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/DeadArgumentElimination.h"
21 #include "llvm/ADT/SmallVector.h"
22 #include "llvm/ADT/Statistic.h"
23 #include "llvm/ADT/StringExtras.h"
24 #include "llvm/IR/CallSite.h"
25 #include "llvm/IR/CallingConv.h"
26 #include "llvm/IR/Constant.h"
27 #include "llvm/IR/DIBuilder.h"
28 #include "llvm/IR/DebugInfo.h"
29 #include "llvm/IR/DerivedTypes.h"
30 #include "llvm/IR/Instructions.h"
31 #include "llvm/IR/IntrinsicInst.h"
32 #include "llvm/IR/LLVMContext.h"
33 #include "llvm/IR/Module.h"
34 #include "llvm/Pass.h"
35 #include "llvm/Support/Debug.h"
36 #include "llvm/Support/raw_ostream.h"
37 #include "llvm/Transforms/IPO.h"
38 #include "llvm/Transforms/Utils/BasicBlockUtils.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 {
54 // DAH uses this to specify a different ID.
55 explicit DAE(char &ID) : ModulePass(ID) {}
58 static char ID; // Pass identification, replacement for typeid
59 DAE() : ModulePass(ID) {
60 initializeDAEPass(*PassRegistry::getPassRegistry());
63 bool runOnModule(Module &M) override {
66 DeadArgumentEliminationPass DAEP(ShouldHackArguments());
67 ModuleAnalysisManager DummyMAM;
68 PreservedAnalyses PA = DAEP.run(M, DummyMAM);
69 return !PA.areAllPreserved();
72 virtual bool ShouldHackArguments() const { return false; }
78 INITIALIZE_PASS(DAE, "deadargelim", "Dead Argument Elimination", false, false)
81 /// DAH - DeadArgumentHacking pass - Same as dead argument elimination, but
82 /// deletes arguments to functions which are external. This is only for use
84 struct DAH : public DAE {
88 bool ShouldHackArguments() const override { return true; }
93 INITIALIZE_PASS(DAH, "deadarghaX0r",
94 "Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)",
97 /// createDeadArgEliminationPass - This pass removes arguments from functions
98 /// which are not used by the body of the function.
100 ModulePass *llvm::createDeadArgEliminationPass() { return new DAE(); }
101 ModulePass *llvm::createDeadArgHackingPass() { return new DAH(); }
103 /// DeleteDeadVarargs - If this is an function that takes a ... list, and if
104 /// llvm.vastart is never called, the varargs list is dead for the function.
105 bool DeadArgumentEliminationPass::DeleteDeadVarargs(Function &Fn) {
106 assert(Fn.getFunctionType()->isVarArg() && "Function isn't varargs!");
107 if (Fn.isDeclaration() || !Fn.hasLocalLinkage()) return false;
109 // Ensure that the function is only directly called.
110 if (Fn.hasAddressTaken())
113 // Don't touch naked functions. The assembly might be using an argument, or
114 // otherwise rely on the frame layout in a way that this analysis will not
116 if (Fn.hasFnAttribute(Attribute::Naked)) {
120 // Okay, we know we can transform this function if safe. Scan its body
121 // looking for calls marked musttail or calls to llvm.vastart.
122 for (BasicBlock &BB : Fn) {
123 for (Instruction &I : BB) {
124 CallInst *CI = dyn_cast<CallInst>(&I);
127 if (CI->isMustTailCall())
129 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(CI)) {
130 if (II->getIntrinsicID() == Intrinsic::vastart)
136 // If we get here, there are no calls to llvm.vastart in the function body,
137 // remove the "..." and adjust all the calls.
139 // Start by computing a new prototype for the function, which is the same as
140 // the old function, but doesn't have isVarArg set.
141 FunctionType *FTy = Fn.getFunctionType();
143 std::vector<Type*> Params(FTy->param_begin(), FTy->param_end());
144 FunctionType *NFTy = FunctionType::get(FTy->getReturnType(),
146 unsigned NumArgs = Params.size();
148 // Create the new function body and insert it into the module...
149 Function *NF = Function::Create(NFTy, Fn.getLinkage());
150 NF->copyAttributesFrom(&Fn);
151 NF->setComdat(Fn.getComdat());
152 Fn.getParent()->getFunctionList().insert(Fn.getIterator(), NF);
155 // Loop over all of the callers of the function, transforming the call sites
156 // to pass in a smaller number of arguments into the new function.
158 std::vector<Value*> Args;
159 for (Value::user_iterator I = Fn.user_begin(), E = Fn.user_end(); I != E; ) {
163 Instruction *Call = CS.getInstruction();
165 // Pass all the same arguments.
166 Args.assign(CS.arg_begin(), CS.arg_begin() + NumArgs);
168 // Drop any attributes that were on the vararg arguments.
