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"
42 #define DEBUG_TYPE "deadargelim"
44 STATISTIC(NumArgumentsEliminated, "Number of unread args removed");
45 STATISTIC(NumRetValsEliminated , "Number of unused return values removed");
46 STATISTIC(NumArgumentsReplacedWithUndef,
47 "Number of unread args replaced with undef");
49 /// DAE - The dead argument elimination pass.
51 class DAE : public ModulePass {
54 /// Struct that represents (part of) either a return value or a function
55 /// argument. Used so that arguments and return values can be used
58 RetOrArg(const Function *F, unsigned Idx, bool IsArg) : F(F), Idx(Idx),
64 /// Make RetOrArg comparable, so we can put it into a map.
65 bool operator<(const RetOrArg &O) const {
66 return std::tie(F, Idx, IsArg) < std::tie(O.F, O.Idx, O.IsArg);
69 /// Make RetOrArg comparable, so we can easily iterate the multimap.
70 bool operator==(const RetOrArg &O) const {
71 return F == O.F && Idx == O.Idx && IsArg == O.IsArg;
74 std::string getDescription() const {
75 return std::string((IsArg ? "Argument #" : "Return value #"))
76 + utostr(Idx) + " of function " + F->getName().str();
80 /// Liveness enum - During our initial pass over the program, we determine
81 /// that things are either alive or maybe alive. We don't mark anything
82 /// explicitly dead (even if we know they are), since anything not alive
83 /// with no registered uses (in Uses) will never be marked alive and will
84 /// thus become dead in the end.
85 enum Liveness { Live, MaybeLive };
87 /// Convenience wrapper
88 RetOrArg CreateRet(const Function *F, unsigned Idx) {
89 return RetOrArg(F, Idx, false);
91 /// Convenience wrapper
92 RetOrArg CreateArg(const Function *F, unsigned Idx) {
93 return RetOrArg(F, Idx, true);
96 typedef std::multimap<RetOrArg, RetOrArg> UseMap;
97 /// This maps a return value or argument to any MaybeLive return values or
98 /// arguments it uses. This allows the MaybeLive values to be marked live
99 /// when any of its users is marked live.
100 /// For example (indices are left out for clarity):
101 /// - Uses[ret F] = ret G
102 /// This means that F calls G, and F returns the value returned by G.
103 /// - Uses[arg F] = ret G
104 /// This means that some function calls G and passes its result as an
106 /// - Uses[ret F] = arg F
107 /// This means that F returns one of its own arguments.
108 /// - Uses[arg F] = arg G
109 /// This means that G calls F and passes one of its own (G's) arguments
113 typedef std::set<RetOrArg> LiveSet;
114 typedef std::set<const Function*> LiveFuncSet;
116 /// This set contains all values that have been determined to be live.
118 /// This set contains all values that are cannot be changed in any way.
119 LiveFuncSet LiveFunctions;
121 typedef SmallVector<RetOrArg, 5> UseVector;
123 // Map each LLVM function to corresponding metadata with debug info. If
124 // the function is replaced with another one, we should patch the pointer
125 // to LLVM function in metadata.
126 // As the code generation for module is finished (and DIBuilder is
127 // finalized) we assume that subprogram descriptors won't be changed, and
128 // they are stored in map for short duration anyway.
129 typedef DenseMap<Function*, DISubprogram> FunctionDIMap;
130 FunctionDIMap 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 = 0);
150 Liveness SurveyUses(const Value *V, UseVector &MaybeLiveUses);
152 void CollectFunctionDIs(Module &M);
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 /// CollectFunctionDIs - Map each function in the module to its debug info
194 void DAE::CollectFunctionDIs(Module &M) {
197 for (Module::named_metadata_iterator I = M.named_metadata_begin(),
198 E = M.named_metadata_end(); I != E; ++I) {
199 NamedMDNode &NMD = *I;
200 for (unsigned MDIndex = 0, MDNum = NMD.getNumOperands();
201 MDIndex < MDNum; ++MDIndex) {
202 MDNode *Node = NMD.getOperand(MDIndex);
203 if (!DIDescriptor(Node).isCompileUnit())
205 DICompileUnit CU(Node);
206 const DIArray &SPs = CU.getSubprograms();
207 for (unsigned SPIndex = 0, SPNum = SPs.getNumElements();
208 SPIndex < SPNum; ++SPIndex) {
209 DISubprogram SP(SPs.getElement(SPIndex));
210 assert((!SP || SP.isSubprogram()) &&
211 "A MDNode in subprograms of a CU should be null or a DISubprogram.");
214 if (Function *F = SP.getFunction())
221 /// DeleteDeadVarargs - If this is an function that takes a ... list, and if
222 /// llvm.vastart is never called, the varargs list is dead for the function.
