1 //===-- DeadArgumentElimination.cpp - Eliminate dead arguments ------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // 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 return values 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 or return values.
18 //===----------------------------------------------------------------------===//
20 #include "llvm/Transforms/IPO.h"
21 #include "llvm/ADT/DenseMap.h"
22 #include "llvm/ADT/SmallVector.h"
23 #include "llvm/ADT/Statistic.h"
24 #include "llvm/ADT/StringExtras.h"
25 #include "llvm/IR/CallSite.h"
26 #include "llvm/IR/CallingConv.h"
27 #include "llvm/IR/Constant.h"
28 #include "llvm/IR/DIBuilder.h"
29 #include "llvm/IR/DebugInfo.h"
30 #include "llvm/IR/DerivedTypes.h"
31 #include "llvm/IR/Instructions.h"
32 #include "llvm/IR/IntrinsicInst.h"
33 #include "llvm/IR/LLVMContext.h"
34 #include "llvm/IR/Module.h"
35 #include "llvm/Pass.h"
36 #include "llvm/Support/Debug.h"
37 #include "llvm/Support/raw_ostream.h"
38 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
44 #define DEBUG_TYPE "deadargelim"
46 STATISTIC(NumArgumentsEliminated, "Number of unread args removed");
47 STATISTIC(NumRetValsEliminated , "Number of unused return values removed");
48 STATISTIC(NumArgumentsReplacedWithUndef,
49 "Number of unread args replaced with undef");
51 /// DAE - The dead argument elimination pass.
53 class DAE : public ModulePass {
56 /// Struct that represents (part of) either a return value or a function
57 /// argument. Used so that arguments and return values can be used
60 RetOrArg(const Function *F, unsigned Idx, bool IsArg) : F(F), Idx(Idx),
66 /// Make RetOrArg comparable, so we can put it into a map.
67 bool operator<(const RetOrArg &O) const {
68 return std::tie(F, Idx, IsArg) < std::tie(O.F, O.Idx, O.IsArg);
71 /// Make RetOrArg comparable, so we can easily iterate the multimap.
72 bool operator==(const RetOrArg &O) const {
73 return F == O.F && Idx == O.Idx && IsArg == O.IsArg;
76 std::string getDescription() const {
77 return (Twine(IsArg ? "Argument #" : "Return value #") + utostr(Idx) +
78 " of function " + F->getName()).str();
82 /// Liveness enum - During our initial pass over the program, we determine
83 /// that things are either alive or maybe alive. We don't mark anything
84 /// explicitly dead (even if we know they are), since anything not alive
85 /// with no registered uses (in Uses) will never be marked alive and will
86 /// thus become dead in the end.
87 enum Liveness { Live, MaybeLive };
89 /// Convenience wrapper
90 RetOrArg CreateRet(const Function *F, unsigned Idx) {
91 return RetOrArg(F, Idx, false);
93 /// Convenience wrapper
94 RetOrArg CreateArg(const Function *F, unsigned Idx) {
95 return RetOrArg(F, Idx, true);
98 typedef std::multimap<RetOrArg, RetOrArg> UseMap;
99 /// This maps a return value or argument to any MaybeLive return values or
100 /// arguments it uses. This allows the MaybeLive values to be marked live
101 /// when any of its users is marked live.
102 /// For example (indices are left out for clarity):
103 /// - Uses[ret F] = ret G
104 /// This means that F calls G, and F returns the value returned by G.
105 /// - Uses[arg F] = ret G
106 /// This means that some function calls G and passes its result as an
108 /// - Uses[ret F] = arg F
109 /// This means that F returns one of its own arguments.
110 /// - Uses[arg F] = arg G
111 /// This means that G calls F and passes one of its own (G's) arguments
115 typedef std::set<RetOrArg> LiveSet;
116 typedef std::set<const Function*> LiveFuncSet;
118 /// This set contains all values that have been determined to be live.
120 /// This set contains all values that are cannot be changed in any way.
121 LiveFuncSet LiveFunctions;
123 typedef SmallVector<RetOrArg, 5> UseVector;
126 // DAH uses this to specify a different ID.
127 explicit DAE(char &ID) : ModulePass(ID) {}
130 static char ID; // Pass identification, replacement for typeid
131 DAE() : ModulePass(ID) {
132 initializeDAEPass(*PassRegistry::getPassRegistry());
135 bool runOnModule(Module &M) override;
137 virtual bool ShouldHackArguments() const { return false; }
140 Liveness MarkIfNotLive(RetOrArg Use, UseVector &MaybeLiveUses);
141 Liveness SurveyUse(const Use *U, UseVector &MaybeLiveUses,
142 unsigned RetValNum = -1U);
143 Liveness SurveyUses(const Value *V, UseVector &MaybeLiveUses);
145 void SurveyFunction(const Function &F);
146 void MarkValue(const RetOrArg &RA, Liveness L,
147 const UseVector &MaybeLiveUses);
148 void MarkLive(const RetOrArg &RA);
149 void MarkLive(const Function &F);
150 void PropagateLiveness(const RetOrArg &RA);
151 bool RemoveDeadStuffFromFunction(Function *F);
152 bool DeleteDeadVarargs(Function &Fn);
153 bool RemoveDeadArgumentsFromCallers(Function &Fn);
159 INITIALIZE_PASS(DAE, "deadargelim", "Dead Argument Elimination", false, false)
162 /// DAH - DeadArgumentHacking pass - Same as dead argument elimination, but
163 /// deletes arguments to functions which are external. This is only for use
165 struct DAH : public DAE {
169 bool ShouldHackArguments() const override { return true; }
174 INITIALIZE_PASS(DAH, "deadarghaX0r",
175 "Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)",
178 /// createDeadArgEliminationPass - This pass removes arguments from functions
179 /// which are not used by the body of the function.
