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/IR/CallSite.h"
27 #include "llvm/IR/CallingConv.h"
28 #include "llvm/IR/Constant.h"
29 #include "llvm/IR/DIBuilder.h"
30 #include "llvm/IR/DebugInfo.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(const Use *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::user_iterator I = Fn.user_begin(), E = Fn.user_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 (Use &U : Fn.uses()) {
386 CallSite CS(U.getUser());
387 if (!CS || !CS.isCallee(&U))
390 // Now go through all unused args and replace them with "undef".
391 for (unsigned I = 0, E = UnusedArgs.size(); I != E; ++I) {
392 unsigned ArgNo = UnusedArgs[I];
394 Value *Arg = CS.getArgument(ArgNo);
395 CS.setArgument(ArgNo, UndefValue::get(Arg->getType()));
396 ++NumArgumentsReplacedWithUndef;
404 /// Convenience function that returns the number of return values. It returns 0
405 /// for void functions and 1 for functions not returning a struct. It returns
406 /// the number of struct elements for functions returning a struct.
407 static unsigned NumRetVals(const Function *F) {
408 if (F->getReturnType()->isVoidTy())
410 else if (StructType *STy = dyn_cast<StructType>(F->getReturnType()))
411 return STy->getNumElements();
416 /// MarkIfNotLive - This checks Use for liveness in LiveValues. If Use is not
417 /// live, it adds Use to the MaybeLiveUses argument. Returns the determined
419 DAE::Liveness DAE::MarkIfNotLive(RetOrArg Use, UseVector &MaybeLiveUses) {
420 // We're live if our use or its Function is already marked as live.
421 if (LiveFunctions.count(Use.F) || LiveValues.count(Use))
424 // We're maybe live otherwise, but remember that we must become live if
426 MaybeLiveUses.push_back(Use);
431 /// SurveyUse - This looks at a single use of an argument or return value
432 /// and determines if it should be alive or not. Adds this use to MaybeLiveUses
433 /// if it causes the used value to become MaybeLive.
435 /// RetValNum is the return value number to use when this use is used in a
436 /// return instruction. This is used in the recursion, you should always leave
438 DAE::Liveness DAE::SurveyUse(const Use *U,
439 UseVector &MaybeLiveUses, unsigned RetValNum) {
440 const User *V = U->getUser();
441 if (const ReturnInst *RI = dyn_cast<ReturnInst>(V)) {
442 // The value is returned from a function. It's only live when the
443 // function's return value is live. We use RetValNum here, for the case
444 // that U is really a use of an insertvalue instruction that uses the
446 RetOrArg Use = CreateRet(RI->getParent()->getParent(), RetValNum);
447 // We might be live, depending on the liveness of Use.
448 return MarkIfNotLive(Use, MaybeLiveUses);
450 if (const InsertValueInst *IV = dyn_cast<InsertValueInst>(V)) {
451 if (U->getOperandNo() != InsertValueInst::getAggregateOperandIndex()
453 // The use we are examining is inserted into an aggregate. Our liveness
454 // depends on all uses of that aggregate, but if it is used as a return
455 // value, only index at which we were inserted counts.
456 RetValNum = *IV->idx_begin();
458 // Note that if we are used as the aggregate operand to the insertvalue,
459 // we don't change RetValNum, but do survey all our uses.
461 Liveness Result = MaybeLive;
462 for (const Use &UU : IV->uses()) {
463 Result = SurveyUse(&UU, MaybeLiveUses, RetValNum);
470 if (ImmutableCallSite CS = V) {
471 const Function *F = CS.getCalledFunction();
473 // Used in a direct call.
475 // Find the argument number. We know for sure that this use is an
476 // argument, since if it was the function argument this would be an
477 // indirect call and the we know can't be looking at a value of the
478 // label type (for the invoke instruction).
479 unsigned ArgNo = CS.getArgumentNo(U);
481 if (ArgNo >= F->getFunctionType()->getNumParams())
482 // The value is passed in through a vararg! Must be live.
