1 //===-- DeadArgumentElimination.cpp - Eliminate dead arguments ------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass deletes dead arguments from internal functions. Dead argument
11 // elimination removes arguments which are directly dead, as well as arguments
12 // only passed into function calls as dead arguments of other functions. This
13 // pass also deletes dead return values in a similar way.
15 // This pass is often useful as a cleanup pass to run after aggressive
16 // interprocedural passes, which add possibly-dead arguments or return values.
18 //===----------------------------------------------------------------------===//
20 #include "llvm/Transforms/IPO.h"
21 #include "llvm/ADT/DenseMap.h"
22 #include "llvm/ADT/SmallVector.h"
23 #include "llvm/ADT/Statistic.h"
24 #include "llvm/ADT/StringExtras.h"
25 #include "llvm/IR/CallSite.h"
26 #include "llvm/IR/CallingConv.h"
27 #include "llvm/IR/Constant.h"
28 #include "llvm/IR/DIBuilder.h"
29 #include "llvm/IR/DebugInfo.h"
30 #include "llvm/IR/DerivedTypes.h"
31 #include "llvm/IR/Instructions.h"
32 #include "llvm/IR/IntrinsicInst.h"
33 #include "llvm/IR/LLVMContext.h"
34 #include "llvm/IR/Module.h"
35 #include "llvm/Pass.h"
36 #include "llvm/Support/Debug.h"
37 #include "llvm/Support/raw_ostream.h"
43 #define DEBUG_TYPE "deadargelim"
45 STATISTIC(NumArgumentsEliminated, "Number of unread args removed");
46 STATISTIC(NumRetValsEliminated , "Number of unused return values removed");
47 STATISTIC(NumArgumentsReplacedWithUndef,
48 "Number of unread args replaced with undef");
50 /// DAE - The dead argument elimination pass.
52 class DAE : public ModulePass {
55 /// Struct that represents (part of) either a return value or a function
56 /// argument. Used so that arguments and return values can be used
59 RetOrArg(const Function *F, unsigned Idx, bool IsArg) : F(F), Idx(Idx),
65 /// Make RetOrArg comparable, so we can put it into a map.
66 bool operator<(const RetOrArg &O) const {
67 return std::tie(F, Idx, IsArg) < std::tie(O.F, O.Idx, O.IsArg);
70 /// Make RetOrArg comparable, so we can easily iterate the multimap.
71 bool operator==(const RetOrArg &O) const {
72 return F == O.F && Idx == O.Idx && IsArg == O.IsArg;
75 std::string getDescription() const {
76 return std::string((IsArg ? "Argument #" : "Return value #"))
77 + utostr(Idx) + " of function " + F->getName().str();
81 /// Liveness enum - During our initial pass over the program, we determine
82 /// that things are either alive or maybe alive. We don't mark anything
83 /// explicitly dead (even if we know they are), since anything not alive
84 /// with no registered uses (in Uses) will never be marked alive and will
85 /// thus become dead in the end.
86 enum Liveness { Live, MaybeLive };
88 /// Convenience wrapper
89 RetOrArg CreateRet(const Function *F, unsigned Idx) {
90 return RetOrArg(F, Idx, false);
92 /// Convenience wrapper
93 RetOrArg CreateArg(const Function *F, unsigned Idx) {
94 return RetOrArg(F, Idx, true);
97 typedef std::multimap<RetOrArg, RetOrArg> UseMap;
98 /// This maps a return value or argument to any MaybeLive return values or
99 /// arguments it uses. This allows the MaybeLive values to be marked live
100 /// when any of its users is marked live.
101 /// For example (indices are left out for clarity):
102 /// - Uses[ret F] = ret G
103 /// This means that F calls G, and F returns the value returned by G.
104 /// - Uses[arg F] = ret G
105 /// This means that some function calls G and passes its result as an
107 /// - Uses[ret F] = arg F
108 /// This means that F returns one of its own arguments.
109 /// - Uses[arg F] = arg G
110 /// This means that G calls F and passes one of its own (G's) arguments
114 typedef std::set<RetOrArg> LiveSet;
115 typedef std::set<const Function*> LiveFuncSet;
117 /// This set contains all values that have been determined to be live.
119 /// This set contains all values that are cannot be changed in any way.
120 LiveFuncSet LiveFunctions;
122 typedef SmallVector<RetOrArg, 5> UseVector;
124 // Map each LLVM function to corresponding metadata with debug info. If
125 // the function is replaced with another one, we should patch the pointer
126 // to LLVM function in metadata.
127 // As the code generation for module is finished (and DIBuilder is
128 // finalized) we assume that subprogram descriptors won't be changed, and
129 // they are stored in map for short duration anyway.
130 typedef DenseMap<Function*, DISubprogram> FunctionDIMap;
131 FunctionDIMap FunctionDIs;
134 // DAH uses this to specify a different ID.
