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 // 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, 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 auto DI = FunctionDIs.find(&Fn);
305 if (DI != FunctionDIs.end()) {
306 DISubprogram *SP = DI->second;
307 SP->replaceFunction(NF);
308 // Ensure the map is updated so it can be reused on non-varargs argument
309 // eliminations of the same function.
310 FunctionDIs.erase(DI);
311 FunctionDIs[NF] = SP;
314 // Fix up any BlockAddresses that refer to the function.
315 Fn.replaceAllUsesWith(ConstantExpr::getBitCast(NF, Fn.getType()));
316 // Delete the bitcast that we just created, so that NF does not
317 // appear to be address-taken.
318 NF->removeDeadConstantUsers();
319 // Finally, nuke the old function.
320 Fn.eraseFromParent();
324 /// RemoveDeadArgumentsFromCallers - Checks if the given function has any
325 /// arguments that are unused, and changes the caller parameters to be undefined
327 bool DAE::RemoveDeadArgumentsFromCallers(Function &Fn)
329 if (Fn.isDeclaration() || Fn.mayBeOverridden())
332 // Functions with local linkage should already have been handled, except the
333 // fragile (variadic) ones which we can improve here.
334 if (Fn.hasLocalLinkage() && !Fn.getFunctionType()->isVarArg())
337 // If a function seen at compile time is not necessarily the one linked to
338 // the binary being built, it is illegal to change the actual arguments
339 // passed to it. These functions can be captured by isWeakForLinker().
340 // *NOTE* that mayBeOverridden() is insufficient for this purpose as it
341 // doesn't include linkage types like AvailableExternallyLinkage and
342 // LinkOnceODRLinkage. Take link_odr* as an example, it indicates a set of
343 // *EQUIVALENT* globals that can be merged at link-time. However, the
344 // semantic of *EQUIVALENT*-functions includes parameters. Changing
345 // parameters breaks this assumption.
347 if (Fn.isWeakForLinker())
353 SmallVector<unsigned, 8> UnusedArgs;
354 for (Function::arg_iterator I = Fn.arg_begin(), E = Fn.arg_end();
358 if (Arg->use_empty() && !Arg->hasByValOrInAllocaAttr())
359 UnusedArgs.push_back(Arg->getArgNo());
362 if (UnusedArgs.empty())
365 bool Changed = false;
367 for (Use &U : Fn.uses()) {
368 CallSite CS(U.getUser());
369 if (!CS || !CS.isCallee(&U))
372 // Now go through all unused args and replace them with "undef".
373 for (unsigned I = 0, E = UnusedArgs.size(); I != E; ++I) {
374 unsigned ArgNo = UnusedArgs[I];
376 Value *Arg = CS.getArgument(ArgNo);
377 CS.setArgument(ArgNo, UndefValue::get(Arg->getType()));
378 ++NumArgumentsReplacedWithUndef;
386 /// Convenience function that returns the number of return values. It returns 0
387 /// for void functions and 1 for functions not returning a struct. It returns
388 /// the number of struct elements for functions returning a struct.
389 static unsigned NumRetVals(const Function *F) {
390 Type *RetTy = F->getReturnType();
391 if (RetTy->isVoidTy())
393 else if (StructType *STy = dyn_cast<StructType>(RetTy))
394 return STy->getNumElements();
395 else if (ArrayType *ATy = dyn_cast<ArrayType>(RetTy))
396 return ATy->getNumElements();
401 /// Returns the sub-type a function will return at a given Idx. Should
402 /// correspond to the result type of an ExtractValue instruction executed with
403 /// just that one Idx (i.e. only top-level structure is considered).
