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"
43 #define DEBUG_TYPE "deadargelim"
45 STATISTIC(NumArgumentsEliminated, "Number of unread args removed");
46 STATISTIC(NumRetValsEliminated , "Number of unused return values removed");
47 STATISTIC(NumArgumentsReplacedWithUndef,
48 "Number of unread args replaced with undef");
50 /// DAE - The dead argument elimination pass.
52 class DAE : public ModulePass {
55 /// Struct that represents (part of) either a return value or a function
56 /// argument. Used so that arguments and return values can be used
59 RetOrArg(const Function *F, unsigned Idx, bool IsArg) : F(F), Idx(Idx),
65 /// Make RetOrArg comparable, so we can put it into a map.
66 bool operator<(const RetOrArg &O) const {
67 return std::tie(F, Idx, IsArg) < std::tie(O.F, O.Idx, O.IsArg);
70 /// Make RetOrArg comparable, so we can easily iterate the multimap.
71 bool operator==(const RetOrArg &O) const {
72 return F == O.F && Idx == O.Idx && IsArg == O.IsArg;
75 std::string getDescription() const {
76 return (Twine(IsArg ? "Argument #" : "Return value #") + utostr(Idx) +
77 " of function " + F->getName()).str();
81 /// Liveness enum - During our initial pass over the program, we determine
82 /// that things are either alive or maybe alive. We don't mark anything
83 /// explicitly dead (even if we know they are), since anything not alive
84 /// with no registered uses (in Uses) will never be marked alive and will
85 /// thus become dead in the end.
86 enum Liveness { Live, MaybeLive };
88 /// Convenience wrapper
89 RetOrArg CreateRet(const Function *F, unsigned Idx) {
90 return RetOrArg(F, Idx, false);
92 /// Convenience wrapper
93 RetOrArg CreateArg(const Function *F, unsigned Idx) {
94 return RetOrArg(F, Idx, true);
97 typedef std::multimap<RetOrArg, RetOrArg> UseMap;
98 /// This maps a return value or argument to any MaybeLive return values or
99 /// arguments it uses. This allows the MaybeLive values to be marked live
100 /// when any of its users is marked live.
101 /// For example (indices are left out for clarity):
102 /// - Uses[ret F] = ret G
103 /// This means that F calls G, and F returns the value returned by G.
104 /// - Uses[arg F] = ret G
105 /// This means that some function calls G and passes its result as an
107 /// - Uses[ret F] = arg F
108 /// This means that F returns one of its own arguments.
109 /// - Uses[arg F] = arg G
110 /// This means that G calls F and passes one of its own (G's) arguments
114 typedef std::set<RetOrArg> LiveSet;
115 typedef std::set<const Function*> LiveFuncSet;
117 /// This set contains all values that have been determined to be live.
119 /// This set contains all values that are cannot be changed in any way.
120 LiveFuncSet LiveFunctions;
122 typedef SmallVector<RetOrArg, 5> UseVector;
124 // Map each LLVM function to corresponding metadata with debug info. If
125 // the function is replaced with another one, we should patch the pointer
126 // to LLVM function in metadata.
127 // As the code generation for module is finished (and DIBuilder is
128 // finalized) we assume that subprogram descriptors won't be changed, and
129 // they are stored in map for short duration anyway.
130 DenseMap<const Function *, DISubprogram *> FunctionDIs;
133 // DAH uses this to specify a different ID.
134 explicit DAE(char &ID) : ModulePass(ID) {}
137 static char ID; // Pass identification, replacement for typeid
138 DAE() : ModulePass(ID) {
139 initializeDAEPass(*PassRegistry::getPassRegistry());
142 bool runOnModule(Module &M) override;
144 virtual bool ShouldHackArguments() const { return false; }
147 Liveness MarkIfNotLive(RetOrArg Use, UseVector &MaybeLiveUses);
148 Liveness SurveyUse(const Use *U, UseVector &MaybeLiveUses,
149 unsigned RetValNum = -1U);
150 Liveness SurveyUses(const Value *V, UseVector &MaybeLiveUses);
152 void SurveyFunction(const Function &F);
153 void MarkValue(const RetOrArg &RA, Liveness L,
154 const UseVector &MaybeLiveUses);
155 void MarkLive(const RetOrArg &RA);
156 void MarkLive(const Function &F);
157 void PropagateLiveness(const RetOrArg &RA);
158 bool RemoveDeadStuffFromFunction(Function *F);
159 bool DeleteDeadVarargs(Function &Fn);
160 bool RemoveDeadArgumentsFromCallers(Function &Fn);
166 INITIALIZE_PASS(DAE, "deadargelim", "Dead Argument Elimination", false, false)
169 /// DAH - DeadArgumentHacking pass - Same as dead argument elimination, but
170 /// deletes arguments to functions which are external. This is only for use
172 struct DAH : public DAE {
176 bool ShouldHackArguments() const override { return true; }
181 INITIALIZE_PASS(DAH, "deadarghaX0r",
182 "Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)",
185 /// createDeadArgEliminationPass - This pass removes arguments from functions
186 /// which are not used by the body of the function.
