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/DIBuilder.h"
27 #include "llvm/DebugInfo.h"
28 #include "llvm/IR/CallingConv.h"
29 #include "llvm/IR/Constant.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/CallSite.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 {
67 else if (Idx != O.Idx)
70 return IsArg < O.IsArg;
73 /// Make RetOrArg comparable, so we can easily iterate the multimap.
74 bool operator==(const RetOrArg &O) const {
75 return F == O.F && Idx == O.Idx && IsArg == O.IsArg;
78 std::string getDescription() const {
79 return std::string((IsArg ? "Argument #" : "Return value #"))
80 + utostr(Idx) + " of function " + F->getName().str();
84 /// Liveness enum - During our initial pass over the program, we determine
85 /// that things are either alive or maybe alive. We don't mark anything
86 /// explicitly dead (even if we know they are), since anything not alive
87 /// with no registered uses (in Uses) will never be marked alive and will
88 /// thus become dead in the end.
89 enum Liveness { Live, MaybeLive };
91 /// Convenience wrapper
92 RetOrArg CreateRet(const Function *F, unsigned Idx) {
93 return RetOrArg(F, Idx, false);
95 /// Convenience wrapper
96 RetOrArg CreateArg(const Function *F, unsigned Idx) {
97 return RetOrArg(F, Idx, true);
100 typedef std::multimap<RetOrArg, RetOrArg> UseMap;
101 /// This maps a return value or argument to any MaybeLive return values or
102 /// arguments it uses. This allows the MaybeLive values to be marked live
103 /// when any of its users is marked live.
104 /// For example (indices are left out for clarity):
105 /// - Uses[ret F] = ret G
106 /// This means that F calls G, and F returns the value returned by G.
107 /// - Uses[arg F] = ret G
108 /// This means that some function calls G and passes its result as an
110 /// - Uses[ret F] = arg F
111 /// This means that F returns one of its own arguments.
112 /// - Uses[arg F] = arg G
113 /// This means that G calls F and passes one of its own (G's) arguments
117 typedef std::set<RetOrArg> LiveSet;
118 typedef std::set<const Function*> LiveFuncSet;
120 /// This set contains all values that have been determined to be live.
122 /// This set contains all values that are cannot be changed in any way.
123 LiveFuncSet LiveFunctions;
125 typedef SmallVector<RetOrArg, 5> UseVector;
127 // Map each LLVM function to corresponding metadata with debug info. If
128 // the function is replaced with another one, we should patch the pointer
129 // to LLVM function in metadata.
130 // As the code generation for module is finished (and DIBuilder is
131 // finalized) we assume that subprogram descriptors won't be changed, and
132 // they are stored in map for short duration anyway.
133 typedef DenseMap<Function*, DISubprogram> FunctionDIMap;
134 FunctionDIMap FunctionDIs;
137 // DAH uses this to specify a different ID.
138 explicit DAE(char &ID) : ModulePass(ID) {}
141 static char ID; // Pass identification, replacement for typeid
142 DAE() : ModulePass(ID) {
143 initializeDAEPass(*PassRegistry::getPassRegistry());
146 bool runOnModule(Module &M);
148 virtual bool ShouldHackArguments() const { return false; }
151 Liveness MarkIfNotLive(RetOrArg Use, UseVector &MaybeLiveUses);
152 Liveness SurveyUse(Value::const_use_iterator U, UseVector &MaybeLiveUses,
153 unsigned RetValNum = 0);
154 Liveness SurveyUses(const Value *V, UseVector &MaybeLiveUses);
156 void CollectFunctionDIs(Module &M);
157 void SurveyFunction(const Function &F);
158 void MarkValue(const RetOrArg &RA, Liveness L,
159 const UseVector &MaybeLiveUses);
160 void MarkLive(const RetOrArg &RA);
161 void MarkLive(const Function &F);
162 void PropagateLiveness(const RetOrArg &RA);
163 bool RemoveDeadStuffFromFunction(Function *F);
164 bool DeleteDeadVarargs(Function &Fn);
165 bool RemoveDeadArgumentsFromCallers(Function &Fn);
171 INITIALIZE_PASS(DAE, "deadargelim", "Dead Argument Elimination", false, false)
174 /// DAH - DeadArgumentHacking pass - Same as dead argument elimination, but
175 /// deletes arguments to functions which are external. This is only for use
177 struct DAH : public DAE {
181 virtual bool ShouldHackArguments() const { return true; }
186 INITIALIZE_PASS(DAH, "deadarghaX0r",
187 "Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)",
190 /// createDeadArgEliminationPass - This pass removes arguments from functions
191 /// which are not used by the body of the function.