404 static Type *getRetComponentType(const Function *F, unsigned Idx) {
405 Type *RetTy = F->getReturnType();
406 assert(!RetTy->isVoidTy() && "void type has no subtype");
408 if (StructType *STy = dyn_cast<StructType>(RetTy))
409 return STy->getElementType(Idx);
410 else if (ArrayType *ATy = dyn_cast<ArrayType>(RetTy))
411 return ATy->getElementType();
416 /// MarkIfNotLive - This checks Use for liveness in LiveValues. If Use is not
417 /// live, it adds Use to the MaybeLiveUses argument. Returns the determined
419 DAE::Liveness DAE::MarkIfNotLive(RetOrArg Use, UseVector &MaybeLiveUses) {
420 // We're live if our use or its Function is already marked as live.
421 if (LiveFunctions.count(Use.F) || LiveValues.count(Use))
424 // We're maybe live otherwise, but remember that we must become live if
426 MaybeLiveUses.push_back(Use);
431 /// SurveyUse - This looks at a single use of an argument or return value
432 /// and determines if it should be alive or not. Adds this use to MaybeLiveUses
433 /// if it causes the used value to become MaybeLive.
435 /// RetValNum is the return value number to use when this use is used in a
436 /// return instruction. This is used in the recursion, you should always leave
438 DAE::Liveness DAE::SurveyUse(const Use *U,
439 UseVector &MaybeLiveUses, unsigned RetValNum) {
440 const User *V = U->getUser();
441 if (const ReturnInst *RI = dyn_cast<ReturnInst>(V)) {
442 // The value is returned from a function. It's only live when the
443 // function's return value is live. We use RetValNum here, for the case
444 // that U is really a use of an insertvalue instruction that uses the
446 const Function *F = RI->getParent()->getParent();
447 if (RetValNum != -1U) {
448 RetOrArg Use = CreateRet(F, RetValNum);
449 // We might be live, depending on the liveness of Use.
450 return MarkIfNotLive(Use, MaybeLiveUses);
452 DAE::Liveness Result = MaybeLive;
453 for (unsigned i = 0; i < NumRetVals(F); ++i) {
454 RetOrArg Use = CreateRet(F, i);
455 // We might be live, depending on the liveness of Use. If any
456 // sub-value is live, then the entire value is considered live. This
457 // is a conservative choice, and better tracking is possible.
458 DAE::Liveness SubResult = MarkIfNotLive(Use, MaybeLiveUses);
465 if (const InsertValueInst *IV = dyn_cast<InsertValueInst>(V)) {
466 if (U->getOperandNo() != InsertValueInst::getAggregateOperandIndex()
468 // The use we are examining is inserted into an aggregate. Our liveness
469 // depends on all uses of that aggregate, but if it is used as a return
470 // value, only index at which we were inserted counts.
471 RetValNum = *IV->idx_begin();
473 // Note that if we are used as the aggregate operand to the insertvalue,
474 // we don't change RetValNum, but do survey all our uses.
476 Liveness Result = MaybeLive;
477 for (const Use &UU : IV->uses()) {
478 Result = SurveyUse(&UU, MaybeLiveUses, RetValNum);
485 if (auto CS = ImmutableCallSite(V)) {
486 const Function *F = CS.getCalledFunction();
488 // Used in a direct call.
490 // Find the argument number. We know for sure that this use is an
491 // argument, since if it was the function argument this would be an
492 // indirect call and the we know can't be looking at a value of the
493 // label type (for the invoke instruction).
494 unsigned ArgNo = CS.getArgumentNo(U);
496 if (ArgNo >= F->getFunctionType()->getNumParams())
497 // The value is passed in through a vararg! Must be live.
500 assert(CS.getArgument(ArgNo)
501 == CS->getOperand(U->getOperandNo())
502 && "Argument is not where we expected it");
504 // Value passed to a normal call. It's only live when the corresponding
505 // argument to the called function turns out live.
506 RetOrArg Use = CreateArg(F, ArgNo);
507 return MarkIfNotLive(Use, MaybeLiveUses);
510 // Used in any other way? Value must be live.
