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;
125 // Map each LLVM function to corresponding metadata with debug info. If
126 // the function is replaced with another one, we should patch the pointer
127 // to LLVM function in metadata.
128 // As the code generation for module is finished (and DIBuilder is
129 // finalized) we assume that subprogram descriptors won't be changed, and
130 // they are stored in map for short duration anyway.
131 DenseMap<const Function *, DISubprogram *> FunctionDIs;
134 // DAH uses this to specify a different ID.
135 explicit DAE(char &ID) : ModulePass(ID) {}
138 static char ID; // Pass identification, replacement for typeid
139 DAE() : ModulePass(ID) {
140 initializeDAEPass(*PassRegistry::getPassRegistry());
143 bool runOnModule(Module &M) override;
145 virtual bool ShouldHackArguments() const { return false; }
148 Liveness MarkIfNotLive(RetOrArg Use, UseVector &MaybeLiveUses);
149 Liveness SurveyUse(const Use *U, UseVector &MaybeLiveUses,
150 unsigned RetValNum = -1U);
151 Liveness SurveyUses(const Value *V, UseVector &MaybeLiveUses);
153 void SurveyFunction(const Function &F);
154 void MarkValue(const RetOrArg &RA, Liveness L,
155 const UseVector &MaybeLiveUses);
156 void MarkLive(const RetOrArg &RA);
157 void MarkLive(const Function &F);
158 void PropagateLiveness(const RetOrArg &RA);
159 bool RemoveDeadStuffFromFunction(Function *F);
160 bool DeleteDeadVarargs(Function &Fn);
161 bool RemoveDeadArgumentsFromCallers(Function &Fn);
167 INITIALIZE_PASS(DAE, "deadargelim", "Dead Argument Elimination", false, false)
170 /// DAH - DeadArgumentHacking pass - Same as dead argument elimination, but
171 /// deletes arguments to functions which are external. This is only for use
173 struct DAH : public DAE {
177 bool ShouldHackArguments() const override { return true; }
182 INITIALIZE_PASS(DAH, "deadarghaX0r",
183 "Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)",
186 /// createDeadArgEliminationPass - This pass removes arguments from functions
187 /// which are not used by the body of the function.
189 ModulePass *llvm::createDeadArgEliminationPass() { return new DAE(); }
190 ModulePass *llvm::createDeadArgHackingPass() { return new DAH(); }
192 /// DeleteDeadVarargs - If this is an function that takes a ... list, and if
193 /// llvm.vastart is never called, the varargs list is dead for the function.
194 bool DAE::DeleteDeadVarargs(Function &Fn) {
195 assert(Fn.getFunctionType()->isVarArg() && "Function isn't varargs!");
196 if (Fn.isDeclaration() || !Fn.hasLocalLinkage()) return false;
198 // Ensure that the function is only directly called.
199 if (Fn.hasAddressTaken())
202 // Don't touch naked functions. The assembly might be using an argument, or
203 // otherwise rely on the frame layout in a way that this analysis will not
205 if (Fn.hasFnAttribute(Attribute::Naked)) {
209 // Okay, we know we can transform this function if safe. Scan its body
210 // looking for calls marked musttail or calls to llvm.vastart.
211 for (Function::iterator BB = Fn.begin(), E = Fn.end(); BB != E; ++BB) {
212 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
213 CallInst *CI = dyn_cast<CallInst>(I);
216 if (CI->isMustTailCall())
218 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(CI)) {
219 if (II->getIntrinsicID() == Intrinsic::vastart)
225 // If we get here, there are no calls to llvm.vastart in the function body,
226 // remove the "..." and adjust all the calls.
228 // Start by computing a new prototype for the function, which is the same as
229 // the old function, but doesn't have isVarArg set.
230 FunctionType *FTy = Fn.getFunctionType();
232 std::vector<Type*> Params(FTy->param_begin(), FTy->param_end());
233 FunctionType *NFTy = FunctionType::get(FTy->getReturnType(),
235 unsigned NumArgs = Params.size();
237 // Create the new function body and insert it into the module...
238 Function *NF = Function::Create(NFTy, Fn.getLinkage());
239 NF->copyAttributesFrom(&Fn);
240 Fn.getParent()->getFunctionList().insert(Fn.getIterator(), NF);
243 // Loop over all of the callers of the function, transforming the call sites
244 // to pass in a smaller number of arguments into the new function.
246 std::vector<Value*> Args;
247 for (Value::user_iterator I = Fn.user_begin(), E = Fn.user_end(); I != E; ) {
251 Instruction *Call = CS.getInstruction();
253 // Pass all the same arguments.
254 Args.assign(CS.arg_begin(), CS.arg_begin() + NumArgs);
256 // Drop any attributes that were on the vararg arguments.