169 AttributeList PAL = CS.getAttributes();
170 if (!PAL.isEmpty() && PAL.getSlotIndex(PAL.getNumSlots() - 1) > NumArgs) {
171 SmallVector<AttributeList, 8> AttributesVec;
172 for (unsigned i = 0; PAL.getSlotIndex(i) <= NumArgs; ++i)
173 AttributesVec.push_back(PAL.getSlotAttributes(i));
174 if (PAL.hasAttributes(AttributeList::FunctionIndex))
175 AttributesVec.push_back(AttributeList::get(Fn.getContext(),
176 AttributeList::FunctionIndex,
177 PAL.getFnAttributes()));
178 PAL = AttributeList::get(Fn.getContext(), AttributesVec);
181 SmallVector<OperandBundleDef, 1> OpBundles;
182 CS.getOperandBundlesAsDefs(OpBundles);
185 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
186 NewCS = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
187 Args, OpBundles, "", Call);
189 NewCS = CallInst::Create(NF, Args, OpBundles, "", Call);
190 cast<CallInst>(NewCS.getInstruction())
191 ->setTailCallKind(cast<CallInst>(Call)->getTailCallKind());
193 NewCS.setCallingConv(CS.getCallingConv());
194 NewCS.setAttributes(PAL);
195 NewCS->setDebugLoc(Call->getDebugLoc());
197 if (Call->extractProfTotalWeight(W))
198 NewCS->setProfWeight(W);
202 if (!Call->use_empty())
203 Call->replaceAllUsesWith(NewCS.getInstruction());
205 NewCS->takeName(Call);
207 // Finally, remove the old call from the program, reducing the use-count of
209 Call->eraseFromParent();
212 // Since we have now created the new function, splice the body of the old
213 // function right into the new function, leaving the old rotting hulk of the
215 NF->getBasicBlockList().splice(NF->begin(), Fn.getBasicBlockList());
217 // Loop over the argument list, transferring uses of the old arguments over to
218 // the new arguments, also transferring over the names as well. While we're at
219 // it, remove the dead arguments from the DeadArguments list.
221 for (Function::arg_iterator I = Fn.arg_begin(), E = Fn.arg_end(),
222 I2 = NF->arg_begin(); I != E; ++I, ++I2) {
223 // Move the name and users over to the new version.
224 I->replaceAllUsesWith(&*I2);
228 // Patch the pointer to LLVM function in debug info descriptor.
229 NF->setSubprogram(Fn.getSubprogram());
231 // Fix up any BlockAddresses that refer to the function.
232 Fn.replaceAllUsesWith(ConstantExpr::getBitCast(NF, Fn.getType()));
233 // Delete the bitcast that we just created, so that NF does not
234 // appear to be address-taken.
235 NF->removeDeadConstantUsers();
236 // Finally, nuke the old function.
237 Fn.eraseFromParent();
241 /// RemoveDeadArgumentsFromCallers - Checks if the given function has any
242 /// arguments that are unused, and changes the caller parameters to be undefined
244 bool DeadArgumentEliminationPass::RemoveDeadArgumentsFromCallers(Function &Fn) {
245 // We cannot change the arguments if this TU does not define the function or
246 // if the linker may choose a function body from another TU, even if the
247 // nominal linkage indicates that other copies of the function have the same
248 // semantics. In the below example, the dead load from %p may not have been
249 // eliminated from the linker-chosen copy of f, so replacing %p with undef
250 // in callers may introduce undefined behavior.
252 // define linkonce_odr void @f(i32* %p) {
256 if (!Fn.hasExactDefinition())
259 // Functions with local linkage should already have been handled, except the
260 // fragile (variadic) ones which we can improve here.
261 if (Fn.hasLocalLinkage() && !Fn.getFunctionType()->isVarArg())
264 // Don't touch naked functions. The assembly might be using an argument, or
265 // otherwise rely on the frame layout in a way that this analysis will not
267 if (Fn.hasFnAttribute(Attribute::Naked))
273 SmallVector<unsigned, 8> UnusedArgs;
274 for (Argument &Arg : Fn.args()) {
275 if (!Arg.hasSwiftErrorAttr() && Arg.use_empty() && !Arg.hasByValOrInAllocaAttr())
276 UnusedArgs.push_back(Arg.getArgNo());
279 if (UnusedArgs.empty())
282 bool Changed = false;
284 for (Use &U : Fn.uses()) {
285 CallSite CS(U.getUser());
286 if (!CS || !CS.isCallee(&U))
289 // Now go through all unused args and replace them with "undef".
