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/IR/Argument.h"
24 #include "llvm/IR/Attributes.h"
25 #include "llvm/IR/BasicBlock.h"
26 #include "llvm/IR/CallSite.h"
27 #include "llvm/IR/Constant.h"
28 #include "llvm/IR/Constants.h"
29 #include "llvm/IR/DerivedTypes.h"
30 #include "llvm/IR/Function.h"
31 #include "llvm/IR/InstrTypes.h"
32 #include "llvm/IR/Instruction.h"
33 #include "llvm/IR/Instructions.h"
34 #include "llvm/IR/IntrinsicInst.h"
35 #include "llvm/IR/Intrinsics.h"
36 #include "llvm/IR/Module.h"
37 #include "llvm/IR/PassManager.h"
38 #include "llvm/IR/Type.h"
39 #include "llvm/IR/Use.h"
40 #include "llvm/IR/User.h"
41 #include "llvm/IR/Value.h"
42 #include "llvm/Pass.h"
43 #include "llvm/Support/Casting.h"
44 #include "llvm/Support/Debug.h"
45 #include "llvm/Support/raw_ostream.h"
46 #include "llvm/Transforms/IPO.h"
47 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
55 #define DEBUG_TYPE "deadargelim"
57 STATISTIC(NumArgumentsEliminated, "Number of unread args removed");
58 STATISTIC(NumRetValsEliminated , "Number of unused return values removed");
59 STATISTIC(NumArgumentsReplacedWithUndef,
60 "Number of unread args replaced with undef");
64 /// DAE - The dead argument elimination pass.
65 class DAE : public ModulePass {
67 // DAH uses this to specify a different ID.
68 explicit DAE(char &ID) : ModulePass(ID) {}
71 static char ID; // Pass identification, replacement for typeid
73 DAE() : ModulePass(ID) {
74 initializeDAEPass(*PassRegistry::getPassRegistry());
77 bool runOnModule(Module &M) override {
80 DeadArgumentEliminationPass DAEP(ShouldHackArguments());
81 ModuleAnalysisManager DummyMAM;
82 PreservedAnalyses PA = DAEP.run(M, DummyMAM);
83 return !PA.areAllPreserved();
86 virtual bool ShouldHackArguments() const { return false; }
89 } // end anonymous namespace
93 INITIALIZE_PASS(DAE, "deadargelim", "Dead Argument Elimination", false, false)
97 /// DAH - DeadArgumentHacking pass - Same as dead argument elimination, but
98 /// deletes arguments to functions which are external. This is only for use
100 struct DAH : public DAE {
105 bool ShouldHackArguments() const override { return true; }
108 } // end anonymous namespace
112 INITIALIZE_PASS(DAH, "deadarghaX0r",
113 "Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)",
116 /// createDeadArgEliminationPass - This pass removes arguments from functions
117 /// which are not used by the body of the function.
118 ModulePass *llvm::createDeadArgEliminationPass() { return new DAE(); }
120 ModulePass *llvm::createDeadArgHackingPass() { return new DAH(); }
122 /// DeleteDeadVarargs - If this is an function that takes a ... list, and if
123 /// llvm.vastart is never called, the varargs list is dead for the function.
124 bool DeadArgumentEliminationPass::DeleteDeadVarargs(Function &Fn) {
125 assert(Fn.getFunctionType()->isVarArg() && "Function isn't varargs!");
126 if (Fn.isDeclaration() || !Fn.hasLocalLinkage()) return false;
128 // Ensure that the function is only directly called.
129 if (Fn.hasAddressTaken())
132 // Don't touch naked functions. The assembly might be using an argument, or
133 // otherwise rely on the frame layout in a way that this analysis will not
135 if (Fn.hasFnAttribute(Attribute::Naked)) {
139 // Okay, we know we can transform this function if safe. Scan its body
140 // looking for calls marked musttail or calls to llvm.vastart.
141 for (BasicBlock &BB : Fn) {
142 for (Instruction &I : BB) {
143 CallInst *CI = dyn_cast<CallInst>(&I);
146 if (CI->isMustTailCall())
148 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(CI)) {
149 if (II->getIntrinsicID() == Intrinsic::vastart)
155 // If we get here, there are no calls to llvm.vastart in the function body,
156 // remove the "..." and adjust all the calls.
158 // Start by computing a new prototype for the function, which is the same as
159 // the old function, but doesn't have isVarArg set.
160 FunctionType *FTy = Fn.getFunctionType();
162 std::vector<Type *> Params(FTy->param_begin(), FTy->param_end());
163 FunctionType *NFTy = FunctionType::get(FTy->getReturnType(),
165 unsigned NumArgs = Params.size();
167 // Create the new function body and insert it into the module...