257 AttributeSet PAL = CS.getAttributes();
258 if (!PAL.isEmpty() && PAL.getSlotIndex(PAL.getNumSlots() - 1) > NumArgs) {
259 SmallVector<AttributeSet, 8> AttributesVec;
260 for (unsigned i = 0; PAL.getSlotIndex(i) <= NumArgs; ++i)
261 AttributesVec.push_back(PAL.getSlotAttributes(i));
262 if (PAL.hasAttributes(AttributeSet::FunctionIndex))
263 AttributesVec.push_back(AttributeSet::get(Fn.getContext(),
264 PAL.getFnAttributes()));
265 PAL = AttributeSet::get(Fn.getContext(), AttributesVec);
269 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
270 New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
272 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
273 cast<InvokeInst>(New)->setAttributes(PAL);
275 New = CallInst::Create(NF, Args, "", Call);
276 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
277 cast<CallInst>(New)->setAttributes(PAL);
278 if (cast<CallInst>(Call)->isTailCall())
279 cast<CallInst>(New)->setTailCall();
281 New->setDebugLoc(Call->getDebugLoc());
285 if (!Call->use_empty())
286 Call->replaceAllUsesWith(New);
290 // Finally, remove the old call from the program, reducing the use-count of
292 Call->eraseFromParent();
295 // Since we have now created the new function, splice the body of the old
296 // function right into the new function, leaving the old rotting hulk of the
298 NF->getBasicBlockList().splice(NF->begin(), Fn.getBasicBlockList());
300 // Loop over the argument list, transferring uses of the old arguments over to
301 // the new arguments, also transferring over the names as well. While we're at
302 // it, remove the dead arguments from the DeadArguments list.
304 for (Function::arg_iterator I = Fn.arg_begin(), E = Fn.arg_end(),
305 I2 = NF->arg_begin(); I != E; ++I, ++I2) {
306 // Move the name and users over to the new version.
307 I->replaceAllUsesWith(&*I2);
311 // Patch the pointer to LLVM function in debug info descriptor.
312 auto DI = FunctionDIs.find(&Fn);
313 if (DI != FunctionDIs.end()) {
314 DISubprogram *SP = DI->second;
315 SP->replaceFunction(NF);
316 // Ensure the map is updated so it can be reused on non-varargs argument
317 // eliminations of the same function.
318 FunctionDIs.erase(DI);
319 FunctionDIs[NF] = SP;
322 // Fix up any BlockAddresses that refer to the function.
323 Fn.replaceAllUsesWith(ConstantExpr::getBitCast(NF, Fn.getType()));
324 // Delete the bitcast that we just created, so that NF does not
325 // appear to be address-taken.
326 NF->removeDeadConstantUsers();
327 // Finally, nuke the old function.
328 Fn.eraseFromParent();
332 /// RemoveDeadArgumentsFromCallers - Checks if the given function has any
333 /// arguments that are unused, and changes the caller parameters to be undefined
335 bool DAE::RemoveDeadArgumentsFromCallers(Function &Fn)
337 // We cannot change the arguments if this TU does not define the function or
338 // if the linker may choose a function body from another TU, even if the
339 // nominal linkage indicates that other copies of the function have the same
340 // semantics. In the below example, the dead load from %p may not have been
341 // eliminated from the linker-chosen copy of f, so replacing %p with undef
342 // in callers may introduce undefined behavior.
344 // define linkonce_odr void @f(i32* %p) {
348 if (!Fn.isStrongDefinitionForLinker())
351 // Functions with local linkage should already have been handled, except the
352 // fragile (variadic) ones which we can improve here.
353 if (Fn.hasLocalLinkage() && !Fn.getFunctionType()->isVarArg())
356 // Don't touch naked functions. The assembly might be using an argument, or
357 // otherwise rely on the frame layout in a way that this analysis will not
359 if (Fn.hasFnAttribute(Attribute::Naked))
365 SmallVector<unsigned, 8> UnusedArgs;
366 for (Argument &Arg : Fn.args()) {
367 if (Arg.use_empty() && !Arg.hasByValOrInAllocaAttr())
368 UnusedArgs.push_back(Arg.getArgNo());
371 if (UnusedArgs.empty())
374 bool Changed = false;
376 for (Use &U : Fn.uses()) {
377 CallSite CS(U.getUser());
378 if (!CS || !CS.isCallee(&U))
381 // Now go through all unused args and replace them with "undef".
382 for (unsigned I = 0, E = UnusedArgs.size(); I != E; ++I) {
383 unsigned ArgNo = UnusedArgs[I];
385 Value *Arg = CS.getArgument(ArgNo);
386 CS.setArgument(ArgNo, UndefValue::get(Arg->getType()));
387 ++NumArgumentsReplacedWithUndef;
395 /// Convenience function that returns the number of return values. It returns 0
396 /// for void functions and 1 for functions not returning a struct. It returns
397 /// the number of struct elements for functions returning a struct.