193 ModulePass *llvm::createDeadArgEliminationPass() { return new DAE(); }
194 ModulePass *llvm::createDeadArgHackingPass() { return new DAH(); }
196 /// CollectFunctionDIs - Map each function in the module to its debug info
198 void DAE::CollectFunctionDIs(Module &M) {
201 for (Module::named_metadata_iterator I = M.named_metadata_begin(),
202 E = M.named_metadata_end(); I != E; ++I) {
203 NamedMDNode &NMD = *I;
204 for (unsigned MDIndex = 0, MDNum = NMD.getNumOperands();
205 MDIndex < MDNum; ++MDIndex) {
206 MDNode *Node = NMD.getOperand(MDIndex);
207 if (!DIDescriptor(Node).isCompileUnit())
209 DICompileUnit CU(Node);
210 const DIArray &SPs = CU.getSubprograms();
211 for (unsigned SPIndex = 0, SPNum = SPs.getNumElements();
212 SPIndex < SPNum; ++SPIndex) {
213 DISubprogram SP(SPs.getElement(SPIndex));
214 assert((!SP || SP.isSubprogram()) &&
215 "A MDNode in subprograms of a CU should be null or a DISubprogram.");
218 if (Function *F = SP.getFunction())
225 /// DeleteDeadVarargs - If this is an function that takes a ... list, and if
226 /// llvm.vastart is never called, the varargs list is dead for the function.
227 bool DAE::DeleteDeadVarargs(Function &Fn) {
228 assert(Fn.getFunctionType()->isVarArg() && "Function isn't varargs!");
229 if (Fn.isDeclaration() || !Fn.hasLocalLinkage()) return false;
231 // Ensure that the function is only directly called.
232 if (Fn.hasAddressTaken())
235 // Okay, we know we can transform this function if safe. Scan its body
236 // looking for calls to llvm.vastart.
237 for (Function::iterator BB = Fn.begin(), E = Fn.end(); BB != E; ++BB) {
238 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
239 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
240 if (II->getIntrinsicID() == Intrinsic::vastart)
246 // If we get here, there are no calls to llvm.vastart in the function body,
247 // remove the "..." and adjust all the calls.
249 // Start by computing a new prototype for the function, which is the same as
250 // the old function, but doesn't have isVarArg set.
251 FunctionType *FTy = Fn.getFunctionType();
253 std::vector<Type*> Params(FTy->param_begin(), FTy->param_end());
254 FunctionType *NFTy = FunctionType::get(FTy->getReturnType(),
256 unsigned NumArgs = Params.size();
258 // Create the new function body and insert it into the module...
259 Function *NF = Function::Create(NFTy, Fn.getLinkage());
260 NF->copyAttributesFrom(&Fn);
261 Fn.getParent()->getFunctionList().insert(&Fn, NF);
264 // Loop over all of the callers of the function, transforming the call sites
265 // to pass in a smaller number of arguments into the new function.
267 std::vector<Value*> Args;
268 for (Value::use_iterator I = Fn.use_begin(), E = Fn.use_end(); I != E; ) {
272 Instruction *Call = CS.getInstruction();
274 // Pass all the same arguments.
275 Args.assign(CS.arg_begin(), CS.arg_begin() + NumArgs);
277 // Drop any attributes that were on the vararg arguments.
278 AttributeSet PAL = CS.getAttributes();
279 if (!PAL.isEmpty() && PAL.getSlotIndex(PAL.getNumSlots() - 1) > NumArgs) {
280 SmallVector<AttributeSet, 8> AttributesVec;
281 for (unsigned i = 0; PAL.getSlotIndex(i) <= NumArgs; ++i)
282 AttributesVec.push_back(PAL.getSlotAttributes(i));
283 if (PAL.hasAttributes(AttributeSet::FunctionIndex))
284 AttributesVec.push_back(AttributeSet::get(Fn.getContext(),
285 PAL.getFnAttributes()));
286 PAL = AttributeSet::get(Fn.getContext(), AttributesVec);
290 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
291 New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
293 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
294 cast<InvokeInst>(New)->setAttributes(PAL);
296 New = CallInst::Create(NF, Args, "", Call);
297 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
298 cast<CallInst>(New)->setAttributes(PAL);
299 if (cast<CallInst>(Call)->isTailCall())
300 cast<CallInst>(New)->setTailCall();
302 New->setDebugLoc(Call->getDebugLoc());
306 if (!Call->use_empty())
307 Call->replaceAllUsesWith(New);
311 // Finally, remove the old call from the program, reducing the use-count of
313 Call->eraseFromParent();
316 // Since we have now created the new function, splice the body of the old
317 // function right into the new function, leaving the old rotting hulk of the
319 NF->getBasicBlockList().splice(NF->begin(), Fn.getBasicBlockList());
321 // Loop over the argument list, transferring uses of the old arguments over to
322 // the new arguments, also transferring over the names as well. While we're at
323 // it, remove the dead arguments from the DeadArguments list.