168 Function *NF = Function::Create(NFTy, Fn.getLinkage());
169 NF->copyAttributesFrom(&Fn);
170 NF->setComdat(Fn.getComdat());
171 Fn.getParent()->getFunctionList().insert(Fn.getIterator(), NF);
174 // Loop over all of the callers of the function, transforming the call sites
175 // to pass in a smaller number of arguments into the new function.
177 std::vector<Value *> Args;
178 for (Value::user_iterator I = Fn.user_begin(), E = Fn.user_end(); I != E; ) {
182 Instruction *Call = CS.getInstruction();
184 // Pass all the same arguments.
185 Args.assign(CS.arg_begin(), CS.arg_begin() + NumArgs);
187 // Drop any attributes that were on the vararg arguments.
188 AttributeList PAL = CS.getAttributes();
189 if (!PAL.isEmpty()) {
190 SmallVector<AttributeSet, 8> ArgAttrs;
191 for (unsigned ArgNo = 0; ArgNo < NumArgs; ++ArgNo)
192 ArgAttrs.push_back(PAL.getParamAttributes(ArgNo));
193 PAL = AttributeList::get(Fn.getContext(), PAL.getFnAttributes(),
194 PAL.getRetAttributes(), ArgAttrs);
197 SmallVector<OperandBundleDef, 1> OpBundles;
198 CS.getOperandBundlesAsDefs(OpBundles);
201 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
202 NewCS = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
203 Args, OpBundles, "", Call);
205 NewCS = CallInst::Create(NF, Args, OpBundles, "", Call);
206 cast<CallInst>(NewCS.getInstruction())
207 ->setTailCallKind(cast<CallInst>(Call)->getTailCallKind());
209 NewCS.setCallingConv(CS.getCallingConv());
210 NewCS.setAttributes(PAL);
211 NewCS->setDebugLoc(Call->getDebugLoc());
213 if (Call->extractProfTotalWeight(W))
214 NewCS->setProfWeight(W);
218 if (!Call->use_empty())
219 Call->replaceAllUsesWith(NewCS.getInstruction());
221 NewCS->takeName(Call);
223 // Finally, remove the old call from the program, reducing the use-count of
225 Call->eraseFromParent();
228 // Since we have now created the new function, splice the body of the old
229 // function right into the new function, leaving the old rotting hulk of the
231 NF->getBasicBlockList().splice(NF->begin(), Fn.getBasicBlockList());
233 // Loop over the argument list, transferring uses of the old arguments over to
234 // the new arguments, also transferring over the names as well. While we're at
235 // it, remove the dead arguments from the DeadArguments list.
236 for (Function::arg_iterator I = Fn.arg_begin(), E = Fn.arg_end(),
237 I2 = NF->arg_begin(); I != E; ++I, ++I2) {
238 // Move the name and users over to the new version.
239 I->replaceAllUsesWith(&*I2);
243 // Clone metadatas from the old function, including debug info descriptor.
244 SmallVector<std::pair<unsigned, MDNode *>, 1> MDs;
245 Fn.getAllMetadata(MDs);
247 NF->addMetadata(MD.first, *MD.second);
249 // Fix up any BlockAddresses that refer to the function.
250 Fn.replaceAllUsesWith(ConstantExpr::getBitCast(NF, Fn.getType()));
251 // Delete the bitcast that we just created, so that NF does not
252 // appear to be address-taken.
253 NF->removeDeadConstantUsers();
254 // Finally, nuke the old function.
255 Fn.eraseFromParent();
259 /// RemoveDeadArgumentsFromCallers - Checks if the given function has any
260 /// arguments that are unused, and changes the caller parameters to be undefined
262 bool DeadArgumentEliminationPass::RemoveDeadArgumentsFromCallers(Function &Fn) {
263 // We cannot change the arguments if this TU does not define the function or
264 // if the linker may choose a function body from another TU, even if the
265 // nominal linkage indicates that other copies of the function have the same
266 // semantics. In the below example, the dead load from %p may not have been
267 // eliminated from the linker-chosen copy of f, so replacing %p with undef
268 // in callers may introduce undefined behavior.
270 // define linkonce_odr void @f(i32* %p) {
274 if (!Fn.hasExactDefinition())
277 // Functions with local linkage should already have been handled, except the
278 // fragile (variadic) ones which we can improve here.
279 if (Fn.hasLocalLinkage() && !Fn.getFunctionType()->isVarArg())
282 // Don't touch naked functions. The assembly might be using an argument, or
283 // otherwise rely on the frame layout in a way that this analysis will not
285 if (Fn.hasFnAttribute(Attribute::Naked))
291 SmallVector<unsigned, 8> UnusedArgs;
292 for (Argument &Arg : Fn.args()) {
293 if (!Arg.hasSwiftErrorAttr() && Arg.use_empty() && !Arg.hasByValOrInAllocaAttr())
294 UnusedArgs.push_back(Arg.getArgNo());
297 if (UnusedArgs.empty())
300 bool Changed = false;
302 for (Use &U : Fn.uses()) {
303 CallSite CS(U.getUser());
304 if (!CS || !CS.isCallee(&U))
307 // Now go through all unused args and replace them with "undef".