514 /// SurveyUses - This looks at all the uses of the given value
515 /// Returns the Liveness deduced from the uses of this value.
517 /// Adds all uses that cause the result to be MaybeLive to MaybeLiveRetUses. If
518 /// the result is Live, MaybeLiveUses might be modified but its content should
519 /// be ignored (since it might not be complete).
520 DAE::Liveness DAE::SurveyUses(const Value *V, UseVector &MaybeLiveUses) {
521 // Assume it's dead (which will only hold if there are no uses at all..).
522 Liveness Result = MaybeLive;
524 for (const Use &U : V->uses()) {
525 Result = SurveyUse(&U, MaybeLiveUses);
532 // SurveyFunction - This performs the initial survey of the specified function,
533 // checking out whether or not it uses any of its incoming arguments or whether
534 // any callers use the return value. This fills in the LiveValues set and Uses
537 // We consider arguments of non-internal functions to be intrinsically alive as
538 // well as arguments to functions which have their "address taken".
540 void DAE::SurveyFunction(const Function &F) {
541 // Functions with inalloca parameters are expecting args in a particular
542 // register and memory layout.
543 if (F.getAttributes().hasAttrSomewhere(Attribute::InAlloca)) {
548 unsigned RetCount = NumRetVals(&F);
549 // Assume all return values are dead
550 typedef SmallVector<Liveness, 5> RetVals;
551 RetVals RetValLiveness(RetCount, MaybeLive);
553 typedef SmallVector<UseVector, 5> RetUses;
554 // These vectors map each return value to the uses that make it MaybeLive, so
555 // we can add those to the Uses map if the return value really turns out to be
556 // MaybeLive. Initialized to a list of RetCount empty lists.
557 RetUses MaybeLiveRetUses(RetCount);
559 for (Function::const_iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
560 if (const ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()))
561 if (RI->getNumOperands() != 0 && RI->getOperand(0)->getType()
562 != F.getFunctionType()->getReturnType()) {
563 // We don't support old style multiple return values.
568 if (!F.hasLocalLinkage() && (!ShouldHackArguments() || F.isIntrinsic())) {
573 DEBUG(dbgs() << "DAE - Inspecting callers for fn: " << F.getName() << "\n");
574 // Keep track of the number of live retvals, so we can skip checks once all
575 // of them turn out to be live.
576 unsigned NumLiveRetVals = 0;
577 // Loop all uses of the function.
578 for (const Use &U : F.uses()) {
579 // If the function is PASSED IN as an argument, its address has been
581 ImmutableCallSite CS(U.getUser());
582 if (!CS || !CS.isCallee(&U)) {
587 // If this use is anything other than a call site, the function is alive.
588 const Instruction *TheCall = CS.getInstruction();
589 if (!TheCall) { // Not a direct call site?
594 // If we end up here, we are looking at a direct call to our function.
596 // Now, check how our return value(s) is/are used in this caller. Don't
597 // bother checking return values if all of them are live already.
598 if (NumLiveRetVals == RetCount)
601 // Check all uses of the return value.
602 for (const Use &U : TheCall->uses()) {
603 if (ExtractValueInst *Ext = dyn_cast<ExtractValueInst>(U.getUser())) {
604 // This use uses a part of our return value, survey the uses of
605 // that part and store the results for this index only.
606 unsigned Idx = *Ext->idx_begin();
607 if (RetValLiveness[Idx] != Live) {
608 RetValLiveness[Idx] = SurveyUses(Ext, MaybeLiveRetUses[Idx]);
609 if (RetValLiveness[Idx] == Live)
613 // Used by something else than extractvalue. Survey, but assume that the
614 // result applies to all sub-values.