181 ModulePass *llvm::createDeadArgEliminationPass() { return new DAE(); }
182 ModulePass *llvm::createDeadArgHackingPass() { return new DAH(); }
184 /// DeleteDeadVarargs - If this is an function that takes a ... list, and if
185 /// llvm.vastart is never called, the varargs list is dead for the function.
186 bool DAE::DeleteDeadVarargs(Function &Fn) {
187 assert(Fn.getFunctionType()->isVarArg() && "Function isn't varargs!");
188 if (Fn.isDeclaration() || !Fn.hasLocalLinkage()) return false;
190 // Ensure that the function is only directly called.
191 if (Fn.hasAddressTaken())
194 // Don't touch naked functions. The assembly might be using an argument, or
195 // otherwise rely on the frame layout in a way that this analysis will not
197 if (Fn.hasFnAttribute(Attribute::Naked)) {
201 // Okay, we know we can transform this function if safe. Scan its body
202 // looking for calls marked musttail or calls to llvm.vastart.
203 for (Function::iterator BB = Fn.begin(), E = Fn.end(); BB != E; ++BB) {
204 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
205 CallInst *CI = dyn_cast<CallInst>(I);
208 if (CI->isMustTailCall())
210 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(CI)) {
211 if (II->getIntrinsicID() == Intrinsic::vastart)
217 // If we get here, there are no calls to llvm.vastart in the function body,
218 // remove the "..." and adjust all the calls.
220 // Start by computing a new prototype for the function, which is the same as
221 // the old function, but doesn't have isVarArg set.
222 FunctionType *FTy = Fn.getFunctionType();
224 std::vector<Type*> Params(FTy->param_begin(), FTy->param_end());
225 FunctionType *NFTy = FunctionType::get(FTy->getReturnType(),
227 unsigned NumArgs = Params.size();
229 // Create the new function body and insert it into the module...
230 Function *NF = Function::Create(NFTy, Fn.getLinkage());
231 NF->copyAttributesFrom(&Fn);
232 Fn.getParent()->getFunctionList().insert(Fn.getIterator(), NF);
235 // Loop over all of the callers of the function, transforming the call sites
236 // to pass in a smaller number of arguments into the new function.
238 std::vector<Value*> Args;
239 for (Value::user_iterator I = Fn.user_begin(), E = Fn.user_end(); I != E; ) {
243 Instruction *Call = CS.getInstruction();
245 // Pass all the same arguments.
246 Args.assign(CS.arg_begin(), CS.arg_begin() + NumArgs);
248 // Drop any attributes that were on the vararg arguments.
249 AttributeSet PAL = CS.getAttributes();
250 if (!PAL.isEmpty() && PAL.getSlotIndex(PAL.getNumSlots() - 1) > NumArgs) {
251 SmallVector<AttributeSet, 8> AttributesVec;
252 for (unsigned i = 0; PAL.getSlotIndex(i) <= NumArgs; ++i)
253 AttributesVec.push_back(PAL.getSlotAttributes(i));
254 if (PAL.hasAttributes(AttributeSet::FunctionIndex))
255 AttributesVec.push_back(AttributeSet::get(Fn.getContext(),
256 PAL.getFnAttributes()));
257 PAL = AttributeSet::get(Fn.getContext(), AttributesVec);
261 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
262 New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
264 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
265 cast<InvokeInst>(New)->setAttributes(PAL);
267 New = CallInst::Create(NF, Args, "", Call);
268 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
269 cast<CallInst>(New)->setAttributes(PAL);
270 if (cast<CallInst>(Call)->isTailCall())
271 cast<CallInst>(New)->setTailCall();
273 New->setDebugLoc(Call->getDebugLoc());
277 if (!Call->use_empty())
278 Call->replaceAllUsesWith(New);
282 // Finally, remove the old call from the program, reducing the use-count of
284 Call->eraseFromParent();
287 // Since we have now created the new function, splice the body of the old
288 // function right into the new function, leaving the old rotting hulk of the
290 NF->getBasicBlockList().splice(NF->begin(), Fn.getBasicBlockList());
292 // Loop over the argument list, transferring uses of the old arguments over to
293 // the new arguments, also transferring over the names as well. While we're at
294 // it, remove the dead arguments from the DeadArguments list.
296 for (Function::arg_iterator I = Fn.arg_begin(), E = Fn.arg_end(),
297 I2 = NF->arg_begin(); I != E; ++I, ++I2) {
298 // Move the name and users over to the new version.