308 for (unsigned I = 0, E = UnusedArgs.size(); I != E; ++I) {
309 unsigned ArgNo = UnusedArgs[I];
311 Value *Arg = CS.getArgument(ArgNo);
312 CS.setArgument(ArgNo, UndefValue::get(Arg->getType()));
313 ++NumArgumentsReplacedWithUndef;
321 /// Convenience function that returns the number of return values. It returns 0
322 /// for void functions and 1 for functions not returning a struct. It returns
323 /// the number of struct elements for functions returning a struct.
324 static unsigned NumRetVals(const Function *F) {
325 Type *RetTy = F->getReturnType();
326 if (RetTy->isVoidTy())
328 else if (StructType *STy = dyn_cast<StructType>(RetTy))
329 return STy->getNumElements();
330 else if (ArrayType *ATy = dyn_cast<ArrayType>(RetTy))
331 return ATy->getNumElements();
336 /// Returns the sub-type a function will return at a given Idx. Should
337 /// correspond to the result type of an ExtractValue instruction executed with
338 /// just that one Idx (i.e. only top-level structure is considered).
339 static Type *getRetComponentType(const Function *F, unsigned Idx) {
340 Type *RetTy = F->getReturnType();
341 assert(!RetTy->isVoidTy() && "void type has no subtype");
343 if (StructType *STy = dyn_cast<StructType>(RetTy))
344 return STy->getElementType(Idx);
345 else if (ArrayType *ATy = dyn_cast<ArrayType>(RetTy))
346 return ATy->getElementType();
351 /// MarkIfNotLive - This checks Use for liveness in LiveValues. If Use is not
352 /// live, it adds Use to the MaybeLiveUses argument. Returns the determined
354 DeadArgumentEliminationPass::Liveness
355 DeadArgumentEliminationPass::MarkIfNotLive(RetOrArg Use,
356 UseVector &MaybeLiveUses) {
357 // We're live if our use or its Function is already marked as live.
358 if (LiveFunctions.count(Use.F) || LiveValues.count(Use))
361 // We're maybe live otherwise, but remember that we must become live if
363 MaybeLiveUses.push_back(Use);
367 /// SurveyUse - This looks at a single use of an argument or return value
368 /// and determines if it should be alive or not. Adds this use to MaybeLiveUses
369 /// if it causes the used value to become MaybeLive.
371 /// RetValNum is the return value number to use when this use is used in a
372 /// return instruction. This is used in the recursion, you should always leave
374 DeadArgumentEliminationPass::Liveness
375 DeadArgumentEliminationPass::SurveyUse(const Use *U, UseVector &MaybeLiveUses,
376 unsigned RetValNum) {
377 const User *V = U->getUser();
378 if (const ReturnInst *RI = dyn_cast<ReturnInst>(V)) {
379 // The value is returned from a function. It's only live when the
380 // function's return value is live. We use RetValNum here, for the case
381 // that U is really a use of an insertvalue instruction that uses the
383 const Function *F = RI->getParent()->getParent();
384 if (RetValNum != -1U) {
385 RetOrArg Use = CreateRet(F, RetValNum);
386 // We might be live, depending on the liveness of Use.
387 return MarkIfNotLive(Use, MaybeLiveUses);
389 DeadArgumentEliminationPass::Liveness Result = MaybeLive;
390 for (unsigned i = 0; i < NumRetVals(F); ++i) {
391 RetOrArg Use = CreateRet(F, i);
392 // We might be live, depending on the liveness of Use. If any
393 // sub-value is live, then the entire value is considered live. This
394 // is a conservative choice, and better tracking is possible.
395 DeadArgumentEliminationPass::Liveness SubResult =
396 MarkIfNotLive(Use, MaybeLiveUses);
403 if (const InsertValueInst *IV = dyn_cast<InsertValueInst>(V)) {
404 if (U->getOperandNo() != InsertValueInst::getAggregateOperandIndex()
406 // The use we are examining is inserted into an aggregate. Our liveness
407 // depends on all uses of that aggregate, but if it is used as a return
408 // value, only index at which we were inserted counts.
409 RetValNum = *IV->idx_begin();
411 // Note that if we are used as the aggregate operand to the insertvalue,
412 // we don't change RetValNum, but do survey all our uses.