398 static unsigned NumRetVals(const Function *F) {
399 Type *RetTy = F->getReturnType();
400 if (RetTy->isVoidTy())
402 else if (StructType *STy = dyn_cast<StructType>(RetTy))
403 return STy->getNumElements();
404 else if (ArrayType *ATy = dyn_cast<ArrayType>(RetTy))
405 return ATy->getNumElements();
410 /// Returns the sub-type a function will return at a given Idx. Should
411 /// correspond to the result type of an ExtractValue instruction executed with
412 /// just that one Idx (i.e. only top-level structure is considered).
413 static Type *getRetComponentType(const Function *F, unsigned Idx) {
414 Type *RetTy = F->getReturnType();
415 assert(!RetTy->isVoidTy() && "void type has no subtype");
417 if (StructType *STy = dyn_cast<StructType>(RetTy))
418 return STy->getElementType(Idx);
419 else if (ArrayType *ATy = dyn_cast<ArrayType>(RetTy))
420 return ATy->getElementType();
425 /// MarkIfNotLive - This checks Use for liveness in LiveValues. If Use is not
426 /// live, it adds Use to the MaybeLiveUses argument. Returns the determined
428 DAE::Liveness DAE::MarkIfNotLive(RetOrArg Use, UseVector &MaybeLiveUses) {
429 // We're live if our use or its Function is already marked as live.
430 if (LiveFunctions.count(Use.F) || LiveValues.count(Use))
433 // We're maybe live otherwise, but remember that we must become live if
435 MaybeLiveUses.push_back(Use);
440 /// SurveyUse - This looks at a single use of an argument or return value
441 /// and determines if it should be alive or not. Adds this use to MaybeLiveUses
442 /// if it causes the used value to become MaybeLive.
444 /// RetValNum is the return value number to use when this use is used in a
445 /// return instruction. This is used in the recursion, you should always leave
447 DAE::Liveness DAE::SurveyUse(const Use *U,
448 UseVector &MaybeLiveUses, unsigned RetValNum) {
449 const User *V = U->getUser();
450 if (const ReturnInst *RI = dyn_cast<ReturnInst>(V)) {
451 // The value is returned from a function. It's only live when the
452 // function's return value is live. We use RetValNum here, for the case
453 // that U is really a use of an insertvalue instruction that uses the
455 const Function *F = RI->getParent()->getParent();
456 if (RetValNum != -1U) {
457 RetOrArg Use = CreateRet(F, RetValNum);
458 // We might be live, depending on the liveness of Use.
459 return MarkIfNotLive(Use, MaybeLiveUses);
461 DAE::Liveness Result = MaybeLive;
462 for (unsigned i = 0; i < NumRetVals(F); ++i) {
463 RetOrArg Use = CreateRet(F, i);
464 // We might be live, depending on the liveness of Use. If any
465 // sub-value is live, then the entire value is considered live. This
466 // is a conservative choice, and better tracking is possible.
467 DAE::Liveness SubResult = MarkIfNotLive(Use, MaybeLiveUses);
474 if (const InsertValueInst *IV = dyn_cast<InsertValueInst>(V)) {
475 if (U->getOperandNo() != InsertValueInst::getAggregateOperandIndex()
477 // The use we are examining is inserted into an aggregate. Our liveness
478 // depends on all uses of that aggregate, but if it is used as a return
479 // value, only index at which we were inserted counts.
480 RetValNum = *IV->idx_begin();
482 // Note that if we are used as the aggregate operand to the insertvalue,
483 // we don't change RetValNum, but do survey all our uses.
485 Liveness Result = MaybeLive;
486 for (const Use &UU : IV->uses()) {
487 Result = SurveyUse(&UU, MaybeLiveUses, RetValNum);
494 if (auto CS = ImmutableCallSite(V)) {
495 const Function *F = CS.getCalledFunction();
497 // Used in a direct call.
499 // Find the argument number. We know for sure that this use is an
500 // argument, since if it was the function argument this would be an
501 // indirect call and the we know can't be looking at a value of the
502 // label type (for the invoke instruction).
503 unsigned ArgNo = CS.getArgumentNo(U);
505 if (ArgNo >= F->getFunctionType()->getNumParams())
506 // The value is passed in through a vararg! Must be live.
509 assert(CS.getArgument(ArgNo)
510 == CS->getOperand(U->getOperandNo())
511 && "Argument is not where we expected it");
513 // Value passed to a normal call. It's only live when the corresponding
514 // argument to the called function turns out live.