615 UseVector MaybeLiveAggregateUses;
616 if (SurveyUse(&U, MaybeLiveAggregateUses) == Live) {
617 NumLiveRetVals = RetCount;
618 RetValLiveness.assign(RetCount, Live);
621 for (unsigned i = 0; i != RetCount; ++i) {
622 if (RetValLiveness[i] != Live)
623 MaybeLiveRetUses[i].append(MaybeLiveAggregateUses.begin(),
624 MaybeLiveAggregateUses.end());
631 // Now we've inspected all callers, record the liveness of our return values.
632 for (unsigned i = 0; i != RetCount; ++i)
633 MarkValue(CreateRet(&F, i), RetValLiveness[i], MaybeLiveRetUses[i]);
635 DEBUG(dbgs() << "DAE - Inspecting args for fn: " << F.getName() << "\n");
637 // Now, check all of our arguments.
639 UseVector MaybeLiveArgUses;
640 for (Function::const_arg_iterator AI = F.arg_begin(),
641 E = F.arg_end(); AI != E; ++AI, ++i) {
643 if (F.getFunctionType()->isVarArg()) {
644 // Variadic functions will already have a va_arg function expanded inside
645 // them, making them potentially very sensitive to ABI changes resulting
646 // from removing arguments entirely, so don't. For example AArch64 handles
647 // register and stack HFAs very differently, and this is reflected in the
648 // IR which has already been generated.
651 // See what the effect of this use is (recording any uses that cause
652 // MaybeLive in MaybeLiveArgUses).
653 Result = SurveyUses(AI, MaybeLiveArgUses);
657 MarkValue(CreateArg(&F, i), Result, MaybeLiveArgUses);
658 // Clear the vector again for the next iteration.
659 MaybeLiveArgUses.clear();
663 /// MarkValue - This function marks the liveness of RA depending on L. If L is
664 /// MaybeLive, it also takes all uses in MaybeLiveUses and records them in Uses,
665 /// such that RA will be marked live if any use in MaybeLiveUses gets marked
667 void DAE::MarkValue(const RetOrArg &RA, Liveness L,
668 const UseVector &MaybeLiveUses) {
670 case Live: MarkLive(RA); break;
673 // Note any uses of this value, so this return value can be
674 // marked live whenever one of the uses becomes live.
675 for (UseVector::const_iterator UI = MaybeLiveUses.begin(),
676 UE = MaybeLiveUses.end(); UI != UE; ++UI)
677 Uses.insert(std::make_pair(*UI, RA));
683 /// MarkLive - Mark the given Function as alive, meaning that it cannot be
684 /// changed in any way. Additionally,
685 /// mark any values that are used as this function's parameters or by its return
686 /// values (according to Uses) live as well.
687 void DAE::MarkLive(const Function &F) {
688 DEBUG(dbgs() << "DAE - Intrinsically live fn: " << F.getName() << "\n");
689 // Mark the function as live.
690 LiveFunctions.insert(&F);
691 // Mark all arguments as live.
692 for (unsigned i = 0, e = F.arg_size(); i != e; ++i)
693 PropagateLiveness(CreateArg(&F, i));
694 // Mark all return values as live.
695 for (unsigned i = 0, e = NumRetVals(&F); i != e; ++i)
696 PropagateLiveness(CreateRet(&F, i));
699 /// MarkLive - Mark the given return value or argument as live. Additionally,
700 /// mark any values that are used by this value (according to Uses) live as
702 void DAE::MarkLive(const RetOrArg &RA) {
703 if (LiveFunctions.count(RA.F))
704 return; // Function was already marked Live.
706 if (!LiveValues.insert(RA).second)
707 return; // We were already marked Live.
709 DEBUG(dbgs() << "DAE - Marking " << RA.getDescription() << " live\n");
710 PropagateLiveness(RA);
713 /// PropagateLiveness - Given that RA is a live value, propagate it's liveness
714 /// to any other values it uses (according to Uses).
715 void DAE::PropagateLiveness(const RetOrArg &RA) {
716 // We don't use upper_bound (or equal_range) here, because our recursive call
717 // to ourselves is likely to cause the upper_bound (which is the first value
718 // not belonging to RA) to become erased and the iterator invalidated.