299 I->replaceAllUsesWith(&*I2);
303 // Patch the pointer to LLVM function in debug info descriptor.
304 NF->setSubprogram(Fn.getSubprogram());
306 // Fix up any BlockAddresses that refer to the function.
307 Fn.replaceAllUsesWith(ConstantExpr::getBitCast(NF, Fn.getType()));
308 // Delete the bitcast that we just created, so that NF does not
309 // appear to be address-taken.
310 NF->removeDeadConstantUsers();
311 // Finally, nuke the old function.
312 Fn.eraseFromParent();
316 /// RemoveDeadArgumentsFromCallers - Checks if the given function has any
317 /// arguments that are unused, and changes the caller parameters to be undefined
319 bool DAE::RemoveDeadArgumentsFromCallers(Function &Fn)
321 // We cannot change the arguments if this TU does not define the function or
322 // if the linker may choose a function body from another TU, even if the
323 // nominal linkage indicates that other copies of the function have the same
324 // semantics. In the below example, the dead load from %p may not have been
325 // eliminated from the linker-chosen copy of f, so replacing %p with undef
326 // in callers may introduce undefined behavior.
328 // define linkonce_odr void @f(i32* %p) {
332 if (!Fn.isStrongDefinitionForLinker())
335 // Functions with local linkage should already have been handled, except the
336 // fragile (variadic) ones which we can improve here.
337 if (Fn.hasLocalLinkage() && !Fn.getFunctionType()->isVarArg())
340 // Don't touch naked functions. The assembly might be using an argument, or
341 // otherwise rely on the frame layout in a way that this analysis will not
343 if (Fn.hasFnAttribute(Attribute::Naked))
349 SmallVector<unsigned, 8> UnusedArgs;
350 for (Argument &Arg : Fn.args()) {
351 if (Arg.use_empty() && !Arg.hasByValOrInAllocaAttr())
352 UnusedArgs.push_back(Arg.getArgNo());
355 if (UnusedArgs.empty())
358 bool Changed = false;
360 for (Use &U : Fn.uses()) {
361 CallSite CS(U.getUser());
362 if (!CS || !CS.isCallee(&U))
365 // Now go through all unused args and replace them with "undef".
366 for (unsigned I = 0, E = UnusedArgs.size(); I != E; ++I) {
367 unsigned ArgNo = UnusedArgs[I];
369 Value *Arg = CS.getArgument(ArgNo);
370 CS.setArgument(ArgNo, UndefValue::get(Arg->getType()));
371 ++NumArgumentsReplacedWithUndef;
379 /// Convenience function that returns the number of return values. It returns 0
380 /// for void functions and 1 for functions not returning a struct. It returns
381 /// the number of struct elements for functions returning a struct.
382 static unsigned NumRetVals(const Function *F) {
383 Type *RetTy = F->getReturnType();
384 if (RetTy->isVoidTy())
386 else if (StructType *STy = dyn_cast<StructType>(RetTy))
387 return STy->getNumElements();
388 else if (ArrayType *ATy = dyn_cast<ArrayType>(RetTy))
389 return ATy->getNumElements();
394 /// Returns the sub-type a function will return at a given Idx. Should
395 /// correspond to the result type of an ExtractValue instruction executed with
396 /// just that one Idx (i.e. only top-level structure is considered).
397 static Type *getRetComponentType(const Function *F, unsigned Idx) {
398 Type *RetTy = F->getReturnType();
399 assert(!RetTy->isVoidTy() && "void type has no subtype");
401 if (StructType *STy = dyn_cast<StructType>(RetTy))
402 return STy->getElementType(Idx);
403 else if (ArrayType *ATy = dyn_cast<ArrayType>(RetTy))
404 return ATy->getElementType();
409 /// MarkIfNotLive - This checks Use for liveness in LiveValues. If Use is not
410 /// live, it adds Use to the MaybeLiveUses argument. Returns the determined
412 DAE::Liveness DAE::MarkIfNotLive(RetOrArg Use, UseVector &MaybeLiveUses) {
413 // We're live if our use or its Function is already marked as live.
414 if (LiveFunctions.count(Use.F) || LiveValues.count(Use))
417 // We're maybe live otherwise, but remember that we must become live if
419 MaybeLiveUses.push_back(Use);
424 /// SurveyUse - This looks at a single use of an argument or return value
425 /// and determines if it should be alive or not. Adds this use to MaybeLiveUses
426 /// if it causes the used value to become MaybeLive.
428 /// RetValNum is the return value number to use when this use is used in a
429 /// return instruction. This is used in the recursion, you should always leave
431 DAE::Liveness DAE::SurveyUse(const Use *U,
432 UseVector &MaybeLiveUses, unsigned RetValNum) {
433 const User *V = U->getUser();
434 if (const ReturnInst *RI = dyn_cast<ReturnInst>(V)) {
435 // The value is returned from a function. It's only live when the
436 // function's return value is live. We use RetValNum here, for the case
437 // that U is really a use of an insertvalue instruction that uses the
439 const Function *F = RI->getParent()->getParent();
440 if (RetValNum != -1U) {
441 RetOrArg Use = CreateRet(F, RetValNum);
442 // We might be live, depending on the liveness of Use.
