1//===- DeadArgumentElimination.cpp - Eliminate dead arguments -------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This pass deletes dead arguments from internal functions. Dead argument
10// elimination removes arguments which are directly dead, as well as arguments
11// only passed into function calls as dead arguments of other functions. This
12// pass also deletes dead return values in a similar way.
13//
14// This pass is often useful as a cleanup pass to run after aggressive
15// interprocedural passes, which add possibly-dead arguments or return values.
16//
17//===----------------------------------------------------------------------===//
18
19#include "llvm/Transforms/IPO/DeadArgumentElimination.h"
20#include "llvm/ADT/SmallVector.h"
21#include "llvm/ADT/Statistic.h"
22#include "llvm/IR/Argument.h"
23#include "llvm/IR/AttributeMask.h"
24#include "llvm/IR/Attributes.h"
25#include "llvm/IR/BasicBlock.h"
26#include "llvm/IR/Constants.h"
27#include "llvm/IR/DIBuilder.h"
28#include "llvm/IR/DerivedTypes.h"
29#include "llvm/IR/Function.h"
30#include "llvm/IR/IRBuilder.h"
31#include "llvm/IR/InstrTypes.h"
32#include "llvm/IR/Instructions.h"
33#include "llvm/IR/IntrinsicInst.h"
34#include "llvm/IR/Intrinsics.h"
35#include "llvm/IR/Module.h"
36#include "llvm/IR/NoFolder.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/InitializePasses.h"
43#include "llvm/Pass.h"
44#include "llvm/Support/Casting.h"
45#include "llvm/Support/Debug.h"
46#include "llvm/Support/raw_ostream.h"
47#include "llvm/Transforms/IPO.h"
48#include "llvm/Transforms/Utils/BasicBlockUtils.h"
49#include <cassert>
50#include <utility>
51#include <vector>
52
53using namespace llvm;
54
55#define DEBUG_TYPE "deadargelim"
56
57STATISTIC(NumArgumentsEliminated, "Number of unread args removed");
58STATISTIC(NumRetValsEliminated, "Number of unused return values removed");
59STATISTIC(NumArgumentsReplacedWithPoison,
60 "Number of unread args replaced with poison");
61
62namespace {
63
64/// The dead argument elimination pass.
65class DAE : public ModulePass {
66protected:
67 // DAH uses this to specify a different ID.
68 explicit DAE(char &ID) : ModulePass(ID) {}
69
70public:
71 static char ID; // Pass identification, replacement for typeid
72
73 DAE() : ModulePass(ID) {
74 initializeDAEPass(*PassRegistry::getPassRegistry());
75 }
76
77 bool runOnModule(Module &M) override {
78 if (skipModule(M))
79 return false;
80 DeadArgumentEliminationPass DAEP(shouldHackArguments());
81 ModuleAnalysisManager DummyMAM;
82 PreservedAnalyses PA = DAEP.run(M, DummyMAM);
83 return !PA.areAllPreserved();
84 }
85
86 virtual bool shouldHackArguments() const { return false; }
87};
88
89bool isMustTailCalleeAnalyzable(const CallBase &CB) {
90 assert(CB.isMustTailCall());
91 return CB.getCalledFunction() && !CB.getCalledFunction()->isDeclaration();
92}
93
94} // end anonymous namespace
95
96char DAE::ID = 0;
97
98INITIALIZE_PASS(DAE, "deadargelim", "Dead Argument Elimination", false, false)
99
100namespace {
101
102/// The DeadArgumentHacking pass, same as dead argument elimination, but deletes
103/// arguments to functions which are external. This is only for use by bugpoint.
104struct DAH : public DAE {
105 static char ID;
106
107 DAH() : DAE(ID) {}
108
109 bool shouldHackArguments() const override { return true; }
110};
111
112} // end anonymous namespace
113
114char DAH::ID = 0;
115
116INITIALIZE_PASS(DAH, "deadarghaX0r",
117 "Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)", false,
118 false)
119
120/// This pass removes arguments from functions which are not used by the body of
121/// the function.
122ModulePass *llvm::createDeadArgEliminationPass() { return new DAE(); }
123
124ModulePass *llvm::createDeadArgHackingPass() { return new DAH(); }
125
126/// If this is an function that takes a ... list, and if llvm.vastart is never
127/// called, the varargs list is dead for the function.
128bool DeadArgumentEliminationPass::deleteDeadVarargs(Function &F) {
129 assert(F.getFunctionType()->isVarArg() && "Function isn't varargs!");
130 if (F.isDeclaration() || !F.hasLocalLinkage())
131 return false;
132
133 // Ensure that the function is only directly called.