135 explicit DAE(char &ID) : ModulePass(ID) {}
138 static char ID; // Pass identification, replacement for typeid
139 DAE() : ModulePass(ID) {
140 initializeDAEPass(*PassRegistry::getPassRegistry());
143 bool runOnModule(Module &M) override;
145 virtual bool ShouldHackArguments() const { return false; }
148 Liveness MarkIfNotLive(RetOrArg Use, UseVector &MaybeLiveUses);
149 Liveness SurveyUse(const Use *U, UseVector &MaybeLiveUses,
150 unsigned RetValNum = 0);
151 Liveness SurveyUses(const Value *V, UseVector &MaybeLiveUses);
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 /// DeleteDeadVarargs - If this is an function that takes a ... list, and if
193 /// llvm.vastart is never called, the varargs list is dead for the function.
194 bool DAE::DeleteDeadVarargs(Function &Fn) {
195 assert(Fn.getFunctionType()->isVarArg() && "Function isn't varargs!");
196 if (Fn.isDeclaration() || !Fn.hasLocalLinkage()) return false;
198 // Ensure that the function is only directly called.
199 if (Fn.hasAddressTaken())
202 // Okay, we know we can transform this function if safe. Scan its body
203 // looking for calls to llvm.vastart.
204 for (Function::iterator BB = Fn.begin(), E = Fn.end(); BB != E; ++BB) {
205 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
206 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
207 if (II->getIntrinsicID() == Intrinsic::vastart)
213 // If we get here, there are no calls to llvm.vastart in the function body,
214 // remove the "..." and adjust all the calls.
216 // Start by computing a new prototype for the function, which is the same as
217 // the old function, but doesn't have isVarArg set.
218 FunctionType *FTy = Fn.getFunctionType();
220 std::vector<Type*> Params(FTy->param_begin(), FTy->param_end());
221 FunctionType *NFTy = FunctionType::get(FTy->getReturnType(),
223 unsigned NumArgs = Params.size();
225 // Create the new function body and insert it into the module...
226 Function *NF = Function::Create(NFTy, Fn.getLinkage());
227 NF->copyAttributesFrom(&Fn);
228 Fn.getParent()->getFunctionList().insert(&Fn, NF);
231 // Loop over all of the callers of the function, transforming the call sites
232 // to pass in a smaller number of arguments into the new function.
234 std::vector<Value*> Args;
235 for (Value::user_iterator I = Fn.user_begin(), E = Fn.user_end(); I != E; ) {
239 Instruction *Call = CS.getInstruction();
241 // Pass all the same arguments.
242 Args.assign(CS.arg_begin(), CS.arg_begin() + NumArgs);
244 // Drop any attributes that were on the vararg arguments.
245 AttributeSet PAL = CS.getAttributes();
246 if (!PAL.isEmpty() && PAL.getSlotIndex(PAL.getNumSlots() - 1) > NumArgs) {
247 SmallVector<AttributeSet, 8> AttributesVec;
248 for (unsigned i = 0; PAL.getSlotIndex(i) <= NumArgs; ++i)
249 AttributesVec.push_back(PAL.getSlotAttributes(i));
250 if (PAL.hasAttributes(AttributeSet::FunctionIndex))
251 AttributesVec.push_back(AttributeSet::get(Fn.getContext(),
252 PAL.getFnAttributes()));
253 PAL = AttributeSet::get(Fn.getContext(), AttributesVec);
257 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
258 New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
260 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
261 cast<InvokeInst>(New)->setAttributes(PAL);
263 New = CallInst::Create(NF, Args, "", Call);
264 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
265 cast<CallInst>(New)->setAttributes(PAL);
266 if (cast<CallInst>(Call)->isTailCall())
267 cast<CallInst>(New)->setTailCall();
269 New->setDebugLoc(Call->getDebugLoc());
273 if (!Call->use_empty())
274 Call->replaceAllUsesWith(New);
278 // Finally, remove the old call from the program, reducing the use-count of
280 Call->eraseFromParent();
283 // Since we have now created the new function, splice the body of the old
284 // function right into the new function, leaving the old rotting hulk of the
286 NF->getBasicBlockList().splice(NF->begin(), Fn.getBasicBlockList());
288 // Loop over the argument list, transferring uses of the old arguments over to
289 // the new arguments, also transferring over the names as well. While we're at
290 // it, remove the dead arguments from the DeadArguments list.
292 for (Function::arg_iterator I = Fn.arg_begin(), E = Fn.arg_end(),
293 I2 = NF->arg_begin(); I != E; ++I, ++I2) {
294 // Move the name and users over to the new version.
295 I->replaceAllUsesWith(I2);
299 // Patch the pointer to LLVM function in debug info descriptor.
300 FunctionDIMap::iterator DI = FunctionDIs.find(&Fn);
301 if (DI != FunctionDIs.end())
302 DI->second.replaceFunction(NF);
304 // Fix up any BlockAddresses that refer to the function.