443 return MarkIfNotLive(Use, MaybeLiveUses);
445 DAE::Liveness Result = MaybeLive;
446 for (unsigned i = 0; i < NumRetVals(F); ++i) {
447 RetOrArg Use = CreateRet(F, i);
448 // We might be live, depending on the liveness of Use. If any
449 // sub-value is live, then the entire value is considered live. This
450 // is a conservative choice, and better tracking is possible.
451 DAE::Liveness SubResult = MarkIfNotLive(Use, MaybeLiveUses);
458 if (const InsertValueInst *IV = dyn_cast<InsertValueInst>(V)) {
459 if (U->getOperandNo() != InsertValueInst::getAggregateOperandIndex()
461 // The use we are examining is inserted into an aggregate. Our liveness
462 // depends on all uses of that aggregate, but if it is used as a return
463 // value, only index at which we were inserted counts.
464 RetValNum = *IV->idx_begin();
466 // Note that if we are used as the aggregate operand to the insertvalue,
467 // we don't change RetValNum, but do survey all our uses.
469 Liveness Result = MaybeLive;
470 for (const Use &UU : IV->uses()) {
471 Result = SurveyUse(&UU, MaybeLiveUses, RetValNum);
478 if (auto CS = ImmutableCallSite(V)) {
479 const Function *F = CS.getCalledFunction();
481 // Used in a direct call.
483 // The function argument is live if it is used as a bundle operand.
484 if (CS.isBundleOperand(U))
487 // Find the argument number. We know for sure that this use is an
488 // argument, since if it was the function argument this would be an
489 // indirect call and the we know can't be looking at a value of the
490 // label type (for the invoke instruction).
491 unsigned ArgNo = CS.getArgumentNo(U);
493 if (ArgNo >= F->getFunctionType()->getNumParams())
494 // The value is passed in through a vararg! Must be live.
497 assert(CS.getArgument(ArgNo)
498 == CS->getOperand(U->getOperandNo())
499 && "Argument is not where we expected it");
501 // Value passed to a normal call. It's only live when the corresponding
502 // argument to the called function turns out live.
503 RetOrArg Use = CreateArg(F, ArgNo);
504 return MarkIfNotLive(Use, MaybeLiveUses);
507 // Used in any other way? Value must be live.
511 /// SurveyUses - This looks at all the uses of the given value
512 /// Returns the Liveness deduced from the uses of this value.
514 /// Adds all uses that cause the result to be MaybeLive to MaybeLiveRetUses. If
515 /// the result is Live, MaybeLiveUses might be modified but its content should
516 /// be ignored (since it might not be complete).
517 DAE::Liveness DAE::SurveyUses(const Value *V, UseVector &MaybeLiveUses) {
518 // Assume it's dead (which will only hold if there are no uses at all..).
519 Liveness Result = MaybeLive;
521 for (const Use &U : V->uses()) {
522 Result = SurveyUse(&U, MaybeLiveUses);
529 // SurveyFunction - This performs the initial survey of the specified function,
530 // checking out whether or not it uses any of its incoming arguments or whether
531 // any callers use the return value. This fills in the LiveValues set and Uses
534 // We consider arguments of non-internal functions to be intrinsically alive as
535 // well as arguments to functions which have their "address taken".
537 void DAE::SurveyFunction(const Function &F) {
538 // Functions with inalloca parameters are expecting args in a particular
539 // register and memory layout.
540 if (F.getAttributes().hasAttrSomewhere(Attribute::InAlloca)) {
545 // Don't touch naked functions. The assembly might be using an argument, or
546 // otherwise rely on the frame layout in a way that this analysis will not
548 if (F.hasFnAttribute(Attribute::Naked)) {
553 unsigned RetCount = NumRetVals(&F);
554 // Assume all return values are dead
555 typedef SmallVector<Liveness, 5> RetVals;
556 RetVals RetValLiveness(RetCount, MaybeLive);
558 typedef SmallVector<UseVector, 5> RetUses;
559 // These vectors map each return value to the uses that make it MaybeLive, so
560 // we can add those to the Uses map if the return value really turns out to be
561 // MaybeLive. Initialized to a list of RetCount empty lists.
562 RetUses MaybeLiveRetUses(RetCount);
564 for (Function::const_iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
565 if (const ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()))
566 if (RI->getNumOperands() != 0 && RI->getOperand(0)->getType()
567 != F.getFunctionType()->getReturnType()) {
568 // We don't support old style multiple return values.
573 if (!F.hasLocalLinkage() && (!ShouldHackArguments() || F.isIntrinsic())) {
578 DEBUG(dbgs() << "DAE - Inspecting callers for fn: " << F.getName() << "\n");
579 // Keep track of the number of live retvals, so we can skip checks once all
580 // of them turn out to be live.
581 unsigned NumLiveRetVals = 0;
582 // Loop all uses of the function.
583 for (const Use &U : F.uses()) {
584 // If the function is PASSED IN as an argument, its address has been
586 ImmutableCallSite CS(U.getUser());
587 if (!CS || !CS.isCallee(&U)) {
592 // If this use is anything other than a call site, the function is alive.