188 ModulePass *llvm::createDeadArgEliminationPass() { return new DAE(); }
189 ModulePass *llvm::createDeadArgHackingPass() { return new DAH(); }
191 /// DeleteDeadVarargs - If this is an function that takes a ... list, and if
192 /// llvm.vastart is never called, the varargs list is dead for the function.
193 bool DAE::DeleteDeadVarargs(Function &Fn) {
194 assert(Fn.getFunctionType()->isVarArg() && "Function isn't varargs!");
195 if (Fn.isDeclaration() || !Fn.hasLocalLinkage()) return false;
197 // Ensure that the function is only directly called.
198 if (Fn.hasAddressTaken())
201 // Don't touch naked functions. The assembly might be using an argument, or
202 // otherwise rely on the frame layout in a way that this analysis will not
204 if (Fn.hasFnAttribute(Attribute::Naked)) {
208 // Okay, we know we can transform this function if safe. Scan its body
209 // looking for calls marked musttail or calls to llvm.vastart.
210 for (Function::iterator BB = Fn.begin(), E = Fn.end(); BB != E; ++BB) {
211 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
212 CallInst *CI = dyn_cast<CallInst>(I);
215 if (CI->isMustTailCall())
217 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(CI)) {
218 if (II->getIntrinsicID() == Intrinsic::vastart)
224 // If we get here, there are no calls to llvm.vastart in the function body,
225 // remove the "..." and adjust all the calls.
227 // Start by computing a new prototype for the function, which is the same as
228 // the old function, but doesn't have isVarArg set.
229 FunctionType *FTy = Fn.getFunctionType();
231 std::vector<Type*> Params(FTy->param_begin(), FTy->param_end());
232 FunctionType *NFTy = FunctionType::get(FTy->getReturnType(),
234 unsigned NumArgs = Params.size();
236 // Create the new function body and insert it into the module...
237 Function *NF = Function::Create(NFTy, Fn.getLinkage());
238 NF->copyAttributesFrom(&Fn);
239 Fn.getParent()->getFunctionList().insert(&Fn, NF);
242 // Loop over all of the callers of the function, transforming the call sites
243 // to pass in a smaller number of arguments into the new function.
245 std::vector<Value*> Args;
246 for (Value::user_iterator I = Fn.user_begin(), E = Fn.user_end(); I != E; ) {
250 Instruction *Call = CS.getInstruction();
252 // Pass all the same arguments.
253 Args.assign(CS.arg_begin(), CS.arg_begin() + NumArgs);
255 // Drop any attributes that were on the vararg arguments.
256 AttributeSet PAL = CS.getAttributes();
257 if (!PAL.isEmpty() && PAL.getSlotIndex(PAL.getNumSlots() - 1) > NumArgs) {
258 SmallVector<AttributeSet, 8> AttributesVec;
259 for (unsigned i = 0; PAL.getSlotIndex(i) <= NumArgs; ++i)
260 AttributesVec.push_back(PAL.getSlotAttributes(i));
261 if (PAL.hasAttributes(AttributeSet::FunctionIndex))
262 AttributesVec.push_back(AttributeSet::get(Fn.getContext(),
263 PAL.getFnAttributes()));
264 PAL = AttributeSet::get(Fn.getContext(), AttributesVec);
268 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
269 New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
271 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
272 cast<InvokeInst>(New)->setAttributes(PAL);
274 New = CallInst::Create(NF, Args, "", Call);
275 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
276 cast<CallInst>(New)->setAttributes(PAL);
277 if (cast<CallInst>(Call)->isTailCall())
278 cast<CallInst>(New)->setTailCall();
280 New->setDebugLoc(Call->getDebugLoc());
284 if (!Call->use_empty())
285 Call->replaceAllUsesWith(New);
289 // Finally, remove the old call from the program, reducing the use-count of
291 Call->eraseFromParent();
294 // Since we have now created the new function, splice the body of the old
295 // function right into the new function, leaving the old rotting hulk of the
297 NF->getBasicBlockList().splice(NF->begin(), Fn.getBasicBlockList());
299 // Loop over the argument list, transferring uses of the old arguments over to
300 // the new arguments, also transferring over the names as well. While we're at
301 // it, remove the dead arguments from the DeadArguments list.
303 for (Function::arg_iterator I = Fn.arg_begin(), E = Fn.arg_end(),
304 I2 = NF->arg_begin(); I != E; ++I, ++I2) {
305 // Move the name and users over to the new version.
306 I->replaceAllUsesWith(I2);
310 // Patch the pointer to LLVM function in debug info descriptor.
311 auto DI = FunctionDIs.find(&Fn);
312 if (DI != FunctionDIs.end()) {
313 DISubprogram *SP = DI->second;
314 SP->replaceFunction(NF);
315 // Ensure the map is updated so it can be reused on non-varargs argument
316 // eliminations of the same function.
317 FunctionDIs.erase(DI);
318 FunctionDIs[NF] = SP;
321 // Fix up any BlockAddresses that refer to the function.