719 UseMap::iterator Begin = Uses.lower_bound(RA);
720 UseMap::iterator E = Uses.end();
722 for (I = Begin; I != E && I->first == RA; ++I)
725 // Erase RA from the Uses map (from the lower bound to wherever we ended up
727 Uses.erase(Begin, I);
730 // RemoveDeadStuffFromFunction - Remove any arguments and return values from F
731 // that are not in LiveValues. Transform the function and all of the callees of
732 // the function to not have these arguments and return values.
734 bool DAE::RemoveDeadStuffFromFunction(Function *F) {
735 // Don't modify fully live functions
736 if (LiveFunctions.count(F))
739 // Start by computing a new prototype for the function, which is the same as
740 // the old function, but has fewer arguments and a different return type.
741 FunctionType *FTy = F->getFunctionType();
742 std::vector<Type*> Params;
744 // Keep track of if we have a live 'returned' argument
745 bool HasLiveReturnedArg = false;
747 // Set up to build a new list of parameter attributes.
748 SmallVector<AttributeSet, 8> AttributesVec;
749 const AttributeSet &PAL = F->getAttributes();
751 // Remember which arguments are still alive.
752 SmallVector<bool, 10> ArgAlive(FTy->getNumParams(), false);
753 // Construct the new parameter list from non-dead arguments. Also construct
754 // a new set of parameter attributes to correspond. Skip the first parameter
755 // attribute, since that belongs to the return value.
757 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
759 RetOrArg Arg = CreateArg(F, i);
760 if (LiveValues.erase(Arg)) {
761 Params.push_back(I->getType());
764 // Get the original parameter attributes (skipping the first one, that is
765 // for the return value.
766 if (PAL.hasAttributes(i + 1)) {
767 AttrBuilder B(PAL, i + 1);
768 if (B.contains(Attribute::Returned))
769 HasLiveReturnedArg = true;
771 push_back(AttributeSet::get(F->getContext(), Params.size(), B));
774 ++NumArgumentsEliminated;
775 DEBUG(dbgs() << "DAE - Removing argument " << i << " (" << I->getName()
776 << ") from " << F->getName() << "\n");
780 // Find out the new return value.
781 Type *RetTy = FTy->getReturnType();
782 Type *NRetTy = nullptr;
783 unsigned RetCount = NumRetVals(F);
785 // -1 means unused, other numbers are the new index
786 SmallVector<int, 5> NewRetIdxs(RetCount, -1);
787 std::vector<Type*> RetTypes;
789 // If there is a function with a live 'returned' argument but a dead return
790 // value, then there are two possible actions:
791 // 1) Eliminate the return value and take off the 'returned' attribute on the
793 // 2) Retain the 'returned' attribute and treat the return value (but not the
794 // entire function) as live so that it is not eliminated.
796 // It's not clear in the general case which option is more profitable because,
797 // even in the absence of explicit uses of the return value, code generation
798 // is free to use the 'returned' attribute to do things like eliding
799 // save/restores of registers across calls. Whether or not this happens is
800 // target and ABI-specific as well as depending on the amount of register
801 // pressure, so there's no good way for an IR-level pass to figure this out.
803 // Fortunately, the only places where 'returned' is currently generated by
804 // the FE are places where 'returned' is basically free and almost always a
805 // performance win, so the second option can just be used always for now.
807 // This should be revisited if 'returned' is ever applied more liberally.
808 if (RetTy->isVoidTy() || HasLiveReturnedArg) {
811 // Look at each of the original return values individually.