414 Liveness Result = MaybeLive;
415 for (const Use &UU : IV->uses()) {
416 Result = SurveyUse(&UU, MaybeLiveUses, RetValNum);
423 if (auto CS = ImmutableCallSite(V)) {
424 const Function *F = CS.getCalledFunction();
426 // Used in a direct call.
428 // The function argument is live if it is used as a bundle operand.
429 if (CS.isBundleOperand(U))
432 // Find the argument number. We know for sure that this use is an
433 // argument, since if it was the function argument this would be an
434 // indirect call and the we know can't be looking at a value of the
435 // label type (for the invoke instruction).
436 unsigned ArgNo = CS.getArgumentNo(U);
438 if (ArgNo >= F->getFunctionType()->getNumParams())
439 // The value is passed in through a vararg! Must be live.
442 assert(CS.getArgument(ArgNo)
443 == CS->getOperand(U->getOperandNo())
444 && "Argument is not where we expected it");
446 // Value passed to a normal call. It's only live when the corresponding
447 // argument to the called function turns out live.
448 RetOrArg Use = CreateArg(F, ArgNo);
449 return MarkIfNotLive(Use, MaybeLiveUses);
452 // Used in any other way? Value must be live.
456 /// SurveyUses - This looks at all the uses of the given value
457 /// Returns the Liveness deduced from the uses of this value.
459 /// Adds all uses that cause the result to be MaybeLive to MaybeLiveRetUses. If
460 /// the result is Live, MaybeLiveUses might be modified but its content should
461 /// be ignored (since it might not be complete).
462 DeadArgumentEliminationPass::Liveness
463 DeadArgumentEliminationPass::SurveyUses(const Value *V,
464 UseVector &MaybeLiveUses) {
465 // Assume it's dead (which will only hold if there are no uses at all..).
466 Liveness Result = MaybeLive;
468 for (const Use &U : V->uses()) {
469 Result = SurveyUse(&U, MaybeLiveUses);
476 // SurveyFunction - This performs the initial survey of the specified function,
477 // checking out whether or not it uses any of its incoming arguments or whether
478 // any callers use the return value. This fills in the LiveValues set and Uses
481 // We consider arguments of non-internal functions to be intrinsically alive as
482 // well as arguments to functions which have their "address taken".
483 void DeadArgumentEliminationPass::SurveyFunction(const Function &F) {
484 // Functions with inalloca parameters are expecting args in a particular
485 // register and memory layout.
486 if (F.getAttributes().hasAttrSomewhere(Attribute::InAlloca)) {
491 // Don't touch naked functions. The assembly might be using an argument, or
492 // otherwise rely on the frame layout in a way that this analysis will not
494 if (F.hasFnAttribute(Attribute::Naked)) {
499 unsigned RetCount = NumRetVals(&F);
501 // Assume all return values are dead
502 using RetVals = SmallVector<Liveness, 5>;
504 RetVals RetValLiveness(RetCount, MaybeLive);
506 using RetUses = SmallVector<UseVector, 5>;
508 // These vectors map each return value to the uses that make it MaybeLive, so
509 // we can add those to the Uses map if the return value really turns out to be
510 // MaybeLive. Initialized to a list of RetCount empty lists.
511 RetUses MaybeLiveRetUses(RetCount);
513 bool HasMustTailCalls = false;
515 for (Function::const_iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
516 if (const ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
517 if (RI->getNumOperands() != 0 && RI->getOperand(0)->getType()
518 != F.getFunctionType()->getReturnType()) {
519 // We don't support old style multiple return values.
525 // If we have any returns of `musttail` results - the signature can't
527 if (BB->getTerminatingMustTailCall() != nullptr)
528 HasMustTailCalls = true;
531 if (HasMustTailCalls) {
532 LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - " << F.getName()
533 << " has musttail calls\n");
536 if (!F.hasLocalLinkage() && (!ShouldHackArguments || F.isIntrinsic())) {
542 dbgs() << "DeadArgumentEliminationPass - Inspecting callers for fn: "
543 << F.getName() << "\n");
544 // Keep track of the number of live retvals, so we can skip checks once all
545 // of them turn out to be live.
546 unsigned NumLiveRetVals = 0;
548 bool HasMustTailCallers = false;
550 // Loop all uses of the function.
551 for (const Use &U : F.uses()) {
552 // If the function is PASSED IN as an argument, its address has been
554 ImmutableCallSite CS(U.getUser());
555 if (!CS || !CS.isCallee(&U)) {
560 // The number of arguments for `musttail` call must match the number of
561 // arguments of the caller
562 if (CS.isMustTailCall())
563 HasMustTailCallers = true;
565 // If this use is anything other than a call site, the function is alive.
566 const Instruction *TheCall = CS.getInstruction();
567 if (!TheCall) { // Not a direct call site?
572 // If we end up here, we are looking at a direct call to our function.