305 Fn.replaceAllUsesWith(ConstantExpr::getBitCast(NF, Fn.getType()));
306 // Delete the bitcast that we just created, so that NF does not
307 // appear to be address-taken.
308 NF->removeDeadConstantUsers();
309 // Finally, nuke the old function.
310 Fn.eraseFromParent();
314 /// RemoveDeadArgumentsFromCallers - Checks if the given function has any
315 /// arguments that are unused, and changes the caller parameters to be undefined
317 bool DAE::RemoveDeadArgumentsFromCallers(Function &Fn)
319 if (Fn.isDeclaration() || Fn.mayBeOverridden())
322 // Functions with local linkage should already have been handled, except the
323 // fragile (variadic) ones which we can improve here.
324 if (Fn.hasLocalLinkage() && !Fn.getFunctionType()->isVarArg())
327 // If a function seen at compile time is not necessarily the one linked to
328 // the binary being built, it is illegal to change the actual arguments
329 // passed to it. These functions can be captured by isWeakForLinker().
330 // *NOTE* that mayBeOverridden() is insufficient for this purpose as it
331 // doesn't include linkage types like AvailableExternallyLinkage and
332 // LinkOnceODRLinkage. Take link_odr* as an example, it indicates a set of
333 // *EQUIVALENT* globals that can be merged at link-time. However, the
334 // semantic of *EQUIVALENT*-functions includes parameters. Changing
335 // parameters breaks this assumption.
337 if (Fn.isWeakForLinker())
343 SmallVector<unsigned, 8> UnusedArgs;
344 for (Function::arg_iterator I = Fn.arg_begin(), E = Fn.arg_end();
348 if (Arg->use_empty() && !Arg->hasByValOrInAllocaAttr())
349 UnusedArgs.push_back(Arg->getArgNo());
352 if (UnusedArgs.empty())
355 bool Changed = false;
357 for (Use &U : Fn.uses()) {
358 CallSite CS(U.getUser());
359 if (!CS || !CS.isCallee(&U))
362 // Now go through all unused args and replace them with "undef".
363 for (unsigned I = 0, E = UnusedArgs.size(); I != E; ++I) {
364 unsigned ArgNo = UnusedArgs[I];
366 Value *Arg = CS.getArgument(ArgNo);
367 CS.setArgument(ArgNo, UndefValue::get(Arg->getType()));
368 ++NumArgumentsReplacedWithUndef;
376 /// Convenience function that returns the number of return values. It returns 0
377 /// for void functions and 1 for functions not returning a struct. It returns
378 /// the number of struct elements for functions returning a struct.
379 static unsigned NumRetVals(const Function *F) {
380 if (F->getReturnType()->isVoidTy())
382 else if (StructType *STy = dyn_cast<StructType>(F->getReturnType()))
383 return STy->getNumElements();
388 /// MarkIfNotLive - This checks Use for liveness in LiveValues. If Use is not
389 /// live, it adds Use to the MaybeLiveUses argument. Returns the determined
391 DAE::Liveness DAE::MarkIfNotLive(RetOrArg Use, UseVector &MaybeLiveUses) {
392 // We're live if our use or its Function is already marked as live.
393 if (LiveFunctions.count(Use.F) || LiveValues.count(Use))
396 // We're maybe live otherwise, but remember that we must become live if
398 MaybeLiveUses.push_back(Use);
403 /// SurveyUse - This looks at a single use of an argument or return value
404 /// and determines if it should be alive or not. Adds this use to MaybeLiveUses
405 /// if it causes the used value to become MaybeLive.
407 /// RetValNum is the return value number to use when this use is used in a
408 /// return instruction. This is used in the recursion, you should always leave
410 DAE::Liveness DAE::SurveyUse(const Use *U,
411 UseVector &MaybeLiveUses, unsigned RetValNum) {
412 const User *V = U->getUser();
413 if (const ReturnInst *RI = dyn_cast<ReturnInst>(V)) {
414 // The value is returned from a function. It's only live when the
415 // function's return value is live. We use RetValNum here, for the case
416 // that U is really a use of an insertvalue instruction that uses the
418 RetOrArg Use = CreateRet(RI->getParent()->getParent(), RetValNum);
419 // We might be live, depending on the liveness of Use.
420 return MarkIfNotLive(Use, MaybeLiveUses);
422 if (const InsertValueInst *IV = dyn_cast<InsertValueInst>(V)) {
423 if (U->getOperandNo() != InsertValueInst::getAggregateOperandIndex()
425 // The use we are examining is inserted into an aggregate. Our liveness
426 // depends on all uses of that aggregate, but if it is used as a return
427 // value, only index at which we were inserted counts.
428 RetValNum = *IV->idx_begin();
430 // Note that if we are used as the aggregate operand to the insertvalue,
431 // we don't change RetValNum, but do survey all our uses.
