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 #define DEBUG_TYPE "deadargelim"
21 #include "llvm/Transforms/IPO.h"
22 #include "llvm/ADT/DenseMap.h"
23 #include "llvm/ADT/SmallVector.h"
24 #include "llvm/ADT/Statistic.h"
25 #include "llvm/ADT/StringExtras.h"
26 #include "llvm/DebugInfo.h"
27 #include "llvm/IR/CallSite.h"
28 #include "llvm/IR/CallingConv.h"
29 #include "llvm/IR/Constant.h"
30 #include "llvm/IR/DIBuilder.h"
31 #include "llvm/IR/DerivedTypes.h"
32 #include "llvm/IR/Instructions.h"
33 #include "llvm/IR/IntrinsicInst.h"
34 #include "llvm/IR/LLVMContext.h"
35 #include "llvm/IR/Module.h"
36 #include "llvm/Pass.h"
37 #include "llvm/Support/Debug.h"
38 #include "llvm/Support/raw_ostream.h"
43 STATISTIC(NumArgumentsEliminated, "Number of unread args removed");
44 STATISTIC(NumRetValsEliminated , "Number of unused return values removed");
45 STATISTIC(NumArgumentsReplacedWithUndef,
46 "Number of unread args replaced with undef");
48 /// DAE - The dead argument elimination pass.
50 class DAE : public ModulePass {
53 /// Struct that represents (part of) either a return value or a function
54 /// argument. Used so that arguments and return values can be used
57 RetOrArg(const Function *F, unsigned Idx, bool IsArg) : F(F), Idx(Idx),
63 /// Make RetOrArg comparable, so we can put it into a map.
64 bool operator<(const RetOrArg &O) const {
65 return std::tie(F, Idx, IsArg) < std::tie(O.F, O.Idx, O.IsArg);
68 /// Make RetOrArg comparable, so we can easily iterate the multimap.
69 bool operator==(const RetOrArg &O) const {
70 return F == O.F && Idx == O.Idx && IsArg == O.IsArg;
73 std::string getDescription() const {
74 return std::string((IsArg ? "Argument #" : "Return value #"))
75 + utostr(Idx) + " of function " + F->getName().str();
79 /// Liveness enum - During our initial pass over the program, we determine
80 /// that things are either alive or maybe alive. We don't mark anything
81 /// explicitly dead (even if we know they are), since anything not alive
82 /// with no registered uses (in Uses) will never be marked alive and will
83 /// thus become dead in the end.
84 enum Liveness { Live, MaybeLive };
86 /// Convenience wrapper
87 RetOrArg CreateRet(const Function *F, unsigned Idx) {
88 return RetOrArg(F, Idx, false);
90 /// Convenience wrapper
91 RetOrArg CreateArg(const Function *F, unsigned Idx) {
92 return RetOrArg(F, Idx, true);
95 typedef std::multimap<RetOrArg, RetOrArg> UseMap;
96 /// This maps a return value or argument to any MaybeLive return values or
97 /// arguments it uses. This allows the MaybeLive values to be marked live
98 /// when any of its users is marked live.
99 /// For example (indices are left out for clarity):
100 /// - Uses[ret F] = ret G
101 /// This means that F calls G, and F returns the value returned by G.
102 /// - Uses[arg F] = ret G
103 /// This means that some function calls G and passes its result as an
105 /// - Uses[ret F] = arg F
106 /// This means that F returns one of its own arguments.
107 /// - Uses[arg F] = arg G
108 /// This means that G calls F and passes one of its own (G's) arguments
112 typedef std::set<RetOrArg> LiveSet;
113 typedef std::set<const Function*> LiveFuncSet;
115 /// This set contains all values that have been determined to be live.
117 /// This set contains all values that are cannot be changed in any way.
118 LiveFuncSet LiveFunctions;
120 typedef SmallVector<RetOrArg, 5> UseVector;
122 // Map each LLVM function to corresponding metadata with debug info. If
123 // the function is replaced with another one, we should patch the pointer
124 // to LLVM function in metadata.
125 // As the code generation for module is finished (and DIBuilder is
126 // finalized) we assume that subprogram descriptors won't be changed, and
127 // they are stored in map for short duration anyway.
128 typedef DenseMap<Function*, DISubprogram> FunctionDIMap;
129 FunctionDIMap FunctionDIs;
132 // DAH uses this to specify a different ID.