515 RetOrArg Use = CreateArg(F, ArgNo);
516 return MarkIfNotLive(Use, MaybeLiveUses);
519 // Used in any other way? Value must be live.
523 /// SurveyUses - This looks at all the uses of the given value
524 /// Returns the Liveness deduced from the uses of this value.
526 /// Adds all uses that cause the result to be MaybeLive to MaybeLiveRetUses. If
527 /// the result is Live, MaybeLiveUses might be modified but its content should
528 /// be ignored (since it might not be complete).
529 DAE::Liveness DAE::SurveyUses(const Value *V, UseVector &MaybeLiveUses) {
530 // Assume it's dead (which will only hold if there are no uses at all..).
531 Liveness Result = MaybeLive;
533 for (const Use &U : V->uses()) {
534 Result = SurveyUse(&U, MaybeLiveUses);
541 // SurveyFunction - This performs the initial survey of the specified function,
542 // checking out whether or not it uses any of its incoming arguments or whether
543 // any callers use the return value. This fills in the LiveValues set and Uses
546 // We consider arguments of non-internal functions to be intrinsically alive as
547 // well as arguments to functions which have their "address taken".
549 void DAE::SurveyFunction(const Function &F) {
550 // Functions with inalloca parameters are expecting args in a particular
551 // register and memory layout.
552 if (F.getAttributes().hasAttrSomewhere(Attribute::InAlloca)) {
557 // Don't touch naked functions. The assembly might be using an argument, or
558 // otherwise rely on the frame layout in a way that this analysis will not
560 if (F.hasFnAttribute(Attribute::Naked)) {
565 unsigned RetCount = NumRetVals(&F);
566 // Assume all return values are dead
567 typedef SmallVector<Liveness, 5> RetVals;
568 RetVals RetValLiveness(RetCount, MaybeLive);
570 typedef SmallVector<UseVector, 5> RetUses;
571 // These vectors map each return value to the uses that make it MaybeLive, so
572 // we can add those to the Uses map if the return value really turns out to be
573 // MaybeLive. Initialized to a list of RetCount empty lists.
574 RetUses MaybeLiveRetUses(RetCount);
576 for (Function::const_iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
577 if (const ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()))
578 if (RI->getNumOperands() != 0 && RI->getOperand(0)->getType()
579 != F.getFunctionType()->getReturnType()) {
580 // We don't support old style multiple return values.
585 if (!F.hasLocalLinkage() && (!ShouldHackArguments() || F.isIntrinsic())) {
590 DEBUG(dbgs() << "DAE - Inspecting callers for fn: " << F.getName() << "\n");
591 // Keep track of the number of live retvals, so we can skip checks once all
592 // of them turn out to be live.
593 unsigned NumLiveRetVals = 0;
594 // Loop all uses of the function.
595 for (const Use &U : F.uses()) {
596 // If the function is PASSED IN as an argument, its address has been
598 ImmutableCallSite CS(U.getUser());
599 if (!CS || !CS.isCallee(&U)) {
604 // If this use is anything other than a call site, the function is alive.
605 const Instruction *TheCall = CS.getInstruction();
606 if (!TheCall) { // Not a direct call site?
611 // If we end up here, we are looking at a direct call to our function.
613 // Now, check how our return value(s) is/are used in this caller. Don't
614 // bother checking return values if all of them are live already.
615 if (NumLiveRetVals == RetCount)
618 // Check all uses of the return value.
619 for (const Use &U : TheCall->uses()) {
620 if (ExtractValueInst *Ext = dyn_cast<ExtractValueInst>(U.getUser())) {
621 // This use uses a part of our return value, survey the uses of
622 // that part and store the results for this index only.
623 unsigned Idx = *Ext->idx_begin();
624 if (RetValLiveness[Idx] != Live) {
625 RetValLiveness[Idx] = SurveyUses(Ext, MaybeLiveRetUses[Idx]);
626 if (RetValLiveness[Idx] == Live)
630 // Used by something else than extractvalue. Survey, but assume that the
631 // result applies to all sub-values.
632 UseVector MaybeLiveAggregateUses;
633 if (SurveyUse(&U, MaybeLiveAggregateUses) == Live) {
634 NumLiveRetVals = RetCount;
635 RetValLiveness.assign(RetCount, Live);
638 for (unsigned i = 0; i != RetCount; ++i) {
639 if (RetValLiveness[i] != Live)
640 MaybeLiveRetUses[i].append(MaybeLiveAggregateUses.begin(),
641 MaybeLiveAggregateUses.end());
648 // Now we've inspected all callers, record the liveness of our return values.
649 for (unsigned i = 0; i != RetCount; ++i)
650 MarkValue(CreateRet(&F, i), RetValLiveness[i], MaybeLiveRetUses[i]);
652 DEBUG(dbgs() << "DAE - Inspecting args for fn: " << F.getName() << "\n");
654 // Now, check all of our arguments.