223 bool DAE::DeleteDeadVarargs(Function &Fn) {
224 assert(Fn.getFunctionType()->isVarArg() && "Function isn't varargs!");
225 if (Fn.isDeclaration() || !Fn.hasLocalLinkage()) return false;
227 // Ensure that the function is only directly called.
228 if (Fn.hasAddressTaken())
231 // Okay, we know we can transform this function if safe. Scan its body
232 // looking for calls to llvm.vastart.
233 for (Function::iterator BB = Fn.begin(), E = Fn.end(); BB != E; ++BB) {
234 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
235 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
236 if (II->getIntrinsicID() == Intrinsic::vastart)
242 // If we get here, there are no calls to llvm.vastart in the function body,
243 // remove the "..." and adjust all the calls.
245 // Start by computing a new prototype for the function, which is the same as
246 // the old function, but doesn't have isVarArg set.
247 FunctionType *FTy = Fn.getFunctionType();
249 std::vector<Type*> Params(FTy->param_begin(), FTy->param_end());
250 FunctionType *NFTy = FunctionType::get(FTy->getReturnType(),
252 unsigned NumArgs = Params.size();
254 // Create the new function body and insert it into the module...
255 Function *NF = Function::Create(NFTy, Fn.getLinkage());
256 NF->copyAttributesFrom(&Fn);
257 Fn.getParent()->getFunctionList().insert(&Fn, NF);
260 // Loop over all of the callers of the function, transforming the call sites
261 // to pass in a smaller number of arguments into the new function.
263 std::vector<Value*> Args;
264 for (Value::user_iterator I = Fn.user_begin(), E = Fn.user_end(); I != E; ) {
268 Instruction *Call = CS.getInstruction();
270 // Pass all the same arguments.
271 Args.assign(CS.arg_begin(), CS.arg_begin() + NumArgs);
273 // Drop any attributes that were on the vararg arguments.
274 AttributeSet PAL = CS.getAttributes();
275 if (!PAL.isEmpty() && PAL.getSlotIndex(PAL.getNumSlots() - 1) > NumArgs) {
276 SmallVector<AttributeSet, 8> AttributesVec;
277 for (unsigned i = 0; PAL.getSlotIndex(i) <= NumArgs; ++i)
278 AttributesVec.push_back(PAL.getSlotAttributes(i));
279 if (PAL.hasAttributes(AttributeSet::FunctionIndex))
280 AttributesVec.push_back(AttributeSet::get(Fn.getContext(),
281 PAL.getFnAttributes()));
282 PAL = AttributeSet::get(Fn.getContext(), AttributesVec);
286 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
287 New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
289 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
290 cast<InvokeInst>(New)->setAttributes(PAL);
292 New = CallInst::Create(NF, Args, "", Call);
293 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
294 cast<CallInst>(New)->setAttributes(PAL);
295 if (cast<CallInst>(Call)->isTailCall())
296 cast<CallInst>(New)->setTailCall();
298 New->setDebugLoc(Call->getDebugLoc());
302 if (!Call->use_empty())
303 Call->replaceAllUsesWith(New);
307 // Finally, remove the old call from the program, reducing the use-count of
309 Call->eraseFromParent();
312 // Since we have now created the new function, splice the body of the old
313 // function right into the new function, leaving the old rotting hulk of the
315 NF->getBasicBlockList().splice(NF->begin(), Fn.getBasicBlockList());
317 // Loop over the argument list, transferring uses of the old arguments over to
318 // the new arguments, also transferring over the names as well. While we're at
319 // it, remove the dead arguments from the DeadArguments list.
321 for (Function::arg_iterator I = Fn.arg_begin(), E = Fn.arg_end(),
322 I2 = NF->arg_begin(); I != E; ++I, ++I2) {
323 // Move the name and users over to the new version.
324 I->replaceAllUsesWith(I2);
328 // Patch the pointer to LLVM function in debug info descriptor.
329 FunctionDIMap::iterator DI = FunctionDIs.find(&Fn);
330 if (DI != FunctionDIs.end())
331 DI->second.replaceFunction(NF);
333 // Fix up any BlockAddresses that refer to the function.
334 Fn.replaceAllUsesWith(ConstantExpr::getBitCast(NF, Fn.getType()));
335 // Delete the bitcast that we just created, so that NF does not
336 // appear to be address-taken.
337 NF->removeDeadConstantUsers();
338 // Finally, nuke the old function.
339 Fn.eraseFromParent();
343 /// RemoveDeadArgumentsFromCallers - Checks if the given function has any
344 /// arguments that are unused, and changes the caller parameters to be undefined
346 bool DAE::RemoveDeadArgumentsFromCallers(Function &Fn)
348 if (Fn.isDeclaration() || Fn.mayBeOverridden())
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 // If a function seen at compile time is not necessarily the one linked to
357 // the binary being built, it is illegal to change the actual arguments
358 // passed to it. These functions can be captured by isWeakForLinker().