485 assert(CS.getArgument(ArgNo)
486 == CS->getOperand(U->getOperandNo())
487 && "Argument is not where we expected it");
489 // Value passed to a normal call. It's only live when the corresponding
490 // argument to the called function turns out live.
491 RetOrArg Use = CreateArg(F, ArgNo);
492 return MarkIfNotLive(Use, MaybeLiveUses);
495 // Used in any other way? Value must be live.
499 /// SurveyUses - This looks at all the uses of the given value
500 /// Returns the Liveness deduced from the uses of this value.
502 /// Adds all uses that cause the result to be MaybeLive to MaybeLiveRetUses. If
503 /// the result is Live, MaybeLiveUses might be modified but its content should
504 /// be ignored (since it might not be complete).
505 DAE::Liveness DAE::SurveyUses(const Value *V, UseVector &MaybeLiveUses) {
506 // Assume it's dead (which will only hold if there are no uses at all..).
507 Liveness Result = MaybeLive;
509 for (const Use &U : V->uses()) {
510 Result = SurveyUse(&U, MaybeLiveUses);
517 // SurveyFunction - This performs the initial survey of the specified function,
518 // checking out whether or not it uses any of its incoming arguments or whether
519 // any callers use the return value. This fills in the LiveValues set and Uses
522 // We consider arguments of non-internal functions to be intrinsically alive as
523 // well as arguments to functions which have their "address taken".
525 void DAE::SurveyFunction(const Function &F) {
526 // Functions with inalloca parameters are expecting args in a particular
527 // register and memory layout.
528 if (F.getAttributes().hasAttrSomewhere(Attribute::InAlloca)) {
533 unsigned RetCount = NumRetVals(&F);
534 // Assume all return values are dead
535 typedef SmallVector<Liveness, 5> RetVals;
536 RetVals RetValLiveness(RetCount, MaybeLive);
538 typedef SmallVector<UseVector, 5> RetUses;
539 // These vectors map each return value to the uses that make it MaybeLive, so
540 // we can add those to the Uses map if the return value really turns out to be
541 // MaybeLive. Initialized to a list of RetCount empty lists.
542 RetUses MaybeLiveRetUses(RetCount);
544 for (Function::const_iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
545 if (const ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()))
546 if (RI->getNumOperands() != 0 && RI->getOperand(0)->getType()
547 != F.getFunctionType()->getReturnType()) {
548 // We don't support old style multiple return values.
553 if (!F.hasLocalLinkage() && (!ShouldHackArguments() || F.isIntrinsic())) {
558 DEBUG(dbgs() << "DAE - Inspecting callers for fn: " << F.getName() << "\n");
559 // Keep track of the number of live retvals, so we can skip checks once all
560 // of them turn out to be live.
561 unsigned NumLiveRetVals = 0;
562 Type *STy = dyn_cast<StructType>(F.getReturnType());
563 // Loop all uses of the function.
564 for (const Use &U : F.uses()) {
565 // If the function is PASSED IN as an argument, its address has been
567 ImmutableCallSite CS(U.getUser());
568 if (!CS || !CS.isCallee(&U)) {
573 // If this use is anything other than a call site, the function is alive.
574 const Instruction *TheCall = CS.getInstruction();
575 if (!TheCall) { // Not a direct call site?
580 // If we end up here, we are looking at a direct call to our function.
582 // Now, check how our return value(s) is/are used in this caller. Don't
583 // bother checking return values if all of them are live already.
584 if (NumLiveRetVals != RetCount) {
586 // Check all uses of the return value.
587 for (const User *U : TheCall->users()) {
588 const ExtractValueInst *Ext = dyn_cast<ExtractValueInst>(U);
589 if (Ext && Ext->hasIndices()) {
590 // This use uses a part of our return value, survey the uses of
591 // that part and store the results for this index only.