133 explicit DAE(char &ID) : ModulePass(ID) {}
136 static char ID; // Pass identification, replacement for typeid
137 DAE() : ModulePass(ID) {
138 initializeDAEPass(*PassRegistry::getPassRegistry());
141 bool runOnModule(Module &M) override;
143 virtual bool ShouldHackArguments() const { return false; }
146 Liveness MarkIfNotLive(RetOrArg Use, UseVector &MaybeLiveUses);
147 Liveness SurveyUse(Value::const_use_iterator U, UseVector &MaybeLiveUses,
148 unsigned RetValNum = 0);
149 Liveness SurveyUses(const Value *V, UseVector &MaybeLiveUses);
151 void CollectFunctionDIs(Module &M);
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 /// CollectFunctionDIs - Map each function in the module to its debug info
193 void DAE::CollectFunctionDIs(Module &M) {
196 for (Module::named_metadata_iterator I = M.named_metadata_begin(),
197 E = M.named_metadata_end(); I != E; ++I) {
198 NamedMDNode &NMD = *I;
199 for (unsigned MDIndex = 0, MDNum = NMD.getNumOperands();
200 MDIndex < MDNum; ++MDIndex) {
201 MDNode *Node = NMD.getOperand(MDIndex);
202 if (!DIDescriptor(Node).isCompileUnit())
204 DICompileUnit CU(Node);
205 const DIArray &SPs = CU.getSubprograms();
206 for (unsigned SPIndex = 0, SPNum = SPs.getNumElements();
207 SPIndex < SPNum; ++SPIndex) {
208 DISubprogram SP(SPs.getElement(SPIndex));
209 assert((!SP || SP.isSubprogram()) &&
210 "A MDNode in subprograms of a CU should be null or a DISubprogram.");
213 if (Function *F = SP.getFunction())
220 /// DeleteDeadVarargs - If this is an function that takes a ... list, and if
221 /// llvm.vastart is never called, the varargs list is dead for the function.
222 bool DAE::DeleteDeadVarargs(Function &Fn) {
223 assert(Fn.getFunctionType()->isVarArg() && "Function isn't varargs!");
224 if (Fn.isDeclaration() || !Fn.hasLocalLinkage()) return false;
226 // Ensure that the function is only directly called.
227 if (Fn.hasAddressTaken())
230 // Okay, we know we can transform this function if safe. Scan its body
231 // looking for calls to llvm.vastart.
232 for (Function::iterator BB = Fn.begin(), E = Fn.end(); BB != E; ++BB) {
233 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
234 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
235 if (II->getIntrinsicID() == Intrinsic::vastart)
241 // If we get here, there are no calls to llvm.vastart in the function body,
242 // remove the "..." and adjust all the calls.
244 // Start by computing a new prototype for the function, which is the same as
245 // the old function, but doesn't have isVarArg set.
246 FunctionType *FTy = Fn.getFunctionType();
248 std::vector<Type*> Params(FTy->param_begin(), FTy->param_end());
249 FunctionType *NFTy = FunctionType::get(FTy->getReturnType(),
251 unsigned NumArgs = Params.size();
253 // Create the new function body and insert it into the module...
254 Function *NF = Function::Create(NFTy, Fn.getLinkage());
255 NF->copyAttributesFrom(&Fn);
256 Fn.getParent()->getFunctionList().insert(&Fn, NF);
259 // Loop over all of the callers of the function, transforming the call sites
260 // to pass in a smaller number of arguments into the new function.
262 std::vector<Value*> Args;
263 for (Value::use_iterator I = Fn.use_begin(), E = Fn.use_end(); I != E; ) {
267 Instruction *Call = CS.getInstruction();
269 // Pass all the same arguments.
270 Args.assign(CS.arg_begin(), CS.arg_begin() + NumArgs);
272 // Drop any attributes that were on the vararg arguments.
273 AttributeSet PAL = CS.getAttributes();
274 if (!PAL.isEmpty() && PAL.getSlotIndex(PAL.getNumSlots() - 1) > NumArgs) {
275 SmallVector<AttributeSet, 8> AttributesVec;
276 for (unsigned i = 0; PAL.getSlotIndex(i) <= NumArgs; ++i)
277 AttributesVec.push_back(PAL.getSlotAttributes(i));
278 if (PAL.hasAttributes(AttributeSet::FunctionIndex))
279 AttributesVec.push_back(AttributeSet::get(Fn.getContext(),
280 PAL.getFnAttributes()));
281 PAL = AttributeSet::get(Fn.getContext(), AttributesVec);
285 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
286 New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
288 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
289 cast<InvokeInst>(New)->setAttributes(PAL);
291 New = CallInst::Create(NF, Args, "", Call);
292 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
293 cast<CallInst>(New)->setAttributes(PAL);
294 if (cast<CallInst>(Call)->isTailCall())
295 cast<CallInst>(New)->setTailCall();
297 New->setDebugLoc(Call->getDebugLoc());
301 if (!Call->use_empty())
302 Call->replaceAllUsesWith(New);
306 // Finally, remove the old call from the program, reducing the use-count of
308 Call->eraseFromParent();
311 // Since we have now created the new function, splice the body of the old
312 // function right into the new function, leaving the old rotting hulk of the
314 NF->getBasicBlockList().splice(NF->begin(), Fn.getBasicBlockList());
316 // Loop over the argument list, transferring uses of the old arguments over to
317 // the new arguments, also transferring over the names as well. While we're at
318 // it, remove the dead arguments from the DeadArguments list.