359 // *NOTE* that mayBeOverridden() is insufficient for this purpose as it
360 // doesn't include linkage types like AvailableExternallyLinkage and
361 // LinkOnceODRLinkage. Take link_odr* as an example, it indicates a set of
362 // *EQUIVALENT* globals that can be merged at link-time. However, the
363 // semantic of *EQUIVALENT*-functions includes parameters. Changing
364 // parameters breaks this assumption.
366 if (Fn.isWeakForLinker())
372 SmallVector<unsigned, 8> UnusedArgs;
373 for (Function::arg_iterator I = Fn.arg_begin(), E = Fn.arg_end();
377 if (Arg->use_empty() && !Arg->hasByValOrInAllocaAttr())
378 UnusedArgs.push_back(Arg->getArgNo());
381 if (UnusedArgs.empty())
384 bool Changed = false;
386 for (Use &U : Fn.uses()) {
387 CallSite CS(U.getUser());
388 if (!CS || !CS.isCallee(&U))
391 // Now go through all unused args and replace them with "undef".
392 for (unsigned I = 0, E = UnusedArgs.size(); I != E; ++I) {
393 unsigned ArgNo = UnusedArgs[I];
395 Value *Arg = CS.getArgument(ArgNo);
396 CS.setArgument(ArgNo, UndefValue::get(Arg->getType()));
397 ++NumArgumentsReplacedWithUndef;
405 /// Convenience function that returns the number of return values. It returns 0
406 /// for void functions and 1 for functions not returning a struct. It returns
407 /// the number of struct elements for functions returning a struct.
408 static unsigned NumRetVals(const Function *F) {
409 if (F->getReturnType()->isVoidTy())
411 else if (StructType *STy = dyn_cast<StructType>(F->getReturnType()))
412 return STy->getNumElements();
417 /// MarkIfNotLive - This checks Use for liveness in LiveValues. If Use is not
418 /// live, it adds Use to the MaybeLiveUses argument. Returns the determined
420 DAE::Liveness DAE::MarkIfNotLive(RetOrArg Use, UseVector &MaybeLiveUses) {
421 // We're live if our use or its Function is already marked as live.
422 if (LiveFunctions.count(Use.F) || LiveValues.count(Use))
425 // We're maybe live otherwise, but remember that we must become live if
427 MaybeLiveUses.push_back(Use);
432 /// SurveyUse - This looks at a single use of an argument or return value
433 /// and determines if it should be alive or not. Adds this use to MaybeLiveUses
434 /// if it causes the used value to become MaybeLive.
436 /// RetValNum is the return value number to use when this use is used in a
437 /// return instruction. This is used in the recursion, you should always leave
439 DAE::Liveness DAE::SurveyUse(const Use *U,
440 UseVector &MaybeLiveUses, unsigned RetValNum) {
441 const User *V = U->getUser();
442 if (const ReturnInst *RI = dyn_cast<ReturnInst>(V)) {
443 // The value is returned from a function. It's only live when the
444 // function's return value is live. We use RetValNum here, for the case
445 // that U is really a use of an insertvalue instruction that uses the
447 RetOrArg Use = CreateRet(RI->getParent()->getParent(), RetValNum);
448 // We might be live, depending on the liveness of Use.
449 return MarkIfNotLive(Use, MaybeLiveUses);
451 if (const InsertValueInst *IV = dyn_cast<InsertValueInst>(V)) {
452 if (U->getOperandNo() != InsertValueInst::getAggregateOperandIndex()
454 // The use we are examining is inserted into an aggregate. Our liveness
455 // depends on all uses of that aggregate, but if it is used as a return
456 // value, only index at which we were inserted counts.
457 RetValNum = *IV->idx_begin();
459 // Note that if we are used as the aggregate operand to the insertvalue,
460 // we don't change RetValNum, but do survey all our uses.
462 Liveness Result = MaybeLive;
463 for (const Use &UU : IV->uses()) {
464 Result = SurveyUse(&UU, MaybeLiveUses, RetValNum);
471 if (ImmutableCallSite CS = V) {
472 const Function *F = CS.getCalledFunction();
474 // Used in a direct call.
476 // Find the argument number. We know for sure that this use is an
477 // argument, since if it was the function argument this would be an
478 // indirect call and the we know can't be looking at a value of the
479 // label type (for the invoke instruction).
480 unsigned ArgNo = CS.getArgumentNo(U);
482 if (ArgNo >= F->getFunctionType()->getNumParams())
483 // The value is passed in through a vararg! Must be live.
486 assert(CS.getArgument(ArgNo)
487 == CS->getOperand(U->getOperandNo())
488 && "Argument is not where we expected it");
490 // Value passed to a normal call. It's only live when the corresponding
491 // argument to the called function turns out live.