592 unsigned Idx = *Ext->idx_begin();
593 if (RetValLiveness[Idx] != Live) {
594 RetValLiveness[Idx] = SurveyUses(Ext, MaybeLiveRetUses[Idx]);
595 if (RetValLiveness[Idx] == Live)
599 // Used by something else than extractvalue. Mark all return
601 for (unsigned i = 0; i != RetCount; ++i )
602 RetValLiveness[i] = Live;
603 NumLiveRetVals = RetCount;
608 // Single return value
609 RetValLiveness[0] = SurveyUses(TheCall, MaybeLiveRetUses[0]);
610 if (RetValLiveness[0] == Live)
611 NumLiveRetVals = RetCount;
616 // Now we've inspected all callers, record the liveness of our return values.
617 for (unsigned i = 0; i != RetCount; ++i)
618 MarkValue(CreateRet(&F, i), RetValLiveness[i], MaybeLiveRetUses[i]);
620 DEBUG(dbgs() << "DAE - Inspecting args for fn: " << F.getName() << "\n");
622 // Now, check all of our arguments.
624 UseVector MaybeLiveArgUses;
625 for (Function::const_arg_iterator AI = F.arg_begin(),
626 E = F.arg_end(); AI != E; ++AI, ++i) {
628 if (F.getFunctionType()->isVarArg()) {
629 // Variadic functions will already have a va_arg function expanded inside
630 // them, making them potentially very sensitive to ABI changes resulting
631 // from removing arguments entirely, so don't. For example AArch64 handles
632 // register and stack HFAs very differently, and this is reflected in the
633 // IR which has already been generated.
636 // See what the effect of this use is (recording any uses that cause
637 // MaybeLive in MaybeLiveArgUses).
638 Result = SurveyUses(AI, MaybeLiveArgUses);
642 MarkValue(CreateArg(&F, i), Result, MaybeLiveArgUses);
643 // Clear the vector again for the next iteration.
644 MaybeLiveArgUses.clear();
648 /// MarkValue - This function marks the liveness of RA depending on L. If L is
649 /// MaybeLive, it also takes all uses in MaybeLiveUses and records them in Uses,
650 /// such that RA will be marked live if any use in MaybeLiveUses gets marked
652 void DAE::MarkValue(const RetOrArg &RA, Liveness L,
653 const UseVector &MaybeLiveUses) {
655 case Live: MarkLive(RA); break;
658 // Note any uses of this value, so this return value can be
659 // marked live whenever one of the uses becomes live.
660 for (UseVector::const_iterator UI = MaybeLiveUses.begin(),
661 UE = MaybeLiveUses.end(); UI != UE; ++UI)
662 Uses.insert(std::make_pair(*UI, RA));
668 /// MarkLive - Mark the given Function as alive, meaning that it cannot be
669 /// changed in any way. Additionally,
670 /// mark any values that are used as this function's parameters or by its return
671 /// values (according to Uses) live as well.
672 void DAE::MarkLive(const Function &F) {
673 DEBUG(dbgs() << "DAE - Intrinsically live fn: " << F.getName() << "\n");
674 // Mark the function as live.
675 LiveFunctions.insert(&F);
676 // Mark all arguments as live.
677 for (unsigned i = 0, e = F.arg_size(); i != e; ++i)
678 PropagateLiveness(CreateArg(&F, i));
679 // Mark all return values as live.
680 for (unsigned i = 0, e = NumRetVals(&F); i != e; ++i)
681 PropagateLiveness(CreateRet(&F, i));
684 /// MarkLive - Mark the given return value or argument as live. Additionally,
685 /// mark any values that are used by this value (according to Uses) live as
687 void DAE::MarkLive(const RetOrArg &RA) {
688 if (LiveFunctions.count(RA.F))
689 return; // Function was already marked Live.
691 if (!LiveValues.insert(RA).second)
692 return; // We were already marked Live.
694 DEBUG(dbgs() << "DAE - Marking " << RA.getDescription() << " live\n");
695 PropagateLiveness(RA);
698 /// PropagateLiveness - Given that RA is a live value, propagate it's liveness
699 /// to any other values it uses (according to Uses).