134 if (F.hasAddressTaken())
135 return false;
136
137 // Don't touch naked functions. The assembly might be using an argument, or
138 // otherwise rely on the frame layout in a way that this analysis will not
139 // see.
140 if (F.hasFnAttribute(Kind: Attribute::Naked)) {
141 return false;
142 }
143
144 // Okay, we know we can transform this function if safe. Scan its body
145 // looking for calls marked musttail or calls to llvm.vastart.
146 for (BasicBlock &BB : F) {
147 for (Instruction &I : BB) {
148 CallInst *CI = dyn_cast<CallInst>(Val: &I);
149 if (!CI)
150 continue;
151 if (CI->isMustTailCall())
152 return false;
153 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(Val: CI)) {
154 if (II->getIntrinsicID() == Intrinsic::vastart)
155 return false;
156 }
157 }
158 }
159
160 // If we get here, there are no calls to llvm.vastart in the function body,
161 // remove the "..." and adjust all the calls.
162
163 // Start by computing a new prototype for the function, which is the same as
164 // the old function, but doesn't have isVarArg set.
165 FunctionType *FTy = F.getFunctionType();
166
167 std::vector<Type *> Params(FTy->param_begin(), FTy->param_end());
168 FunctionType *NFTy = FunctionType::get(Result: FTy->getReturnType(), Params, isVarArg: false);
169 unsigned NumArgs = Params.size();
170
171 // Create the new function body and insert it into the module...
172 Function *NF = Function::Create(Ty: NFTy, Linkage: F.getLinkage(), AddrSpace: F.getAddressSpace());
173 NF->copyAttributesFrom(Src: &F);
174 NF->setComdat(F.getComdat());
175 F.getParent()->getFunctionList().insert(where: F.getIterator(), New: NF);
176 NF->takeName(V: &F);
177 NF->IsNewDbgInfoFormat = F.IsNewDbgInfoFormat;
178
179 // Loop over all the callers of the function, transforming the call sites
180 // to pass in a smaller number of arguments into the new function.
181 //
182 std::vector<Value *> Args;
183 for (User *U : llvm::make_early_inc_range(Range: F.users())) {
184 CallBase *CB = dyn_cast<CallBase>(Val: U);
185 if (!CB)
186 continue;
187
188 // Pass all the same arguments.
189 Args.assign(first: CB->arg_begin(), last: CB->arg_begin() + NumArgs);
190
191 // Drop any attributes that were on the vararg arguments.
192 AttributeList PAL = CB->getAttributes();
193 if (!PAL.isEmpty()) {
194 SmallVector<AttributeSet, 8> ArgAttrs;
195 for (unsigned ArgNo = 0; ArgNo < NumArgs; ++ArgNo)
196 ArgAttrs.push_back(Elt: PAL.getParamAttrs(ArgNo));
197 PAL = AttributeList::get(C&: F.getContext(), FnAttrs: PAL.getFnAttrs(),
198 RetAttrs: PAL.getRetAttrs(), ArgAttrs);
199 }
200
201 SmallVector<OperandBundleDef, 1> OpBundles;
202 CB->getOperandBundlesAsDefs(Defs&: OpBundles);
203
204 CallBase *NewCB = nullptr;
205 if (InvokeInst *II = dyn_cast<InvokeInst>(Val: CB)) {
206 NewCB = InvokeInst::Create(Func: NF, IfNormal: II->getNormalDest(), IfException: II->getUnwindDest(),
207 Args, Bundles: OpBundles, NameStr: "", InsertBefore: CB->getIterator());
208 } else {
209 NewCB = CallInst::Create(Func: NF, Args, Bundles: OpBundles, NameStr: "", InsertBefore: CB->getIterator());
210 cast<CallInst>(Val: NewCB)->setTailCallKind(
211 cast<CallInst>(Val: CB)->getTailCallKind());
212 }
213 NewCB->setCallingConv(CB->getCallingConv());
214 NewCB->setAttributes(PAL);
215 NewCB->copyMetadata(SrcInst: *CB, WL: {LLVMContext::MD_prof, LLVMContext::MD_dbg});
216
217 Args.clear();
218
219 if (!CB->use_empty())
220 CB->replaceAllUsesWith(V: NewCB);
221
222 NewCB->takeName(V: CB);
223
224 // Finally, remove the old call from the program, reducing the use-count of
225 // F.
226 CB->eraseFromParent();
227 }
228
229 // Since we have now created the new function, splice the body of the old
230 // function right into the new function, leaving the old rotting hulk of the
231 // function empty.