656 UseVector MaybeLiveArgUses;
657 for (Function::const_arg_iterator AI = F.arg_begin(),
658 E = F.arg_end(); AI != E; ++AI, ++i) {
660 if (F.getFunctionType()->isVarArg()) {
661 // Variadic functions will already have a va_arg function expanded inside
662 // them, making them potentially very sensitive to ABI changes resulting
663 // from removing arguments entirely, so don't. For example AArch64 handles
664 // register and stack HFAs very differently, and this is reflected in the
665 // IR which has already been generated.
668 // See what the effect of this use is (recording any uses that cause
669 // MaybeLive in MaybeLiveArgUses).
670 Result = SurveyUses(&*AI, MaybeLiveArgUses);
674 MarkValue(CreateArg(&F, i), Result, MaybeLiveArgUses);
675 // Clear the vector again for the next iteration.
676 MaybeLiveArgUses.clear();
680 /// MarkValue - This function marks the liveness of RA depending on L. If L is
681 /// MaybeLive, it also takes all uses in MaybeLiveUses and records them in Uses,
682 /// such that RA will be marked live if any use in MaybeLiveUses gets marked
684 void DAE::MarkValue(const RetOrArg &RA, Liveness L,
685 const UseVector &MaybeLiveUses) {
687 case Live: MarkLive(RA); break;
690 // Note any uses of this value, so this return value can be
691 // marked live whenever one of the uses becomes live.
692 for (UseVector::const_iterator UI = MaybeLiveUses.begin(),
693 UE = MaybeLiveUses.end(); UI != UE; ++UI)
694 Uses.insert(std::make_pair(*UI, RA));
700 /// MarkLive - Mark the given Function as alive, meaning that it cannot be
701 /// changed in any way. Additionally,
702 /// mark any values that are used as this function's parameters or by its return
703 /// values (according to Uses) live as well.
704 void DAE::MarkLive(const Function &F) {
705 DEBUG(dbgs() << "DAE - Intrinsically live fn: " << F.getName() << "\n");
706 // Mark the function as live.
707 LiveFunctions.insert(&F);
708 // Mark all arguments as live.
709 for (unsigned i = 0, e = F.arg_size(); i != e; ++i)
710 PropagateLiveness(CreateArg(&F, i));
711 // Mark all return values as live.
712 for (unsigned i = 0, e = NumRetVals(&F); i != e; ++i)
713 PropagateLiveness(CreateRet(&F, i));
716 /// MarkLive - Mark the given return value or argument as live. Additionally,
717 /// mark any values that are used by this value (according to Uses) live as
719 void DAE::MarkLive(const RetOrArg &RA) {
720 if (LiveFunctions.count(RA.F))
721 return; // Function was already marked Live.
723 if (!LiveValues.insert(RA).second)
724 return; // We were already marked Live.
726 DEBUG(dbgs() << "DAE - Marking " << RA.getDescription() << " live\n");
727 PropagateLiveness(RA);
730 /// PropagateLiveness - Given that RA is a live value, propagate it's liveness
731 /// to any other values it uses (according to Uses).
732 void DAE::PropagateLiveness(const RetOrArg &RA) {
733 // We don't use upper_bound (or equal_range) here, because our recursive call
734 // to ourselves is likely to cause the upper_bound (which is the first value
735 // not belonging to RA) to become erased and the iterator invalidated.
736 UseMap::iterator Begin = Uses.lower_bound(RA);
737 UseMap::iterator E = Uses.end();
739 for (I = Begin; I != E && I->first == RA; ++I)
742 // Erase RA from the Uses map (from the lower bound to wherever we ended up
744 Uses.erase(Begin, I);
747 // RemoveDeadStuffFromFunction - Remove any arguments and return values from F
748 // that are not in LiveValues. Transform the function and all of the callees of
749 // the function to not have these arguments and return values.
751 bool DAE::RemoveDeadStuffFromFunction(Function *F) {
752 // Don't modify fully live functions
753 if (LiveFunctions.count(F))
756 // Start by computing a new prototype for the function, which is the same as
757 // the old function, but has fewer arguments and a different return type.
758 FunctionType *FTy = F->getFunctionType();
759 std::vector<Type*> Params;
761 // Keep track of if we have a live 'returned' argument
762 bool HasLiveReturnedArg = false;
764 // Set up to build a new list of parameter attributes.
765 SmallVector<AttributeSet, 8> AttributesVec;
766 const AttributeSet &PAL = F->getAttributes();
768 // Remember which arguments are still alive.