290 for (unsigned I = 0, E = UnusedArgs.size(); I != E; ++I) {
291 unsigned ArgNo = UnusedArgs[I];
293 Value *Arg = CS.getArgument(ArgNo);
294 CS.setArgument(ArgNo, UndefValue::get(Arg->getType()));
295 ++NumArgumentsReplacedWithUndef;
303 /// Convenience function that returns the number of return values. It returns 0
304 /// for void functions and 1 for functions not returning a struct. It returns
305 /// the number of struct elements for functions returning a struct.
306 static unsigned NumRetVals(const Function *F) {
307 Type *RetTy = F->getReturnType();
308 if (RetTy->isVoidTy())
310 else if (StructType *STy = dyn_cast<StructType>(RetTy))
311 return STy->getNumElements();
312 else if (ArrayType *ATy = dyn_cast<ArrayType>(RetTy))
313 return ATy->getNumElements();
318 /// Returns the sub-type a function will return at a given Idx. Should
319 /// correspond to the result type of an ExtractValue instruction executed with
320 /// just that one Idx (i.e. only top-level structure is considered).
321 static Type *getRetComponentType(const Function *F, unsigned Idx) {
322 Type *RetTy = F->getReturnType();
323 assert(!RetTy->isVoidTy() && "void type has no subtype");
325 if (StructType *STy = dyn_cast<StructType>(RetTy))
326 return STy->getElementType(Idx);
327 else if (ArrayType *ATy = dyn_cast<ArrayType>(RetTy))
328 return ATy->getElementType();
333 /// MarkIfNotLive - This checks Use for liveness in LiveValues. If Use is not
334 /// live, it adds Use to the MaybeLiveUses argument. Returns the determined
336 DeadArgumentEliminationPass::Liveness
337 DeadArgumentEliminationPass::MarkIfNotLive(RetOrArg Use,
338 UseVector &MaybeLiveUses) {
339 // We're live if our use or its Function is already marked as live.
340 if (LiveFunctions.count(Use.F) || LiveValues.count(Use))
343 // We're maybe live otherwise, but remember that we must become live if
345 MaybeLiveUses.push_back(Use);
350 /// SurveyUse - This looks at a single use of an argument or return value
351 /// and determines if it should be alive or not. Adds this use to MaybeLiveUses
352 /// if it causes the used value to become MaybeLive.
354 /// RetValNum is the return value number to use when this use is used in a
355 /// return instruction. This is used in the recursion, you should always leave
357 DeadArgumentEliminationPass::Liveness
358 DeadArgumentEliminationPass::SurveyUse(const Use *U, UseVector &MaybeLiveUses,
359 unsigned RetValNum) {
360 const User *V = U->getUser();
361 if (const ReturnInst *RI = dyn_cast<ReturnInst>(V)) {
362 // The value is returned from a function. It's only live when the
363 // function's return value is live. We use RetValNum here, for the case
364 // that U is really a use of an insertvalue instruction that uses the
366 const Function *F = RI->getParent()->getParent();
367 if (RetValNum != -1U) {
368 RetOrArg Use = CreateRet(F, RetValNum);
369 // We might be live, depending on the liveness of Use.
370 return MarkIfNotLive(Use, MaybeLiveUses);
372 DeadArgumentEliminationPass::Liveness Result = MaybeLive;
373 for (unsigned i = 0; i < NumRetVals(F); ++i) {
374 RetOrArg Use = CreateRet(F, i);
375 // We might be live, depending on the liveness of Use. If any
376 // sub-value is live, then the entire value is considered live. This
377 // is a conservative choice, and better tracking is possible.
378 DeadArgumentEliminationPass::Liveness SubResult =
379 MarkIfNotLive(Use, MaybeLiveUses);
386 if (const InsertValueInst *IV = dyn_cast<InsertValueInst>(V)) {
387 if (U->getOperandNo() != InsertValueInst::getAggregateOperandIndex()
389 // The use we are examining is inserted into an aggregate. Our liveness
390 // depends on all uses of that aggregate, but if it is used as a return
391 // value, only index at which we were inserted counts.
392 RetValNum = *IV->idx_begin();
394 // Note that if we are used as the aggregate operand to the insertvalue,
395 // we don't change RetValNum, but do survey all our uses.
397 Liveness Result = MaybeLive;
398 for (const Use &UU : IV->uses()) {
399 Result = SurveyUse(&UU, MaybeLiveUses, RetValNum);
406 if (auto CS = ImmutableCallSite(V)) {
407 const Function *F = CS.getCalledFunction();
409 // Used in a direct call.
411 // The function argument is live if it is used as a bundle operand.
412 if (CS.isBundleOperand(U))
415 // Find the argument number. We know for sure that this use is an
416 // argument, since if it was the function argument this would be an
417 // indirect call and the we know can't be looking at a value of the
418 // label type (for the invoke instruction).