320 for (Function::arg_iterator I = Fn.arg_begin(), E = Fn.arg_end(),
321 I2 = NF->arg_begin(); I != E; ++I, ++I2) {
322 // Move the name and users over to the new version.
323 I->replaceAllUsesWith(I2);
327 // Patch the pointer to LLVM function in debug info descriptor.
328 FunctionDIMap::iterator DI = FunctionDIs.find(&Fn);
329 if (DI != FunctionDIs.end())
330 DI->second.replaceFunction(NF);
332 // Fix up any BlockAddresses that refer to the function.
333 Fn.replaceAllUsesWith(ConstantExpr::getBitCast(NF, Fn.getType()));
334 // Delete the bitcast that we just created, so that NF does not
335 // appear to be address-taken.
336 NF->removeDeadConstantUsers();
337 // Finally, nuke the old function.
338 Fn.eraseFromParent();
342 /// RemoveDeadArgumentsFromCallers - Checks if the given function has any
343 /// arguments that are unused, and changes the caller parameters to be undefined
345 bool DAE::RemoveDeadArgumentsFromCallers(Function &Fn)
347 if (Fn.isDeclaration() || Fn.mayBeOverridden())
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 // If a function seen at compile time is not necessarily the one linked to
356 // the binary being built, it is illegal to change the actual arguments
357 // passed to it. These functions can be captured by isWeakForLinker().
358 // *NOTE* that mayBeOverridden() is insufficient for this purpose as it
359 // doesn't include linkage types like AvailableExternallyLinkage and
360 // LinkOnceODRLinkage. Take link_odr* as an example, it indicates a set of
361 // *EQUIVALENT* globals that can be merged at link-time. However, the
362 // semantic of *EQUIVALENT*-functions includes parameters. Changing
363 // parameters breaks this assumption.
365 if (Fn.isWeakForLinker())
371 SmallVector<unsigned, 8> UnusedArgs;
372 for (Function::arg_iterator I = Fn.arg_begin(), E = Fn.arg_end();
376 if (Arg->use_empty() && !Arg->hasByValOrInAllocaAttr())
377 UnusedArgs.push_back(Arg->getArgNo());
380 if (UnusedArgs.empty())
383 bool Changed = false;
385 for (Function::use_iterator I = Fn.use_begin(), E = Fn.use_end();
388 if (!CS || !CS.isCallee(I))
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(Value::const_use_iterator U,
440 UseVector &MaybeLiveUses, unsigned RetValNum) {
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 (Value::const_use_iterator I = IV->use_begin(),
464 E = V->use_end(); I != E; ++I) {
465 Result = SurveyUse(I, MaybeLiveUses, RetValNum);
472 if (ImmutableCallSite CS = V) {
473 const Function *F = CS.getCalledFunction();
475 // Used in a direct call.
477 // Find the argument number. We know for sure that this use is an
478 // argument, since if it was the function argument this would be an
479 // indirect call and the we know can't be looking at a value of the
480 // label type (for the invoke instruction).
481 unsigned ArgNo = CS.getArgumentNo(U);
483 if (ArgNo >= F->getFunctionType()->getNumParams())
484 // The value is passed in through a vararg! Must be live.
487 assert(CS.getArgument(ArgNo)
488 == CS->getOperand(U.getOperandNo())
489 && "Argument is not where we expected it");
491 // Value passed to a normal call. It's only live when the corresponding
492 // argument to the called function turns out live.
493 RetOrArg Use = CreateArg(F, ArgNo);
494 return MarkIfNotLive(Use, MaybeLiveUses);
497 // Used in any other way? Value must be live.
501 /// SurveyUses - This looks at all the uses of the given value
502 /// Returns the Liveness deduced from the uses of this value.
504 /// Adds all uses that cause the result to be MaybeLive to MaybeLiveRetUses. If
505 /// the result is Live, MaybeLiveUses might be modified but its content should
506 /// be ignored (since it might not be complete).