232 NF->splice(ToIt: NF->begin(), FromF: &F);
233
234 // Loop over the argument list, transferring uses of the old arguments over to
235 // the new arguments, also transferring over the names as well. While we're
236 // at it, remove the dead arguments from the DeadArguments list.
237 for (Function::arg_iterator I = F.arg_begin(), E = F.arg_end(),
238 I2 = NF->arg_begin();
239 I != E; ++I, ++I2) {
240 // Move the name and users over to the new version.
241 I->replaceAllUsesWith(V: &*I2);
242 I2->takeName(V: &*I);
243 }
244
245 // Clone metadata from the old function, including debug info descriptor.
246 SmallVector<std::pair<unsigned, MDNode *>, 1> MDs;
247 F.getAllMetadata(MDs);
248 for (auto [KindID, Node] : MDs)
249 NF->addMetadata(KindID, MD&: *Node);
250
251 // Fix up any BlockAddresses that refer to the function.
252 F.replaceAllUsesWith(V: NF);
253 // Delete the bitcast that we just created, so that NF does not
254 // appear to be address-taken.
255 NF->removeDeadConstantUsers();
256 // Finally, nuke the old function.
257 F.eraseFromParent();
258 return true;
259}
260
261/// Checks if the given function has any arguments that are unused, and changes
262/// the caller parameters to be poison instead.
263bool DeadArgumentEliminationPass::removeDeadArgumentsFromCallers(Function &F) {
264 // We cannot change the arguments if this TU does not define the function or
265 // if the linker may choose a function body from another TU, even if the
266 // nominal linkage indicates that other copies of the function have the same
267 // semantics. In the below example, the dead load from %p may not have been
268 // eliminated from the linker-chosen copy of f, so replacing %p with poison
269 // in callers may introduce undefined behavior.
270 //
271 // define linkonce_odr void @f(i32* %p) {
272 // %v = load i32 %p
273 // ret void
274 // }
275 if (!F.hasExactDefinition())
276 return false;
277
278 // Functions with local linkage should already have been handled, except if
279 // they are fully alive (e.g., called indirectly) and except for the fragile
280 // (variadic) ones. In these cases, we may still be able to improve their
281 // statically known call sites.
282 if ((F.hasLocalLinkage() && !LiveFunctions.count(x: &F)) &&
283 !F.getFunctionType()->isVarArg())
284 return false;
285
286 // Don't touch naked functions. The assembly might be using an argument, or
287 // otherwise rely on the frame layout in a way that this analysis will not
288 // see.
289 if (F.hasFnAttribute(Kind: Attribute::Naked))
290 return false;
291
292 if (F.use_empty())
293 return false;
294
295 SmallVector<unsigned, 8> UnusedArgs;
296 bool Changed = false;
297
298 AttributeMask UBImplyingAttributes =
299 AttributeFuncs::getUBImplyingAttributes();
300 for (Argument &Arg : F.args()) {
301 if (!Arg.hasSwiftErrorAttr() && Arg.use_empty() &&
302 !Arg.hasPassPointeeByValueCopyAttr()) {
303 if (Arg.isUsedByMetadata()) {
304 Arg.replaceAllUsesWith(V: PoisonValue::get(T: Arg.getType()));
305 Changed = true;
306 }
307 UnusedArgs.push_back(Elt: Arg.getArgNo());
308 F.removeParamAttrs(ArgNo: Arg.getArgNo(), Attrs: UBImplyingAttributes);
309 }
310 }
311
312 if (UnusedArgs.empty())
313 return false;
314
315 for (Use &U : F.uses()) {
316 CallBase *CB = dyn_cast<CallBase>(Val: U.getUser());
317 if (!CB || !CB->isCallee(U: &U) ||
318 CB->getFunctionType() != F.getFunctionType())
319 continue;
320
321 // Now go through all unused args and replace them with poison.
322 for (unsigned ArgNo : UnusedArgs) {
323 Value *Arg = CB->getArgOperand(i: ArgNo);
324 CB->setArgOperand(i: ArgNo, v: PoisonValue::get(T: Arg->getType()));
325 CB->removeParamAttrs(ArgNo, AttrsToRemove: UBImplyingAttributes);
326
327 ++NumArgumentsReplacedWithPoison;
328 Changed = true;
329 }
330 }
331
332 return Changed;
333}
334
335/// Convenience function that returns the number of return values. It returns 0
336/// for void functions and 1 for functions not returning a struct. It returns
337/// the number of struct elements for functions returning a struct.
