)
260 CallSites.insert(std::make_pair(&F, CallSite::get(*I)));
262 if (RetValLiveness == MaybeLive)
263 for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
265 InstructionsToInspect.push_back(cast<Instruction>(*UI));
269 // isMaybeLiveArgumentNowLive - Check to see if Arg is alive. At this point, we
270 // know that the only uses of Arg are to be passed in as an argument to a
271 // function call or return. Check to see if the formal argument passed in is in
272 // the LiveArguments set. If so, return true.
274 bool DAE::isMaybeLiveArgumentNowLive(Argument *Arg) {
275 for (Value::use_iterator I = Arg->use_begin(), E = Arg->use_end(); I!=E; ++I){
276 if (isa<ReturnInst>(*I)) {
277 if (LiveRetVal.count(Arg->getParent())) return true;
281 CallSite CS = CallSite::get(*I);
283 // We know that this can only be used for direct calls...
284 Function *Callee = CS.getCalledFunction();
286 // Loop over all of the arguments (because Arg may be passed into the call
287 // multiple times) and check to see if any are now alive...
288 CallSite::arg_iterator CSAI = CS.arg_begin();
289 for (Function::arg_iterator AI = Callee->arg_begin(), E = Callee->arg_end();
290 AI != E; ++AI, ++CSAI)
291 // If this is the argument we are looking for, check to see if it's alive
292 if (*CSAI == Arg && LiveArguments.count(AI))
298 /// MarkArgumentLive - The MaybeLive argument 'Arg' is now known to be alive.
299 /// Mark it live in the specified sets and recursively mark arguments in callers
300 /// live that are needed to pass in a value.
302 void DAE::MarkArgumentLive(Argument *Arg) {
303 std::set<Argument*>::iterator It = MaybeLiveArguments.lower_bound(Arg);
304 if (It == MaybeLiveArguments.end() || *It != Arg) return;
306 DEBUG(std::cerr << " MaybeLive argument now live: " << Arg->getName()<<"\n");
307 MaybeLiveArguments.erase(It);
308 LiveArguments.insert(Arg);
310 // Loop over all of the call sites of the function, making any arguments
311 // passed in to provide a value for this argument live as necessary.
313 Function *Fn = Arg->getParent();
314 unsigned ArgNo = std::distance(Fn->arg_begin(), Function::arg_iterator(Arg));
316 std::multimap<Function*, CallSite>::iterator I = CallSites.lower_bound(Fn);
317 for (; I != CallSites.end() && I->first == Fn; ++I) {
318 CallSite CS = I->second;
319 Value *ArgVal = *(CS.arg_begin()+ArgNo);
320 if (Argument *ActualArg = dyn_cast<Argument>(ArgVal)) {
321 MarkArgumentLive(ActualArg);
323 // If the value passed in at this call site is a return value computed by
324 // some other call site, make sure to mark the return value at the other
325 // call site as being needed.
326 CallSite ArgCS = CallSite::get(ArgVal);
327 if (ArgCS.getInstruction())
328 if (Function *Fn = ArgCS.getCalledFunction())
334 /// MarkArgumentLive - The MaybeLive return value for the specified function is
335 /// now known to be alive. Propagate this fact to the return instructions which
337 void DAE::MarkRetValLive(Function *F) {
338 assert(F && "Shame shame, we can't have null pointers here!");
340 // Check to see if we already knew it was live
341 std::set<Function*>::iterator I = MaybeLiveRetVal.lower_bound(F);
342 if (I == MaybeLiveRetVal.end() || *I != F) return; // It's already alive!
344 DEBUG(std::cerr << " MaybeLive retval now live: " << F->getName() << "\n");
346 MaybeLiveRetVal.erase(I);
347 LiveRetVal.insert(F); // It is now known to be live!
349 // Loop over all of the functions, noticing that the return value is now live.