507 DAE::Liveness DAE::SurveyUses(const Value *V, UseVector &MaybeLiveUses) {
508 // Assume it's dead (which will only hold if there are no uses at all..).
509 Liveness Result = MaybeLive;
511 for (Value::const_use_iterator I = V->use_begin(),
512 E = V->use_end(); I != E; ++I) {
513 Result = SurveyUse(I, MaybeLiveUses);
520 // SurveyFunction - This performs the initial survey of the specified function,
521 // checking out whether or not it uses any of its incoming arguments or whether
522 // any callers use the return value. This fills in the LiveValues set and Uses
525 // We consider arguments of non-internal functions to be intrinsically alive as
526 // well as arguments to functions which have their "address taken".
528 void DAE::SurveyFunction(const Function &F) {
529 // Functions with inalloca parameters are expecting args in a particular
530 // register and memory layout.
531 if (F.getAttributes().hasAttrSomewhere(Attribute::InAlloca)) {
536 unsigned RetCount = NumRetVals(&F);
537 // Assume all return values are dead
538 typedef SmallVector<Liveness, 5> RetVals;
539 RetVals RetValLiveness(RetCount, MaybeLive);
541 typedef SmallVector<UseVector, 5> RetUses;
542 // These vectors map each return value to the uses that make it MaybeLive, so
543 // we can add those to the Uses map if the return value really turns out to be
544 // MaybeLive. Initialized to a list of RetCount empty lists.
545 RetUses MaybeLiveRetUses(RetCount);
547 for (Function::const_iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
548 if (const ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()))
549 if (RI->getNumOperands() != 0 && RI->getOperand(0)->getType()
550 != F.getFunctionType()->getReturnType()) {
551 // We don't support old style multiple return values.
556 if (!F.hasLocalLinkage() && (!ShouldHackArguments() || F.isIntrinsic())) {
561 DEBUG(dbgs() << "DAE - Inspecting callers for fn: " << F.getName() << "\n");
562 // Keep track of the number of live retvals, so we can skip checks once all
563 // of them turn out to be live.
564 unsigned NumLiveRetVals = 0;
565 Type *STy = dyn_cast<StructType>(F.getReturnType());
566 // Loop all uses of the function.
567 for (Value::const_use_iterator I = F.use_begin(), E = F.use_end();
569 // If the function is PASSED IN as an argument, its address has been
571 ImmutableCallSite CS(*I);
572 if (!CS || !CS.isCallee(I)) {
577 // If this use is anything other than a call site, the function is alive.
578 const Instruction *TheCall = CS.getInstruction();
579 if (!TheCall) { // Not a direct call site?
584 // If we end up here, we are looking at a direct call to our function.
586 // Now, check how our return value(s) is/are used in this caller. Don't
587 // bother checking return values if all of them are live already.
588 if (NumLiveRetVals != RetCount) {
590 // Check all uses of the return value.
591 for (Value::const_use_iterator I = TheCall->use_begin(),
592 E = TheCall->use_end(); I != E; ++I) {
593 const ExtractValueInst *Ext = dyn_cast<ExtractValueInst>(*I);
594 if (Ext && Ext->hasIndices()) {
595 // This use uses a part of our return value, survey the uses of
596 // that part and store the results for this index only.
597 unsigned Idx = *Ext->idx_begin();
598 if (RetValLiveness[Idx] != Live) {
599 RetValLiveness[Idx] = SurveyUses(Ext, MaybeLiveRetUses[Idx]);
600 if (RetValLiveness[Idx] == Live)
604 // Used by something else than extractvalue. Mark all return
606 for (unsigned i = 0; i != RetCount; ++i )
607 RetValLiveness[i] = Live;
608 NumLiveRetVals = RetCount;
613 // Single return value
614 RetValLiveness[0] = SurveyUses(TheCall, MaybeLiveRetUses[0]);
615 if (RetValLiveness[0] == Live)
616 NumLiveRetVals = RetCount;
621 // Now we've inspected all callers, record the liveness of our return values.
622 for (unsigned i = 0; i != RetCount; ++i)
623 MarkValue(CreateRet(&F, i), RetValLiveness[i], MaybeLiveRetUses[i]);
625 DEBUG(dbgs() << "DAE - Inspecting args for fn: " << F.getName() << "\n");
627 // Now, check all of our arguments.