322 Fn.replaceAllUsesWith(ConstantExpr::getBitCast(NF, Fn.getType()));
323 // Delete the bitcast that we just created, so that NF does not
324 // appear to be address-taken.
325 NF->removeDeadConstantUsers();
326 // Finally, nuke the old function.
327 Fn.eraseFromParent();
331 /// RemoveDeadArgumentsFromCallers - Checks if the given function has any
332 /// arguments that are unused, and changes the caller parameters to be undefined
334 bool DAE::RemoveDeadArgumentsFromCallers(Function &Fn)
336 // We cannot change the arguments if this TU does not define the function or
337 // if the linker may choose a function body from another TU, even if the
338 // nominal linkage indicates that other copies of the function have the same
339 // semantics. In the below example, the dead load from %p may not have been
340 // eliminated from the linker-chosen copy of f, so replacing %p with undef
341 // in callers may introduce undefined behavior.
343 // define linkonce_odr void @f(i32* %p) {
347 if (!Fn.isStrongDefinitionForLinker())
350 // Functions with local linkage should already have been handled, except the
351 // fragile (variadic) ones which we can improve here.
352 if (Fn.hasLocalLinkage() && !Fn.getFunctionType()->isVarArg())
355 // Don't touch naked functions. The assembly might be using an argument, or
356 // otherwise rely on the frame layout in a way that this analysis will not
358 if (Fn.hasFnAttribute(Attribute::Naked))
364 SmallVector<unsigned, 8> UnusedArgs;
365 for (Function::arg_iterator I = Fn.arg_begin(), E = Fn.arg_end();
369 if (Arg->use_empty() && !Arg->hasByValOrInAllocaAttr())
370 UnusedArgs.push_back(Arg->getArgNo());
373 if (UnusedArgs.empty())
376 bool Changed = false;
378 for (Use &U : Fn.uses()) {
379 CallSite CS(U.getUser());
380 if (!CS || !CS.isCallee(&U))
383 // Now go through all unused args and replace them with "undef".
384 for (unsigned I = 0, E = UnusedArgs.size(); I != E; ++I) {
385 unsigned ArgNo = UnusedArgs[I];
387 Value *Arg = CS.getArgument(ArgNo);
388 CS.setArgument(ArgNo, UndefValue::get(Arg->getType()));
389 ++NumArgumentsReplacedWithUndef;
397 /// Convenience function that returns the number of return values. It returns 0
398 /// for void functions and 1 for functions not returning a struct. It returns
399 /// the number of struct elements for functions returning a struct.
400 static unsigned NumRetVals(const Function *F) {
401 Type *RetTy = F->getReturnType();
402 if (RetTy->isVoidTy())
404 else if (StructType *STy = dyn_cast<StructType>(RetTy))
405 return STy->getNumElements();
406 else if (ArrayType *ATy = dyn_cast<ArrayType>(RetTy))
407 return ATy->getNumElements();
412 /// Returns the sub-type a function will return at a given Idx. Should
413 /// correspond to the result type of an ExtractValue instruction executed with
414 /// just that one Idx (i.e. only top-level structure is considered).
415 static Type *getRetComponentType(const Function *F, unsigned Idx) {
416 Type *RetTy = F->getReturnType();
417 assert(!RetTy->isVoidTy() && "void type has no subtype");
419 if (StructType *STy = dyn_cast<StructType>(RetTy))
420 return STy->getElementType(Idx);
421 else if (ArrayType *ATy = dyn_cast<ArrayType>(RetTy))
422 return ATy->getElementType();
427 /// MarkIfNotLive - This checks Use for liveness in LiveValues. If Use is not
428 /// live, it adds Use to the MaybeLiveUses argument. Returns the determined
430 DAE::Liveness DAE::MarkIfNotLive(RetOrArg Use, UseVector &MaybeLiveUses) {
431 // We're live if our use or its Function is already marked as live.
432 if (LiveFunctions.count(Use.F) || LiveValues.count(Use))
435 // We're maybe live otherwise, but remember that we must become live if
437 MaybeLiveUses.push_back(Use);
442 /// SurveyUse - This looks at a single use of an argument or return value
443 /// and determines if it should be alive or not. Adds this use to MaybeLiveUses
444 /// if it causes the used value to become MaybeLive.
446 /// RetValNum is the return value number to use when this use is used in a
447 /// return instruction. This is used in the recursion, you should always leave
449 DAE::Liveness DAE::SurveyUse(const Use *U,
450 UseVector &MaybeLiveUses, unsigned RetValNum) {
451 const User *V = U->getUser();
452 if (const ReturnInst *RI = dyn_cast<ReturnInst>(V)) {
453 // The value is returned from a function. It's only live when the
454 // function's return value is live. We use RetValNum here, for the case
455 // that U is really a use of an insertvalue instruction that uses the
457 const Function *F = RI->getParent()->getParent();
458 if (RetValNum != -1U) {
459 RetOrArg Use = CreateRet(F, RetValNum);
460 // We might be live, depending on the liveness of Use.