325 for (Function::arg_iterator I = Fn.arg_begin(), E = Fn.arg_end(),
326 I2 = NF->arg_begin(); I != E; ++I, ++I2) {
327 // Move the name and users over to the new version.
328 I->replaceAllUsesWith(I2);
332 // Patch the pointer to LLVM function in debug info descriptor.
333 FunctionDIMap::iterator DI = FunctionDIs.find(&Fn);
334 if (DI != FunctionDIs.end())
335 DI->second.replaceFunction(NF);
337 // Fix up any BlockAddresses that refer to the function.
338 Fn.replaceAllUsesWith(ConstantExpr::getBitCast(NF, Fn.getType()));
339 // Delete the bitcast that we just created, so that NF does not
340 // appear to be address-taken.
341 NF->removeDeadConstantUsers();
342 // Finally, nuke the old function.
343 Fn.eraseFromParent();
347 /// RemoveDeadArgumentsFromCallers - Checks if the given function has any
348 /// arguments that are unused, and changes the caller parameters to be undefined
350 bool DAE::RemoveDeadArgumentsFromCallers(Function &Fn)
352 if (Fn.isDeclaration() || Fn.mayBeOverridden())
355 // Functions with local linkage should already have been handled, except the
356 // fragile (variadic) ones which we can improve here.
357 if (Fn.hasLocalLinkage() && !Fn.getFunctionType()->isVarArg())
360 // If a function seen at compile time is not necessarily the one linked to
361 // the binary being built, it is illegal to change the actual arguments
362 // passed to it. These functions can be captured by isWeakForLinker().
363 // *NOTE* that mayBeOverridden() is insufficient for this purpose as it
364 // doesn't include linkage types like AvailableExternallyLinkage and
365 // LinkOnceODRLinkage. Take link_odr* as an example, it indicates a set of
366 // *EQUIVALENT* globals that can be merged at link-time. However, the
367 // semantic of *EQUIVALENT*-functions includes parameters. Changing
368 // parameters breaks this assumption.
370 if (Fn.isWeakForLinker())
376 SmallVector<unsigned, 8> UnusedArgs;
377 for (Function::arg_iterator I = Fn.arg_begin(), E = Fn.arg_end();
381 if (Arg->use_empty() && !Arg->hasByValAttr())
382 UnusedArgs.push_back(Arg->getArgNo());
385 if (UnusedArgs.empty())
388 bool Changed = false;
390 for (Function::use_iterator I = Fn.use_begin(), E = Fn.use_end();
393 if (!CS || !CS.isCallee(I))
396 // Now go through all unused args and replace them with "undef".
397 for (unsigned I = 0, E = UnusedArgs.size(); I != E; ++I) {
398 unsigned ArgNo = UnusedArgs[I];
400 Value *Arg = CS.getArgument(ArgNo);
401 CS.setArgument(ArgNo, UndefValue::get(Arg->getType()));
402 ++NumArgumentsReplacedWithUndef;
410 /// Convenience function that returns the number of return values. It returns 0
411 /// for void functions and 1 for functions not returning a struct. It returns
412 /// the number of struct elements for functions returning a struct.
413 static unsigned NumRetVals(const Function *F) {
414 if (F->getReturnType()->isVoidTy())
416 else if (StructType *STy = dyn_cast<StructType>(F->getReturnType()))
417 return STy->getNumElements();
422 /// MarkIfNotLive - This checks Use for liveness in LiveValues. If Use is not
423 /// live, it adds Use to the MaybeLiveUses argument. Returns the determined
425 DAE::Liveness DAE::MarkIfNotLive(RetOrArg Use, UseVector &MaybeLiveUses) {
426 // We're live if our use or its Function is already marked as live.
427 if (LiveFunctions.count(Use.F) || LiveValues.count(Use))
430 // We're maybe live otherwise, but remember that we must become live if
432 MaybeLiveUses.push_back(Use);
437 /// SurveyUse - This looks at a single use of an argument or return value
438 /// and determines if it should be alive or not. Adds this use to MaybeLiveUses
439 /// if it causes the used value to become MaybeLive.
441 /// RetValNum is the return value number to use when this use is used in a
442 /// return instruction. This is used in the recursion, you should always leave
444 DAE::Liveness DAE::SurveyUse(Value::const_use_iterator U,
445 UseVector &MaybeLiveUses, unsigned RetValNum) {
447 if (const ReturnInst *RI = dyn_cast<ReturnInst>(V)) {
448 // The value is returned from a function. It's only live when the
449 // function's return value is live. We use RetValNum here, for the case
450 // that U is really a use of an insertvalue instruction that uses the
452 RetOrArg Use = CreateRet(RI->getParent()->getParent(), RetValNum);
453 // We might be live, depending on the liveness of Use.