629 UseVector MaybeLiveArgUses;
630 for (Function::const_arg_iterator AI = F.arg_begin(),
631 E = F.arg_end(); AI != E; ++AI, ++i) {
633 if (F.getFunctionType()->isVarArg()) {
634 // Variadic functions will already have a va_arg function expanded inside
635 // them, making them potentially very sensitive to ABI changes resulting
636 // from removing arguments entirely, so don't. For example AArch64 handles
637 // register and stack HFAs very differently, and this is reflected in the
638 // IR which has already been generated.
641 // See what the effect of this use is (recording any uses that cause
642 // MaybeLive in MaybeLiveArgUses).
643 Result = SurveyUses(AI, MaybeLiveArgUses);
647 MarkValue(CreateArg(&F, i), Result, MaybeLiveArgUses);
648 // Clear the vector again for the next iteration.
649 MaybeLiveArgUses.clear();
653 /// MarkValue - This function marks the liveness of RA depending on L. If L is
654 /// MaybeLive, it also takes all uses in MaybeLiveUses and records them in Uses,
655 /// such that RA will be marked live if any use in MaybeLiveUses gets marked
657 void DAE::MarkValue(const RetOrArg &RA, Liveness L,
658 const UseVector &MaybeLiveUses) {
660 case Live: MarkLive(RA); break;
663 // Note any uses of this value, so this return value can be
664 // marked live whenever one of the uses becomes live.
665 for (UseVector::const_iterator UI = MaybeLiveUses.begin(),
666 UE = MaybeLiveUses.end(); UI != UE; ++UI)
667 Uses.insert(std::make_pair(*UI, RA));
673 /// MarkLive - Mark the given Function as alive, meaning that it cannot be
674 /// changed in any way. Additionally,
675 /// mark any values that are used as this function's parameters or by its return
676 /// values (according to Uses) live as well.
677 void DAE::MarkLive(const Function &F) {
678 DEBUG(dbgs() << "DAE - Intrinsically live fn: " << F.getName() << "\n");
679 // Mark the function as live.
680 LiveFunctions.insert(&F);
681 // Mark all arguments as live.
682 for (unsigned i = 0, e = F.arg_size(); i != e; ++i)
683 PropagateLiveness(CreateArg(&F, i));
684 // Mark all return values as live.
685 for (unsigned i = 0, e = NumRetVals(&F); i != e; ++i)
686 PropagateLiveness(CreateRet(&F, i));
689 /// MarkLive - Mark the given return value or argument as live. Additionally,
690 /// mark any values that are used by this value (according to Uses) live as
692 void DAE::MarkLive(const RetOrArg &RA) {
693 if (LiveFunctions.count(RA.F))
694 return; // Function was already marked Live.
696 if (!LiveValues.insert(RA).second)
697 return; // We were already marked Live.
699 DEBUG(dbgs() << "DAE - Marking " << RA.getDescription() << " live\n");
700 PropagateLiveness(RA);
703 /// PropagateLiveness - Given that RA is a live value, propagate it's liveness
704 /// to any other values it uses (according to Uses).
705 void DAE::PropagateLiveness(const RetOrArg &RA) {
706 // We don't use upper_bound (or equal_range) here, because our recursive call
707 // to ourselves is likely to cause the upper_bound (which is the first value
708 // not belonging to RA) to become erased and the iterator invalidated.
709 UseMap::iterator Begin = Uses.lower_bound(RA);
710 UseMap::iterator E = Uses.end();
712 for (I = Begin; I != E && I->first == RA; ++I)
715 // Erase RA from the Uses map (from the lower bound to wherever we ended up
717 Uses.erase(Begin, I);
720 // RemoveDeadStuffFromFunction - Remove any arguments and return values from F
721 // that are not in LiveValues. Transform the function and all of the callees of
722 // the function to not have these arguments and return values.
724 bool DAE::RemoveDeadStuffFromFunction(Function *F) {
725 // Don't modify fully live functions
726 if (LiveFunctions.count(F))
729 // Start by computing a new prototype for the function, which is the same as
730 // the old function, but has fewer arguments and a different return type.
731 FunctionType *FTy = F->getFunctionType();
732 std::vector<Type*> Params;
734 // Keep track of if we have a live 'returned' argument
735 bool HasLiveReturnedArg = false;
737 // Set up to build a new list of parameter attributes.
738 SmallVector<AttributeSet, 8> AttributesVec;
739 const AttributeSet &PAL = F->getAttributes();
741 // Remember which arguments are still alive.
742 SmallVector<bool, 10> ArgAlive(FTy->getNumParams(), false);
743 // Construct the new parameter list from non-dead arguments. Also construct
744 // a new set of parameter attributes to correspond. Skip the first parameter
745 // attribute, since that belongs to the return value.