593 const Instruction *TheCall = CS.getInstruction();
594 if (!TheCall) { // Not a direct call site?
599 // If we end up here, we are looking at a direct call to our function.
601 // Now, check how our return value(s) is/are used in this caller. Don't
602 // bother checking return values if all of them are live already.
603 if (NumLiveRetVals == RetCount)
606 // Check all uses of the return value.
607 for (const Use &U : TheCall->uses()) {
608 if (ExtractValueInst *Ext = dyn_cast<ExtractValueInst>(U.getUser())) {
609 // This use uses a part of our return value, survey the uses of
610 // that part and store the results for this index only.
611 unsigned Idx = *Ext->idx_begin();
612 if (RetValLiveness[Idx] != Live) {
613 RetValLiveness[Idx] = SurveyUses(Ext, MaybeLiveRetUses[Idx]);
614 if (RetValLiveness[Idx] == Live)
618 // Used by something else than extractvalue. Survey, but assume that the
619 // result applies to all sub-values.
620 UseVector MaybeLiveAggregateUses;
621 if (SurveyUse(&U, MaybeLiveAggregateUses) == Live) {
622 NumLiveRetVals = RetCount;
623 RetValLiveness.assign(RetCount, Live);
626 for (unsigned i = 0; i != RetCount; ++i) {
627 if (RetValLiveness[i] != Live)
628 MaybeLiveRetUses[i].append(MaybeLiveAggregateUses.begin(),
629 MaybeLiveAggregateUses.end());
636 // Now we've inspected all callers, record the liveness of our return values.
637 for (unsigned i = 0; i != RetCount; ++i)
638 MarkValue(CreateRet(&F, i), RetValLiveness[i], MaybeLiveRetUses[i]);
640 DEBUG(dbgs() << "DAE - Inspecting args for fn: " << F.getName() << "\n");
642 // Now, check all of our arguments.
644 UseVector MaybeLiveArgUses;
645 for (Function::const_arg_iterator AI = F.arg_begin(),
646 E = F.arg_end(); AI != E; ++AI, ++i) {
648 if (F.getFunctionType()->isVarArg()) {
649 // Variadic functions will already have a va_arg function expanded inside
650 // them, making them potentially very sensitive to ABI changes resulting
651 // from removing arguments entirely, so don't. For example AArch64 handles
652 // register and stack HFAs very differently, and this is reflected in the
653 // IR which has already been generated.
656 // See what the effect of this use is (recording any uses that cause
657 // MaybeLive in MaybeLiveArgUses).
658 Result = SurveyUses(&*AI, MaybeLiveArgUses);
662 MarkValue(CreateArg(&F, i), Result, MaybeLiveArgUses);
663 // Clear the vector again for the next iteration.
664 MaybeLiveArgUses.clear();
668 /// MarkValue - This function marks the liveness of RA depending on L. If L is
669 /// MaybeLive, it also takes all uses in MaybeLiveUses and records them in Uses,
670 /// such that RA will be marked live if any use in MaybeLiveUses gets marked
672 void DAE::MarkValue(const RetOrArg &RA, Liveness L,
673 const UseVector &MaybeLiveUses) {
675 case Live: MarkLive(RA); break;
678 // Note any uses of this value, so this return value can be
679 // marked live whenever one of the uses becomes live.
680 for (UseVector::const_iterator UI = MaybeLiveUses.begin(),
681 UE = MaybeLiveUses.end(); UI != UE; ++UI)
682 Uses.insert(std::make_pair(*UI, RA));
688 /// MarkLive - Mark the given Function as alive, meaning that it cannot be
689 /// changed in any way. Additionally,
690 /// mark any values that are used as this function's parameters or by its return
691 /// values (according to Uses) live as well.
692 void DAE::MarkLive(const Function &F) {
693 DEBUG(dbgs() << "DAE - Intrinsically live fn: " << F.getName() << "\n");
694 // Mark the function as live.
695 LiveFunctions.insert(&F);
696 // Mark all arguments as live.
697 for (unsigned i = 0, e = F.arg_size(); i != e; ++i)
698 PropagateLiveness(CreateArg(&F, i));
699 // Mark all return values as live.
700 for (unsigned i = 0, e = NumRetVals(&F); i != e; ++i)
701 PropagateLiveness(CreateRet(&F, i));
704 /// MarkLive - Mark the given return value or argument as live. Additionally,
705 /// mark any values that are used by this value (according to Uses) live as
707 void DAE::MarkLive(const RetOrArg &RA) {
708 if (LiveFunctions.count(RA.F))
709 return; // Function was already marked Live.
711 if (!LiveValues.insert(RA).second)
712 return; // We were already marked Live.
714 DEBUG(dbgs() << "DAE - Marking " << RA.getDescription() << " live\n");
715 PropagateLiveness(RA);
718 /// PropagateLiveness - Given that RA is a live value, propagate it's liveness
719 /// to any other values it uses (according to Uses).
720 void DAE::PropagateLiveness(const RetOrArg &RA) {
721 // We don't use upper_bound (or equal_range) here, because our recursive call
722 // to ourselves is likely to cause the upper_bound (which is the first value
723 // not belonging to RA) to become erased and the iterator invalidated.