492 RetOrArg Use = CreateArg(F, ArgNo);
493 return MarkIfNotLive(Use, MaybeLiveUses);
496 // Used in any other way? Value must be live.
500 /// SurveyUses - This looks at all the uses of the given value
501 /// Returns the Liveness deduced from the uses of this value.
503 /// Adds all uses that cause the result to be MaybeLive to MaybeLiveRetUses. If
504 /// the result is Live, MaybeLiveUses might be modified but its content should
505 /// be ignored (since it might not be complete).
506 DAE::Liveness DAE::SurveyUses(const Value *V, UseVector &MaybeLiveUses) {
507 // Assume it's dead (which will only hold if there are no uses at all..).
508 Liveness Result = MaybeLive;
510 for (const Use &U : V->uses()) {
511 Result = SurveyUse(&U, MaybeLiveUses);
518 // SurveyFunction - This performs the initial survey of the specified function,
519 // checking out whether or not it uses any of its incoming arguments or whether
520 // any callers use the return value. This fills in the LiveValues set and Uses
523 // We consider arguments of non-internal functions to be intrinsically alive as
524 // well as arguments to functions which have their "address taken".
526 void DAE::SurveyFunction(const Function &F) {
527 // Functions with inalloca parameters are expecting args in a particular
528 // register and memory layout.
529 if (F.getAttributes().hasAttrSomewhere(Attribute::InAlloca)) {
534 unsigned RetCount = NumRetVals(&F);
535 // Assume all return values are dead
536 typedef SmallVector<Liveness, 5> RetVals;
537 RetVals RetValLiveness(RetCount, MaybeLive);
539 typedef SmallVector<UseVector, 5> RetUses;
540 // These vectors map each return value to the uses that make it MaybeLive, so
541 // we can add those to the Uses map if the return value really turns out to be
542 // MaybeLive. Initialized to a list of RetCount empty lists.
543 RetUses MaybeLiveRetUses(RetCount);
545 for (Function::const_iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
546 if (const ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()))
547 if (RI->getNumOperands() != 0 && RI->getOperand(0)->getType()
548 != F.getFunctionType()->getReturnType()) {
549 // We don't support old style multiple return values.
554 if (!F.hasLocalLinkage() && (!ShouldHackArguments() || F.isIntrinsic())) {
559 DEBUG(dbgs() << "DAE - Inspecting callers for fn: " << F.getName() << "\n");
560 // Keep track of the number of live retvals, so we can skip checks once all
561 // of them turn out to be live.
562 unsigned NumLiveRetVals = 0;
563 Type *STy = dyn_cast<StructType>(F.getReturnType());
564 // Loop all uses of the function.
565 for (const Use &U : F.uses()) {
566 // If the function is PASSED IN as an argument, its address has been
568 ImmutableCallSite CS(U.getUser());
569 if (!CS || !CS.isCallee(&U)) {
574 // If this use is anything other than a call site, the function is alive.
575 const Instruction *TheCall = CS.getInstruction();
576 if (!TheCall) { // Not a direct call site?
581 // If we end up here, we are looking at a direct call to our function.
583 // Now, check how our return value(s) is/are used in this caller. Don't
584 // bother checking return values if all of them are live already.
585 if (NumLiveRetVals != RetCount) {
587 // Check all uses of the return value.
588 for (const User *U : TheCall->users()) {
589 const ExtractValueInst *Ext = dyn_cast<ExtractValueInst>(U);
590 if (Ext && Ext->hasIndices()) {
591 // This use uses a part of our return value, survey the uses of
592 // that part and store the results for this index only.
593 unsigned Idx = *Ext->idx_begin();
594 if (RetValLiveness[Idx] != Live) {
595 RetValLiveness[Idx] = SurveyUses(Ext, MaybeLiveRetUses[Idx]);
596 if (RetValLiveness[Idx] == Live)
600 // Used by something else than extractvalue. Mark all return
602 for (unsigned i = 0; i != RetCount; ++i )
603 RetValLiveness[i] = Live;
604 NumLiveRetVals = RetCount;
609 // Single return value
610 RetValLiveness[0] = SurveyUses(TheCall, MaybeLiveRetUses[0]);
611 if (RetValLiveness[0] == Live)
612 NumLiveRetVals = RetCount;
617 // Now we've inspected all callers, record the liveness of our return values.
618 for (unsigned i = 0; i != RetCount; ++i)
619 MarkValue(CreateRet(&F, i), RetValLiveness[i], MaybeLiveRetUses[i]);
621 DEBUG(dbgs() << "DAE - Inspecting args for fn: " << F.getName() << "\n");
623 // Now, check all of our arguments.