747 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
749 RetOrArg Arg = CreateArg(F, i);
750 if (LiveValues.erase(Arg)) {
751 Params.push_back(I->getType());
754 // Get the original parameter attributes (skipping the first one, that is
755 // for the return value.
756 if (PAL.hasAttributes(i + 1)) {
757 AttrBuilder B(PAL, i + 1);
758 if (B.contains(Attribute::Returned))
759 HasLiveReturnedArg = true;
761 push_back(AttributeSet::get(F->getContext(), Params.size(), B));
764 ++NumArgumentsEliminated;
765 DEBUG(dbgs() << "DAE - Removing argument " << i << " (" << I->getName()
766 << ") from " << F->getName() << "\n");
770 // Find out the new return value.
771 Type *RetTy = FTy->getReturnType();
773 unsigned RetCount = NumRetVals(F);
775 // -1 means unused, other numbers are the new index
776 SmallVector<int, 5> NewRetIdxs(RetCount, -1);
777 std::vector<Type*> RetTypes;
779 // If there is a function with a live 'returned' argument but a dead return
780 // value, then there are two possible actions:
781 // 1) Eliminate the return value and take off the 'returned' attribute on the
783 // 2) Retain the 'returned' attribute and treat the return value (but not the
784 // entire function) as live so that it is not eliminated.
786 // It's not clear in the general case which option is more profitable because,
787 // even in the absence of explicit uses of the return value, code generation
788 // is free to use the 'returned' attribute to do things like eliding
789 // save/restores of registers across calls. Whether or not this happens is
790 // target and ABI-specific as well as depending on the amount of register
791 // pressure, so there's no good way for an IR-level pass to figure this out.
793 // Fortunately, the only places where 'returned' is currently generated by
794 // the FE are places where 'returned' is basically free and almost always a
795 // performance win, so the second option can just be used always for now.
797 // This should be revisited if 'returned' is ever applied more liberally.
798 if (RetTy->isVoidTy() || HasLiveReturnedArg) {
801 StructType *STy = dyn_cast<StructType>(RetTy);
803 // Look at each of the original return values individually.
804 for (unsigned i = 0; i != RetCount; ++i) {
805 RetOrArg Ret = CreateRet(F, i);
806 if (LiveValues.erase(Ret)) {
807 RetTypes.push_back(STy->getElementType(i));
808 NewRetIdxs[i] = RetTypes.size() - 1;
810 ++NumRetValsEliminated;
811 DEBUG(dbgs() << "DAE - Removing return value " << i << " from "
812 << F->getName() << "\n");
816 // We used to return a single value.
817 if (LiveValues.erase(CreateRet(F, 0))) {
818 RetTypes.push_back(RetTy);
821 DEBUG(dbgs() << "DAE - Removing return value from " << F->getName()
823 ++NumRetValsEliminated;
825 if (RetTypes.size() > 1)
826 // More than one return type? Return a struct with them. Also, if we used
827 // to return a struct and didn't change the number of return values,
828 // return a struct again. This prevents changing {something} into
829 // something and {} into void.
830 // Make the new struct packed if we used to return a packed struct
832 NRetTy = StructType::get(STy->getContext(), RetTypes, STy->isPacked());
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())
853 AttributeSet::get(NRetTy->getContext(), AttributeSet::ReturnIndex,
854 AttrBuilder(RAttrs, AttributeSet::ReturnIndex).
855 removeAttributes(AttributeFuncs::
856 typeIncompatible(NRetTy, AttributeSet::ReturnIndex),
857 AttributeSet::ReturnIndex));
859 assert(!AttrBuilder(RAttrs, AttributeSet::ReturnIndex).
860 hasAttributes(AttributeFuncs::
861 typeIncompatible(NRetTy, AttributeSet::ReturnIndex),
862 AttributeSet::ReturnIndex) &&
863 "Return attributes no longer compatible?");
865 if (RAttrs.hasAttributes(AttributeSet::ReturnIndex))
866 AttributesVec.push_back(AttributeSet::get(NRetTy->getContext(), RAttrs));
868 if (PAL.hasAttributes(AttributeSet::FunctionIndex))
869 AttributesVec.push_back(AttributeSet::get(F->getContext(),
870 PAL.getFnAttributes()));
872 // Reconstruct the AttributesList based on the vector we constructed.
873 AttributeSet NewPAL = AttributeSet::get(F->getContext(), AttributesVec);
875 // Create the new function type based on the recomputed parameters.
876 FunctionType *NFTy = FunctionType::get(NRetTy, Params, FTy->isVarArg());
882 // Create the new function body and insert it into the module...