461 return MarkIfNotLive(Use, MaybeLiveUses);
463 DAE::Liveness Result = MaybeLive;
464 for (unsigned i = 0; i < NumRetVals(F); ++i) {
465 RetOrArg Use = CreateRet(F, i);
466 // We might be live, depending on the liveness of Use. If any
467 // sub-value is live, then the entire value is considered live. This
468 // is a conservative choice, and better tracking is possible.
469 DAE::Liveness SubResult = MarkIfNotLive(Use, MaybeLiveUses);
476 if (const InsertValueInst *IV = dyn_cast<InsertValueInst>(V)) {
477 if (U->getOperandNo() != InsertValueInst::getAggregateOperandIndex()
479 // The use we are examining is inserted into an aggregate. Our liveness
480 // depends on all uses of that aggregate, but if it is used as a return
481 // value, only index at which we were inserted counts.
482 RetValNum = *IV->idx_begin();
484 // Note that if we are used as the aggregate operand to the insertvalue,
485 // we don't change RetValNum, but do survey all our uses.
487 Liveness Result = MaybeLive;
488 for (const Use &UU : IV->uses()) {
489 Result = SurveyUse(&UU, MaybeLiveUses, RetValNum);
496 if (auto CS = ImmutableCallSite(V)) {
497 const Function *F = CS.getCalledFunction();
499 // Used in a direct call.
501 // Find the argument number. We know for sure that this use is an
502 // argument, since if it was the function argument this would be an
503 // indirect call and the we know can't be looking at a value of the
504 // label type (for the invoke instruction).
505 unsigned ArgNo = CS.getArgumentNo(U);
507 if (ArgNo >= F->getFunctionType()->getNumParams())
508 // The value is passed in through a vararg! Must be live.
511 assert(CS.getArgument(ArgNo)
512 == CS->getOperand(U->getOperandNo())
513 && "Argument is not where we expected it");
515 // Value passed to a normal call. It's only live when the corresponding
516 // argument to the called function turns out live.
517 RetOrArg Use = CreateArg(F, ArgNo);
518 return MarkIfNotLive(Use, MaybeLiveUses);
521 // Used in any other way? Value must be live.
525 /// SurveyUses - This looks at all the uses of the given value
526 /// Returns the Liveness deduced from the uses of this value.
528 /// Adds all uses that cause the result to be MaybeLive to MaybeLiveRetUses. If
529 /// the result is Live, MaybeLiveUses might be modified but its content should
530 /// be ignored (since it might not be complete).
531 DAE::Liveness DAE::SurveyUses(const Value *V, UseVector &MaybeLiveUses) {
532 // Assume it's dead (which will only hold if there are no uses at all..).
533 Liveness Result = MaybeLive;
535 for (const Use &U : V->uses()) {
536 Result = SurveyUse(&U, MaybeLiveUses);
543 // SurveyFunction - This performs the initial survey of the specified function,
544 // checking out whether or not it uses any of its incoming arguments or whether
545 // any callers use the return value. This fills in the LiveValues set and Uses
548 // We consider arguments of non-internal functions to be intrinsically alive as
549 // well as arguments to functions which have their "address taken".
551 void DAE::SurveyFunction(const Function &F) {
552 // Functions with inalloca parameters are expecting args in a particular
553 // register and memory layout.
554 if (F.getAttributes().hasAttrSomewhere(Attribute::InAlloca)) {
559 // Don't touch naked functions. The assembly might be using an argument, or
560 // otherwise rely on the frame layout in a way that this analysis will not
562 if (F.hasFnAttribute(Attribute::Naked)) {
567 unsigned RetCount = NumRetVals(&F);
568 // Assume all return values are dead
569 typedef SmallVector<Liveness, 5> RetVals;
570 RetVals RetValLiveness(RetCount, MaybeLive);
572 typedef SmallVector<UseVector, 5> RetUses;
573 // These vectors map each return value to the uses that make it MaybeLive, so
574 // we can add those to the Uses map if the return value really turns out to be
575 // MaybeLive. Initialized to a list of RetCount empty lists.
576 RetUses MaybeLiveRetUses(RetCount);
578 for (Function::const_iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
579 if (const ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()))
580 if (RI->getNumOperands() != 0 && RI->getOperand(0)->getType()
581 != F.getFunctionType()->getReturnType()) {
582 // We don't support old style multiple return values.
587 if (!F.hasLocalLinkage() && (!ShouldHackArguments() || F.isIntrinsic())) {
592 DEBUG(dbgs() << "DAE - Inspecting callers for fn: " << F.getName() << "\n");
593 // Keep track of the number of live retvals, so we can skip checks once all
594 // of them turn out to be live.
595 unsigned NumLiveRetVals = 0;
596 // Loop all uses of the function.
597 for (const Use &U : F.uses()) {
598 // If the function is PASSED IN as an argument, its address has been
600 ImmutableCallSite CS(U.getUser());
601 if (!CS || !CS.isCallee(&U)) {
606 // If this use is anything other than a call site, the function is alive.
607 const Instruction *TheCall = CS.getInstruction();
608 if (!TheCall) { // Not a direct call site?
613 // If we end up here, we are looking at a direct call to our function.