769 SmallVector<bool, 10> ArgAlive(FTy->getNumParams(), false);
770 // Construct the new parameter list from non-dead arguments. Also construct
771 // a new set of parameter attributes to correspond. Skip the first parameter
772 // attribute, since that belongs to the return value.
774 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
776 RetOrArg Arg = CreateArg(F, i);
777 if (LiveValues.erase(Arg)) {
778 Params.push_back(I->getType());
781 // Get the original parameter attributes (skipping the first one, that is
782 // for the return value.
783 if (PAL.hasAttributes(i + 1)) {
784 AttrBuilder B(PAL, i + 1);
785 if (B.contains(Attribute::Returned))
786 HasLiveReturnedArg = true;
788 push_back(AttributeSet::get(F->getContext(), Params.size(), B));
791 ++NumArgumentsEliminated;
792 DEBUG(dbgs() << "DAE - Removing argument " << i << " (" << I->getName()
793 << ") from " << F->getName() << "\n");
797 // Find out the new return value.
798 Type *RetTy = FTy->getReturnType();
799 Type *NRetTy = nullptr;
800 unsigned RetCount = NumRetVals(F);
802 // -1 means unused, other numbers are the new index
803 SmallVector<int, 5> NewRetIdxs(RetCount, -1);
804 std::vector<Type*> RetTypes;
806 // If there is a function with a live 'returned' argument but a dead return
807 // value, then there are two possible actions:
808 // 1) Eliminate the return value and take off the 'returned' attribute on the
810 // 2) Retain the 'returned' attribute and treat the return value (but not the
811 // entire function) as live so that it is not eliminated.
813 // It's not clear in the general case which option is more profitable because,
814 // even in the absence of explicit uses of the return value, code generation
815 // is free to use the 'returned' attribute to do things like eliding
816 // save/restores of registers across calls. Whether or not this happens is
817 // target and ABI-specific as well as depending on the amount of register
818 // pressure, so there's no good way for an IR-level pass to figure this out.
820 // Fortunately, the only places where 'returned' is currently generated by
821 // the FE are places where 'returned' is basically free and almost always a
822 // performance win, so the second option can just be used always for now.
824 // This should be revisited if 'returned' is ever applied more liberally.
825 if (RetTy->isVoidTy() || HasLiveReturnedArg) {
828 // Look at each of the original return values individually.
829 for (unsigned i = 0; i != RetCount; ++i) {
830 RetOrArg Ret = CreateRet(F, i);
831 if (LiveValues.erase(Ret)) {
832 RetTypes.push_back(getRetComponentType(F, i));
833 NewRetIdxs[i] = RetTypes.size() - 1;
835 ++NumRetValsEliminated;
836 DEBUG(dbgs() << "DAE - Removing return value " << i << " from "
837 << F->getName() << "\n");
840 if (RetTypes.size() > 1) {
841 // More than one return type? Reduce it down to size.
842 if (StructType *STy = dyn_cast<StructType>(RetTy)) {
843 // Make the new struct packed if we used to return a packed struct
845 NRetTy = StructType::get(STy->getContext(), RetTypes, STy->isPacked());
847 assert(isa<ArrayType>(RetTy) && "unexpected multi-value return");
848 NRetTy = ArrayType::get(RetTypes[0], RetTypes.size());
850 } else if (RetTypes.size() == 1)
851 // One return type? Just a simple value then, but only if we didn't use to
852 // return a struct with that simple value before.
853 NRetTy = RetTypes.front();
854 else if (RetTypes.size() == 0)
855 // No return types? Make it void, but only if we didn't use to return {}.
856 NRetTy = Type::getVoidTy(F->getContext());
859 assert(NRetTy && "No new return type found?");
861 // The existing function return attributes.
862 AttributeSet RAttrs = PAL.getRetAttributes();
864 // Remove any incompatible attributes, but only if we removed all return
865 // values. Otherwise, ensure that we don't have any conflicting attributes
866 // here. Currently, this should not be possible, but special handling might be
867 // required when new return value attributes are added.
868 if (NRetTy->isVoidTy())
869 RAttrs = RAttrs.removeAttributes(NRetTy->getContext(),
870 AttributeSet::ReturnIndex,
871 AttributeFuncs::typeIncompatible(NRetTy));
873 assert(!AttrBuilder(RAttrs, AttributeSet::ReturnIndex).
874 overlaps(AttributeFuncs::typeIncompatible(NRetTy)) &&
875 "Return attributes no longer compatible?");
877 if (RAttrs.hasAttributes(AttributeSet::ReturnIndex))
878 AttributesVec.push_back(AttributeSet::get(NRetTy->getContext(), RAttrs));
880 if (PAL.hasAttributes(AttributeSet::FunctionIndex))
881 AttributesVec.push_back(AttributeSet::get(F->getContext(),
882 PAL.getFnAttributes()));
884 // Reconstruct the AttributesList based on the vector we constructed.