812 for (unsigned i = 0; i != RetCount; ++i) {
813 RetOrArg Ret = CreateRet(F, i);
814 if (LiveValues.erase(Ret)) {
815 RetTypes.push_back(getRetComponentType(F, i));
816 NewRetIdxs[i] = RetTypes.size() - 1;
818 ++NumRetValsEliminated;
819 DEBUG(dbgs() << "DAE - Removing return value " << i << " from "
820 << F->getName() << "\n");
823 if (RetTypes.size() > 1) {
824 // More than one return type? Reduce it down to size.
825 if (StructType *STy = dyn_cast<StructType>(RetTy)) {
826 // Make the new struct packed if we used to return a packed struct
828 NRetTy = StructType::get(STy->getContext(), RetTypes, STy->isPacked());
830 assert(isa<ArrayType>(RetTy) && "unexpected multi-value return");
831 NRetTy = ArrayType::get(RetTypes[0], RetTypes.size());
833 } else if (RetTypes.size() == 1)
834 // One return type? Just a simple value then, but only if we didn't use to
835 // return a struct with that simple value before.
836 NRetTy = RetTypes.front();
837 else if (RetTypes.size() == 0)
838 // No return types? Make it void, but only if we didn't use to return {}.
839 NRetTy = Type::getVoidTy(F->getContext());
842 assert(NRetTy && "No new return type found?");
844 // The existing function return attributes.
845 AttributeSet RAttrs = PAL.getRetAttributes();
847 // Remove any incompatible attributes, but only if we removed all return
848 // values. Otherwise, ensure that we don't have any conflicting attributes
849 // here. Currently, this should not be possible, but special handling might be
850 // required when new return value attributes are added.
851 if (NRetTy->isVoidTy())
852 RAttrs = RAttrs.removeAttributes(NRetTy->getContext(),
853 AttributeSet::ReturnIndex,
854 AttributeFuncs::typeIncompatible(NRetTy));
856 assert(!AttrBuilder(RAttrs, AttributeSet::ReturnIndex).
857 overlaps(AttributeFuncs::typeIncompatible(NRetTy)) &&
858 "Return attributes no longer compatible?");
860 if (RAttrs.hasAttributes(AttributeSet::ReturnIndex))
861 AttributesVec.push_back(AttributeSet::get(NRetTy->getContext(), RAttrs));
863 if (PAL.hasAttributes(AttributeSet::FunctionIndex))
864 AttributesVec.push_back(AttributeSet::get(F->getContext(),
865 PAL.getFnAttributes()));
867 // Reconstruct the AttributesList based on the vector we constructed.
868 AttributeSet NewPAL = AttributeSet::get(F->getContext(), AttributesVec);
870 // Create the new function type based on the recomputed parameters.
871 FunctionType *NFTy = FunctionType::get(NRetTy, Params, FTy->isVarArg());
877 // Create the new function body and insert it into the module...
878 Function *NF = Function::Create(NFTy, F->getLinkage());
879 NF->copyAttributesFrom(F);
880 NF->setAttributes(NewPAL);
881 // Insert the new function before the old function, so we won't be processing
883 F->getParent()->getFunctionList().insert(F, NF);
886 // Loop over all of the callers of the function, transforming the call sites
887 // to pass in a smaller number of arguments into the new function.
889 std::vector<Value*> Args;
890 while (!F->use_empty()) {
891 CallSite CS(F->user_back());
892 Instruction *Call = CS.getInstruction();
894 AttributesVec.clear();
895 const AttributeSet &CallPAL = CS.getAttributes();
897 // The call return attributes.
898 AttributeSet RAttrs = CallPAL.getRetAttributes();
900 // Adjust in case the function was changed to return void.
901 RAttrs = RAttrs.removeAttributes(NRetTy->getContext(),
902 AttributeSet::ReturnIndex,
903 AttributeFuncs::typeIncompatible(NF->getReturnType()));
904 if (RAttrs.hasAttributes(AttributeSet::ReturnIndex))
905 AttributesVec.push_back(AttributeSet::get(NF->getContext(), RAttrs));
907 // Declare these outside of the loops, so we can reuse them for the second
908 // loop, which loops the varargs.