700 void DAE::PropagateLiveness(const RetOrArg &RA) {
701 // We don't use upper_bound (or equal_range) here, because our recursive call
702 // to ourselves is likely to cause the upper_bound (which is the first value
703 // not belonging to RA) to become erased and the iterator invalidated.
704 UseMap::iterator Begin = Uses.lower_bound(RA);
705 UseMap::iterator E = Uses.end();
707 for (I = Begin; I != E && I->first == RA; ++I)
710 // Erase RA from the Uses map (from the lower bound to wherever we ended up
712 Uses.erase(Begin, I);
715 // RemoveDeadStuffFromFunction - Remove any arguments and return values from F
716 // that are not in LiveValues. Transform the function and all of the callees of
717 // the function to not have these arguments and return values.
719 bool DAE::RemoveDeadStuffFromFunction(Function *F) {
720 // Don't modify fully live functions
721 if (LiveFunctions.count(F))
724 // Start by computing a new prototype for the function, which is the same as
725 // the old function, but has fewer arguments and a different return type.
726 FunctionType *FTy = F->getFunctionType();
727 std::vector<Type*> Params;
729 // Keep track of if we have a live 'returned' argument
730 bool HasLiveReturnedArg = false;
732 // Set up to build a new list of parameter attributes.
733 SmallVector<AttributeSet, 8> AttributesVec;
734 const AttributeSet &PAL = F->getAttributes();
736 // Remember which arguments are still alive.
737 SmallVector<bool, 10> ArgAlive(FTy->getNumParams(), false);
738 // Construct the new parameter list from non-dead arguments. Also construct
739 // a new set of parameter attributes to correspond. Skip the first parameter
740 // attribute, since that belongs to the return value.
742 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
744 RetOrArg Arg = CreateArg(F, i);
745 if (LiveValues.erase(Arg)) {
746 Params.push_back(I->getType());
749 // Get the original parameter attributes (skipping the first one, that is
750 // for the return value.
751 if (PAL.hasAttributes(i + 1)) {
752 AttrBuilder B(PAL, i + 1);
753 if (B.contains(Attribute::Returned))
754 HasLiveReturnedArg = true;
756 push_back(AttributeSet::get(F->getContext(), Params.size(), B));
759 ++NumArgumentsEliminated;
760 DEBUG(dbgs() << "DAE - Removing argument " << i << " (" << I->getName()
761 << ") from " << F->getName() << "\n");
765 // Find out the new return value.
766 Type *RetTy = FTy->getReturnType();
768 unsigned RetCount = NumRetVals(F);
770 // -1 means unused, other numbers are the new index
771 SmallVector<int, 5> NewRetIdxs(RetCount, -1);
772 std::vector<Type*> RetTypes;
774 // If there is a function with a live 'returned' argument but a dead return
775 // value, then there are two possible actions:
776 // 1) Eliminate the return value and take off the 'returned' attribute on the
778 // 2) Retain the 'returned' attribute and treat the return value (but not the
779 // entire function) as live so that it is not eliminated.
781 // It's not clear in the general case which option is more profitable because,
782 // even in the absence of explicit uses of the return value, code generation
783 // is free to use the 'returned' attribute to do things like eliding
784 // save/restores of registers across calls. Whether or not this happens is
785 // target and ABI-specific as well as depending on the amount of register
786 // pressure, so there's no good way for an IR-level pass to figure this out.
788 // Fortunately, the only places where 'returned' is currently generated by
789 // the FE are places where 'returned' is basically free and almost always a
790 // performance win, so the second option can just be used always for now.
792 // This should be revisited if 'returned' is ever applied more liberally.
793 if (RetTy->isVoidTy() || HasLiveReturnedArg) {
796 StructType *STy = dyn_cast<StructType>(RetTy);
798 // Look at each of the original return values individually.
799 for (unsigned i = 0; i != RetCount; ++i) {
800 RetOrArg Ret = CreateRet(F, i);
801 if (LiveValues.erase(Ret)) {
802 RetTypes.push_back(STy->getElementType(i));
803 NewRetIdxs[i] = RetTypes.size() - 1;
805 ++NumRetValsEliminated;
806 DEBUG(dbgs() << "DAE - Removing return value " << i << " from "
807 << F->getName() << "\n");
811 // We used to return a single value.