615 // Now, check how our return value(s) is/are used in this caller. Don't
616 // bother checking return values if all of them are live already.
617 if (NumLiveRetVals == RetCount)
620 // Check all uses of the return value.
621 for (const Use &U : TheCall->uses()) {
622 if (ExtractValueInst *Ext = dyn_cast<ExtractValueInst>(U.getUser())) {
623 // This use uses a part of our return value, survey the uses of
624 // that part and store the results for this index only.
625 unsigned Idx = *Ext->idx_begin();
626 if (RetValLiveness[Idx] != Live) {
627 RetValLiveness[Idx] = SurveyUses(Ext, MaybeLiveRetUses[Idx]);
628 if (RetValLiveness[Idx] == Live)
632 // Used by something else than extractvalue. Survey, but assume that the
633 // result applies to all sub-values.
634 UseVector MaybeLiveAggregateUses;
635 if (SurveyUse(&U, MaybeLiveAggregateUses) == Live) {
636 NumLiveRetVals = RetCount;
637 RetValLiveness.assign(RetCount, Live);
640 for (unsigned i = 0; i != RetCount; ++i) {
641 if (RetValLiveness[i] != Live)
642 MaybeLiveRetUses[i].append(MaybeLiveAggregateUses.begin(),
643 MaybeLiveAggregateUses.end());
650 // Now we've inspected all callers, record the liveness of our return values.
651 for (unsigned i = 0; i != RetCount; ++i)
652 MarkValue(CreateRet(&F, i), RetValLiveness[i], MaybeLiveRetUses[i]);
654 DEBUG(dbgs() << "DAE - Inspecting args for fn: " << F.getName() << "\n");
656 // Now, check all of our arguments.
658 UseVector MaybeLiveArgUses;
659 for (Function::const_arg_iterator AI = F.arg_begin(),
660 E = F.arg_end(); AI != E; ++AI, ++i) {
662 if (F.getFunctionType()->isVarArg()) {
663 // Variadic functions will already have a va_arg function expanded inside
664 // them, making them potentially very sensitive to ABI changes resulting
665 // from removing arguments entirely, so don't. For example AArch64 handles
666 // register and stack HFAs very differently, and this is reflected in the
667 // IR which has already been generated.
670 // See what the effect of this use is (recording any uses that cause
671 // MaybeLive in MaybeLiveArgUses).
672 Result = SurveyUses(AI, MaybeLiveArgUses);
676 MarkValue(CreateArg(&F, i), Result, MaybeLiveArgUses);
677 // Clear the vector again for the next iteration.
678 MaybeLiveArgUses.clear();
682 /// MarkValue - This function marks the liveness of RA depending on L. If L is
683 /// MaybeLive, it also takes all uses in MaybeLiveUses and records them in Uses,
684 /// such that RA will be marked live if any use in MaybeLiveUses gets marked
686 void DAE::MarkValue(const RetOrArg &RA, Liveness L,
687 const UseVector &MaybeLiveUses) {
689 case Live: MarkLive(RA); break;
692 // Note any uses of this value, so this return value can be
693 // marked live whenever one of the uses becomes live.
694 for (UseVector::const_iterator UI = MaybeLiveUses.begin(),
695 UE = MaybeLiveUses.end(); UI != UE; ++UI)
696 Uses.insert(std::make_pair(*UI, RA));
702 /// MarkLive - Mark the given Function as alive, meaning that it cannot be
703 /// changed in any way. Additionally,
704 /// mark any values that are used as this function's parameters or by its return
705 /// values (according to Uses) live as well.
706 void DAE::MarkLive(const Function &F) {
707 DEBUG(dbgs() << "DAE - Intrinsically live fn: " << F.getName() << "\n");
708 // Mark the function as live.
709 LiveFunctions.insert(&F);
710 // Mark all arguments as live.
711 for (unsigned i = 0, e = F.arg_size(); i != e; ++i)
712 PropagateLiveness(CreateArg(&F, i));
713 // Mark all return values as live.
714 for (unsigned i = 0, e = NumRetVals(&F); i != e; ++i)
715 PropagateLiveness(CreateRet(&F, i));
718 /// MarkLive - Mark the given return value or argument as live. Additionally,
719 /// mark any values that are used by this value (according to Uses) live as
721 void DAE::MarkLive(const RetOrArg &RA) {
722 if (LiveFunctions.count(RA.F))
723 return; // Function was already marked Live.
725 if (!LiveValues.insert(RA).second)
726 return; // We were already marked Live.
728 DEBUG(dbgs() << "DAE - Marking " << RA.getDescription() << " live\n");
729 PropagateLiveness(RA);
732 /// PropagateLiveness - Given that RA is a live value, propagate it's liveness
733 /// to any other values it uses (according to Uses).
734 void DAE::PropagateLiveness(const RetOrArg &RA) {
735 // We don't use upper_bound (or equal_range) here, because our recursive call
736 // to ourselves is likely to cause the upper_bound (which is the first value
737 // not belonging to RA) to become erased and the iterator invalidated.