724 UseMap::iterator Begin = Uses.lower_bound(RA);
725 UseMap::iterator E = Uses.end();
727 for (I = Begin; I != E && I->first == RA; ++I)
730 // Erase RA from the Uses map (from the lower bound to wherever we ended up
732 Uses.erase(Begin, I);
735 // RemoveDeadStuffFromFunction - Remove any arguments and return values from F
736 // that are not in LiveValues. Transform the function and all of the callees of
737 // the function to not have these arguments and return values.
739 bool DAE::RemoveDeadStuffFromFunction(Function *F) {
740 // Don't modify fully live functions
741 if (LiveFunctions.count(F))
744 // Start by computing a new prototype for the function, which is the same as
745 // the old function, but has fewer arguments and a different return type.
746 FunctionType *FTy = F->getFunctionType();
747 std::vector<Type*> Params;
749 // Keep track of if we have a live 'returned' argument
750 bool HasLiveReturnedArg = false;
752 // Set up to build a new list of parameter attributes.
753 SmallVector<AttributeSet, 8> AttributesVec;
754 const AttributeSet &PAL = F->getAttributes();
756 // Remember which arguments are still alive.
757 SmallVector<bool, 10> ArgAlive(FTy->getNumParams(), false);
758 // Construct the new parameter list from non-dead arguments. Also construct
759 // a new set of parameter attributes to correspond. Skip the first parameter
760 // attribute, since that belongs to the return value.
762 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
764 RetOrArg Arg = CreateArg(F, i);
765 if (LiveValues.erase(Arg)) {
766 Params.push_back(I->getType());
769 // Get the original parameter attributes (skipping the first one, that is
770 // for the return value.
771 if (PAL.hasAttributes(i + 1)) {
772 AttrBuilder B(PAL, i + 1);
773 if (B.contains(Attribute::Returned))
774 HasLiveReturnedArg = true;
776 push_back(AttributeSet::get(F->getContext(), Params.size(), B));
779 ++NumArgumentsEliminated;
780 DEBUG(dbgs() << "DAE - Removing argument " << i << " (" << I->getName()
781 << ") from " << F->getName() << "\n");
785 // Find out the new return value.
786 Type *RetTy = FTy->getReturnType();
787 Type *NRetTy = nullptr;
788 unsigned RetCount = NumRetVals(F);
790 // -1 means unused, other numbers are the new index
791 SmallVector<int, 5> NewRetIdxs(RetCount, -1);
792 std::vector<Type*> RetTypes;
794 // If there is a function with a live 'returned' argument but a dead return
795 // value, then there are two possible actions:
796 // 1) Eliminate the return value and take off the 'returned' attribute on the
798 // 2) Retain the 'returned' attribute and treat the return value (but not the
799 // entire function) as live so that it is not eliminated.
801 // It's not clear in the general case which option is more profitable because,
802 // even in the absence of explicit uses of the return value, code generation
803 // is free to use the 'returned' attribute to do things like eliding
804 // save/restores of registers across calls. Whether or not this happens is
805 // target and ABI-specific as well as depending on the amount of register
806 // pressure, so there's no good way for an IR-level pass to figure this out.
808 // Fortunately, the only places where 'returned' is currently generated by
809 // the FE are places where 'returned' is basically free and almost always a
810 // performance win, so the second option can just be used always for now.
812 // This should be revisited if 'returned' is ever applied more liberally.
813 if (RetTy->isVoidTy() || HasLiveReturnedArg) {
816 // Look at each of the original return values individually.
817 for (unsigned i = 0; i != RetCount; ++i) {
818 RetOrArg Ret = CreateRet(F, i);
819 if (LiveValues.erase(Ret)) {
820 RetTypes.push_back(getRetComponentType(F, i));
821 NewRetIdxs[i] = RetTypes.size() - 1;
823 ++NumRetValsEliminated;
824 DEBUG(dbgs() << "DAE - Removing return value " << i << " from "
825 << F->getName() << "\n");
828 if (RetTypes.size() > 1) {
829 // More than one return type? Reduce it down to size.
830 if (StructType *STy = dyn_cast<StructType>(RetTy)) {
831 // Make the new struct packed if we used to return a packed struct
833 NRetTy = StructType::get(STy->getContext(), RetTypes, STy->isPacked());
835 assert(isa<ArrayType>(RetTy) && "unexpected multi-value return");
836 NRetTy = ArrayType::get(RetTypes[0], RetTypes.size());
838 } else if (RetTypes.size() == 1)
839 // One return type? Just a simple value then, but only if we didn't use to
840 // return a struct with that simple value before.
841 NRetTy = RetTypes.front();
842 else if (RetTypes.size() == 0)
843 // No return types? Make it void, but only if we didn't use to return {}.
844 NRetTy = Type::getVoidTy(F->getContext());
847 assert(NRetTy && "No new return type found?");
849 // The existing function return attributes.