885 AttributeSet NewPAL = AttributeSet::get(F->getContext(), AttributesVec);
887 // Create the new function type based on the recomputed parameters.
888 FunctionType *NFTy = FunctionType::get(NRetTy, Params, FTy->isVarArg());
894 // Create the new function body and insert it into the module...
895 Function *NF = Function::Create(NFTy, F->getLinkage());
896 NF->copyAttributesFrom(F);
897 NF->setAttributes(NewPAL);
898 // Insert the new function before the old function, so we won't be processing
900 F->getParent()->getFunctionList().insert(F->getIterator(), NF);
903 // Loop over all of the callers of the function, transforming the call sites
904 // to pass in a smaller number of arguments into the new function.
906 std::vector<Value*> Args;
907 while (!F->use_empty()) {
908 CallSite CS(F->user_back());
909 Instruction *Call = CS.getInstruction();
911 AttributesVec.clear();
912 const AttributeSet &CallPAL = CS.getAttributes();
914 // The call return attributes.
915 AttributeSet RAttrs = CallPAL.getRetAttributes();
917 // Adjust in case the function was changed to return void.
918 RAttrs = RAttrs.removeAttributes(NRetTy->getContext(),
919 AttributeSet::ReturnIndex,
920 AttributeFuncs::typeIncompatible(NF->getReturnType()));
921 if (RAttrs.hasAttributes(AttributeSet::ReturnIndex))
922 AttributesVec.push_back(AttributeSet::get(NF->getContext(), RAttrs));
924 // Declare these outside of the loops, so we can reuse them for the second
925 // loop, which loops the varargs.
926 CallSite::arg_iterator I = CS.arg_begin();
928 // Loop over those operands, corresponding to the normal arguments to the
929 // original function, and add those that are still alive.
930 for (unsigned e = FTy->getNumParams(); i != e; ++I, ++i)
933 // Get original parameter attributes, but skip return attributes.
934 if (CallPAL.hasAttributes(i + 1)) {
935 AttrBuilder B(CallPAL, i + 1);
936 // If the return type has changed, then get rid of 'returned' on the
937 // call site. The alternative is to make all 'returned' attributes on
938 // call sites keep the return value alive just like 'returned'
939 // attributes on function declaration but it's less clearly a win
940 // and this is not an expected case anyway
941 if (NRetTy != RetTy && B.contains(Attribute::Returned))
942 B.removeAttribute(Attribute::Returned);
944 push_back(AttributeSet::get(F->getContext(), Args.size(), B));
948 // Push any varargs arguments on the list. Don't forget their attributes.
949 for (CallSite::arg_iterator E = CS.arg_end(); I != E; ++I, ++i) {
951 if (CallPAL.hasAttributes(i + 1)) {
952 AttrBuilder B(CallPAL, i + 1);
954 push_back(AttributeSet::get(F->getContext(), Args.size(), B));
958 if (CallPAL.hasAttributes(AttributeSet::FunctionIndex))
959 AttributesVec.push_back(AttributeSet::get(Call->getContext(),
960 CallPAL.getFnAttributes()));
962 // Reconstruct the AttributesList based on the vector we constructed.
963 AttributeSet NewCallPAL = AttributeSet::get(F->getContext(), AttributesVec);
966 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
967 New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
968 Args, "", Call->getParent());
969 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
970 cast<InvokeInst>(New)->setAttributes(NewCallPAL);
972 New = CallInst::Create(NF, Args, "", Call);
973 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
974 cast<CallInst>(New)->setAttributes(NewCallPAL);
975 if (cast<CallInst>(Call)->isTailCall())
976 cast<CallInst>(New)->setTailCall();
978 New->setDebugLoc(Call->getDebugLoc());
982 if (!Call->use_empty()) {
983 if (New->getType() == Call->getType()) {
984 // Return type not changed? Just replace users then.
985 Call->replaceAllUsesWith(New);
987 } else if (New->getType()->isVoidTy()) {
988 // Our return value has uses, but they will get removed later on.
989 // Replace by null for now.
990 if (!Call->getType()->isX86_MMXTy())
991 Call->replaceAllUsesWith(Constant::getNullValue(Call->getType()));
993 assert((RetTy->isStructTy() || RetTy->isArrayTy()) &&
994 "Return type changed, but not into a void. The old return type"
995 " must have been a struct or an array!");
996 Instruction *InsertPt = Call;
997 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
998 BasicBlock *NewEdge = SplitEdge(New->getParent(), II->getNormalDest());
999 InsertPt = &*NewEdge->getFirstInsertionPt();
1002 // We used to return a struct or array. Instead of doing smart stuff
1003 // with all the uses, we will just rebuild it using extract/insertvalue
1004 // chaining and let instcombine clean that up.