738 UseMap::iterator Begin = Uses.lower_bound(RA);
739 UseMap::iterator E = Uses.end();
741 for (I = Begin; I != E && I->first == RA; ++I)
744 // Erase RA from the Uses map (from the lower bound to wherever we ended up
746 Uses.erase(Begin, I);
749 // RemoveDeadStuffFromFunction - Remove any arguments and return values from F
750 // that are not in LiveValues. Transform the function and all of the callees of
751 // the function to not have these arguments and return values.
753 bool DAE::RemoveDeadStuffFromFunction(Function *F) {
754 // Don't modify fully live functions
755 if (LiveFunctions.count(F))
758 // Start by computing a new prototype for the function, which is the same as
759 // the old function, but has fewer arguments and a different return type.
760 FunctionType *FTy = F->getFunctionType();
761 std::vector<Type*> Params;
763 // Keep track of if we have a live 'returned' argument
764 bool HasLiveReturnedArg = false;
766 // Set up to build a new list of parameter attributes.
767 SmallVector<AttributeSet, 8> AttributesVec;
768 const AttributeSet &PAL = F->getAttributes();
770 // Remember which arguments are still alive.
771 SmallVector<bool, 10> ArgAlive(FTy->getNumParams(), false);
772 // Construct the new parameter list from non-dead arguments. Also construct
773 // a new set of parameter attributes to correspond. Skip the first parameter
774 // attribute, since that belongs to the return value.
776 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
778 RetOrArg Arg = CreateArg(F, i);
779 if (LiveValues.erase(Arg)) {
780 Params.push_back(I->getType());
783 // Get the original parameter attributes (skipping the first one, that is
784 // for the return value.
785 if (PAL.hasAttributes(i + 1)) {
786 AttrBuilder B(PAL, i + 1);
787 if (B.contains(Attribute::Returned))
788 HasLiveReturnedArg = true;
790 push_back(AttributeSet::get(F->getContext(), Params.size(), B));
793 ++NumArgumentsEliminated;
794 DEBUG(dbgs() << "DAE - Removing argument " << i << " (" << I->getName()
795 << ") from " << F->getName() << "\n");
799 // Find out the new return value.
800 Type *RetTy = FTy->getReturnType();
801 Type *NRetTy = nullptr;
802 unsigned RetCount = NumRetVals(F);
804 // -1 means unused, other numbers are the new index
805 SmallVector<int, 5> NewRetIdxs(RetCount, -1);
806 std::vector<Type*> RetTypes;
808 // If there is a function with a live 'returned' argument but a dead return
809 // value, then there are two possible actions:
810 // 1) Eliminate the return value and take off the 'returned' attribute on the
812 // 2) Retain the 'returned' attribute and treat the return value (but not the
813 // entire function) as live so that it is not eliminated.
815 // It's not clear in the general case which option is more profitable because,
816 // even in the absence of explicit uses of the return value, code generation
817 // is free to use the 'returned' attribute to do things like eliding
818 // save/restores of registers across calls. Whether or not this happens is
819 // target and ABI-specific as well as depending on the amount of register
820 // pressure, so there's no good way for an IR-level pass to figure this out.
822 // Fortunately, the only places where 'returned' is currently generated by
823 // the FE are places where 'returned' is basically free and almost always a
824 // performance win, so the second option can just be used always for now.
826 // This should be revisited if 'returned' is ever applied more liberally.
827 if (RetTy->isVoidTy() || HasLiveReturnedArg) {
830 // Look at each of the original return values individually.
831 for (unsigned i = 0; i != RetCount; ++i) {
832 RetOrArg Ret = CreateRet(F, i);
833 if (LiveValues.erase(Ret)) {
834 RetTypes.push_back(getRetComponentType(F, i));
835 NewRetIdxs[i] = RetTypes.size() - 1;
837 ++NumRetValsEliminated;
838 DEBUG(dbgs() << "DAE - Removing return value " << i << " from "
839 << F->getName() << "\n");
842 if (RetTypes.size() > 1) {
843 // More than one return type? Reduce it down to size.
844 if (StructType *STy = dyn_cast<StructType>(RetTy)) {
845 // Make the new struct packed if we used to return a packed struct
847 NRetTy = StructType::get(STy->getContext(), RetTypes, STy->isPacked());
849 assert(isa<ArrayType>(RetTy) && "unexpected multi-value return");
850 NRetTy = ArrayType::get(RetTypes[0], RetTypes.size());
852 } else if (RetTypes.size() == 1)
853 // One return type? Just a simple value then, but only if we didn't use to
854 // return a struct with that simple value before.
855 NRetTy = RetTypes.front();
856 else if (RetTypes.size() == 0)
857 // No return types? Make it void, but only if we didn't use to return {}.
858 NRetTy = Type::getVoidTy(F->getContext());
861 assert(NRetTy && "No new return type found?");
863 // The existing function return attributes.
864 AttributeSet RAttrs = PAL.getRetAttributes();
866 // Remove any incompatible attributes, but only if we removed all return
867 // values. Otherwise, ensure that we don't have any conflicting attributes
868 // here. Currently, this should not be possible, but special handling might be
869 // required when new return value attributes are added.