812 if (LiveValues.erase(CreateRet(F, 0))) {
813 RetTypes.push_back(RetTy);
816 DEBUG(dbgs() << "DAE - Removing return value from " << F->getName()
818 ++NumRetValsEliminated;
820 if (RetTypes.size() > 1)
821 // More than one return type? Return a struct with them. Also, if we used
822 // to return a struct and didn't change the number of return values,
823 // return a struct again. This prevents changing {something} into
824 // something and {} into void.
825 // Make the new struct packed if we used to return a packed struct
827 NRetTy = StructType::get(STy->getContext(), RetTypes, STy->isPacked());
828 else if (RetTypes.size() == 1)
829 // One return type? Just a simple value then, but only if we didn't use to
830 // return a struct with that simple value before.
831 NRetTy = RetTypes.front();
832 else if (RetTypes.size() == 0)
833 // No return types? Make it void, but only if we didn't use to return {}.
834 NRetTy = Type::getVoidTy(F->getContext());
837 assert(NRetTy && "No new return type found?");
839 // The existing function return attributes.
840 AttributeSet RAttrs = PAL.getRetAttributes();
842 // Remove any incompatible attributes, but only if we removed all return
843 // values. Otherwise, ensure that we don't have any conflicting attributes
844 // here. Currently, this should not be possible, but special handling might be
845 // required when new return value attributes are added.
846 if (NRetTy->isVoidTy())
848 AttributeSet::get(NRetTy->getContext(), AttributeSet::ReturnIndex,
849 AttrBuilder(RAttrs, AttributeSet::ReturnIndex).
850 removeAttributes(AttributeFuncs::
851 typeIncompatible(NRetTy, AttributeSet::ReturnIndex),
852 AttributeSet::ReturnIndex));
854 assert(!AttrBuilder(RAttrs, AttributeSet::ReturnIndex).
855 hasAttributes(AttributeFuncs::
856 typeIncompatible(NRetTy, AttributeSet::ReturnIndex),
857 AttributeSet::ReturnIndex) &&
858 "Return attributes no longer compatible?");
860 if (RAttrs.hasAttributes(AttributeSet::ReturnIndex))
861 AttributesVec.push_back(AttributeSet::get(NRetTy->getContext(), RAttrs));
863 if (PAL.hasAttributes(AttributeSet::FunctionIndex))
864 AttributesVec.push_back(AttributeSet::get(F->getContext(),
865 PAL.getFnAttributes()));
867 // Reconstruct the AttributesList based on the vector we constructed.
868 AttributeSet NewPAL = AttributeSet::get(F->getContext(), AttributesVec);
870 // Create the new function type based on the recomputed parameters.
871 FunctionType *NFTy = FunctionType::get(NRetTy, Params, FTy->isVarArg());
877 // Create the new function body and insert it into the module...
878 Function *NF = Function::Create(NFTy, F->getLinkage());
879 NF->copyAttributesFrom(F);
880 NF->setAttributes(NewPAL);
881 // Insert the new function before the old function, so we won't be processing
883 F->getParent()->getFunctionList().insert(F, NF);
886 // Loop over all of the callers of the function, transforming the call sites
887 // to pass in a smaller number of arguments into the new function.
889 std::vector<Value*> Args;
890 while (!F->use_empty()) {
891 CallSite CS(F->user_back());
892 Instruction *Call = CS.getInstruction();
894 AttributesVec.clear();
895 const AttributeSet &CallPAL = CS.getAttributes();
897 // The call return attributes.
898 AttributeSet RAttrs = CallPAL.getRetAttributes();
900 // Adjust in case the function was changed to return void.