454 return MarkIfNotLive(Use, MaybeLiveUses);
456 if (const InsertValueInst *IV = dyn_cast<InsertValueInst>(V)) {
457 if (U.getOperandNo() != InsertValueInst::getAggregateOperandIndex()
459 // The use we are examining is inserted into an aggregate. Our liveness
460 // depends on all uses of that aggregate, but if it is used as a return
461 // value, only index at which we were inserted counts.
462 RetValNum = *IV->idx_begin();
464 // Note that if we are used as the aggregate operand to the insertvalue,
465 // we don't change RetValNum, but do survey all our uses.
467 Liveness Result = MaybeLive;
468 for (Value::const_use_iterator I = IV->use_begin(),
469 E = V->use_end(); I != E; ++I) {
470 Result = SurveyUse(I, MaybeLiveUses, RetValNum);
477 if (ImmutableCallSite CS = V) {
478 const Function *F = CS.getCalledFunction();
480 // Used in a direct call.
482 // Find the argument number. We know for sure that this use is an
483 // argument, since if it was the function argument this would be an
484 // indirect call and the we know can't be looking at a value of the
485 // label type (for the invoke instruction).
486 unsigned ArgNo = CS.getArgumentNo(U);
488 if (ArgNo >= F->getFunctionType()->getNumParams())
489 // The value is passed in through a vararg! Must be live.
492 assert(CS.getArgument(ArgNo)
493 == CS->getOperand(U.getOperandNo())
494 && "Argument is not where we expected it");
496 // Value passed to a normal call. It's only live when the corresponding
497 // argument to the called function turns out live.
498 RetOrArg Use = CreateArg(F, ArgNo);
499 return MarkIfNotLive(Use, MaybeLiveUses);
502 // Used in any other way? Value must be live.
506 /// SurveyUses - This looks at all the uses of the given value
507 /// Returns the Liveness deduced from the uses of this value.
509 /// Adds all uses that cause the result to be MaybeLive to MaybeLiveRetUses. If
510 /// the result is Live, MaybeLiveUses might be modified but its content should
511 /// be ignored (since it might not be complete).
512 DAE::Liveness DAE::SurveyUses(const Value *V, UseVector &MaybeLiveUses) {
513 // Assume it's dead (which will only hold if there are no uses at all..).
514 Liveness Result = MaybeLive;
516 for (Value::const_use_iterator I = V->use_begin(),
517 E = V->use_end(); I != E; ++I) {
518 Result = SurveyUse(I, MaybeLiveUses);
525 // SurveyFunction - This performs the initial survey of the specified function,
526 // checking out whether or not it uses any of its incoming arguments or whether
527 // any callers use the return value. This fills in the LiveValues set and Uses
530 // We consider arguments of non-internal functions to be intrinsically alive as
531 // well as arguments to functions which have their "address taken".
533 void DAE::SurveyFunction(const Function &F) {
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 (Value::const_use_iterator I = F.use_begin(), E = F.use_end();
567 // If the function is PASSED IN as an argument, its address has been
569 ImmutableCallSite CS(*I);
570 if (!CS || !CS.isCallee(I)) {
575 // If this use is anything other than a call site, the function is alive.
576 const Instruction *TheCall = CS.getInstruction();
577 if (!TheCall) { // Not a direct call site?
582 // If we end up here, we are looking at a direct call to our function.
584 // Now, check how our return value(s) is/are used in this caller. Don't
585 // bother checking return values if all of them are live already.
586 if (NumLiveRetVals != RetCount) {
588 // Check all uses of the return value.
589 for (Value::const_use_iterator I = TheCall->use_begin(),
590 E = TheCall->use_end(); I != E; ++I) {
591 const ExtractValueInst *Ext = dyn_cast<ExtractValueInst>(*I);
592 if (Ext && Ext->hasIndices()) {
593 // This use uses a part of our return value, survey the uses of
594 // that part and store the results for this index only.
595 unsigned Idx = *Ext->idx_begin();
596 if (RetValLiveness[Idx] != Live) {
597 RetValLiveness[Idx] = SurveyUses(Ext, MaybeLiveRetUses[Idx]);
598 if (RetValLiveness[Idx] == Live)
602 // Used by something else than extractvalue. Mark all return
604 for (unsigned i = 0; i != RetCount; ++i )
605 RetValLiveness[i] = Live;
606 NumLiveRetVals = RetCount;
611 // Single return value
612 RetValLiveness[0] = SurveyUses(TheCall, MaybeLiveRetUses[0]);
613 if (RetValLiveness[0] == Live)
614 NumLiveRetVals = RetCount;
619 // Now we've inspected all callers, record the liveness of our return values.
620 for (unsigned i = 0; i != RetCount; ++i)
621 MarkValue(CreateRet(&F, i), RetValLiveness[i], MaybeLiveRetUses[i]);
623 DEBUG(dbgs() << "DAE - Inspecting args for fn: " << F.getName() << "\n");
625 // Now, check all of our arguments.