625 UseVector MaybeLiveArgUses;
626 for (Function::const_arg_iterator AI = F.arg_begin(),
627 E = F.arg_end(); AI != E; ++AI, ++i) {
629 if (F.getFunctionType()->isVarArg()) {
630 // Variadic functions will already have a va_arg function expanded inside
631 // them, making them potentially very sensitive to ABI changes resulting
632 // from removing arguments entirely, so don't. For example AArch64 handles
633 // register and stack HFAs very differently, and this is reflected in the
634 // IR which has already been generated.
637 // See what the effect of this use is (recording any uses that cause
638 // MaybeLive in MaybeLiveArgUses).
639 Result = SurveyUses(AI, MaybeLiveArgUses);
643 MarkValue(CreateArg(&F, i), Result, MaybeLiveArgUses);
644 // Clear the vector again for the next iteration.
645 MaybeLiveArgUses.clear();
649 /// MarkValue - This function marks the liveness of RA depending on L. If L is
650 /// MaybeLive, it also takes all uses in MaybeLiveUses and records them in Uses,
651 /// such that RA will be marked live if any use in MaybeLiveUses gets marked
653 void DAE::MarkValue(const RetOrArg &RA, Liveness L,
654 const UseVector &MaybeLiveUses) {
656 case Live: MarkLive(RA); break;
659 // Note any uses of this value, so this return value can be
660 // marked live whenever one of the uses becomes live.
661 for (UseVector::const_iterator UI = MaybeLiveUses.begin(),
662 UE = MaybeLiveUses.end(); UI != UE; ++UI)
663 Uses.insert(std::make_pair(*UI, RA));
669 /// MarkLive - Mark the given Function as alive, meaning that it cannot be
670 /// changed in any way. Additionally,
671 /// mark any values that are used as this function's parameters or by its return
672 /// values (according to Uses) live as well.
673 void DAE::MarkLive(const Function &F) {
674 DEBUG(dbgs() << "DAE - Intrinsically live fn: " << F.getName() << "\n");
675 // Mark the function as live.
676 LiveFunctions.insert(&F);
677 // Mark all arguments as live.
678 for (unsigned i = 0, e = F.arg_size(); i != e; ++i)
679 PropagateLiveness(CreateArg(&F, i));
680 // Mark all return values as live.
681 for (unsigned i = 0, e = NumRetVals(&F); i != e; ++i)
682 PropagateLiveness(CreateRet(&F, i));
685 /// MarkLive - Mark the given return value or argument as live. Additionally,
686 /// mark any values that are used by this value (according to Uses) live as
688 void DAE::MarkLive(const RetOrArg &RA) {
689 if (LiveFunctions.count(RA.F))
690 return; // Function was already marked Live.
692 if (!LiveValues.insert(RA).second)
693 return; // We were already marked Live.
695 DEBUG(dbgs() << "DAE - Marking " << RA.getDescription() << " live\n");
696 PropagateLiveness(RA);
699 /// PropagateLiveness - Given that RA is a live value, propagate it's liveness
700 /// to any other values it uses (according to Uses).
701 void DAE::PropagateLiveness(const RetOrArg &RA) {
702 // We don't use upper_bound (or equal_range) here, because our recursive call
703 // to ourselves is likely to cause the upper_bound (which is the first value
704 // not belonging to RA) to become erased and the iterator invalidated.
705 UseMap::iterator Begin = Uses.lower_bound(RA);
706 UseMap::iterator E = Uses.end();
708 for (I = Begin; I != E && I->first == RA; ++I)
711 // Erase RA from the Uses map (from the lower bound to wherever we ended up
713 Uses.erase(Begin, I);
716 // RemoveDeadStuffFromFunction - Remove any arguments and return values from F
717 // that are not in LiveValues. Transform the function and all of the callees of
718 // the function to not have these arguments and return values.
720 bool DAE::RemoveDeadStuffFromFunction(Function *F) {
721 // Don't modify fully live functions
722 if (LiveFunctions.count(F))
725 // Start by computing a new prototype for the function, which is the same as
726 // the old function, but has fewer arguments and a different return type.
727 FunctionType *FTy = F->getFunctionType();
728 std::vector<Type*> Params;
730 // Keep track of if we have a live 'returned' argument
731 bool HasLiveReturnedArg = false;
733 // Set up to build a new list of parameter attributes.
734 SmallVector<AttributeSet, 8> AttributesVec;
735 const AttributeSet &PAL = F->getAttributes();
737 // Remember which arguments are still alive.
738 SmallVector<bool, 10> ArgAlive(FTy->getNumParams(), false);
739 // Construct the new parameter list from non-dead arguments. Also construct
740 // a new set of parameter attributes to correspond. Skip the first parameter
741 // attribute, since that belongs to the return value.
743 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
745 RetOrArg Arg = CreateArg(F, i);
746 if (LiveValues.erase(Arg)) {
747 Params.push_back(I->getType());
750 // Get the original parameter attributes (skipping the first one, that is
751 // for the return value.