902 AttributeSet::get(NF->getContext(), AttributeSet::ReturnIndex,
903 AttrBuilder(RAttrs, AttributeSet::ReturnIndex).
904 removeAttributes(AttributeFuncs::
905 typeIncompatible(NF->getReturnType(),
906 AttributeSet::ReturnIndex),
907 AttributeSet::ReturnIndex));
908 if (RAttrs.hasAttributes(AttributeSet::ReturnIndex))
909 AttributesVec.push_back(AttributeSet::get(NF->getContext(), RAttrs));
911 // Declare these outside of the loops, so we can reuse them for the second
912 // loop, which loops the varargs.
913 CallSite::arg_iterator I = CS.arg_begin();
915 // Loop over those operands, corresponding to the normal arguments to the
916 // original function, and add those that are still alive.
917 for (unsigned e = FTy->getNumParams(); i != e; ++I, ++i)
920 // Get original parameter attributes, but skip return attributes.
921 if (CallPAL.hasAttributes(i + 1)) {
922 AttrBuilder B(CallPAL, i + 1);
923 // If the return type has changed, then get rid of 'returned' on the
924 // call site. The alternative is to make all 'returned' attributes on
925 // call sites keep the return value alive just like 'returned'
926 // attributes on function declaration but it's less clearly a win
927 // and this is not an expected case anyway
928 if (NRetTy != RetTy && B.contains(Attribute::Returned))
929 B.removeAttribute(Attribute::Returned);
931 push_back(AttributeSet::get(F->getContext(), Args.size(), B));
935 // Push any varargs arguments on the list. Don't forget their attributes.
936 for (CallSite::arg_iterator E = CS.arg_end(); I != E; ++I, ++i) {
938 if (CallPAL.hasAttributes(i + 1)) {
939 AttrBuilder B(CallPAL, i + 1);
941 push_back(AttributeSet::get(F->getContext(), Args.size(), B));
945 if (CallPAL.hasAttributes(AttributeSet::FunctionIndex))
946 AttributesVec.push_back(AttributeSet::get(Call->getContext(),
947 CallPAL.getFnAttributes()));
949 // Reconstruct the AttributesList based on the vector we constructed.
950 AttributeSet NewCallPAL = AttributeSet::get(F->getContext(), AttributesVec);
953 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
954 New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
956 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
957 cast<InvokeInst>(New)->setAttributes(NewCallPAL);
959 New = CallInst::Create(NF, Args, "", Call);
960 cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
961 cast<CallInst>(New)->setAttributes(NewCallPAL);
962 if (cast<CallInst>(Call)->isTailCall())
963 cast<CallInst>(New)->setTailCall();
965 New->setDebugLoc(Call->getDebugLoc());
969 if (!Call->use_empty()) {
970 if (New->getType() == Call->getType()) {
971 // Return type not changed? Just replace users then.
972 Call->replaceAllUsesWith(New);
974 } else if (New->getType()->isVoidTy()) {
975 // Our return value has uses, but they will get removed later on.
976 // Replace by null for now.
977 if (!Call->getType()->isX86_MMXTy())
978 Call->replaceAllUsesWith(Constant::getNullValue(Call->getType()));
980 assert(RetTy->isStructTy() &&
981 "Return type changed, but not into a void. The old return type"
982 " must have been a struct!");
983 Instruction *InsertPt = Call;
984 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
985 BasicBlock::iterator IP = II->getNormalDest()->begin();
986 while (isa<PHINode>(IP)) ++IP;
990 // We used to return a struct. Instead of doing smart stuff with all the
991 // uses of this struct, we will just rebuild it using
992 // extract/insertvalue chaining and let instcombine clean that up.
994 // Start out building up our return value from undef
995 Value *RetVal = UndefValue::get(RetTy);
996 for (unsigned i = 0; i != RetCount; ++i)
997 if (NewRetIdxs[i] != -1) {
999 if (RetTypes.size() > 1)
1000 // We are still returning a struct, so extract the value from our
1002 V = ExtractValueInst::Create(New, NewRetIdxs[i], "newret",
1005 // We are now returning a single element, so just insert that
1007 // Insert the value at the old position
1008 RetVal = InsertValueInst::Create(RetVal, V, i, "oldret", InsertPt);
1010 // Now, replace all uses of the old call instruction with the return
1012 Call->replaceAllUsesWith(RetVal);
1013 New->takeName(Call);
1017 // Finally, remove the old call from the program, reducing the use-count of
1019 Call->eraseFromParent();
1022 // Since we have now created the new function, splice the body of the old
1023 // function right into the new function, leaving the old rotting hulk of the
1025 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
1027 // Loop over the argument list, transferring uses of the old arguments over to
1028 // the new arguments, also transferring over the names as well.