419 unsigned ArgNo = CS.getArgumentNo(U);
421 if (ArgNo >= F->getFunctionType()->getNumParams())
422 // The value is passed in through a vararg! Must be live.
425 assert(CS.getArgument(ArgNo)
426 == CS->getOperand(U->getOperandNo())
427 && "Argument is not where we expected it");
429 // Value passed to a normal call. It's only live when the corresponding
430 // argument to the called function turns out live.
431 RetOrArg Use = CreateArg(F, ArgNo);
432 return MarkIfNotLive(Use, MaybeLiveUses);
435 // Used in any other way? Value must be live.
439 /// SurveyUses - This looks at all the uses of the given value
440 /// Returns the Liveness deduced from the uses of this value.
442 /// Adds all uses that cause the result to be MaybeLive to MaybeLiveRetUses. If
443 /// the result is Live, MaybeLiveUses might be modified but its content should
444 /// be ignored (since it might not be complete).
445 DeadArgumentEliminationPass::Liveness
446 DeadArgumentEliminationPass::SurveyUses(const Value *V,
447 UseVector &MaybeLiveUses) {
448 // Assume it's dead (which will only hold if there are no uses at all..).
449 Liveness Result = MaybeLive;
451 for (const Use &U : V->uses()) {
452 Result = SurveyUse(&U, MaybeLiveUses);
459 // SurveyFunction - This performs the initial survey of the specified function,
460 // checking out whether or not it uses any of its incoming arguments or whether
461 // any callers use the return value. This fills in the LiveValues set and Uses
464 // We consider arguments of non-internal functions to be intrinsically alive as
465 // well as arguments to functions which have their "address taken".
467 void DeadArgumentEliminationPass::SurveyFunction(const Function &F) {
468 // Functions with inalloca parameters are expecting args in a particular
469 // register and memory layout.
470 if (F.getAttributes().hasAttrSomewhere(Attribute::InAlloca)) {
475 // Don't touch naked functions. The assembly might be using an argument, or
476 // otherwise rely on the frame layout in a way that this analysis will not
478 if (F.hasFnAttribute(Attribute::Naked)) {
483 unsigned RetCount = NumRetVals(&F);
484 // Assume all return values are dead
485 typedef SmallVector<Liveness, 5> RetVals;
486 RetVals RetValLiveness(RetCount, MaybeLive);
488 typedef SmallVector<UseVector, 5> RetUses;
489 // These vectors map each return value to the uses that make it MaybeLive, so
490 // we can add those to the Uses map if the return value really turns out to be
491 // MaybeLive. Initialized to a list of RetCount empty lists.
492 RetUses MaybeLiveRetUses(RetCount);
494 for (Function::const_iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
495 if (const ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()))
496 if (RI->getNumOperands() != 0 && RI->getOperand(0)->getType()
497 != F.getFunctionType()->getReturnType()) {
498 // We don't support old style multiple return values.
503 if (!F.hasLocalLinkage() && (!ShouldHackArguments || F.isIntrinsic())) {
508 DEBUG(dbgs() << "DeadArgumentEliminationPass - Inspecting callers for fn: "
509 << F.getName() << "\n");
510 // Keep track of the number of live retvals, so we can skip checks once all
511 // of them turn out to be live.
512 unsigned NumLiveRetVals = 0;
513 // Loop all uses of the function.
514 for (const Use &U : F.uses()) {
515 // If the function is PASSED IN as an argument, its address has been
517 ImmutableCallSite CS(U.getUser());
518 if (!CS || !CS.isCallee(&U)) {
523 // If this use is anything other than a call site, the function is alive.
524 const Instruction *TheCall = CS.getInstruction();
525 if (!TheCall) { // Not a direct call site?
530 // If we end up here, we are looking at a direct call to our function.
532 // Now, check how our return value(s) is/are used in this caller. Don't
533 // bother checking return values if all of them are live already.
534 if (NumLiveRetVals == RetCount)
537 // Check all uses of the return value.
538 for (const Use &U : TheCall->uses()) {
539 if (ExtractValueInst *Ext = dyn_cast<ExtractValueInst>(U.getUser())) {
540 // This use uses a part of our return value, survey the uses of
541 // that part and store the results for this index only.
542 unsigned Idx = *Ext->idx_begin();
543 if (RetValLiveness[Idx] != Live) {
544 RetValLiveness[Idx] = SurveyUses(Ext, MaybeLiveRetUses[Idx]);
545 if (RetValLiveness[Idx] == Live)
549 // Used by something else than extractvalue. Survey, but assume that the
550 // result applies to all sub-values.