752 if (PAL.hasAttributes(i + 1)) {
753 AttrBuilder B(PAL, i + 1);
754 if (B.contains(Attribute::Returned))
755 HasLiveReturnedArg = true;
757 push_back(AttributeSet::get(F->getContext(), Params.size(), B));
760 ++NumArgumentsEliminated;
761 DEBUG(dbgs() << "DAE - Removing argument " << i << " (" << I->getName()
762 << ") from " << F->getName() << "\n");
766 // Find out the new return value.
767 Type *RetTy = FTy->getReturnType();
768 Type *NRetTy = nullptr;
769 unsigned RetCount = NumRetVals(F);
771 // -1 means unused, other numbers are the new index
772 SmallVector<int, 5> NewRetIdxs(RetCount, -1);
773 std::vector<Type*> RetTypes;
775 // If there is a function with a live 'returned' argument but a dead return
776 // value, then there are two possible actions:
777 // 1) Eliminate the return value and take off the 'returned' attribute on the
779 // 2) Retain the 'returned' attribute and treat the return value (but not the
780 // entire function) as live so that it is not eliminated.
782 // It's not clear in the general case which option is more profitable because,
783 // even in the absence of explicit uses of the return value, code generation
784 // is free to use the 'returned' attribute to do things like eliding
785 // save/restores of registers across calls. Whether or not this happens is
786 // target and ABI-specific as well as depending on the amount of register
787 // pressure, so there's no good way for an IR-level pass to figure this out.
789 // Fortunately, the only places where 'returned' is currently generated by
790 // the FE are places where 'returned' is basically free and almost always a
791 // performance win, so the second option can just be used always for now.
793 // This should be revisited if 'returned' is ever applied more liberally.
794 if (RetTy->isVoidTy() || HasLiveReturnedArg) {
797 StructType *STy = dyn_cast<StructType>(RetTy);
799 // Look at each of the original return values individually.
800 for (unsigned i = 0; i != RetCount; ++i) {
801 RetOrArg Ret = CreateRet(F, i);
802 if (LiveValues.erase(Ret)) {
803 RetTypes.push_back(STy->getElementType(i));
804 NewRetIdxs[i] = RetTypes.size() - 1;
806 ++NumRetValsEliminated;
807 DEBUG(dbgs() << "DAE - Removing return value " << i << " from "
808 << F->getName() << "\n");
812 // We used to return a single value.
813 if (LiveValues.erase(CreateRet(F, 0))) {
814 RetTypes.push_back(RetTy);
817 DEBUG(dbgs() << "DAE - Removing return value from " << F->getName()
819 ++NumRetValsEliminated;
821 if (RetTypes.size() > 1)
822 // More than one return type? Return a struct with them. Also, if we used
823 // to return a struct and didn't change the number of return values,
824 // return a struct again. This prevents changing {something} into
825 // something and {} into void.
826 // Make the new struct packed if we used to return a packed struct
828 NRetTy = StructType::get(STy->getContext(), RetTypes, STy->isPacked());
829 else if (RetTypes.size() == 1)
830 // One return type? Just a simple value then, but only if we didn't use to
831 // return a struct with that simple value before.
832 NRetTy = RetTypes.front();
833 else if (RetTypes.size() == 0)
834 // No return types? Make it void, but only if we didn't use to return {}.
835 NRetTy = Type::getVoidTy(F->getContext());
838 assert(NRetTy && "No new return type found?");
840 // The existing function return attributes.
841 AttributeSet RAttrs = PAL.getRetAttributes();
843 // Remove any incompatible attributes, but only if we removed all return
844 // values. Otherwise, ensure that we don't have any conflicting attributes
845 // here. Currently, this should not be possible, but special handling might be
846 // required when new return value attributes are added.
847 if (NRetTy->isVoidTy())
849 AttributeSet::get(NRetTy->getContext(), AttributeSet::ReturnIndex,
850 AttrBuilder(RAttrs, AttributeSet::ReturnIndex).
851 removeAttributes(AttributeFuncs::
852 typeIncompatible(NRetTy, AttributeSet::ReturnIndex),
853 AttributeSet::ReturnIndex));
855 assert(!AttrBuilder(RAttrs, AttributeSet::ReturnIndex).
856 hasAttributes(AttributeFuncs::
857 typeIncompatible(NRetTy, AttributeSet::ReturnIndex),
858 AttributeSet::ReturnIndex) &&
859 "Return attributes no longer compatible?");
861 if (RAttrs.hasAttributes(AttributeSet::ReturnIndex))
862 AttributesVec.push_back(AttributeSet::get(NRetTy->getContext(), RAttrs));
864 if (PAL.hasAttributes(AttributeSet::FunctionIndex))
865 AttributesVec.push_back(AttributeSet::get(F->getContext(),
866 PAL.getFnAttributes()));
868 // Reconstruct the AttributesList based on the vector we constructed.