1030 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(),
1031 I2 = NF->arg_begin(); I != E; ++I, ++i)
1033 // If this is a live argument, move the name and users over to the new
1035 I->replaceAllUsesWith(I2);
1039 // If this argument is dead, replace any uses of it with null constants
1040 // (these are guaranteed to become unused later on).
1041 if (!I->getType()->isX86_MMXTy())
1042 I->replaceAllUsesWith(Constant::getNullValue(I->getType()));
1045 // If we change the return value of the function we must rewrite any return
1046 // instructions. Check this now.
1047 if (F->getReturnType() != NF->getReturnType())
1048 for (Function::iterator BB = NF->begin(), E = NF->end(); BB != E; ++BB)
1049 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
1052 if (NFTy->getReturnType()->isVoidTy()) {
1055 assert (RetTy->isStructTy());
1056 // The original return value was a struct, insert
1057 // extractvalue/insertvalue chains to extract only the values we need
1058 // to return and insert them into our new result.
1059 // This does generate messy code, but we'll let it to instcombine to
1061 Value *OldRet = RI->getOperand(0);
1062 // Start out building up our return value from undef
1063 RetVal = UndefValue::get(NRetTy);
1064 for (unsigned i = 0; i != RetCount; ++i)
1065 if (NewRetIdxs[i] != -1) {
1066 ExtractValueInst *EV = ExtractValueInst::Create(OldRet, i,
1068 if (RetTypes.size() > 1) {
1069 // We're still returning a struct, so reinsert the value into
1070 // our new return value at the new index
1072 RetVal = InsertValueInst::Create(RetVal, EV, NewRetIdxs[i],
1075 // We are now only returning a simple value, so just return the
1081 // Replace the return instruction with one returning the new return
1082 // value (possibly 0 if we became void).
1083 ReturnInst::Create(F->getContext(), RetVal, RI);
1084 BB->getInstList().erase(RI);
1087 // Patch the pointer to LLVM function in debug info descriptor.
1088 FunctionDIMap::iterator DI = FunctionDIs.find(F);
1089 if (DI != FunctionDIs.end())
1090 DI->second.replaceFunction(NF);
1092 // Now that the old function is dead, delete it.
1093 F->eraseFromParent();
1098 bool DAE::runOnModule(Module &M) {
1099 bool Changed = false;
1101 // Collect debug info descriptors for functions.
1102 CollectFunctionDIs(M);
1104 // First pass: Do a simple check to see if any functions can have their "..."
1105 // removed. We can do this if they never call va_start. This loop cannot be
1106 // fused with the next loop, because deleting a function invalidates
1107 // information computed while surveying other functions.
1108 DEBUG(dbgs() << "DAE - Deleting dead varargs\n");
1109 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1111 if (F.getFunctionType()->isVarArg())
1112 Changed |= DeleteDeadVarargs(F);
1115 // Second phase:loop through the module, determining which arguments are live.
1116 // We assume all arguments are dead unless proven otherwise (allowing us to
1117 // determine that dead arguments passed into recursive functions are dead).
1119 DEBUG(dbgs() << "DAE - Determining liveness\n");
1120 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
1123 // Now, remove all dead arguments and return values from each function in
1125 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1126 // Increment now, because the function will probably get removed (ie.
1127 // replaced by a new one).
1129 Changed |= RemoveDeadStuffFromFunction(F);
1132 // Finally, look for any unused parameters in functions with non-local
1133 // linkage and replace the passed in parameters with undef.
1134 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
1137 Changed |= RemoveDeadArgumentsFromCallers(F);