574 // Now, check how our return value(s) is/are used in this caller. Don't
575 // bother checking return values if all of them are live already.
576 if (NumLiveRetVals == RetCount)
579 // Check all uses of the return value.
580 for (const Use &U : TheCall->uses()) {
581 if (ExtractValueInst *Ext = dyn_cast<ExtractValueInst>(U.getUser())) {
582 // This use uses a part of our return value, survey the uses of
583 // that part and store the results for this index only.
584 unsigned Idx = *Ext->idx_begin();
585 if (RetValLiveness[Idx] != Live) {
586 RetValLiveness[Idx] = SurveyUses(Ext, MaybeLiveRetUses[Idx]);
587 if (RetValLiveness[Idx] == Live)
591 // Used by something else than extractvalue. Survey, but assume that the
592 // result applies to all sub-values.
593 UseVector MaybeLiveAggregateUses;
594 if (SurveyUse(&U, MaybeLiveAggregateUses) == Live) {
595 NumLiveRetVals = RetCount;
596 RetValLiveness.assign(RetCount, Live);
599 for (unsigned i = 0; i != RetCount; ++i) {
600 if (RetValLiveness[i] != Live)
601 MaybeLiveRetUses[i].append(MaybeLiveAggregateUses.begin(),
602 MaybeLiveAggregateUses.end());
609 if (HasMustTailCallers) {
610 LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - " << F.getName()
611 << " has musttail callers\n");
614 // Now we've inspected all callers, record the liveness of our return values.
615 for (unsigned i = 0; i != RetCount; ++i)
616 MarkValue(CreateRet(&F, i), RetValLiveness[i], MaybeLiveRetUses[i]);
618 LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - Inspecting args for fn: "
619 << F.getName() << "\n");
621 // Now, check all of our arguments.
623 UseVector MaybeLiveArgUses;
624 for (Function::const_arg_iterator AI = F.arg_begin(),
625 E = F.arg_end(); AI != E; ++AI, ++i) {
627 if (F.getFunctionType()->isVarArg() || HasMustTailCallers ||
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.
635 // `musttail` calls to this function restrict argument removal attempts.
636 // The signature of the caller must match the signature of the function.
638 // `musttail` calls in this function prevents us from changing its
642 // See what the effect of this use is (recording any uses that cause
643 // MaybeLive in MaybeLiveArgUses).
644 Result = SurveyUses(&*AI, MaybeLiveArgUses);
648 MarkValue(CreateArg(&F, i), Result, MaybeLiveArgUses);
649 // Clear the vector again for the next iteration.
650 MaybeLiveArgUses.clear();
654 /// MarkValue - This function marks the liveness of RA depending on L. If L is
655 /// MaybeLive, it also takes all uses in MaybeLiveUses and records them in Uses,
656 /// such that RA will be marked live if any use in MaybeLiveUses gets marked
658 void DeadArgumentEliminationPass::MarkValue(const RetOrArg &RA, Liveness L,
659 const UseVector &MaybeLiveUses) {
665 // Note any uses of this value, so this return value can be
666 // marked live whenever one of the uses becomes live.
667 for (const auto &MaybeLiveUse : MaybeLiveUses)
668 Uses.insert(std::make_pair(MaybeLiveUse, RA));
673 /// MarkLive - Mark the given Function as alive, meaning that it cannot be
674 /// changed in any way. Additionally,
675 /// mark any values that are used as this function's parameters or by its return
676 /// values (according to Uses) live as well.
677 void DeadArgumentEliminationPass::MarkLive(const Function &F) {
678 LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - Intrinsically live fn: "
679 << F.getName() << "\n");
680 // Mark the function as live.
681 LiveFunctions.insert(&F);
682 // Mark all arguments as live.
683 for (unsigned i = 0, e = F.arg_size(); i != e; ++i)
684 PropagateLiveness(CreateArg(&F, i));
685 // Mark all return values as live.
686 for (unsigned i = 0, e = NumRetVals(&F); i != e; ++i)
687 PropagateLiveness(CreateRet(&F, i));
690 /// MarkLive - Mark the given return value or argument as live. Additionally,
691 /// mark any values that are used by this value (according to Uses) live as
693 void DeadArgumentEliminationPass::MarkLive(const RetOrArg &RA) {
694 if (LiveFunctions.count(RA.F))
695 return; // Function was already marked Live.
697 if (!LiveValues.insert(RA).second)
698 return; // We were already marked Live.
700 LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - Marking "
701 << RA.getDescription() << " live\n");
702 PropagateLiveness(RA);
705 /// PropagateLiveness - Given that RA is a live value, propagate it's liveness
706 /// to any other values it uses (according to Uses).