350 for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
351 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()))
352 MarkReturnInstArgumentLive(RI);
355 void DAE::MarkReturnInstArgumentLive(ReturnInst *RI) {
356 Value *Op = RI->getOperand(0);
357 if (Argument *A = dyn_cast<Argument>(Op)) {
359 } else if (CallInst *CI = dyn_cast<CallInst>(Op)) {
360 if (Function *F = CI->getCalledFunction())
362 } else if (InvokeInst *II = dyn_cast<InvokeInst>(Op)) {
363 if (Function *F = II->getCalledFunction())
368 // RemoveDeadArgumentsFromFunction - We know that F has dead arguments, as
369 // specified by the DeadArguments list. Transform the function and all of the
370 // callees of the function to not have these arguments.
372 void DAE::RemoveDeadArgumentsFromFunction(Function *F) {
373 // Start by computing a new prototype for the function, which is the same as
374 // the old function, but has fewer arguments.
375 const FunctionType *FTy = F->getFunctionType();
376 std::vector<const Type*> Params;
378 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E; ++I)
379 if (!DeadArguments.count(I))
380 Params.push_back(I->getType());
382 const Type *RetTy = FTy->getReturnType();
383 if (DeadRetVal.count(F)) {
384 RetTy = Type::VoidTy;
388 // Work around LLVM bug PR56: the CWriter cannot emit varargs functions which
389 // have zero fixed arguments.
391 // FIXME: once this bug is fixed in the CWriter, this hack should be removed.
393 bool ExtraArgHack = false;
394 if (Params.empty() && FTy->isVarArg()) {
396 Params.push_back(Type::IntTy);
399 FunctionType *NFTy = FunctionType::get(RetTy, Params, FTy->isVarArg());
401 // Create the new function body and insert it into the module...
402 Function *NF = new Function(NFTy, F->getLinkage(), F->getName());
403 NF->setCallingConv(F->getCallingConv());
404 F->getParent()->getFunctionList().insert(F, NF);
406 // Loop over all of the callers of the function, transforming the call sites
407 // to pass in a smaller number of arguments into the new function.
409 std::vector<Value*> Args;
410 while (!F->use_empty()) {
411 CallSite CS = CallSite::get(F->use_back());
412 Instruction *Call = CS.getInstruction();
414 // Loop over the operands, deleting dead ones...
415 CallSite::arg_iterator AI = CS.arg_begin();
416 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
418 if (!DeadArguments.count(I)) // Remove operands for dead arguments
422 Args.push_back(Constant::getNullValue(Type::IntTy));
424 // Push any varargs arguments on the list
425 for (; AI != CS.arg_end(); ++AI)
429 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
430 New = new InvokeInst(NF, II->getNormalDest(), II->getUnwindDest(),
432 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
434 New = new CallInst(NF, Args, "", Call);
435 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
436 if (cast<CallInst>(Call)->isTailCall())
437 cast<CallInst>(New)->setTailCall();
441 if (!Call->use_empty()) {
442 if (New->getType() == Type::VoidTy)
443 Call->replaceAllUsesWith(Constant::getNullValue(Call->getType()));
445 Call->replaceAllUsesWith(New);
446 std::string Name = Call->getName();
452 // Finally, remove the old call from the program, reducing the use-count of
454 Call->getParent()->getInstList().erase(Call);
457 // Since we have now created the new function, splice the body of the old
458 // function right into the new function, leaving the old rotting hulk of the
460 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
462 // Loop over the argument list, transfering uses of the old arguments over to
463 // the new arguments, also transfering over the names as well. While we're at
464 // it, remove the dead arguments from the DeadArguments list.
466 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(),
467 I2 = NF->arg_begin();
469 if (!DeadArguments.count(I)) {
470 // If this is a live argument, move the name and users over to the new
472 I->replaceAllUsesWith(I2);
473 I2->setName(I->getName());
476 // If this argument is dead, replace any uses of it with null constants
477 // (these are guaranteed to only be operands to call instructions which
478 // will later be simplified).
479 I->replaceAllUsesWith(Constant::getNullValue(I->getType()));
480 DeadArguments.erase(I);
483 // If we change the return value of the function we must rewrite any return
484 // instructions. Check this now.