909 CallSite::arg_iterator I = CS.arg_begin();
911 // Loop over those operands, corresponding to the normal arguments to the
912 // original function, and add those that are still alive.
913 for (unsigned e = FTy->getNumParams(); i != e; ++I, ++i)
916 // Get original parameter attributes, but skip return attributes.
917 if (CallPAL.hasAttributes(i + 1)) {
918 AttrBuilder B(CallPAL, i + 1);
919 // If the return type has changed, then get rid of 'returned' on the
920 // call site. The alternative is to make all 'returned' attributes on
921 // call sites keep the return value alive just like 'returned'
922 // attributes on function declaration but it's less clearly a win
923 // and this is not an expected case anyway
924 if (NRetTy != RetTy && B.contains(Attribute::Returned))
925 B.removeAttribute(Attribute::Returned);
927 push_back(AttributeSet::get(F->getContext(), Args.size(), B));
931 // Push any varargs arguments on the list. Don't forget their attributes.
932 for (CallSite::arg_iterator E = CS.arg_end(); I != E; ++I, ++i) {
934 if (CallPAL.hasAttributes(i + 1)) {
935 AttrBuilder B(CallPAL, i + 1);
937 push_back(AttributeSet::get(F->getContext(), Args.size(), B));
941 if (CallPAL.hasAttributes(AttributeSet::FunctionIndex))
942 AttributesVec.push_back(AttributeSet::get(Call->getContext(),
943 CallPAL.getFnAttributes()));
945 // Reconstruct the AttributesList based on the vector we constructed.
946 AttributeSet NewCallPAL = AttributeSet::get(F->getContext(), AttributesVec);
949 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
950 New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
952 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
953 cast<InvokeInst>(New)->setAttributes(NewCallPAL);
955 New = CallInst::Create(NF, Args, "", Call);
956 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
957 cast<CallInst>(New)->setAttributes(NewCallPAL);
958 if (cast<CallInst>(Call)->isTailCall())
959 cast<CallInst>(New)->setTailCall();
961 New->setDebugLoc(Call->getDebugLoc());
965 if (!Call->use_empty()) {
966 if (New->getType() == Call->getType()) {
967 // Return type not changed? Just replace users then.
968 Call->replaceAllUsesWith(New);
970 } else if (New->getType()->isVoidTy()) {
971 // Our return value has uses, but they will get removed later on.
972 // Replace by null for now.
973 if (!Call->getType()->isX86_MMXTy())
974 Call->replaceAllUsesWith(Constant::getNullValue(Call->getType()));
976 assert((RetTy->isStructTy() || RetTy->isArrayTy()) &&
977 "Return type changed, but not into a void. The old return type"
978 " must have been a struct or an array!");
979 Instruction *InsertPt = Call;
980 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
981 BasicBlock::iterator IP = II->getNormalDest()->begin();
982 while (isa<PHINode>(IP)) ++IP;
986 // We used to return a struct or array. Instead of doing smart stuff
987 // with all the uses, we will just rebuild it using extract/insertvalue
988 // chaining and let instcombine clean that up.
990 // Start out building up our return value from undef
991 Value *RetVal = UndefValue::get(RetTy);
992 for (unsigned i = 0; i != RetCount; ++i)
993 if (NewRetIdxs[i] != -1) {
995 if (RetTypes.size() > 1)
996 // We are still returning a struct, so extract the value from our
998 V = ExtractValueInst::Create(New, NewRetIdxs[i], "newret",
1001 // We are now returning a single element, so just insert that
1003 // Insert the value at the old position
1004 RetVal = InsertValueInst::Create(RetVal, V, i, "oldret", InsertPt);
1006 // Now, replace all uses of the old call instruction with the return
1008 Call->replaceAllUsesWith(RetVal);
1009 New->takeName(Call);
1013 // Finally, remove the old call from the program, reducing the use-count of
1015 Call->eraseFromParent();
1018 // Since we have now created the new function, splice the body of the old
1019 // function right into the new function, leaving the old rotting hulk of the
1021 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
1023 // Loop over the argument list, transferring uses of the old arguments over to
1024 // the new arguments, also transferring over the names as well.