707 void DeadArgumentEliminationPass::PropagateLiveness(const RetOrArg &RA) {
708 // We don't use upper_bound (or equal_range) here, because our recursive call
709 // to ourselves is likely to cause the upper_bound (which is the first value
710 // not belonging to RA) to become erased and the iterator invalidated.
711 UseMap::iterator Begin = Uses.lower_bound(RA);
712 UseMap::iterator E = Uses.end();
714 for (I = Begin; I != E && I->first == RA; ++I)
717 // Erase RA from the Uses map (from the lower bound to wherever we ended up
719 Uses.erase(Begin, I);
722 // RemoveDeadStuffFromFunction - Remove any arguments and return values from F
723 // that are not in LiveValues. Transform the function and all of the callees of
724 // the function to not have these arguments and return values.
726 bool DeadArgumentEliminationPass::RemoveDeadStuffFromFunction(Function *F) {
727 // Don't modify fully live functions
728 if (LiveFunctions.count(F))
731 // Start by computing a new prototype for the function, which is the same as
732 // the old function, but has fewer arguments and a different return type.
733 FunctionType *FTy = F->getFunctionType();
734 std::vector<Type*> Params;
736 // Keep track of if we have a live 'returned' argument
737 bool HasLiveReturnedArg = false;
739 // Set up to build a new list of parameter attributes.
740 SmallVector<AttributeSet, 8> ArgAttrVec;
741 const AttributeList &PAL = F->getAttributes();
743 // Remember which arguments are still alive.
744 SmallVector<bool, 10> ArgAlive(FTy->getNumParams(), false);
745 // Construct the new parameter list from non-dead arguments. Also construct
746 // a new set of parameter attributes to correspond. Skip the first parameter
747 // attribute, since that belongs to the return value.
749 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
751 RetOrArg Arg = CreateArg(F, i);
752 if (LiveValues.erase(Arg)) {
753 Params.push_back(I->getType());
755 ArgAttrVec.push_back(PAL.getParamAttributes(i));
756 HasLiveReturnedArg |= PAL.hasParamAttribute(i, Attribute::Returned);
758 ++NumArgumentsEliminated;
759 LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - Removing argument "
760 << i << " (" << I->getName() << ") from "
761 << F->getName() << "\n");
765 // Find out the new return value.
766 Type *RetTy = FTy->getReturnType();
767 Type *NRetTy = nullptr;
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 // Look at each of the original return values individually.
797 for (unsigned i = 0; i != RetCount; ++i) {
798 RetOrArg Ret = CreateRet(F, i);
799 if (LiveValues.erase(Ret)) {
800 RetTypes.push_back(getRetComponentType(F, i));
801 NewRetIdxs[i] = RetTypes.size() - 1;
803 ++NumRetValsEliminated;
805 dbgs() << "DeadArgumentEliminationPass - Removing return value "
806 << i << " from " << F->getName() << "\n");
809 if (RetTypes.size() > 1) {
810 // More than one return type? Reduce it down to size.
811 if (StructType *STy = dyn_cast<StructType>(RetTy)) {
812 // Make the new struct packed if we used to return a packed struct
814 NRetTy = StructType::get(STy->getContext(), RetTypes, STy->isPacked());
816 assert(isa<ArrayType>(RetTy) && "unexpected multi-value return");
817 NRetTy = ArrayType::get(RetTypes[0], RetTypes.size());
819 } else if (RetTypes.size() == 1)
820 // One return type? Just a simple value then, but only if we didn't use to
821 // return a struct with that simple value before.
822 NRetTy = RetTypes.front();
823 else if (RetTypes.empty())
824 // No return types? Make it void, but only if we didn't use to return {}.
825 NRetTy = Type::getVoidTy(F->getContext());
828 assert(NRetTy && "No new return type found?");
830 // The existing function return attributes.
831 AttrBuilder RAttrs(PAL.getRetAttributes());
833 // Remove any incompatible attributes, but only if we removed all return
834 // values. Otherwise, ensure that we don't have any conflicting attributes
835 // here. Currently, this should not be possible, but special handling might be
836 // required when new return value attributes are added.
837 if (NRetTy->isVoidTy())
838 RAttrs.remove(AttributeFuncs::typeIncompatible(NRetTy));
840 assert(!RAttrs.overlaps(AttributeFuncs::typeIncompatible(NRetTy)) &&
841 "Return attributes no longer compatible?");
843 AttributeSet RetAttrs = AttributeSet::get(F->getContext(), RAttrs);
845 // Strip allocsize attributes. They might refer to the deleted arguments.
846 AttributeSet FnAttrs = PAL.getFnAttributes().removeAttribute(
847 F->getContext(), Attribute::AllocSize);
849 // Reconstruct the AttributesList based on the vector we constructed.
850 assert(ArgAttrVec.size() == Params.size());
851 AttributeList NewPAL =
852 AttributeList::get(F->getContext(), FnAttrs, RetAttrs, ArgAttrVec);
854 // Create the new function type based on the recomputed parameters.