850 AttributeSet RAttrs = PAL.getRetAttributes();
852 // Remove any incompatible attributes, but only if we removed all return
853 // values. Otherwise, ensure that we don't have any conflicting attributes
854 // here. Currently, this should not be possible, but special handling might be
855 // required when new return value attributes are added.
856 if (NRetTy->isVoidTy())
857 RAttrs = RAttrs.removeAttributes(NRetTy->getContext(),
858 AttributeSet::ReturnIndex,
859 AttributeFuncs::typeIncompatible(NRetTy));
861 assert(!AttrBuilder(RAttrs, AttributeSet::ReturnIndex).
862 overlaps(AttributeFuncs::typeIncompatible(NRetTy)) &&
863 "Return attributes no longer compatible?");
865 if (RAttrs.hasAttributes(AttributeSet::ReturnIndex))
866 AttributesVec.push_back(AttributeSet::get(NRetTy->getContext(), RAttrs));
868 if (PAL.hasAttributes(AttributeSet::FunctionIndex))
869 AttributesVec.push_back(AttributeSet::get(F->getContext(),
870 PAL.getFnAttributes()));
872 // Reconstruct the AttributesList based on the vector we constructed.
873 AttributeSet NewPAL = AttributeSet::get(F->getContext(), AttributesVec);
875 // Create the new function type based on the recomputed parameters.
876 FunctionType *NFTy = FunctionType::get(NRetTy, Params, FTy->isVarArg());
882 // Create the new function body and insert it into the module...
883 Function *NF = Function::Create(NFTy, F->getLinkage());
884 NF->copyAttributesFrom(F);
885 NF->setAttributes(NewPAL);
886 // Insert the new function before the old function, so we won't be processing
888 F->getParent()->getFunctionList().insert(F->getIterator(), NF);
891 // Loop over all of the callers of the function, transforming the call sites
892 // to pass in a smaller number of arguments into the new function.
894 std::vector<Value*> Args;
895 while (!F->use_empty()) {
896 CallSite CS(F->user_back());
897 Instruction *Call = CS.getInstruction();
899 AttributesVec.clear();
900 const AttributeSet &CallPAL = CS.getAttributes();
902 // The call return attributes.
903 AttributeSet RAttrs = CallPAL.getRetAttributes();
905 // Adjust in case the function was changed to return void.
906 RAttrs = RAttrs.removeAttributes(NRetTy->getContext(),
907 AttributeSet::ReturnIndex,
908 AttributeFuncs::typeIncompatible(NF->getReturnType()));
909 if (RAttrs.hasAttributes(AttributeSet::ReturnIndex))
910 AttributesVec.push_back(AttributeSet::get(NF->getContext(), RAttrs));
912 // Declare these outside of the loops, so we can reuse them for the second
913 // loop, which loops the varargs.
914 CallSite::arg_iterator I = CS.arg_begin();
916 // Loop over those operands, corresponding to the normal arguments to the
917 // original function, and add those that are still alive.
918 for (unsigned e = FTy->getNumParams(); i != e; ++I, ++i)
921 // Get original parameter attributes, but skip return attributes.
922 if (CallPAL.hasAttributes(i + 1)) {
923 AttrBuilder B(CallPAL, i + 1);
924 // If the return type has changed, then get rid of 'returned' on the
925 // call site. The alternative is to make all 'returned' attributes on
926 // call sites keep the return value alive just like 'returned'
927 // attributes on function declaration but it's less clearly a win
928 // and this is not an expected case anyway
929 if (NRetTy != RetTy && B.contains(Attribute::Returned))
930 B.removeAttribute(Attribute::Returned);
932 push_back(AttributeSet::get(F->getContext(), Args.size(), B));
936 // Push any varargs arguments on the list. Don't forget their attributes.
937 for (CallSite::arg_iterator E = CS.arg_end(); I != E; ++I, ++i) {
939 if (CallPAL.hasAttributes(i + 1)) {
940 AttrBuilder B(CallPAL, i + 1);
942 push_back(AttributeSet::get(F->getContext(), Args.size(), B));
946 if (CallPAL.hasAttributes(AttributeSet::FunctionIndex))
947 AttributesVec.push_back(AttributeSet::get(Call->getContext(),
948 CallPAL.getFnAttributes()));
950 // Reconstruct the AttributesList based on the vector we constructed.
951 AttributeSet NewCallPAL = AttributeSet::get(F->getContext(), AttributesVec);
954 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
955 New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
956 Args, "", Call->getParent());
957 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
958 cast<InvokeInst>(New)->setAttributes(NewCallPAL);
960 New = CallInst::Create(NF, Args, "", Call);
961 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
962 cast<CallInst>(New)->setAttributes(NewCallPAL);
963 if (cast<CallInst>(Call)->isTailCall())
964 cast<CallInst>(New)->setTailCall();
966 New->setDebugLoc(Call->getDebugLoc());
970 if (!Call->use_empty()) {
971 if (New->getType() == Call->getType()) {
972 // Return type not changed? Just replace users then.
973 Call->replaceAllUsesWith(New);
975 } else if (New->getType()->isVoidTy()) {
976 // Our return value has uses, but they will get removed later on.
977 // Replace by null for now.