870 if (NRetTy->isVoidTy())
871 RAttrs = RAttrs.removeAttributes(NRetTy->getContext(),
872 AttributeSet::ReturnIndex,
873 AttributeFuncs::typeIncompatible(NRetTy));
875 assert(!AttrBuilder(RAttrs, AttributeSet::ReturnIndex).
876 overlaps(AttributeFuncs::typeIncompatible(NRetTy)) &&
877 "Return attributes no longer compatible?");
879 if (RAttrs.hasAttributes(AttributeSet::ReturnIndex))
880 AttributesVec.push_back(AttributeSet::get(NRetTy->getContext(), RAttrs));
882 if (PAL.hasAttributes(AttributeSet::FunctionIndex))
883 AttributesVec.push_back(AttributeSet::get(F->getContext(),
884 PAL.getFnAttributes()));
886 // Reconstruct the AttributesList based on the vector we constructed.
887 AttributeSet NewPAL = AttributeSet::get(F->getContext(), AttributesVec);
889 // Create the new function type based on the recomputed parameters.
890 FunctionType *NFTy = FunctionType::get(NRetTy, Params, FTy->isVarArg());
896 // Create the new function body and insert it into the module...
897 Function *NF = Function::Create(NFTy, F->getLinkage());
898 NF->copyAttributesFrom(F);
899 NF->setAttributes(NewPAL);
900 // Insert the new function before the old function, so we won't be processing
902 F->getParent()->getFunctionList().insert(F, NF);
905 // Loop over all of the callers of the function, transforming the call sites
906 // to pass in a smaller number of arguments into the new function.
908 std::vector<Value*> Args;
909 while (!F->use_empty()) {
910 CallSite CS(F->user_back());
911 Instruction *Call = CS.getInstruction();
913 AttributesVec.clear();
914 const AttributeSet &CallPAL = CS.getAttributes();
916 // The call return attributes.
917 AttributeSet RAttrs = CallPAL.getRetAttributes();
919 // Adjust in case the function was changed to return void.
920 RAttrs = RAttrs.removeAttributes(NRetTy->getContext(),
921 AttributeSet::ReturnIndex,
922 AttributeFuncs::typeIncompatible(NF->getReturnType()));
923 if (RAttrs.hasAttributes(AttributeSet::ReturnIndex))
924 AttributesVec.push_back(AttributeSet::get(NF->getContext(), RAttrs));
926 // Declare these outside of the loops, so we can reuse them for the second
927 // loop, which loops the varargs.
928 CallSite::arg_iterator I = CS.arg_begin();
930 // Loop over those operands, corresponding to the normal arguments to the
931 // original function, and add those that are still alive.
932 for (unsigned e = FTy->getNumParams(); i != e; ++I, ++i)
935 // Get original parameter attributes, but skip return attributes.
936 if (CallPAL.hasAttributes(i + 1)) {
937 AttrBuilder B(CallPAL, i + 1);
938 // If the return type has changed, then get rid of 'returned' on the
939 // call site. The alternative is to make all 'returned' attributes on
940 // call sites keep the return value alive just like 'returned'
941 // attributes on function declaration but it's less clearly a win
942 // and this is not an expected case anyway
943 if (NRetTy != RetTy && B.contains(Attribute::Returned))
944 B.removeAttribute(Attribute::Returned);
946 push_back(AttributeSet::get(F->getContext(), Args.size(), B));
950 // Push any varargs arguments on the list. Don't forget their attributes.
951 for (CallSite::arg_iterator E = CS.arg_end(); I != E; ++I, ++i) {
953 if (CallPAL.hasAttributes(i + 1)) {
954 AttrBuilder B(CallPAL, i + 1);
956 push_back(AttributeSet::get(F->getContext(), Args.size(), B));
960 if (CallPAL.hasAttributes(AttributeSet::FunctionIndex))
961 AttributesVec.push_back(AttributeSet::get(Call->getContext(),
962 CallPAL.getFnAttributes()));
964 // Reconstruct the AttributesList based on the vector we constructed.
965 AttributeSet NewCallPAL = AttributeSet::get(F->getContext(), AttributesVec);
968 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
969 New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
971 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
972 cast<InvokeInst>(New)->setAttributes(NewCallPAL);
974 New = CallInst::Create(NF, Args, "", Call);
975 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
976 cast<CallInst>(New)->setAttributes(NewCallPAL);
977 if (cast<CallInst>(Call)->isTailCall())
978 cast<CallInst>(New)->setTailCall();
980 New->setDebugLoc(Call->getDebugLoc());
984 if (!Call->use_empty()) {
985 if (New->getType() == Call->getType()) {
986 // Return type not changed? Just replace users then.
987 Call->replaceAllUsesWith(New);
989 } else if (New->getType()->isVoidTy()) {
990 // Our return value has uses, but they will get removed later on.
991 // Replace by null for now.