485 if (F->getReturnType() != NF->getReturnType())
486 for (Function::iterator BB = NF->begin(), E = NF->end(); BB != E; ++BB)
487 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
488 new ReturnInst(0, RI);
489 BB->getInstList().erase(RI);
492 // Now that the old function is dead, delete it.
493 F->getParent()->getFunctionList().erase(F);
496 bool DAE::runOnModule(Module &M) {
497 // First phase: loop through the module, determining which arguments are live.
498 // We assume all arguments are dead unless proven otherwise (allowing us to
499 // determine that dead arguments passed into recursive functions are dead).
501 DEBUG(std::cerr << "DAE - Determining liveness\n");
502 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
505 // Loop over the instructions to inspect, propagating liveness among arguments
506 // and return values which are MaybeLive.
508 while (!InstructionsToInspect.empty()) {
509 Instruction *I = InstructionsToInspect.back();
510 InstructionsToInspect.pop_back();
512 if (ReturnInst *RI = dyn_cast<ReturnInst>(I)) {
513 // For return instructions, we just have to check to see if the return
514 // value for the current function is known now to be alive. If so, any
515 // arguments used by it are now alive, and any call instruction return
516 // value is alive as well.
517 if (LiveRetVal.count(RI->getParent()->getParent()))
518 MarkReturnInstArgumentLive(RI);
521 CallSite CS = CallSite::get(I);
522 assert(CS.getInstruction() && "Unknown instruction for the I2I list!");
524 Function *Callee = CS.getCalledFunction();
526 // If we found a call or invoke instruction on this list, that means that
527 // an argument of the function is a call instruction. If the argument is
528 // live, then the return value of the called instruction is now live.
530 CallSite::arg_iterator AI = CS.arg_begin(); // ActualIterator
531 for (Function::arg_iterator FI = Callee->arg_begin(),
532 E = Callee->arg_end(); FI != E; ++AI, ++FI) {
533 // If this argument is another call...
534 CallSite ArgCS = CallSite::get(*AI);
535 if (ArgCS.getInstruction() && LiveArguments.count(FI))
536 if (Function *Callee = ArgCS.getCalledFunction())
537 MarkRetValLive(Callee);
542 // Now we loop over all of the MaybeLive arguments, promoting them to be live
543 // arguments if one of the calls that uses the arguments to the calls they are
544 // passed into requires them to be live. Of course this could make other
545 // arguments live, so process callers recursively.
547 // Because elements can be removed from the MaybeLiveArguments set, copy it to
548 // a temporary vector.
550 std::vector<Argument*> TmpArgList(MaybeLiveArguments.begin(),
551 MaybeLiveArguments.end());
552 for (unsigned i = 0, e = TmpArgList.size(); i != e; ++i) {
553 Argument *MLA = TmpArgList[i];
554 if (MaybeLiveArguments.count(MLA) &&
555 isMaybeLiveArgumentNowLive(MLA))
556 MarkArgumentLive(MLA);
559 // Recover memory early...
562 // At this point, we know that all arguments in DeadArguments and
563 // MaybeLiveArguments are dead. If the two sets are empty, there is nothing
565 if (MaybeLiveArguments.empty() && DeadArguments.empty() &&
566 MaybeLiveRetVal.empty() && DeadRetVal.empty())
569 // Otherwise, compact into one set, and start eliminating the arguments from
571 DeadArguments.insert(MaybeLiveArguments.begin(), MaybeLiveArguments.end());
572 MaybeLiveArguments.clear();
573 DeadRetVal.insert(MaybeLiveRetVal.begin(), MaybeLiveRetVal.end());
574 MaybeLiveRetVal.clear();
576 LiveArguments.clear();
579 NumArgumentsEliminated += DeadArguments.size();
580 NumRetValsEliminated += DeadRetVal.size();
581 while (!DeadArguments.empty())
582 RemoveDeadArgumentsFromFunction((*DeadArguments.begin())->getParent());
584 while (!DeadRetVal.empty())
585 RemoveDeadArgumentsFromFunction(*DeadRetVal.begin());