433 Liveness Result = MaybeLive;
434 for (const Use &UU : IV->uses()) {
435 Result = SurveyUse(&UU, MaybeLiveUses, RetValNum);
442 if (ImmutableCallSite CS = V) {
443 const Function *F = CS.getCalledFunction();
445 // Used in a direct call.
447 // Find the argument number. We know for sure that this use is an
448 // argument, since if it was the function argument this would be an
449 // indirect call and the we know can't be looking at a value of the
450 // label type (for the invoke instruction).
451 unsigned ArgNo = CS.getArgumentNo(U);
453 if (ArgNo >= F->getFunctionType()->getNumParams())
454 // The value is passed in through a vararg! Must be live.
457 assert(CS.getArgument(ArgNo)
458 == CS->getOperand(U->getOperandNo())
459 && "Argument is not where we expected it");
461 // Value passed to a normal call. It's only live when the corresponding
462 // argument to the called function turns out live.
463 RetOrArg Use = CreateArg(F, ArgNo);
464 return MarkIfNotLive(Use, MaybeLiveUses);
467 // Used in any other way? Value must be live.
471 /// SurveyUses - This looks at all the uses of the given value
472 /// Returns the Liveness deduced from the uses of this value.
474 /// Adds all uses that cause the result to be MaybeLive to MaybeLiveRetUses. If
475 /// the result is Live, MaybeLiveUses might be modified but its content should
476 /// be ignored (since it might not be complete).
477 DAE::Liveness DAE::SurveyUses(const Value *V, UseVector &MaybeLiveUses) {
478 // Assume it's dead (which will only hold if there are no uses at all..).
479 Liveness Result = MaybeLive;
481 for (const Use &U : V->uses()) {
482 Result = SurveyUse(&U, MaybeLiveUses);
489 // SurveyFunction - This performs the initial survey of the specified function,
490 // checking out whether or not it uses any of its incoming arguments or whether
491 // any callers use the return value. This fills in the LiveValues set and Uses
494 // We consider arguments of non-internal functions to be intrinsically alive as
495 // well as arguments to functions which have their "address taken".
497 void DAE::SurveyFunction(const Function &F) {
498 // Functions with inalloca parameters are expecting args in a particular
499 // register and memory layout.
500 if (F.getAttributes().hasAttrSomewhere(Attribute::InAlloca)) {
505 unsigned RetCount = NumRetVals(&F);
506 // Assume all return values are dead
507 typedef SmallVector<Liveness, 5> RetVals;
508 RetVals RetValLiveness(RetCount, MaybeLive);
510 typedef SmallVector<UseVector, 5> RetUses;
511 // These vectors map each return value to the uses that make it MaybeLive, so
512 // we can add those to the Uses map if the return value really turns out to be
513 // MaybeLive. Initialized to a list of RetCount empty lists.
514 RetUses MaybeLiveRetUses(RetCount);
516 for (Function::const_iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
517 if (const ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()))
518 if (RI->getNumOperands() != 0 && RI->getOperand(0)->getType()
519 != F.getFunctionType()->getReturnType()) {
520 // We don't support old style multiple return values.
525 if (!F.hasLocalLinkage() && (!ShouldHackArguments() || F.isIntrinsic())) {
530 DEBUG(dbgs() << "DAE - Inspecting callers for fn: " << F.getName() << "\n");
531 // Keep track of the number of live retvals, so we can skip checks once all
532 // of them turn out to be live.
533 unsigned NumLiveRetVals = 0;
534 Type *STy = dyn_cast<StructType>(F.getReturnType());
535 // Loop all uses of the function.
536 for (const Use &U : F.uses()) {
537 // If the function is PASSED IN as an argument, its address has been
539 ImmutableCallSite CS(U.getUser());
540 if (!CS || !CS.isCallee(&U)) {
545 // If this use is anything other than a call site, the function is alive.
546 const Instruction *TheCall = CS.getInstruction();
547 if (!TheCall) { // Not a direct call site?
552 // If we end up here, we are looking at a direct call to our function.
554 // Now, check how our return value(s) is/are used in this caller. Don't
555 // bother checking return values if all of them are live already.
556 if (NumLiveRetVals != RetCount) {
558 // Check all uses of the return value.
559 for (const User *U : TheCall->users()) {
560 const ExtractValueInst *Ext = dyn_cast<ExtractValueInst>(U);
561 if (Ext && Ext->hasIndices()) {
562 // This use uses a part of our return value, survey the uses of
563 // that part and store the results for this index only.
564 unsigned Idx = *Ext->idx_begin();
565 if (RetValLiveness[Idx] != Live) {
566 RetValLiveness[Idx] = SurveyUses(Ext, MaybeLiveRetUses[Idx]);
567 if (RetValLiveness[Idx] == Live)
571 // Used by something else than extractvalue. Mark all return
573 for (unsigned i = 0; i != RetCount; ++i )
574 RetValLiveness[i] = Live;
575 NumLiveRetVals = RetCount;
580 // Single return value
581 RetValLiveness[0] = SurveyUses(TheCall, MaybeLiveRetUses[0]);
582 if (RetValLiveness[0] == Live)
583 NumLiveRetVals = RetCount;
588 // Now we've inspected all callers, record the liveness of our return values.