883 Function *NF = Function::Create(NFTy, F->getLinkage());
884 NF->copyAttributesFrom(F);
885 NF->setAttributes(NewPAL);
886 // Insert the new function before the old function, so we won't be processing
888 F->getParent()->getFunctionList().insert(F, NF);
891 // Loop over all of the callers of the function, transforming the call sites
892 // to pass in a smaller number of arguments into the new function.
894 std::vector<Value*> Args;
895 while (!F->use_empty()) {
896 CallSite CS(F->use_back());
897 Instruction *Call = CS.getInstruction();
899 AttributesVec.clear();
900 const AttributeSet &CallPAL = CS.getAttributes();
902 // The call return attributes.
903 AttributeSet RAttrs = CallPAL.getRetAttributes();
905 // Adjust in case the function was changed to return void.
907 AttributeSet::get(NF->getContext(), AttributeSet::ReturnIndex,
908 AttrBuilder(RAttrs, AttributeSet::ReturnIndex).
909 removeAttributes(AttributeFuncs::
910 typeIncompatible(NF->getReturnType(),
911 AttributeSet::ReturnIndex),
912 AttributeSet::ReturnIndex));
913 if (RAttrs.hasAttributes(AttributeSet::ReturnIndex))
914 AttributesVec.push_back(AttributeSet::get(NF->getContext(), RAttrs));
916 // Declare these outside of the loops, so we can reuse them for the second
917 // loop, which loops the varargs.
918 CallSite::arg_iterator I = CS.arg_begin();
920 // Loop over those operands, corresponding to the normal arguments to the
921 // original function, and add those that are still alive.
922 for (unsigned e = FTy->getNumParams(); i != e; ++I, ++i)
925 // Get original parameter attributes, but skip return attributes.
926 if (CallPAL.hasAttributes(i + 1)) {
927 AttrBuilder B(CallPAL, i + 1);
928 // If the return type has changed, then get rid of 'returned' on the
929 // call site. The alternative is to make all 'returned' attributes on
930 // call sites keep the return value alive just like 'returned'
931 // attributes on function declaration but it's less clearly a win
932 // and this is not an expected case anyway
933 if (NRetTy != RetTy && B.contains(Attribute::Returned))
934 B.removeAttribute(Attribute::Returned);
936 push_back(AttributeSet::get(F->getContext(), Args.size(), B));
940 // Push any varargs arguments on the list. Don't forget their attributes.
941 for (CallSite::arg_iterator E = CS.arg_end(); I != E; ++I, ++i) {
943 if (CallPAL.hasAttributes(i + 1)) {
944 AttrBuilder B(CallPAL, i + 1);
946 push_back(AttributeSet::get(F->getContext(), Args.size(), B));
950 if (CallPAL.hasAttributes(AttributeSet::FunctionIndex))
951 AttributesVec.push_back(AttributeSet::get(Call->getContext(),
952 CallPAL.getFnAttributes()));
954 // Reconstruct the AttributesList based on the vector we constructed.
955 AttributeSet NewCallPAL = AttributeSet::get(F->getContext(), AttributesVec);
958 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
959 New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
961 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
962 cast<InvokeInst>(New)->setAttributes(NewCallPAL);
964 New = CallInst::Create(NF, Args, "", Call);
965 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
966 cast<CallInst>(New)->setAttributes(NewCallPAL);
967 if (cast<CallInst>(Call)->isTailCall())
968 cast<CallInst>(New)->setTailCall();
970 New->setDebugLoc(Call->getDebugLoc());
974 if (!Call->use_empty()) {
975 if (New->getType() == Call->getType()) {
976 // Return type not changed? Just replace users then.
977 Call->replaceAllUsesWith(New);
979 } else if (New->getType()->isVoidTy()) {
980 // Our return value has uses, but they will get removed later on.
981 // Replace by null for now.
982 if (!Call->getType()->isX86_MMXTy())
983 Call->replaceAllUsesWith(Constant::getNullValue(Call->getType()));
985 assert(RetTy->isStructTy() &&
986 "Return type changed, but not into a void. The old return type"
987 " must have been a struct!");
988 Instruction *InsertPt = Call;
989 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
990 BasicBlock::iterator IP = II->getNormalDest()->begin();
991 while (isa<PHINode>(IP)) ++IP;
995 // We used to return a struct. Instead of doing smart stuff with all the
996 // uses of this struct, we will just rebuild it using
997 // extract/insertvalue chaining and let instcombine clean that up.