855 FunctionType *NFTy = FunctionType::get(NRetTy, Params, FTy->isVarArg());
861 // Create the new function body and insert it into the module...
862 Function *NF = Function::Create(NFTy, F->getLinkage());
863 NF->copyAttributesFrom(F);
864 NF->setComdat(F->getComdat());
865 NF->setAttributes(NewPAL);
866 // Insert the new function before the old function, so we won't be processing
868 F->getParent()->getFunctionList().insert(F->getIterator(), NF);
871 // Loop over all of the callers of the function, transforming the call sites
872 // to pass in a smaller number of arguments into the new function.
873 std::vector<Value*> Args;
874 while (!F->use_empty()) {
875 CallSite CS(F->user_back());
876 Instruction *Call = CS.getInstruction();
879 const AttributeList &CallPAL = CS.getAttributes();
881 // Adjust the call return attributes in case the function was changed to
883 AttrBuilder RAttrs(CallPAL.getRetAttributes());
884 RAttrs.remove(AttributeFuncs::typeIncompatible(NRetTy));
885 AttributeSet RetAttrs = AttributeSet::get(F->getContext(), RAttrs);
887 // Declare these outside of the loops, so we can reuse them for the second
888 // loop, which loops the varargs.
889 CallSite::arg_iterator I = CS.arg_begin();
891 // Loop over those operands, corresponding to the normal arguments to the
892 // original function, and add those that are still alive.
893 for (unsigned e = FTy->getNumParams(); i != e; ++I, ++i)
896 // Get original parameter attributes, but skip return attributes.
897 AttributeSet Attrs = CallPAL.getParamAttributes(i);
898 if (NRetTy != RetTy && Attrs.hasAttribute(Attribute::Returned)) {
899 // If the return type has changed, then get rid of 'returned' on the
900 // call site. The alternative is to make all 'returned' attributes on
901 // call sites keep the return value alive just like 'returned'
902 // attributes on function declaration but it's less clearly a win and
903 // this is not an expected case anyway
904 ArgAttrVec.push_back(AttributeSet::get(
906 AttrBuilder(Attrs).removeAttribute(Attribute::Returned)));
908 // Otherwise, use the original attributes.
909 ArgAttrVec.push_back(Attrs);
913 // Push any varargs arguments on the list. Don't forget their attributes.
914 for (CallSite::arg_iterator E = CS.arg_end(); I != E; ++I, ++i) {
916 ArgAttrVec.push_back(CallPAL.getParamAttributes(i));
919 // Reconstruct the AttributesList based on the vector we constructed.
920 assert(ArgAttrVec.size() == Args.size());
922 // Again, be sure to remove any allocsize attributes, since their indices
923 // may now be incorrect.
924 AttributeSet FnAttrs = CallPAL.getFnAttributes().removeAttribute(
925 F->getContext(), Attribute::AllocSize);
927 AttributeList NewCallPAL = AttributeList::get(
928 F->getContext(), FnAttrs, RetAttrs, ArgAttrVec);
930 SmallVector<OperandBundleDef, 1> OpBundles;
931 CS.getOperandBundlesAsDefs(OpBundles);
934 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
935 NewCS = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
936 Args, OpBundles, "", Call->getParent());
938 NewCS = CallInst::Create(NF, Args, OpBundles, "", Call);
939 cast<CallInst>(NewCS.getInstruction())
940 ->setTailCallKind(cast<CallInst>(Call)->getTailCallKind());
942 NewCS.setCallingConv(CS.getCallingConv());
943 NewCS.setAttributes(NewCallPAL);
944 NewCS->setDebugLoc(Call->getDebugLoc());
946 if (Call->extractProfTotalWeight(W))
947 NewCS->setProfWeight(W);
951 Instruction *New = NewCS.getInstruction();
952 if (!Call->use_empty()) {
953 if (New->getType() == Call->getType()) {
954 // Return type not changed? Just replace users then.
955 Call->replaceAllUsesWith(New);
957 } else if (New->getType()->isVoidTy()) {
958 // Our return value has uses, but they will get removed later on.
959 // Replace by null for now.
960 if (!Call->getType()->isX86_MMXTy())
961 Call->replaceAllUsesWith(Constant::getNullValue(Call->getType()));
963 assert((RetTy->isStructTy() || RetTy->isArrayTy()) &&
964 "Return type changed, but not into a void. The old return type"
965 " must have been a struct or an array!");
966 Instruction *InsertPt = Call;
967 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
968 BasicBlock *NewEdge = SplitEdge(New->getParent(), II->getNormalDest());
969 InsertPt = &*NewEdge->getFirstInsertionPt();
972 // We used to return a struct or array. Instead of doing smart stuff
973 // with all the uses, we will just rebuild it using extract/insertvalue
974 // chaining and let instcombine clean that up.