589 for (unsigned i = 0; i != RetCount; ++i)
590 MarkValue(CreateRet(&F, i), RetValLiveness[i], MaybeLiveRetUses[i]);
592 DEBUG(dbgs() << "DAE - Inspecting args for fn: " << F.getName() << "\n");
594 // Now, check all of our arguments.
596 UseVector MaybeLiveArgUses;
597 for (Function::const_arg_iterator AI = F.arg_begin(),
598 E = F.arg_end(); AI != E; ++AI, ++i) {
600 if (F.getFunctionType()->isVarArg()) {
601 // Variadic functions will already have a va_arg function expanded inside
602 // them, making them potentially very sensitive to ABI changes resulting
603 // from removing arguments entirely, so don't. For example AArch64 handles
604 // register and stack HFAs very differently, and this is reflected in the
605 // IR which has already been generated.
608 // See what the effect of this use is (recording any uses that cause
609 // MaybeLive in MaybeLiveArgUses).
610 Result = SurveyUses(AI, MaybeLiveArgUses);
614 MarkValue(CreateArg(&F, i), Result, MaybeLiveArgUses);
615 // Clear the vector again for the next iteration.
616 MaybeLiveArgUses.clear();
620 /// MarkValue - This function marks the liveness of RA depending on L. If L is
621 /// MaybeLive, it also takes all uses in MaybeLiveUses and records them in Uses,
622 /// such that RA will be marked live if any use in MaybeLiveUses gets marked
624 void DAE::MarkValue(const RetOrArg &RA, Liveness L,
625 const UseVector &MaybeLiveUses) {
627 case Live: MarkLive(RA); break;
630 // Note any uses of this value, so this return value can be
631 // marked live whenever one of the uses becomes live.
632 for (UseVector::const_iterator UI = MaybeLiveUses.begin(),
633 UE = MaybeLiveUses.end(); UI != UE; ++UI)
634 Uses.insert(std::make_pair(*UI, RA));
640 /// MarkLive - Mark the given Function as alive, meaning that it cannot be
641 /// changed in any way. Additionally,
642 /// mark any values that are used as this function's parameters or by its return
643 /// values (according to Uses) live as well.
644 void DAE::MarkLive(const Function &F) {
645 DEBUG(dbgs() << "DAE - Intrinsically live fn: " << F.getName() << "\n");
646 // Mark the function as live.
647 LiveFunctions.insert(&F);
648 // Mark all arguments as live.
649 for (unsigned i = 0, e = F.arg_size(); i != e; ++i)
650 PropagateLiveness(CreateArg(&F, i));
651 // Mark all return values as live.
652 for (unsigned i = 0, e = NumRetVals(&F); i != e; ++i)
653 PropagateLiveness(CreateRet(&F, i));
656 /// MarkLive - Mark the given return value or argument as live. Additionally,
657 /// mark any values that are used by this value (according to Uses) live as
659 void DAE::MarkLive(const RetOrArg &RA) {
660 if (LiveFunctions.count(RA.F))
661 return; // Function was already marked Live.
663 if (!LiveValues.insert(RA).second)
664 return; // We were already marked Live.
666 DEBUG(dbgs() << "DAE - Marking " << RA.getDescription() << " live\n");
667 PropagateLiveness(RA);
670 /// PropagateLiveness - Given that RA is a live value, propagate it's liveness
671 /// to any other values it uses (according to Uses).
672 void DAE::PropagateLiveness(const RetOrArg &RA) {
673 // We don't use upper_bound (or equal_range) here, because our recursive call
674 // to ourselves is likely to cause the upper_bound (which is the first value
675 // not belonging to RA) to become erased and the iterator invalidated.
676 UseMap::iterator Begin = Uses.lower_bound(RA);
677 UseMap::iterator E = Uses.end();
679 for (I = Begin; I != E && I->first == RA; ++I)
682 // Erase RA from the Uses map (from the lower bound to wherever we ended up
684 Uses.erase(Begin, I);
687 // RemoveDeadStuffFromFunction - Remove any arguments and return values from F
688 // that are not in LiveValues. Transform the function and all of the callees of
689 // the function to not have these arguments and return values.
691 bool DAE::RemoveDeadStuffFromFunction(Function *F) {
692 // Don't modify fully live functions
693 if (LiveFunctions.count(F))
696 // Start by computing a new prototype for the function, which is the same as
697 // the old function, but has fewer arguments and a different return type.
698 FunctionType *FTy = F->getFunctionType();
699 std::vector<Type*> Params;
701 // Keep track of if we have a live 'returned' argument
702 bool HasLiveReturnedArg = false;
704 // Set up to build a new list of parameter attributes.
705 SmallVector<AttributeSet, 8> AttributesVec;
706 const AttributeSet &PAL = F->getAttributes();
708 // Remember which arguments are still alive.