992 if (!Call->getType()->isX86_MMXTy())
993 Call->replaceAllUsesWith(Constant::getNullValue(Call->getType()));
995 assert((RetTy->isStructTy() || RetTy->isArrayTy()) &&
996 "Return type changed, but not into a void. The old return type"
997 " must have been a struct or an array!");
998 Instruction *InsertPt = Call;
999 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
1000 BasicBlock::iterator IP = II->getNormalDest()->begin();
1001 while (isa<PHINode>(IP)) ++IP;
1005 // We used to return a struct or array. Instead of doing smart stuff
1006 // with all the uses, we will just rebuild it using extract/insertvalue
1007 // chaining and let instcombine clean that up.
1009 // Start out building up our return value from undef
1010 Value *RetVal = UndefValue::get(RetTy);
1011 for (unsigned i = 0; i != RetCount; ++i)
1012 if (NewRetIdxs[i] != -1) {
1014 if (RetTypes.size() > 1)
1015 // We are still returning a struct, so extract the value from our
1017 V = ExtractValueInst::Create(New, NewRetIdxs[i], "newret",
1020 // We are now returning a single element, so just insert that
1022 // Insert the value at the old position
1023 RetVal = InsertValueInst::Create(RetVal, V, i, "oldret", InsertPt);
1025 // Now, replace all uses of the old call instruction with the return
1027 Call->replaceAllUsesWith(RetVal);
1028 New->takeName(Call);
1032 // Finally, remove the old call from the program, reducing the use-count of
1034 Call->eraseFromParent();
1037 // Since we have now created the new function, splice the body of the old
1038 // function right into the new function, leaving the old rotting hulk of the
1040 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
1042 // Loop over the argument list, transferring uses of the old arguments over to
1043 // the new arguments, also transferring over the names as well.
1045 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(),
1046 I2 = NF->arg_begin(); I != E; ++I, ++i)
1048 // If this is a live argument, move the name and users over to the new
1050 I->replaceAllUsesWith(I2);
1054 // If this argument is dead, replace any uses of it with null constants
1055 // (these are guaranteed to become unused later on).
1056 if (!I->getType()->isX86_MMXTy())
1057 I->replaceAllUsesWith(Constant::getNullValue(I->getType()));
1060 // If we change the return value of the function we must rewrite any return
1061 // instructions. Check this now.
1062 if (F->getReturnType() != NF->getReturnType())
1063 for (Function::iterator BB = NF->begin(), E = NF->end(); BB != E; ++BB)
1064 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
1067 if (NFTy->getReturnType()->isVoidTy()) {
1070 assert(RetTy->isStructTy() || RetTy->isArrayTy());
1071 // The original return value was a struct or array, insert
1072 // extractvalue/insertvalue chains to extract only the values we need
1073 // to return and insert them into our new result.
1074 // This does generate messy code, but we'll let it to instcombine to
1076 Value *OldRet = RI->getOperand(0);
1077 // Start out building up our return value from undef
1078 RetVal = UndefValue::get(NRetTy);
1079 for (unsigned i = 0; i != RetCount; ++i)
1080 if (NewRetIdxs[i] != -1) {
1081 ExtractValueInst *EV = ExtractValueInst::Create(OldRet, i,
1083 if (RetTypes.size() > 1) {
1084 // We're still returning a struct, so reinsert the value into
1085 // our new return value at the new index
1087 RetVal = InsertValueInst::Create(RetVal, EV, NewRetIdxs[i],
1090 // We are now only returning a simple value, so just return the
1096 // Replace the return instruction with one returning the new return
1097 // value (possibly 0 if we became void).
1098 ReturnInst::Create(F->getContext(), RetVal, RI);
1099 BB->getInstList().erase(RI);
1102 // Patch the pointer to LLVM function in debug info descriptor.
1103 auto DI = FunctionDIs.find(F);
1104 if (DI != FunctionDIs.end())
1105 DI->second->replaceFunction(NF);
1107 // Now that the old function is dead, delete it.
1108 F->eraseFromParent();
1113 bool DAE::runOnModule(Module &M) {
1114 bool Changed = false;
1116 // Collect debug info descriptors for functions.
1117 FunctionDIs = makeSubprogramMap(M);
1119 // First pass: Do a simple check to see if any functions can have their "..."
1120 // removed. We can do this if they never call va_start. This loop cannot be
1121 // fused with the next loop, because deleting a function invalidates
1122 // information computed while surveying other functions.
1123 DEBUG(dbgs() << "DAE - Deleting dead varargs\n");
1124 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1126 if (F.getFunctionType()->isVarArg())
1127 Changed |= DeleteDeadVarargs(F);
1130 // Second phase:loop through the module, determining which arguments are live.
1131 // We assume all arguments are dead unless proven otherwise (allowing us to
1132 // determine that dead arguments passed into recursive functions are dead).
1134 DEBUG(dbgs() << "DAE - Determining liveness\n");
1138 // Now, remove all dead arguments and return values from each function in
1140 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1141 // Increment now, because the function will probably get removed (ie.
1142 // replaced by a new one).
1144 Changed |= RemoveDeadStuffFromFunction(F);
1147 // Finally, look for any unused parameters in functions with non-local
1148 // linkage and replace the passed in parameters with undef.
1150 Changed |= RemoveDeadArgumentsFromCallers(F);