551 UseVector MaybeLiveAggregateUses;
552 if (SurveyUse(&U, MaybeLiveAggregateUses) == Live) {
553 NumLiveRetVals = RetCount;
554 RetValLiveness.assign(RetCount, Live);
557 for (unsigned i = 0; i != RetCount; ++i) {
558 if (RetValLiveness[i] != Live)
559 MaybeLiveRetUses[i].append(MaybeLiveAggregateUses.begin(),
560 MaybeLiveAggregateUses.end());
567 // Now we've inspected all callers, record the liveness of our return values.
568 for (unsigned i = 0; i != RetCount; ++i)
569 MarkValue(CreateRet(&F, i), RetValLiveness[i], MaybeLiveRetUses[i]);
571 DEBUG(dbgs() << "DeadArgumentEliminationPass - Inspecting args for fn: "
572 << F.getName() << "\n");
574 // Now, check all of our arguments.
576 UseVector MaybeLiveArgUses;
577 for (Function::const_arg_iterator AI = F.arg_begin(),
578 E = F.arg_end(); AI != E; ++AI, ++i) {
580 if (F.getFunctionType()->isVarArg()) {
581 // Variadic functions will already have a va_arg function expanded inside
582 // them, making them potentially very sensitive to ABI changes resulting
583 // from removing arguments entirely, so don't. For example AArch64 handles
584 // register and stack HFAs very differently, and this is reflected in the
585 // IR which has already been generated.
588 // See what the effect of this use is (recording any uses that cause
589 // MaybeLive in MaybeLiveArgUses).
590 Result = SurveyUses(&*AI, MaybeLiveArgUses);
594 MarkValue(CreateArg(&F, i), Result, MaybeLiveArgUses);
595 // Clear the vector again for the next iteration.
596 MaybeLiveArgUses.clear();
600 /// MarkValue - This function marks the liveness of RA depending on L. If L is
601 /// MaybeLive, it also takes all uses in MaybeLiveUses and records them in Uses,
602 /// such that RA will be marked live if any use in MaybeLiveUses gets marked
604 void DeadArgumentEliminationPass::MarkValue(const RetOrArg &RA, Liveness L,
605 const UseVector &MaybeLiveUses) {
607 case Live: MarkLive(RA); break;
610 // Note any uses of this value, so this return value can be
611 // marked live whenever one of the uses becomes live.
612 for (const auto &MaybeLiveUse : MaybeLiveUses)
613 Uses.insert(std::make_pair(MaybeLiveUse, RA));
619 /// MarkLive - Mark the given Function as alive, meaning that it cannot be
620 /// changed in any way. Additionally,
621 /// mark any values that are used as this function's parameters or by its return
622 /// values (according to Uses) live as well.
623 void DeadArgumentEliminationPass::MarkLive(const Function &F) {
624 DEBUG(dbgs() << "DeadArgumentEliminationPass - Intrinsically live fn: "
625 << F.getName() << "\n");
626 // Mark the function as live.
627 LiveFunctions.insert(&F);
628 // Mark all arguments as live.
629 for (unsigned i = 0, e = F.arg_size(); i != e; ++i)
630 PropagateLiveness(CreateArg(&F, i));
631 // Mark all return values as live.
632 for (unsigned i = 0, e = NumRetVals(&F); i != e; ++i)
633 PropagateLiveness(CreateRet(&F, i));
636 /// MarkLive - Mark the given return value or argument as live. Additionally,
637 /// mark any values that are used by this value (according to Uses) live as
639 void DeadArgumentEliminationPass::MarkLive(const RetOrArg &RA) {
640 if (LiveFunctions.count(RA.F))
641 return; // Function was already marked Live.
643 if (!LiveValues.insert(RA).second)
644 return; // We were already marked Live.
646 DEBUG(dbgs() << "DeadArgumentEliminationPass - Marking "
647 << RA.getDescription() << " live\n");
648 PropagateLiveness(RA);
651 /// PropagateLiveness - Given that RA is a live value, propagate it's liveness
652 /// to any other values it uses (according to Uses).
653 void DeadArgumentEliminationPass::PropagateLiveness(const RetOrArg &RA) {
654 // We don't use upper_bound (or equal_range) here, because our recursive call
655 // to ourselves is likely to cause the upper_bound (which is the first value
656 // not belonging to RA) to become erased and the iterator invalidated.
657 UseMap::iterator Begin = Uses.lower_bound(RA);
658 UseMap::iterator E = Uses.end();
660 for (I = Begin; I != E && I->first == RA; ++I)
663 // Erase RA from the Uses map (from the lower bound to wherever we ended up
665 Uses.erase(Begin, I);
668 // RemoveDeadStuffFromFunction - Remove any arguments and return values from F
669 // that are not in LiveValues. Transform the function and all of the callees of
670 // the function to not have these arguments and return values.