978 if (!Call->getType()->isX86_MMXTy())
979 Call->replaceAllUsesWith(Constant::getNullValue(Call->getType()));
981 assert((RetTy->isStructTy() || RetTy->isArrayTy()) &&
982 "Return type changed, but not into a void. The old return type"
983 " must have been a struct or an array!");
984 Instruction *InsertPt = Call;
985 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
986 BasicBlock *NewEdge = SplitEdge(New->getParent(), II->getNormalDest());
987 InsertPt = &*NewEdge->getFirstInsertionPt();
990 // We used to return a struct or array. Instead of doing smart stuff
991 // with all the uses, we will just rebuild it using extract/insertvalue
992 // chaining and let instcombine clean that up.
994 // Start out building up our return value from undef
995 Value *RetVal = UndefValue::get(RetTy);
996 for (unsigned i = 0; i != RetCount; ++i)
997 if (NewRetIdxs[i] != -1) {
999 if (RetTypes.size() > 1)
1000 // We are still returning a struct, so extract the value from our
1002 V = ExtractValueInst::Create(New, NewRetIdxs[i], "newret",
1005 // We are now returning a single element, so just insert that
1007 // Insert the value at the old position
1008 RetVal = InsertValueInst::Create(RetVal, V, i, "oldret", InsertPt);
1010 // Now, replace all uses of the old call instruction with the return
1012 Call->replaceAllUsesWith(RetVal);
1013 New->takeName(Call);
1017 // Finally, remove the old call from the program, reducing the use-count of
1019 Call->eraseFromParent();
1022 // Since we have now created the new function, splice the body of the old
1023 // function right into the new function, leaving the old rotting hulk of the
1025 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
1027 // Loop over the argument list, transferring uses of the old arguments over to
1028 // the new arguments, also transferring over the names as well.
1030 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(),
1031 I2 = NF->arg_begin(); I != E; ++I, ++i)
1033 // If this is a live argument, move the name and users over to the new
1035 I->replaceAllUsesWith(&*I2);
1039 // If this argument is dead, replace any uses of it with null constants
1040 // (these are guaranteed to become unused later on).
1041 if (!I->getType()->isX86_MMXTy())
1042 I->replaceAllUsesWith(Constant::getNullValue(I->getType()));
1045 // If we change the return value of the function we must rewrite any return
1046 // instructions. Check this now.
1047 if (F->getReturnType() != NF->getReturnType())
1048 for (Function::iterator BB = NF->begin(), E = NF->end(); BB != E; ++BB)
1049 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
1052 if (NFTy->getReturnType()->isVoidTy()) {
1055 assert(RetTy->isStructTy() || RetTy->isArrayTy());
1056 // The original return value was a struct or array, insert
1057 // extractvalue/insertvalue chains to extract only the values we need
1058 // to return and insert them into our new result.
1059 // This does generate messy code, but we'll let it to instcombine to
1061 Value *OldRet = RI->getOperand(0);
1062 // Start out building up our return value from undef
1063 RetVal = UndefValue::get(NRetTy);
1064 for (unsigned i = 0; i != RetCount; ++i)
1065 if (NewRetIdxs[i] != -1) {
1066 ExtractValueInst *EV = ExtractValueInst::Create(OldRet, i,
1068 if (RetTypes.size() > 1) {
1069 // We're still returning a struct, so reinsert the value into
1070 // our new return value at the new index
1072 RetVal = InsertValueInst::Create(RetVal, EV, NewRetIdxs[i],
1075 // We are now only returning a simple value, so just return the
1081 // Replace the return instruction with one returning the new return
1082 // value (possibly 0 if we became void).
1083 ReturnInst::Create(F->getContext(), RetVal, RI);
1084 BB->getInstList().erase(RI);
1087 // Patch the pointer to LLVM function in debug info descriptor.
1088 NF->setSubprogram(F->getSubprogram());
1090 // Now that the old function is dead, delete it.
1091 F->eraseFromParent();
1096 bool DAE::runOnModule(Module &M) {
1097 bool Changed = false;
1099 // First pass: Do a simple check to see if any functions can have their "..."
1100 // removed. We can do this if they never call va_start. This loop cannot be
1101 // fused with the next loop, because deleting a function invalidates
1102 // information computed while surveying other functions.
1103 DEBUG(dbgs() << "DAE - Deleting dead varargs\n");
1104 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1106 if (F.getFunctionType()->isVarArg())
1107 Changed |= DeleteDeadVarargs(F);
1110 // Second phase:loop through the module, determining which arguments are live.
1111 // We assume all arguments are dead unless proven otherwise (allowing us to
1112 // determine that dead arguments passed into recursive functions are dead).
1114 DEBUG(dbgs() << "DAE - Determining liveness\n");
1118 // Now, remove all dead arguments and return values from each function in
1120 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1121 // Increment now, because the function will probably get removed (ie.
1122 // replaced by a new one).
1123 Function *F = &*I++;
1124 Changed |= RemoveDeadStuffFromFunction(F);
1127 // Finally, look for any unused parameters in functions with non-local
1128 // linkage and replace the passed in parameters with undef.
1130 Changed |= RemoveDeadArgumentsFromCallers(F);