627 UseVector MaybeLiveArgUses;
628 for (Function::const_arg_iterator AI = F.arg_begin(),
629 E = F.arg_end(); AI != E; ++AI, ++i) {
631 if (F.getFunctionType()->isVarArg()) {
632 // Variadic functions will already have a va_arg function expanded inside
633 // them, making them potentially very sensitive to ABI changes resulting
634 // from removing arguments entirely, so don't. For example AArch64 handles
635 // register and stack HFAs very differently, and this is reflected in the
636 // IR which has already been generated.
639 // See what the effect of this use is (recording any uses that cause
640 // MaybeLive in MaybeLiveArgUses).
641 Result = SurveyUses(AI, MaybeLiveArgUses);
645 MarkValue(CreateArg(&F, i), Result, MaybeLiveArgUses);
646 // Clear the vector again for the next iteration.
647 MaybeLiveArgUses.clear();
651 /// MarkValue - This function marks the liveness of RA depending on L. If L is
652 /// MaybeLive, it also takes all uses in MaybeLiveUses and records them in Uses,
653 /// such that RA will be marked live if any use in MaybeLiveUses gets marked
655 void DAE::MarkValue(const RetOrArg &RA, Liveness L,
656 const UseVector &MaybeLiveUses) {
658 case Live: MarkLive(RA); break;
661 // Note any uses of this value, so this return value can be
662 // marked live whenever one of the uses becomes live.
663 for (UseVector::const_iterator UI = MaybeLiveUses.begin(),
664 UE = MaybeLiveUses.end(); UI != UE; ++UI)
665 Uses.insert(std::make_pair(*UI, RA));
671 /// MarkLive - Mark the given Function as alive, meaning that it cannot be
672 /// changed in any way. Additionally,
673 /// mark any values that are used as this function's parameters or by its return
674 /// values (according to Uses) live as well.
675 void DAE::MarkLive(const Function &F) {
676 DEBUG(dbgs() << "DAE - Intrinsically live fn: " << F.getName() << "\n");
677 // Mark the function as live.
678 LiveFunctions.insert(&F);
679 // Mark all arguments as live.
680 for (unsigned i = 0, e = F.arg_size(); i != e; ++i)
681 PropagateLiveness(CreateArg(&F, i));
682 // Mark all return values as live.
683 for (unsigned i = 0, e = NumRetVals(&F); i != e; ++i)
684 PropagateLiveness(CreateRet(&F, i));
687 /// MarkLive - Mark the given return value or argument as live. Additionally,
688 /// mark any values that are used by this value (according to Uses) live as
690 void DAE::MarkLive(const RetOrArg &RA) {
691 if (LiveFunctions.count(RA.F))
692 return; // Function was already marked Live.
694 if (!LiveValues.insert(RA).second)
695 return; // We were already marked Live.
697 DEBUG(dbgs() << "DAE - Marking " << RA.getDescription() << " live\n");
698 PropagateLiveness(RA);
701 /// PropagateLiveness - Given that RA is a live value, propagate it's liveness
702 /// to any other values it uses (according to Uses).
703 void DAE::PropagateLiveness(const RetOrArg &RA) {
704 // We don't use upper_bound (or equal_range) here, because our recursive call
705 // to ourselves is likely to cause the upper_bound (which is the first value
706 // not belonging to RA) to become erased and the iterator invalidated.
707 UseMap::iterator Begin = Uses.lower_bound(RA);
708 UseMap::iterator E = Uses.end();
710 for (I = Begin; I != E && I->first == RA; ++I)
713 // Erase RA from the Uses map (from the lower bound to wherever we ended up
715 Uses.erase(Begin, I);
718 // RemoveDeadStuffFromFunction - Remove any arguments and return values from F
719 // that are not in LiveValues. Transform the function and all of the callees of
720 // the function to not have these arguments and return values.
722 bool DAE::RemoveDeadStuffFromFunction(Function *F) {
723 // Don't modify fully live functions
724 if (LiveFunctions.count(F))
727 // Start by computing a new prototype for the function, which is the same as
728 // the old function, but has fewer arguments and a different return type.
729 FunctionType *FTy = F->getFunctionType();
730 std::vector<Type*> Params;
732 // Keep track of if we have a live 'returned' argument
733 bool HasLiveReturnedArg = false;
735 // Set up to build a new list of parameter attributes.
736 SmallVector<AttributeSet, 8> AttributesVec;
737 const AttributeSet &PAL = F->getAttributes();
739 // Remember which arguments are still alive.
740 SmallVector<bool, 10> ArgAlive(FTy->getNumParams(), false);
741 // Construct the new parameter list from non-dead arguments. Also construct
742 // a new set of parameter attributes to correspond. Skip the first parameter
743 // attribute, since that belongs to the return value.