672 bool DeadArgumentEliminationPass::RemoveDeadStuffFromFunction(Function *F) {
673 // Don't modify fully live functions
674 if (LiveFunctions.count(F))
677 // Start by computing a new prototype for the function, which is the same as
678 // the old function, but has fewer arguments and a different return type.
679 FunctionType *FTy = F->getFunctionType();
680 std::vector<Type*> Params;
682 // Keep track of if we have a live 'returned' argument
683 bool HasLiveReturnedArg = false;
685 // Set up to build a new list of parameter attributes.
686 SmallVector<AttributeSet, 8> ArgAttrVec;
687 const AttributeList &PAL = F->getAttributes();
689 // Remember which arguments are still alive.
690 SmallVector<bool, 10> ArgAlive(FTy->getNumParams(), false);
691 // Construct the new parameter list from non-dead arguments. Also construct
692 // a new set of parameter attributes to correspond. Skip the first parameter
693 // attribute, since that belongs to the return value.
695 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
697 RetOrArg Arg = CreateArg(F, i);
698 if (LiveValues.erase(Arg)) {
699 Params.push_back(I->getType());
701 ArgAttrVec.push_back(PAL.getParamAttributes(i));
702 HasLiveReturnedArg |= PAL.hasParamAttribute(i, Attribute::Returned);
704 ++NumArgumentsEliminated;
705 DEBUG(dbgs() << "DeadArgumentEliminationPass - Removing argument " << i
706 << " (" << I->getName() << ") from " << F->getName()
711 // Find out the new return value.
712 Type *RetTy = FTy->getReturnType();
713 Type *NRetTy = nullptr;
714 unsigned RetCount = NumRetVals(F);
716 // -1 means unused, other numbers are the new index
717 SmallVector<int, 5> NewRetIdxs(RetCount, -1);
718 std::vector<Type*> RetTypes;
720 // If there is a function with a live 'returned' argument but a dead return
721 // value, then there are two possible actions:
722 // 1) Eliminate the return value and take off the 'returned' attribute on the
724 // 2) Retain the 'returned' attribute and treat the return value (but not the
725 // entire function) as live so that it is not eliminated.
727 // It's not clear in the general case which option is more profitable because,
728 // even in the absence of explicit uses of the return value, code generation
729 // is free to use the 'returned' attribute to do things like eliding
730 // save/restores of registers across calls. Whether or not this happens is
731 // target and ABI-specific as well as depending on the amount of register
732 // pressure, so there's no good way for an IR-level pass to figure this out.
734 // Fortunately, the only places where 'returned' is currently generated by
735 // the FE are places where 'returned' is basically free and almost always a
736 // performance win, so the second option can just be used always for now.
738 // This should be revisited if 'returned' is ever applied more liberally.
739 if (RetTy->isVoidTy() || HasLiveReturnedArg) {
742 // Look at each of the original return values individually.
743 for (unsigned i = 0; i != RetCount; ++i) {
744 RetOrArg Ret = CreateRet(F, i);
745 if (LiveValues.erase(Ret)) {
746 RetTypes.push_back(getRetComponentType(F, i));
747 NewRetIdxs[i] = RetTypes.size() - 1;
749 ++NumRetValsEliminated;
750 DEBUG(dbgs() << "DeadArgumentEliminationPass - Removing return value "
751 << i << " from " << F->getName() << "\n");
754 if (RetTypes.size() > 1) {
755 // More than one return type? Reduce it down to size.
756 if (StructType *STy = dyn_cast<StructType>(RetTy)) {
757 // Make the new struct packed if we used to return a packed struct
759 NRetTy = StructType::get(STy->getContext(), RetTypes, STy->isPacked());
761 assert(isa<ArrayType>(RetTy) && "unexpected multi-value return");
762 NRetTy = ArrayType::get(RetTypes[0], RetTypes.size());
764 } else if (RetTypes.size() == 1)
765 // One return type? Just a simple value then, but only if we didn't use to
766 // return a struct with that simple value before.
767 NRetTy = RetTypes.front();
768 else if (RetTypes.size() == 0)
769 // No return types? Make it void, but only if we didn't use to return {}.
770 NRetTy = Type::getVoidTy(F->getContext());
773 assert(NRetTy && "No new return type found?");
775 // The existing function return attributes.
776 AttrBuilder RAttrs(PAL.getRetAttributes());
778 // Remove any incompatible attributes, but only if we removed all return
779 // values. Otherwise, ensure that we don't have any conflicting attributes
780 // here. Currently, this should not be possible, but special handling might be
781 // required when new return value attributes are added.
782 if (NRetTy->isVoidTy())
783 RAttrs.remove(AttributeFuncs::typeIncompatible(NRetTy));
785 assert(!RAttrs.overlaps(AttributeFuncs::typeIncompatible(NRetTy)) &&
786 "Return attributes no longer compatible?");
788 AttributeSet RetAttrs = AttributeSet::get(F->getContext(), RAttrs);
790 // Reconstruct the AttributesList based on the vector we constructed.