976 // Start out building up our return value from undef
977 Value *RetVal = UndefValue::get(RetTy);
978 for (unsigned i = 0; i != RetCount; ++i)
979 if (NewRetIdxs[i] != -1) {
981 if (RetTypes.size() > 1)
982 // We are still returning a struct, so extract the value from our
984 V = ExtractValueInst::Create(New, NewRetIdxs[i], "newret",
987 // We are now returning a single element, so just insert that
989 // Insert the value at the old position
990 RetVal = InsertValueInst::Create(RetVal, V, i, "oldret", InsertPt);
992 // Now, replace all uses of the old call instruction with the return
994 Call->replaceAllUsesWith(RetVal);
999 // Finally, remove the old call from the program, reducing the use-count of
1001 Call->eraseFromParent();
1004 // Since we have now created the new function, splice the body of the old
1005 // function right into the new function, leaving the old rotting hulk of the
1007 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
1009 // Loop over the argument list, transferring uses of the old arguments over to
1010 // the new arguments, also transferring over the names as well.
1012 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(),
1013 I2 = NF->arg_begin(); I != E; ++I, ++i)
1015 // If this is a live argument, move the name and users over to the new
1017 I->replaceAllUsesWith(&*I2);
1021 // If this argument is dead, replace any uses of it with null constants
1022 // (these are guaranteed to become unused later on).
1023 if (!I->getType()->isX86_MMXTy())
1024 I->replaceAllUsesWith(Constant::getNullValue(I->getType()));
1027 // If we change the return value of the function we must rewrite any return
1028 // instructions. Check this now.
1029 if (F->getReturnType() != NF->getReturnType())
1030 for (BasicBlock &BB : *NF)
1031 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB.getTerminator())) {
1034 if (NFTy->getReturnType()->isVoidTy()) {
1037 assert(RetTy->isStructTy() || RetTy->isArrayTy());
1038 // The original return value was a struct or array, insert
1039 // extractvalue/insertvalue chains to extract only the values we need
1040 // to return and insert them into our new result.
1041 // This does generate messy code, but we'll let it to instcombine to
1043 Value *OldRet = RI->getOperand(0);
1044 // Start out building up our return value from undef
1045 RetVal = UndefValue::get(NRetTy);
1046 for (unsigned i = 0; i != RetCount; ++i)
1047 if (NewRetIdxs[i] != -1) {
1048 ExtractValueInst *EV = ExtractValueInst::Create(OldRet, i,
1050 if (RetTypes.size() > 1) {
1051 // We're still returning a struct, so reinsert the value into
1052 // our new return value at the new index
1054 RetVal = InsertValueInst::Create(RetVal, EV, NewRetIdxs[i],
1057 // We are now only returning a simple value, so just return the
1063 // Replace the return instruction with one returning the new return
1064 // value (possibly 0 if we became void).
1065 ReturnInst::Create(F->getContext(), RetVal, RI);
1066 BB.getInstList().erase(RI);
1069 // Clone metadatas from the old function, including debug info descriptor.
1070 SmallVector<std::pair<unsigned, MDNode *>, 1> MDs;
1071 F->getAllMetadata(MDs);
1073 NF->addMetadata(MD.first, *MD.second);
1075 // Now that the old function is dead, delete it.
1076 F->eraseFromParent();
1081 PreservedAnalyses DeadArgumentEliminationPass::run(Module &M,
1082 ModuleAnalysisManager &) {
1083 bool Changed = false;
1085 // First pass: Do a simple check to see if any functions can have their "..."
1086 // removed. We can do this if they never call va_start. This loop cannot be
1087 // fused with the next loop, because deleting a function invalidates
1088 // information computed while surveying other functions.
1089 LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - Deleting dead varargs\n");
1090 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1092 if (F.getFunctionType()->isVarArg())
1093 Changed |= DeleteDeadVarargs(F);
1096 // Second phase:loop through the module, determining which arguments are live.
1097 // We assume all arguments are dead unless proven otherwise (allowing us to
1098 // determine that dead arguments passed into recursive functions are dead).
1100 LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - Determining liveness\n");
1104 // Now, remove all dead arguments and return values from each function in
1106 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
1107 // Increment now, because the function will probably get removed (ie.
1108 // replaced by a new one).
1109 Function *F = &*I++;
1110 Changed |= RemoveDeadStuffFromFunction(F);
1113 // Finally, look for any unused parameters in functions with non-local
1114 // linkage and replace the passed in parameters with undef.
1116 Changed |= RemoveDeadArgumentsFromCallers(F);
1119 return PreservedAnalyses::all();
1120 return PreservedAnalyses::none();