745 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
747 RetOrArg Arg = CreateArg(F, i);
748 if (LiveValues.erase(Arg)) {
749 Params.push_back(I->getType());
752 // Get the original parameter attributes (skipping the first one, that is
753 // for the return value.
754 if (PAL.hasAttributes(i + 1)) {
755 AttrBuilder B(PAL, i + 1);
756 if (B.contains(Attribute::Returned))
757 HasLiveReturnedArg = true;
759 push_back(AttributeSet::get(F->getContext(), Params.size(), B));
762 ++NumArgumentsEliminated;
763 DEBUG(dbgs() << "DAE - Removing argument " << i << " (" << I->getName()
764 << ") from " << F->getName() << "\n");
768 // Find out the new return value.
769 Type *RetTy = FTy->getReturnType();
771 unsigned RetCount = NumRetVals(F);
773 // -1 means unused, other numbers are the new index
774 SmallVector<int, 5> NewRetIdxs(RetCount, -1);
775 std::vector<Type*> RetTypes;
777 // If there is a function with a live 'returned' argument but a dead return
778 // value, then there are two possible actions:
779 // 1) Eliminate the return value and take off the 'returned' attribute on the
781 // 2) Retain the 'returned' attribute and treat the return value (but not the
782 // entire function) as live so that it is not eliminated.
784 // It's not clear in the general case which option is more profitable because,
785 // even in the absence of explicit uses of the return value, code generation
786 // is free to use the 'returned' attribute to do things like eliding
787 // save/restores of registers across calls. Whether or not this happens is
788 // target and ABI-specific as well as depending on the amount of register
789 // pressure, so there's no good way for an IR-level pass to figure this out.
791 // Fortunately, the only places where 'returned' is currently generated by
792 // the FE are places where 'returned' is basically free and almost always a
793 // performance win, so the second option can just be used always for now.
795 // This should be revisited if 'returned' is ever applied more liberally.
796 if (RetTy->isVoidTy() || HasLiveReturnedArg) {
799 StructType *STy = dyn_cast<StructType>(RetTy);
801 // Look at each of the original return values individually.
802 for (unsigned i = 0; i != RetCount; ++i) {
803 RetOrArg Ret = CreateRet(F, i);
804 if (LiveValues.erase(Ret)) {
805 RetTypes.push_back(STy->getElementType(i));
806 NewRetIdxs[i] = RetTypes.size() - 1;
808 ++NumRetValsEliminated;
809 DEBUG(dbgs() << "DAE - Removing return value " << i << " from "
810 << F->getName() << "\n");
814 // We used to return a single value.
815 if (LiveValues.erase(CreateRet(F, 0))) {
816 RetTypes.push_back(RetTy);
819 DEBUG(dbgs() << "DAE - Removing return value from " << F->getName()
821 ++NumRetValsEliminated;
823 if (RetTypes.size() > 1)
824 // More than one return type? Return a struct with them. Also, if we used
825 // to return a struct and didn't change the number of return values,
826 // return a struct again. This prevents changing {something} into
827 // something and {} into void.
828 // Make the new struct packed if we used to return a packed struct
830 NRetTy = StructType::get(STy->getContext(), RetTypes, STy->isPacked());
831 else if (RetTypes.size() == 1)
832 // One return type? Just a simple value then, but only if we didn't use to
833 // return a struct with that simple value before.
834 NRetTy = RetTypes.front();
835 else if (RetTypes.size() == 0)
836 // No return types? Make it void, but only if we didn't use to return {}.
837 NRetTy = Type::getVoidTy(F->getContext());
840 assert(NRetTy && "No new return type found?");
842 // The existing function return attributes.
843 AttributeSet RAttrs = PAL.getRetAttributes();
845 // Remove any incompatible attributes, but only if we removed all return
846 // values. Otherwise, ensure that we don't have any conflicting attributes
847 // here. Currently, this should not be possible, but special handling might be
848 // required when new return value attributes are added.
849 if (NRetTy->isVoidTy())
851 AttributeSet::get(NRetTy->getContext(), AttributeSet::ReturnIndex,
852 AttrBuilder(RAttrs, AttributeSet::ReturnIndex).
853 removeAttributes(AttributeFuncs::
854 typeIncompatible(NRetTy, AttributeSet::ReturnIndex),
855 AttributeSet::ReturnIndex));
857 assert(!AttrBuilder(RAttrs, AttributeSet::ReturnIndex).
858 hasAttributes(AttributeFuncs::
859 typeIncompatible(NRetTy, AttributeSet::ReturnIndex),
860 AttributeSet::ReturnIndex) &&
861 "Return attributes no longer compatible?");
863 if (RAttrs.hasAttributes(AttributeSet::ReturnIndex))
864 AttributesVec.push_back(AttributeSet::get(NRetTy->getContext(), RAttrs));
866 if (PAL.hasAttributes(AttributeSet::FunctionIndex))
867 AttributesVec.push_back(AttributeSet::get(F->getContext(),
868 PAL.getFnAttributes()));
870 // Reconstruct the AttributesList based on the vector we constructed.