791 assert(ArgAttrVec.size() == Params.size());
792 AttributeList NewPAL = AttributeList::get(
793 F->getContext(), PAL.getFnAttributes(), RetAttrs, ArgAttrVec);
795 // Create the new function type based on the recomputed parameters.
796 FunctionType *NFTy = FunctionType::get(NRetTy, Params, FTy->isVarArg());
802 // Create the new function body and insert it into the module...
803 Function *NF = Function::Create(NFTy, F->getLinkage());
804 NF->copyAttributesFrom(F);
805 NF->setComdat(F->getComdat());
806 NF->setAttributes(NewPAL);
807 // Insert the new function before the old function, so we won't be processing
809 F->getParent()->getFunctionList().insert(F->getIterator(), NF);
812 // Loop over all of the callers of the function, transforming the call sites
813 // to pass in a smaller number of arguments into the new function.
815 std::vector<Value*> Args;
816 while (!F->use_empty()) {
817 CallSite CS(F->user_back());
818 Instruction *Call = CS.getInstruction();
821 const AttributeList &CallPAL = CS.getAttributes();
823 // Adjust the call return attributes in case the function was changed to
825 AttrBuilder RAttrs(CallPAL.getRetAttributes());
826 RAttrs.remove(AttributeFuncs::typeIncompatible(NRetTy));
827 AttributeSet RetAttrs = AttributeSet::get(F->getContext(), RAttrs);
829 // Declare these outside of the loops, so we can reuse them for the second
830 // loop, which loops the varargs.
831 CallSite::arg_iterator I = CS.arg_begin();
833 // Loop over those operands, corresponding to the normal arguments to the
834 // original function, and add those that are still alive.
835 for (unsigned e = FTy->getNumParams(); i != e; ++I, ++i)
838 // Get original parameter attributes, but skip return attributes.
839 AttributeSet Attrs = CallPAL.getParamAttributes(i);
840 if (NRetTy != RetTy && Attrs.hasAttribute(Attribute::Returned)) {
841 // If the return type has changed, then get rid of 'returned' on the
842 // call site. The alternative is to make all 'returned' attributes on
843 // call sites keep the return value alive just like 'returned'
844 // attributes on function declaration but it's less clearly a win and
845 // this is not an expected case anyway
846 ArgAttrVec.push_back(AttributeSet::get(
848 AttrBuilder(Attrs).removeAttribute(Attribute::Returned)));
850 // Otherwise, use the original attributes.
851 ArgAttrVec.push_back(Attrs);
855 // Push any varargs arguments on the list. Don't forget their attributes.
856 for (CallSite::arg_iterator E = CS.arg_end(); I != E; ++I, ++i) {
858 ArgAttrVec.push_back(CallPAL.getParamAttributes(i));
861 // Reconstruct the AttributesList based on the vector we constructed.
862 assert(ArgAttrVec.size() == Args.size());
863 AttributeList NewCallPAL = AttributeList::get(
864 F->getContext(), CallPAL.getFnAttributes(), RetAttrs, ArgAttrVec);
866 SmallVector<OperandBundleDef, 1> OpBundles;
867 CS.getOperandBundlesAsDefs(OpBundles);
870 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
871 NewCS = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
872 Args, OpBundles, "", Call->getParent());
874 NewCS = CallInst::Create(NF, Args, OpBundles, "", Call);
875 cast<CallInst>(NewCS.getInstruction())
876 ->setTailCallKind(cast<CallInst>(Call)->getTailCallKind());
878 NewCS.setCallingConv(CS.getCallingConv());
879 NewCS.setAttributes(NewCallPAL);
880 NewCS->setDebugLoc(Call->getDebugLoc());
882 if (Call->extractProfTotalWeight(W))
883 NewCS->setProfWeight(W);
887 Instruction *New = NewCS.getInstruction();
888 if (!Call->use_empty()) {
889 if (New->getType() == Call->getType()) {
890 // Return type not changed? Just replace users then.
891 Call->replaceAllUsesWith(New);
893 } else if (New->getType()->isVoidTy()) {
894 // Our return value has uses, but they will get removed later on.
895 // Replace by null for now.
896 if (!Call->getType()->isX86_MMXTy())
897 Call->replaceAllUsesWith(Constant::getNullValue(Call->getType()));
899 assert((RetTy->isStructTy() || RetTy->isArrayTy()) &&
900 "Return type changed, but not into a void. The old return type"
901 " must have been a struct or an array!");
902 Instruction *InsertPt = Call;
903 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
904 BasicBlock *NewEdge = SplitEdge(New->getParent(), II->getNormalDest());
905 InsertPt = &*NewEdge->getFirstInsertionPt();
908 // We used to return a struct or array. Instead of doing smart stuff
909 // with all the uses, we will just rebuild it using extract/insertvalue
910 // chaining and let instcombine clean that up.