999 // Start out building up our return value from undef
1000 Value *RetVal = UndefValue::get(RetTy);
1001 for (unsigned i = 0; i != RetCount; ++i)
1002 if (NewRetIdxs[i] != -1) {
1004 if (RetTypes.size() > 1)
1005 // We are still returning a struct, so extract the value from our
1007 V = ExtractValueInst::Create(New, NewRetIdxs[i], "newret",
1010 // We are now returning a single element, so just insert that
1012 // Insert the value at the old position
1013 RetVal = InsertValueInst::Create(RetVal, V, i, "oldret", InsertPt);
1015 // Now, replace all uses of the old call instruction with the return
1017 Call->replaceAllUsesWith(RetVal);
1018 New->takeName(Call);
1022 // Finally, remove the old call from the program, reducing the use-count of
1024 Call->eraseFromParent();
1027 // Since we have now created the new function, splice the body of the old
1028 // function right into the new function, leaving the old rotting hulk of the
1030 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
1032 // Loop over the argument list, transferring uses of the old arguments over to
1033 // the new arguments, also transferring over the names as well.
1035 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(),
1036 I2 = NF->arg_begin(); I != E; ++I, ++i)
1038 // If this is a live argument, move the name and users over to the new
1040 I->replaceAllUsesWith(I2);
1044 // If this argument is dead, replace any uses of it with null constants
1045 // (these are guaranteed to become unused later on).
1046 if (!I->getType()->isX86_MMXTy())
1047 I->replaceAllUsesWith(Constant::getNullValue(I->getType()));
1050 // If we change the return value of the function we must rewrite any return
1051 // instructions. Check this now.
1052 if (F->getReturnType() != NF->getReturnType())
1053 for (Function::iterator BB = NF->begin(), E = NF->end(); BB != E; ++BB)
1054 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
1057 if (NFTy->getReturnType()->isVoidTy()) {
1060 assert (RetTy->isStructTy());
1061 // The original return value was a struct, insert
1062 // extractvalue/insertvalue chains to extract only the values we need
1063 // to return and insert them into our new result.
1064 // This does generate messy code, but we'll let it to instcombine to
1066 Value *OldRet = RI->getOperand(0);
1067 // Start out building up our return value from undef
1068 RetVal = UndefValue::get(NRetTy);
1069 for (unsigned i = 0; i != RetCount; ++i)
1070 if (NewRetIdxs[i] != -1) {
1071 ExtractValueInst *EV = ExtractValueInst::Create(OldRet, i,
1073 if (RetTypes.size() > 1) {
1074 // We're still returning a struct, so reinsert the value into
1075 // our new return value at the new index
1077 RetVal = InsertValueInst::Create(RetVal, EV, NewRetIdxs[i],
1080 // We are now only returning a simple value, so just return the
1086 // Replace the return instruction with one returning the new return
1087 // value (possibly 0 if we became void).
1088 ReturnInst::Create(F->getContext(), RetVal, RI);
1089 BB->getInstList().erase(RI);
1092 // Patch the pointer to LLVM function in debug info descriptor.
1093 FunctionDIMap::iterator DI = FunctionDIs.find(F);
1094 if (DI != FunctionDIs.end())
1095 DI->second.replaceFunction(NF);
1097 // Now that the old function is dead, delete it.
1098 F->eraseFromParent();
1103 bool DAE::runOnModule(Module &M) {
1104 bool Changed = false;
1106 // Collect debug info descriptors for functions.
1107 CollectFunctionDIs(M);
1109 // First pass: Do a simple check to see if any functions can have their "..."
1110 // removed. We can do this if they never call va_start. This loop cannot be
1111 // fused with the next loop, because deleting a function invalidates
1112 // information computed while surveying other functions.
1113 DEBUG(dbgs() << "DAE - Deleting dead varargs\n");
1114 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1116 if (F.getFunctionType()->isVarArg())
1117 Changed |= DeleteDeadVarargs(F);
1120 // Second phase:loop through the module, determining which arguments are live.
1121 // We assume all arguments are dead unless proven otherwise (allowing us to
1122 // determine that dead arguments passed into recursive functions are dead).
1124 DEBUG(dbgs() << "DAE - Determining liveness\n");
1125 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
1128 // Now, remove all dead arguments and return values from each function in
1130 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1131 // Increment now, because the function will probably get removed (ie.
1132 // replaced by a new one).
1134 Changed |= RemoveDeadStuffFromFunction(F);
1137 // Finally, look for any unused parameters in functions with non-local
1138 // linkage and replace the passed in parameters with undef.
1139 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
1142 Changed |= RemoveDeadArgumentsFromCallers(F);