1006 // Start out building up our return value from undef
1007 Value *RetVal = UndefValue::get(RetTy);
1008 for (unsigned i = 0; i != RetCount; ++i)
1009 if (NewRetIdxs[i] != -1) {
1011 if (RetTypes.size() > 1)
1012 // We are still returning a struct, so extract the value from our
1014 V = ExtractValueInst::Create(New, NewRetIdxs[i], "newret",
1017 // We are now returning a single element, so just insert that
1019 // Insert the value at the old position
1020 RetVal = InsertValueInst::Create(RetVal, V, i, "oldret", InsertPt);
1022 // Now, replace all uses of the old call instruction with the return
1024 Call->replaceAllUsesWith(RetVal);
1025 New->takeName(Call);
1029 // Finally, remove the old call from the program, reducing the use-count of
1031 Call->eraseFromParent();
1034 // Since we have now created the new function, splice the body of the old
1035 // function right into the new function, leaving the old rotting hulk of the
1037 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
1039 // Loop over the argument list, transferring uses of the old arguments over to
1040 // the new arguments, also transferring over the names as well.
1042 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(),
1043 I2 = NF->arg_begin(); I != E; ++I, ++i)
1045 // If this is a live argument, move the name and users over to the new
1047 I->replaceAllUsesWith(&*I2);
1051 // If this argument is dead, replace any uses of it with null constants
1052 // (these are guaranteed to become unused later on).
1053 if (!I->getType()->isX86_MMXTy())
1054 I->replaceAllUsesWith(Constant::getNullValue(I->getType()));
1057 // If we change the return value of the function we must rewrite any return
1058 // instructions. Check this now.
1059 if (F->getReturnType() != NF->getReturnType())
1060 for (Function::iterator BB = NF->begin(), E = NF->end(); BB != E; ++BB)
1061 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
1064 if (NFTy->getReturnType()->isVoidTy()) {
1067 assert(RetTy->isStructTy() || RetTy->isArrayTy());
1068 // The original return value was a struct or array, insert
1069 // extractvalue/insertvalue chains to extract only the values we need
1070 // to return and insert them into our new result.
1071 // This does generate messy code, but we'll let it to instcombine to
1073 Value *OldRet = RI->getOperand(0);
1074 // Start out building up our return value from undef
1075 RetVal = UndefValue::get(NRetTy);
1076 for (unsigned i = 0; i != RetCount; ++i)
1077 if (NewRetIdxs[i] != -1) {
1078 ExtractValueInst *EV = ExtractValueInst::Create(OldRet, i,
1080 if (RetTypes.size() > 1) {
1081 // We're still returning a struct, so reinsert the value into
1082 // our new return value at the new index
1084 RetVal = InsertValueInst::Create(RetVal, EV, NewRetIdxs[i],
1087 // We are now only returning a simple value, so just return the
1093 // Replace the return instruction with one returning the new return
1094 // value (possibly 0 if we became void).
1095 ReturnInst::Create(F->getContext(), RetVal, RI);
1096 BB->getInstList().erase(RI);
1099 // Patch the pointer to LLVM function in debug info descriptor.
1100 auto DI = FunctionDIs.find(F);
1101 if (DI != FunctionDIs.end())
1102 DI->second->replaceFunction(NF);
1104 // Now that the old function is dead, delete it.
1105 F->eraseFromParent();
1110 bool DAE::runOnModule(Module &M) {
1111 bool Changed = false;
1113 // Collect debug info descriptors for functions.
1114 FunctionDIs = makeSubprogramMap(M);
1116 // First pass: Do a simple check to see if any functions can have their "..."
1117 // removed. We can do this if they never call va_start. This loop cannot be
1118 // fused with the next loop, because deleting a function invalidates
1119 // information computed while surveying other functions.
1120 DEBUG(dbgs() << "DAE - Deleting dead varargs\n");
1121 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1123 if (F.getFunctionType()->isVarArg())
1124 Changed |= DeleteDeadVarargs(F);
1127 // Second phase:loop through the module, determining which arguments are live.
1128 // We assume all arguments are dead unless proven otherwise (allowing us to
1129 // determine that dead arguments passed into recursive functions are dead).
1131 DEBUG(dbgs() << "DAE - Determining liveness\n");
1135 // Now, remove all dead arguments and return values from each function in
1137 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1138 // Increment now, because the function will probably get removed (ie.
1139 // replaced by a new one).
1140 Function *F = &*I++;
1141 Changed |= RemoveDeadStuffFromFunction(F);
1144 // Finally, look for any unused parameters in functions with non-local
1145 // linkage and replace the passed in parameters with undef.
1147 Changed |= RemoveDeadArgumentsFromCallers(F);