912 // Start out building up our return value from undef
913 Value *RetVal = UndefValue::get(RetTy);
914 for (unsigned i = 0; i != RetCount; ++i)
915 if (NewRetIdxs[i] != -1) {
917 if (RetTypes.size() > 1)
918 // We are still returning a struct, so extract the value from our
920 V = ExtractValueInst::Create(New, NewRetIdxs[i], "newret",
923 // We are now returning a single element, so just insert that
925 // Insert the value at the old position
926 RetVal = InsertValueInst::Create(RetVal, V, i, "oldret", InsertPt);
928 // Now, replace all uses of the old call instruction with the return
930 Call->replaceAllUsesWith(RetVal);
935 // Finally, remove the old call from the program, reducing the use-count of
937 Call->eraseFromParent();
940 // Since we have now created the new function, splice the body of the old
941 // function right into the new function, leaving the old rotting hulk of the
943 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
945 // Loop over the argument list, transferring uses of the old arguments over to
946 // the new arguments, also transferring over the names as well.
948 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(),
949 I2 = NF->arg_begin(); I != E; ++I, ++i)
951 // If this is a live argument, move the name and users over to the new
953 I->replaceAllUsesWith(&*I2);
957 // If this argument is dead, replace any uses of it with null constants
958 // (these are guaranteed to become unused later on).
959 if (!I->getType()->isX86_MMXTy())
960 I->replaceAllUsesWith(Constant::getNullValue(I->getType()));
963 // If we change the return value of the function we must rewrite any return
964 // instructions. Check this now.
965 if (F->getReturnType() != NF->getReturnType())
966 for (BasicBlock &BB : *NF)
967 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB.getTerminator())) {
970 if (NFTy->getReturnType()->isVoidTy()) {
973 assert(RetTy->isStructTy() || RetTy->isArrayTy());
974 // The original return value was a struct or array, insert
975 // extractvalue/insertvalue chains to extract only the values we need
976 // to return and insert them into our new result.
977 // This does generate messy code, but we'll let it to instcombine to
979 Value *OldRet = RI->getOperand(0);
980 // Start out building up our return value from undef
981 RetVal = UndefValue::get(NRetTy);
982 for (unsigned i = 0; i != RetCount; ++i)
983 if (NewRetIdxs[i] != -1) {
984 ExtractValueInst *EV = ExtractValueInst::Create(OldRet, i,
986 if (RetTypes.size() > 1) {
987 // We're still returning a struct, so reinsert the value into
988 // our new return value at the new index
990 RetVal = InsertValueInst::Create(RetVal, EV, NewRetIdxs[i],
993 // We are now only returning a simple value, so just return the
999 // Replace the return instruction with one returning the new return
1000 // value (possibly 0 if we became void).
1001 ReturnInst::Create(F->getContext(), RetVal, RI);
1002 BB.getInstList().erase(RI);
1005 // Patch the pointer to LLVM function in debug info descriptor.
1006 NF->setSubprogram(F->getSubprogram());
1008 // Now that the old function is dead, delete it.
1009 F->eraseFromParent();
1014 PreservedAnalyses DeadArgumentEliminationPass::run(Module &M,
1015 ModuleAnalysisManager &) {
1016 bool Changed = false;
1018 // First pass: Do a simple check to see if any functions can have their "..."
1019 // removed. We can do this if they never call va_start. This loop cannot be
1020 // fused with the next loop, because deleting a function invalidates
1021 // information computed while surveying other functions.
1022 DEBUG(dbgs() << "DeadArgumentEliminationPass - Deleting dead varargs\n");
1023 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1025 if (F.getFunctionType()->isVarArg())
1026 Changed |= DeleteDeadVarargs(F);
1029 // Second phase:loop through the module, determining which arguments are live.
1030 // We assume all arguments are dead unless proven otherwise (allowing us to
1031 // determine that dead arguments passed into recursive functions are dead).
1033 DEBUG(dbgs() << "DeadArgumentEliminationPass - Determining liveness\n");
1037 // Now, remove all dead arguments and return values from each function in
1039 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1040 // Increment now, because the function will probably get removed (ie.
1041 // replaced by a new one).
1042 Function *F = &*I++;
1043 Changed |= RemoveDeadStuffFromFunction(F);
1046 // Finally, look for any unused parameters in functions with non-local
1047 // linkage and replace the passed in parameters with undef.
1049 Changed |= RemoveDeadArgumentsFromCallers(F);
1052 return PreservedAnalyses::all();
1053 return PreservedAnalyses::none();