709 SmallVector<bool, 10> ArgAlive(FTy->getNumParams(), false);
710 // Construct the new parameter list from non-dead arguments. Also construct
711 // a new set of parameter attributes to correspond. Skip the first parameter
712 // attribute, since that belongs to the return value.
714 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
716 RetOrArg Arg = CreateArg(F, i);
717 if (LiveValues.erase(Arg)) {
718 Params.push_back(I->getType());
721 // Get the original parameter attributes (skipping the first one, that is
722 // for the return value.
723 if (PAL.hasAttributes(i + 1)) {
724 AttrBuilder B(PAL, i + 1);
725 if (B.contains(Attribute::Returned))
726 HasLiveReturnedArg = true;
728 push_back(AttributeSet::get(F->getContext(), Params.size(), B));
731 ++NumArgumentsEliminated;
732 DEBUG(dbgs() << "DAE - Removing argument " << i << " (" << I->getName()
733 << ") from " << F->getName() << "\n");
737 // Find out the new return value.
738 Type *RetTy = FTy->getReturnType();
739 Type *NRetTy = nullptr;
740 unsigned RetCount = NumRetVals(F);
742 // -1 means unused, other numbers are the new index
743 SmallVector<int, 5> NewRetIdxs(RetCount, -1);
744 std::vector<Type*> RetTypes;
746 // If there is a function with a live 'returned' argument but a dead return
747 // value, then there are two possible actions:
748 // 1) Eliminate the return value and take off the 'returned' attribute on the
750 // 2) Retain the 'returned' attribute and treat the return value (but not the
751 // entire function) as live so that it is not eliminated.
753 // It's not clear in the general case which option is more profitable because,
754 // even in the absence of explicit uses of the return value, code generation
755 // is free to use the 'returned' attribute to do things like eliding
756 // save/restores of registers across calls. Whether or not this happens is
757 // target and ABI-specific as well as depending on the amount of register
758 // pressure, so there's no good way for an IR-level pass to figure this out.
760 // Fortunately, the only places where 'returned' is currently generated by
761 // the FE are places where 'returned' is basically free and almost always a
762 // performance win, so the second option can just be used always for now.
764 // This should be revisited if 'returned' is ever applied more liberally.
765 if (RetTy->isVoidTy() || HasLiveReturnedArg) {
768 StructType *STy = dyn_cast<StructType>(RetTy);
770 // Look at each of the original return values individually.
771 for (unsigned i = 0; i != RetCount; ++i) {
772 RetOrArg Ret = CreateRet(F, i);
773 if (LiveValues.erase(Ret)) {
774 RetTypes.push_back(STy->getElementType(i));
775 NewRetIdxs[i] = RetTypes.size() - 1;
777 ++NumRetValsEliminated;
778 DEBUG(dbgs() << "DAE - Removing return value " << i << " from "
779 << F->getName() << "\n");
783 // We used to return a single value.
784 if (LiveValues.erase(CreateRet(F, 0))) {
785 RetTypes.push_back(RetTy);
788 DEBUG(dbgs() << "DAE - Removing return value from " << F->getName()
790 ++NumRetValsEliminated;
792 if (RetTypes.size() > 1)
793 // More than one return type? Return a struct with them. Also, if we used
794 // to return a struct and didn't change the number of return values,
795 // return a struct again. This prevents changing {something} into
796 // something and {} into void.
797 // Make the new struct packed if we used to return a packed struct
799 NRetTy = StructType::get(STy->getContext(), RetTypes, STy->isPacked());
800 else if (RetTypes.size() == 1)
801 // One return type? Just a simple value then, but only if we didn't use to
802 // return a struct with that simple value before.
803 NRetTy = RetTypes.front();
804 else if (RetTypes.size() == 0)
805 // No return types? Make it void, but only if we didn't use to return {}.
806 NRetTy = Type::getVoidTy(F->getContext());
809 assert(NRetTy && "No new return type found?");
811 // The existing function return attributes.
812 AttributeSet RAttrs = PAL.getRetAttributes();
814 // Remove any incompatible attributes, but only if we removed all return
815 // values. Otherwise, ensure that we don't have any conflicting attributes
816 // here. Currently, this should not be possible, but special handling might be
817 // required when new return value attributes are added.
818 if (NRetTy->isVoidTy())
820 AttributeSet::get(NRetTy->getContext(), AttributeSet::ReturnIndex,
821 AttrBuilder(RAttrs, AttributeSet::ReturnIndex).
822 removeAttributes(AttributeFuncs::
823 typeIncompatible(NRetTy, AttributeSet::ReturnIndex),
824 AttributeSet::ReturnIndex));
826 assert(!AttrBuilder(RAttrs, AttributeSet::ReturnIndex).