871 AttributeSet NewPAL = AttributeSet::get(F->getContext(), AttributesVec);
873 // Create the new function type based on the recomputed parameters.
874 FunctionType *NFTy = FunctionType::get(NRetTy, Params, FTy->isVarArg());
880 // Create the new function body and insert it into the module...
881 Function *NF = Function::Create(NFTy, F->getLinkage());
882 NF->copyAttributesFrom(F);
883 NF->setAttributes(NewPAL);
884 // Insert the new function before the old function, so we won't be processing
886 F->getParent()->getFunctionList().insert(F, NF);
889 // Loop over all of the callers of the function, transforming the call sites
890 // to pass in a smaller number of arguments into the new function.
892 std::vector<Value*> Args;
893 while (!F->use_empty()) {
894 CallSite CS(F->use_back());
895 Instruction *Call = CS.getInstruction();
897 AttributesVec.clear();
898 const AttributeSet &CallPAL = CS.getAttributes();
900 // The call return attributes.
901 AttributeSet RAttrs = CallPAL.getRetAttributes();
903 // Adjust in case the function was changed to return void.
905 AttributeSet::get(NF->getContext(), AttributeSet::ReturnIndex,
906 AttrBuilder(RAttrs, AttributeSet::ReturnIndex).
907 removeAttributes(AttributeFuncs::
908 typeIncompatible(NF->getReturnType(),
909 AttributeSet::ReturnIndex),
910 AttributeSet::ReturnIndex));
911 if (RAttrs.hasAttributes(AttributeSet::ReturnIndex))
912 AttributesVec.push_back(AttributeSet::get(NF->getContext(), RAttrs));
914 // Declare these outside of the loops, so we can reuse them for the second
915 // loop, which loops the varargs.
916 CallSite::arg_iterator I = CS.arg_begin();
918 // Loop over those operands, corresponding to the normal arguments to the
919 // original function, and add those that are still alive.
920 for (unsigned e = FTy->getNumParams(); i != e; ++I, ++i)
923 // Get original parameter attributes, but skip return attributes.
924 if (CallPAL.hasAttributes(i + 1)) {
925 AttrBuilder B(CallPAL, i + 1);
926 // If the return type has changed, then get rid of 'returned' on the
927 // call site. The alternative is to make all 'returned' attributes on
928 // call sites keep the return value alive just like 'returned'
929 // attributes on function declaration but it's less clearly a win
930 // and this is not an expected case anyway
931 if (NRetTy != RetTy && B.contains(Attribute::Returned))
932 B.removeAttribute(Attribute::Returned);
934 push_back(AttributeSet::get(F->getContext(), Args.size(), B));
938 // Push any varargs arguments on the list. Don't forget their attributes.
939 for (CallSite::arg_iterator E = CS.arg_end(); I != E; ++I, ++i) {
941 if (CallPAL.hasAttributes(i + 1)) {
942 AttrBuilder B(CallPAL, i + 1);
944 push_back(AttributeSet::get(F->getContext(), Args.size(), B));
948 if (CallPAL.hasAttributes(AttributeSet::FunctionIndex))
949 AttributesVec.push_back(AttributeSet::get(Call->getContext(),
950 CallPAL.getFnAttributes()));
952 // Reconstruct the AttributesList based on the vector we constructed.
953 AttributeSet NewCallPAL = AttributeSet::get(F->getContext(), AttributesVec);
956 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
957 New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
959 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
960 cast<InvokeInst>(New)->setAttributes(NewCallPAL);
962 New = CallInst::Create(NF, Args, "", Call);
963 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
964 cast<CallInst>(New)->setAttributes(NewCallPAL);
965 if (cast<CallInst>(Call)->isTailCall())
966 cast<CallInst>(New)->setTailCall();
968 New->setDebugLoc(Call->getDebugLoc());
972 if (!Call->use_empty()) {
973 if (New->getType() == Call->getType()) {
974 // Return type not changed? Just replace users then.
975 Call->replaceAllUsesWith(New);
977 } else if (New->getType()->isVoidTy()) {
978 // Our return value has uses, but they will get removed later on.
979 // Replace by null for now.
980 if (!Call->getType()->isX86_MMXTy())
981 Call->replaceAllUsesWith(Constant::getNullValue(Call->getType()));
983 assert(RetTy->isStructTy() &&
984 "Return type changed, but not into a void. The old return type"
985 " must have been a struct!");
986 Instruction *InsertPt = Call;
987 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
988 BasicBlock::iterator IP = II->getNormalDest()->begin();
989 while (isa<PHINode>(IP)) ++IP;
993 // We used to return a struct. Instead of doing smart stuff with all the
994 // uses of this struct, we will just rebuild it using
995 // extract/insertvalue chaining and let instcombine clean that up.