1026 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(),
1027 I2 = NF->arg_begin(); I != E; ++I, ++i)
1029 // If this is a live argument, move the name and users over to the new
1031 I->replaceAllUsesWith(I2);
1035 // If this argument is dead, replace any uses of it with null constants
1036 // (these are guaranteed to become unused later on).
1037 if (!I->getType()->isX86_MMXTy())
1038 I->replaceAllUsesWith(Constant::getNullValue(I->getType()));
1041 // If we change the return value of the function we must rewrite any return
1042 // instructions. Check this now.
1043 if (F->getReturnType() != NF->getReturnType())
1044 for (Function::iterator BB = NF->begin(), E = NF->end(); BB != E; ++BB)
1045 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
1048 if (NFTy->getReturnType()->isVoidTy()) {
1051 assert(RetTy->isStructTy() || RetTy->isArrayTy());
1052 // The original return value was a struct or array, insert
1053 // extractvalue/insertvalue chains to extract only the values we need
1054 // to return and insert them into our new result.
1055 // This does generate messy code, but we'll let it to instcombine to
1057 Value *OldRet = RI->getOperand(0);
1058 // Start out building up our return value from undef
1059 RetVal = UndefValue::get(NRetTy);
1060 for (unsigned i = 0; i != RetCount; ++i)
1061 if (NewRetIdxs[i] != -1) {
1062 ExtractValueInst *EV = ExtractValueInst::Create(OldRet, i,
1064 if (RetTypes.size() > 1) {
1065 // We're still returning a struct, so reinsert the value into
1066 // our new return value at the new index
1068 RetVal = InsertValueInst::Create(RetVal, EV, NewRetIdxs[i],
1071 // We are now only returning a simple value, so just return the
1077 // Replace the return instruction with one returning the new return
1078 // value (possibly 0 if we became void).
1079 ReturnInst::Create(F->getContext(), RetVal, RI);
1080 BB->getInstList().erase(RI);
1083 // Patch the pointer to LLVM function in debug info descriptor.
1084 auto DI = FunctionDIs.find(F);
1085 if (DI != FunctionDIs.end())
1086 DI->second->replaceFunction(NF);
1088 // Now that the old function is dead, delete it.
1089 F->eraseFromParent();
1094 bool DAE::runOnModule(Module &M) {
1095 bool Changed = false;
1097 // Collect debug info descriptors for functions.
1098 FunctionDIs = makeSubprogramMap(M);
1100 // First pass: Do a simple check to see if any functions can have their "..."
1101 // removed. We can do this if they never call va_start. This loop cannot be
1102 // fused with the next loop, because deleting a function invalidates
1103 // information computed while surveying other functions.
1104 DEBUG(dbgs() << "DAE - Deleting dead varargs\n");
1105 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1107 if (F.getFunctionType()->isVarArg())
1108 Changed |= DeleteDeadVarargs(F);
1111 // Second phase:loop through the module, determining which arguments are live.
1112 // We assume all arguments are dead unless proven otherwise (allowing us to
1113 // determine that dead arguments passed into recursive functions are dead).
1115 DEBUG(dbgs() << "DAE - Determining liveness\n");
1119 // Now, remove all dead arguments and return values from each function in
1121 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1122 // Increment now, because the function will probably get removed (ie.
1123 // replaced by a new one).
1125 Changed |= RemoveDeadStuffFromFunction(F);
1128 // Finally, look for any unused parameters in functions with non-local
1129 // linkage and replace the passed in parameters with undef.
1131 Changed |= RemoveDeadArgumentsFromCallers(F);