827 hasAttributes(AttributeFuncs::
828 typeIncompatible(NRetTy, AttributeSet::ReturnIndex),
829 AttributeSet::ReturnIndex) &&
830 "Return attributes no longer compatible?");
832 if (RAttrs.hasAttributes(AttributeSet::ReturnIndex))
833 AttributesVec.push_back(AttributeSet::get(NRetTy->getContext(), RAttrs));
835 if (PAL.hasAttributes(AttributeSet::FunctionIndex))
836 AttributesVec.push_back(AttributeSet::get(F->getContext(),
837 PAL.getFnAttributes()));
839 // Reconstruct the AttributesList based on the vector we constructed.
840 AttributeSet NewPAL = AttributeSet::get(F->getContext(), AttributesVec);
842 // Create the new function type based on the recomputed parameters.
843 FunctionType *NFTy = FunctionType::get(NRetTy, Params, FTy->isVarArg());
849 // Create the new function body and insert it into the module...
850 Function *NF = Function::Create(NFTy, F->getLinkage());
851 NF->copyAttributesFrom(F);
852 NF->setAttributes(NewPAL);
853 // Insert the new function before the old function, so we won't be processing
855 F->getParent()->getFunctionList().insert(F, NF);
858 // Loop over all of the callers of the function, transforming the call sites
859 // to pass in a smaller number of arguments into the new function.
861 std::vector<Value*> Args;
862 while (!F->use_empty()) {
863 CallSite CS(F->user_back());
864 Instruction *Call = CS.getInstruction();
866 AttributesVec.clear();
867 const AttributeSet &CallPAL = CS.getAttributes();
869 // The call return attributes.
870 AttributeSet RAttrs = CallPAL.getRetAttributes();
872 // Adjust in case the function was changed to return void.
874 AttributeSet::get(NF->getContext(), AttributeSet::ReturnIndex,
875 AttrBuilder(RAttrs, AttributeSet::ReturnIndex).
876 removeAttributes(AttributeFuncs::
877 typeIncompatible(NF->getReturnType(),
878 AttributeSet::ReturnIndex),
879 AttributeSet::ReturnIndex));
880 if (RAttrs.hasAttributes(AttributeSet::ReturnIndex))
881 AttributesVec.push_back(AttributeSet::get(NF->getContext(), RAttrs));
883 // Declare these outside of the loops, so we can reuse them for the second
884 // loop, which loops the varargs.
885 CallSite::arg_iterator I = CS.arg_begin();
887 // Loop over those operands, corresponding to the normal arguments to the
888 // original function, and add those that are still alive.
889 for (unsigned e = FTy->getNumParams(); i != e; ++I, ++i)
892 // Get original parameter attributes, but skip return attributes.
893 if (CallPAL.hasAttributes(i + 1)) {
894 AttrBuilder B(CallPAL, i + 1);
895 // If the return type has changed, then get rid of 'returned' on the
896 // call site. The alternative is to make all 'returned' attributes on
897 // call sites keep the return value alive just like 'returned'
898 // attributes on function declaration but it's less clearly a win
899 // and this is not an expected case anyway
900 if (NRetTy != RetTy && B.contains(Attribute::Returned))
901 B.removeAttribute(Attribute::Returned);
903 push_back(AttributeSet::get(F->getContext(), Args.size(), B));
907 // Push any varargs arguments on the list. Don't forget their attributes.
908 for (CallSite::arg_iterator E = CS.arg_end(); I != E; ++I, ++i) {
910 if (CallPAL.hasAttributes(i + 1)) {
911 AttrBuilder B(CallPAL, i + 1);
913 push_back(AttributeSet::get(F->getContext(), Args.size(), B));
917 if (CallPAL.hasAttributes(AttributeSet::FunctionIndex))
918 AttributesVec.push_back(AttributeSet::get(Call->getContext(),
919 CallPAL.getFnAttributes()));
921 // Reconstruct the AttributesList based on the vector we constructed.
922 AttributeSet NewCallPAL = AttributeSet::get(F->getContext(), AttributesVec);
925 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
926 New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
928 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
929 cast<InvokeInst>(New)->setAttributes(NewCallPAL);
931 New = CallInst::Create(NF, Args, "", Call);
932 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
933 cast<CallInst>(New)->setAttributes(NewCallPAL);
934 if (cast<CallInst>(Call)->isTailCall())
935 cast<CallInst>(New)->setTailCall();
937 New->setDebugLoc(Call->getDebugLoc());
941 if (!Call->use_empty()) {
942 if (New->getType() == Call->getType()) {
943 // Return type not changed? Just replace users then.
944 Call->replaceAllUsesWith(New);
946 } else if (New->getType()->isVoidTy()) {
947 // Our return value has uses, but they will get removed later on.
948 // Replace by null for now.