997 // Start out building up our return value from undef
998 Value *RetVal = UndefValue::get(RetTy);
999 for (unsigned i = 0; i != RetCount; ++i)
1000 if (NewRetIdxs[i] != -1) {
1002 if (RetTypes.size() > 1)
1003 // We are still returning a struct, so extract the value from our
1005 V = ExtractValueInst::Create(New, NewRetIdxs[i], "newret",
1008 // We are now returning a single element, so just insert that
1010 // Insert the value at the old position
1011 RetVal = InsertValueInst::Create(RetVal, V, i, "oldret", InsertPt);
1013 // Now, replace all uses of the old call instruction with the return
1015 Call->replaceAllUsesWith(RetVal);
1016 New->takeName(Call);
1020 // Finally, remove the old call from the program, reducing the use-count of
1022 Call->eraseFromParent();
1025 // Since we have now created the new function, splice the body of the old
1026 // function right into the new function, leaving the old rotting hulk of the
1028 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
1030 // Loop over the argument list, transferring uses of the old arguments over to
1031 // the new arguments, also transferring over the names as well.
1033 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(),
1034 I2 = NF->arg_begin(); I != E; ++I, ++i)
1036 // If this is a live argument, move the name and users over to the new
1038 I->replaceAllUsesWith(I2);
1042 // If this argument is dead, replace any uses of it with null constants
1043 // (these are guaranteed to become unused later on).
1044 if (!I->getType()->isX86_MMXTy())
1045 I->replaceAllUsesWith(Constant::getNullValue(I->getType()));
1048 // If we change the return value of the function we must rewrite any return
1049 // instructions. Check this now.
1050 if (F->getReturnType() != NF->getReturnType())
1051 for (Function::iterator BB = NF->begin(), E = NF->end(); BB != E; ++BB)
1052 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
1055 if (NFTy->getReturnType()->isVoidTy()) {
1058 assert (RetTy->isStructTy());
1059 // The original return value was a struct, insert
1060 // extractvalue/insertvalue chains to extract only the values we need
1061 // to return and insert them into our new result.
1062 // This does generate messy code, but we'll let it to instcombine to
1064 Value *OldRet = RI->getOperand(0);
1065 // Start out building up our return value from undef
1066 RetVal = UndefValue::get(NRetTy);
1067 for (unsigned i = 0; i != RetCount; ++i)
1068 if (NewRetIdxs[i] != -1) {
1069 ExtractValueInst *EV = ExtractValueInst::Create(OldRet, i,
1071 if (RetTypes.size() > 1) {
1072 // We're still returning a struct, so reinsert the value into
1073 // our new return value at the new index
1075 RetVal = InsertValueInst::Create(RetVal, EV, NewRetIdxs[i],
1078 // We are now only returning a simple value, so just return the
1084 // Replace the return instruction with one returning the new return
1085 // value (possibly 0 if we became void).
1086 ReturnInst::Create(F->getContext(), RetVal, RI);
1087 BB->getInstList().erase(RI);
1090 // Patch the pointer to LLVM function in debug info descriptor.
1091 FunctionDIMap::iterator DI = FunctionDIs.find(F);
1092 if (DI != FunctionDIs.end())
1093 DI->second.replaceFunction(NF);
1095 // Now that the old function is dead, delete it.
1096 F->eraseFromParent();
1101 bool DAE::runOnModule(Module &M) {
1102 bool Changed = false;
1104 // Collect debug info descriptors for functions.
1105 CollectFunctionDIs(M);
1107 // First pass: Do a simple check to see if any functions can have their "..."
1108 // removed. We can do this if they never call va_start. This loop cannot be
1109 // fused with the next loop, because deleting a function invalidates
1110 // information computed while surveying other functions.
1111 DEBUG(dbgs() << "DAE - Deleting dead varargs\n");
1112 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1114 if (F.getFunctionType()->isVarArg())
1115 Changed |= DeleteDeadVarargs(F);
1118 // Second phase:loop through the module, determining which arguments are live.
1119 // We assume all arguments are dead unless proven otherwise (allowing us to
1120 // determine that dead arguments passed into recursive functions are dead).
1122 DEBUG(dbgs() << "DAE - Determining liveness\n");
1123 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
1126 // Now, remove all dead arguments and return values from each function in
1128 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1129 // Increment now, because the function will probably get removed (ie.
1130 // replaced by a new one).
1132 Changed |= RemoveDeadStuffFromFunction(F);
1135 // Finally, look for any unused parameters in functions with non-local
1136 // linkage and replace the passed in parameters with undef.
1137 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
1140 Changed |= RemoveDeadArgumentsFromCallers(F);