869 AttributeSet NewPAL = AttributeSet::get(F->getContext(), AttributesVec);
871 // Create the new function type based on the recomputed parameters.
872 FunctionType *NFTy = FunctionType::get(NRetTy, Params, FTy->isVarArg());
878 // Create the new function body and insert it into the module...
879 Function *NF = Function::Create(NFTy, F->getLinkage());
880 NF->copyAttributesFrom(F);
881 NF->setAttributes(NewPAL);
882 // Insert the new function before the old function, so we won't be processing
884 F->getParent()->getFunctionList().insert(F, NF);
887 // Loop over all of the callers of the function, transforming the call sites
888 // to pass in a smaller number of arguments into the new function.
890 std::vector<Value*> Args;
891 while (!F->use_empty()) {
892 CallSite CS(F->user_back());
893 Instruction *Call = CS.getInstruction();
895 AttributesVec.clear();
896 const AttributeSet &CallPAL = CS.getAttributes();
898 // The call return attributes.
899 AttributeSet RAttrs = CallPAL.getRetAttributes();
901 // Adjust in case the function was changed to return void.
903 AttributeSet::get(NF->getContext(), AttributeSet::ReturnIndex,
904 AttrBuilder(RAttrs, AttributeSet::ReturnIndex).
905 removeAttributes(AttributeFuncs::
906 typeIncompatible(NF->getReturnType(),
907 AttributeSet::ReturnIndex),
908 AttributeSet::ReturnIndex));
909 if (RAttrs.hasAttributes(AttributeSet::ReturnIndex))
910 AttributesVec.push_back(AttributeSet::get(NF->getContext(), RAttrs));
912 // Declare these outside of the loops, so we can reuse them for the second
913 // loop, which loops the varargs.
914 CallSite::arg_iterator I = CS.arg_begin();
916 // Loop over those operands, corresponding to the normal arguments to the
917 // original function, and add those that are still alive.
918 for (unsigned e = FTy->getNumParams(); i != e; ++I, ++i)
921 // Get original parameter attributes, but skip return attributes.
922 if (CallPAL.hasAttributes(i + 1)) {
923 AttrBuilder B(CallPAL, i + 1);
924 // If the return type has changed, then get rid of 'returned' on the
925 // call site. The alternative is to make all 'returned' attributes on
926 // call sites keep the return value alive just like 'returned'
927 // attributes on function declaration but it's less clearly a win
928 // and this is not an expected case anyway
929 if (NRetTy != RetTy && B.contains(Attribute::Returned))
930 B.removeAttribute(Attribute::Returned);
932 push_back(AttributeSet::get(F->getContext(), Args.size(), B));
936 // Push any varargs arguments on the list. Don't forget their attributes.
937 for (CallSite::arg_iterator E = CS.arg_end(); I != E; ++I, ++i) {
939 if (CallPAL.hasAttributes(i + 1)) {
940 AttrBuilder B(CallPAL, i + 1);
942 push_back(AttributeSet::get(F->getContext(), Args.size(), B));
946 if (CallPAL.hasAttributes(AttributeSet::FunctionIndex))
947 AttributesVec.push_back(AttributeSet::get(Call->getContext(),
948 CallPAL.getFnAttributes()));
950 // Reconstruct the AttributesList based on the vector we constructed.
951 AttributeSet NewCallPAL = AttributeSet::get(F->getContext(), AttributesVec);
954 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
955 New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
957 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
958 cast<InvokeInst>(New)->setAttributes(NewCallPAL);
960 New = CallInst::Create(NF, Args, "", Call);
961 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
962 cast<CallInst>(New)->setAttributes(NewCallPAL);
963 if (cast<CallInst>(Call)->isTailCall())
964 cast<CallInst>(New)->setTailCall();
966 New->setDebugLoc(Call->getDebugLoc());
970 if (!Call->use_empty()) {
971 if (New->getType() == Call->getType()) {
972 // Return type not changed? Just replace users then.
973 Call->replaceAllUsesWith(New);
975 } else if (New->getType()->isVoidTy()) {
976 // Our return value has uses, but they will get removed later on.
977 // Replace by null for now.
978 if (!Call->getType()->isX86_MMXTy())
979 Call->replaceAllUsesWith(Constant::getNullValue(Call->getType()));
981 assert(RetTy->isStructTy() &&
982 "Return type changed, but not into a void. The old return type"
983 " must have been a struct!");
984 Instruction *InsertPt = Call;
985 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
986 BasicBlock::iterator IP = II->getNormalDest()->begin();
987 while (isa<PHINode>(IP)) ++IP;
991 // We used to return a struct. Instead of doing smart stuff with all the
992 // uses of this struct, we will just rebuild it using
993 // extract/insertvalue chaining and let instcombine clean that up.