949 if (!Call->getType()->isX86_MMXTy())
950 Call->replaceAllUsesWith(Constant::getNullValue(Call->getType()));
952 assert(RetTy->isStructTy() &&
953 "Return type changed, but not into a void. The old return type"
954 " must have been a struct!");
955 Instruction *InsertPt = Call;
956 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
957 BasicBlock::iterator IP = II->getNormalDest()->begin();
958 while (isa<PHINode>(IP)) ++IP;
962 // We used to return a struct. Instead of doing smart stuff with all the
963 // uses of this struct, we will just rebuild it using
964 // extract/insertvalue chaining and let instcombine clean that up.
966 // Start out building up our return value from undef
967 Value *RetVal = UndefValue::get(RetTy);
968 for (unsigned i = 0; i != RetCount; ++i)
969 if (NewRetIdxs[i] != -1) {
971 if (RetTypes.size() > 1)
972 // We are still returning a struct, so extract the value from our
974 V = ExtractValueInst::Create(New, NewRetIdxs[i], "newret",
977 // We are now returning a single element, so just insert that
979 // Insert the value at the old position
980 RetVal = InsertValueInst::Create(RetVal, V, i, "oldret", InsertPt);
982 // Now, replace all uses of the old call instruction with the return
984 Call->replaceAllUsesWith(RetVal);
989 // Finally, remove the old call from the program, reducing the use-count of
991 Call->eraseFromParent();
994 // Since we have now created the new function, splice the body of the old
995 // function right into the new function, leaving the old rotting hulk of the
997 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
999 // Loop over the argument list, transferring uses of the old arguments over to
1000 // the new arguments, also transferring over the names as well.
1002 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(),
1003 I2 = NF->arg_begin(); I != E; ++I, ++i)
1005 // If this is a live argument, move the name and users over to the new
1007 I->replaceAllUsesWith(I2);
1011 // If this argument is dead, replace any uses of it with null constants
1012 // (these are guaranteed to become unused later on).
1013 if (!I->getType()->isX86_MMXTy())
1014 I->replaceAllUsesWith(Constant::getNullValue(I->getType()));
1017 // If we change the return value of the function we must rewrite any return
1018 // instructions. Check this now.
1019 if (F->getReturnType() != NF->getReturnType())
1020 for (Function::iterator BB = NF->begin(), E = NF->end(); BB != E; ++BB)
1021 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
1024 if (NFTy->getReturnType()->isVoidTy()) {
1027 assert (RetTy->isStructTy());
1028 // The original return value was a struct, insert
1029 // extractvalue/insertvalue chains to extract only the values we need
1030 // to return and insert them into our new result.
1031 // This does generate messy code, but we'll let it to instcombine to
1033 Value *OldRet = RI->getOperand(0);
1034 // Start out building up our return value from undef
1035 RetVal = UndefValue::get(NRetTy);
1036 for (unsigned i = 0; i != RetCount; ++i)
1037 if (NewRetIdxs[i] != -1) {
1038 ExtractValueInst *EV = ExtractValueInst::Create(OldRet, i,
1040 if (RetTypes.size() > 1) {
1041 // We're still returning a struct, so reinsert the value into
1042 // our new return value at the new index
1044 RetVal = InsertValueInst::Create(RetVal, EV, NewRetIdxs[i],
1047 // We are now only returning a simple value, so just return the
1053 // Replace the return instruction with one returning the new return
1054 // value (possibly 0 if we became void).
1055 ReturnInst::Create(F->getContext(), RetVal, RI);
1056 BB->getInstList().erase(RI);
1059 // Patch the pointer to LLVM function in debug info descriptor.
1060 FunctionDIMap::iterator DI = FunctionDIs.find(F);
1061 if (DI != FunctionDIs.end())
1062 DI->second.replaceFunction(NF);
1064 // Now that the old function is dead, delete it.
1065 F->eraseFromParent();
1070 bool DAE::runOnModule(Module &M) {
1071 bool Changed = false;
1073 // Collect debug info descriptors for functions.
1074 FunctionDIs = makeSubprogramMap(M);
1076 // First pass: Do a simple check to see if any functions can have their "..."
1077 // removed. We can do this if they never call va_start. This loop cannot be
1078 // fused with the next loop, because deleting a function invalidates
1079 // information computed while surveying other functions.
1080 DEBUG(dbgs() << "DAE - Deleting dead varargs\n");
1081 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1083 if (F.getFunctionType()->isVarArg())
1084 Changed |= DeleteDeadVarargs(F);
1087 // Second phase:loop through the module, determining which arguments are live.
1088 // We assume all arguments are dead unless proven otherwise (allowing us to
1089 // determine that dead arguments passed into recursive functions are dead).
1091 DEBUG(dbgs() << "DAE - Determining liveness\n");
1092 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
1095 // Now, remove all dead arguments and return values from each function in
1097 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1098 // Increment now, because the function will probably get removed (ie.
1099 // replaced by a new one).
1101 Changed |= RemoveDeadStuffFromFunction(F);
1104 // Finally, look for any unused parameters in functions with non-local
1105 // linkage and replace the passed in parameters with undef.
1106 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
1109 Changed |= RemoveDeadArgumentsFromCallers(F);