995 // Start out building up our return value from undef
996 Value *RetVal = UndefValue::get(RetTy);
997 for (unsigned i = 0; i != RetCount; ++i)
998 if (NewRetIdxs[i] != -1) {
1000 if (RetTypes.size() > 1)
1001 // We are still returning a struct, so extract the value from our
1003 V = ExtractValueInst::Create(New, NewRetIdxs[i], "newret",
1006 // We are now returning a single element, so just insert that
1008 // Insert the value at the old position
1009 RetVal = InsertValueInst::Create(RetVal, V, i, "oldret", InsertPt);
1011 // Now, replace all uses of the old call instruction with the return
1013 Call->replaceAllUsesWith(RetVal);
1014 New->takeName(Call);
1018 // Finally, remove the old call from the program, reducing the use-count of
1020 Call->eraseFromParent();
1023 // Since we have now created the new function, splice the body of the old
1024 // function right into the new function, leaving the old rotting hulk of the
1026 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
1028 // Loop over the argument list, transferring uses of the old arguments over to
1029 // the new arguments, also transferring over the names as well.
1031 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(),
1032 I2 = NF->arg_begin(); I != E; ++I, ++i)
1034 // If this is a live argument, move the name and users over to the new
1036 I->replaceAllUsesWith(I2);
1040 // If this argument is dead, replace any uses of it with null constants
1041 // (these are guaranteed to become unused later on).
1042 if (!I->getType()->isX86_MMXTy())
1043 I->replaceAllUsesWith(Constant::getNullValue(I->getType()));
1046 // If we change the return value of the function we must rewrite any return
1047 // instructions. Check this now.
1048 if (F->getReturnType() != NF->getReturnType())
1049 for (Function::iterator BB = NF->begin(), E = NF->end(); BB != E; ++BB)
1050 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
1053 if (NFTy->getReturnType()->isVoidTy()) {
1056 assert (RetTy->isStructTy());
1057 // The original return value was a struct, insert
1058 // extractvalue/insertvalue chains to extract only the values we need
1059 // to return and insert them into our new result.
1060 // This does generate messy code, but we'll let it to instcombine to
1062 Value *OldRet = RI->getOperand(0);
1063 // Start out building up our return value from undef
1064 RetVal = UndefValue::get(NRetTy);
1065 for (unsigned i = 0; i != RetCount; ++i)
1066 if (NewRetIdxs[i] != -1) {
1067 ExtractValueInst *EV = ExtractValueInst::Create(OldRet, i,
1069 if (RetTypes.size() > 1) {
1070 // We're still returning a struct, so reinsert the value into
1071 // our new return value at the new index
1073 RetVal = InsertValueInst::Create(RetVal, EV, NewRetIdxs[i],
1076 // We are now only returning a simple value, so just return the
1082 // Replace the return instruction with one returning the new return
1083 // value (possibly 0 if we became void).
1084 ReturnInst::Create(F->getContext(), RetVal, RI);
1085 BB->getInstList().erase(RI);
1088 // Patch the pointer to LLVM function in debug info descriptor.
1089 FunctionDIMap::iterator DI = FunctionDIs.find(F);
1090 if (DI != FunctionDIs.end())
1091 DI->second.replaceFunction(NF);
1093 // Now that the old function is dead, delete it.
1094 F->eraseFromParent();
1099 bool DAE::runOnModule(Module &M) {
1100 bool Changed = false;
1102 // Collect debug info descriptors for functions.
1103 CollectFunctionDIs(M);
1105 // First pass: Do a simple check to see if any functions can have their "..."
1106 // removed. We can do this if they never call va_start. This loop cannot be
1107 // fused with the next loop, because deleting a function invalidates
1108 // information computed while surveying other functions.
1109 DEBUG(dbgs() << "DAE - Deleting dead varargs\n");
1110 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1112 if (F.getFunctionType()->isVarArg())
1113 Changed |= DeleteDeadVarargs(F);
1116 // Second phase:loop through the module, determining which arguments are live.
1117 // We assume all arguments are dead unless proven otherwise (allowing us to
1118 // determine that dead arguments passed into recursive functions are dead).
1120 DEBUG(dbgs() << "DAE - Determining liveness\n");
1121 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
1124 // Now, remove all dead arguments and return values from each function in
1126 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1127 // Increment now, because the function will probably get removed (ie.
1128 // replaced by a new one).
1130 Changed |= RemoveDeadStuffFromFunction(F);
1133 // Finally, look for any unused parameters in functions with non-local
1134 // linkage and replace the passed in parameters with undef.
1135 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
1138 Changed |= RemoveDeadArgumentsFromCallers(F);