338static unsigned numRetVals(const Function *F) {
339 Type *RetTy = F->getReturnType();
340 if (RetTy->isVoidTy())
341 return 0;
342 if (StructType *STy = dyn_cast<StructType>(Val: RetTy))
343 return STy->getNumElements();
344 if (ArrayType *ATy = dyn_cast<ArrayType>(Val: RetTy))
345 return ATy->getNumElements();
346 return 1;
347}
348
349/// Returns the sub-type a function will return at a given Idx. Should
350/// correspond to the result type of an ExtractValue instruction executed with
351/// just that one Idx (i.e. only top-level structure is considered).
352static Type *getRetComponentType(const Function *F, unsigned Idx) {
353 Type *RetTy = F->getReturnType();
354 assert(!RetTy->isVoidTy() && "void type has no subtype");
355
356 if (StructType *STy = dyn_cast<StructType>(Val: RetTy))
357 return STy->getElementType(N: Idx);
358 if (ArrayType *ATy = dyn_cast<ArrayType>(Val: RetTy))
359 return ATy->getElementType();
360 return RetTy;
361}
362
363/// Checks Use for liveness in LiveValues. If Use is not live, it adds Use to
364/// the MaybeLiveUses argument. Returns the determined liveness of Use.
365DeadArgumentEliminationPass::Liveness
366DeadArgumentEliminationPass::markIfNotLive(RetOrArg Use,
367 UseVector &MaybeLiveUses) {
368 // We're live if our use or its Function is already marked as live.
369 if (isLive(RA: Use))
370 return Live;
371
372 // We're maybe live otherwise, but remember that we must become live if
373 // Use becomes live.
374 MaybeLiveUses.push_back(Elt: Use);
375 return MaybeLive;
376}
377
378/// Looks at a single use of an argument or return value and determines if it
379/// should be alive or not. Adds this use to MaybeLiveUses if it causes the
380/// used value to become MaybeLive.
381///
382/// RetValNum is the return value number to use when this use is used in a
383/// return instruction. This is used in the recursion, you should always leave
384/// it at 0.
385DeadArgumentEliminationPass::Liveness
386DeadArgumentEliminationPass::surveyUse(const Use *U, UseVector &MaybeLiveUses,
387 unsigned RetValNum) {
388 const User *V = U->getUser();
389 if (const ReturnInst *RI = dyn_cast<ReturnInst>(Val: V)) {
390 // The value is returned from a function. It's only live when the
391 // function's return value is live. We use RetValNum here, for the case
392 // that U is really a use of an insertvalue instruction that uses the
393 // original Use.
394 const Function *F = RI->getParent()->getParent();
395 if (RetValNum != -1U) {
396 RetOrArg Use = createRet(F, Idx: RetValNum);
397 // We might be live, depending on the liveness of Use.
398 return markIfNotLive(Use, MaybeLiveUses);
399 }
400
401 DeadArgumentEliminationPass::Liveness Result = MaybeLive;
402 for (unsigned Ri = 0; Ri < numRetVals(F); ++Ri) {
403 RetOrArg Use = createRet(F, Idx: Ri);
404 // We might be live, depending on the liveness of Use. If any
405 // sub-value is live, then the entire value is considered live. This
406 // is a conservative choice, and better tracking is possible.
407 DeadArgumentEliminationPass::Liveness SubResult =
408 markIfNotLive(Use, MaybeLiveUses);
409 if (Result != Live)
410 Result = SubResult;
411 }
412 return Result;
413 }
414
415 if (const InsertValueInst *IV = dyn_cast<InsertValueInst>(Val: V)) {
416 if (U->getOperandNo() != InsertValueInst::getAggregateOperandIndex() &&
417 IV->hasIndices())
418 // The use we are examining is inserted into an aggregate. Our liveness
419 // depends on all uses of that aggregate, but if it is used as a return
420 // value, only index at which we were inserted counts.
421 RetValNum = *IV->idx_begin();
422
423 // Note that if we are used as the aggregate operand to the insertvalue,
424 // we don't change RetValNum, but do survey all our uses.
425
426 Liveness Result = MaybeLive;
427 for (const Use &UU : IV->uses()) {
428 Result = surveyUse(U: &UU, MaybeLiveUses, RetValNum);
429 if (Result == Live)
430 break;
431 }
432 return Result;
433 }
434
435 if (const auto *CB = dyn_cast<CallBase>(Val: V)) {
436 const Function *F = CB->getCalledFunction();
437 if (F) {
438 // Used in a direct call.
439
440 // The function argument is live if it is used as a bundle operand.
441 if (CB->isBundleOperand(U))
442 return Live;
443
444 // Find the argument number. We know for sure that this use is an
445 // argument, since if it was the function argument this would be an
446 // indirect call and that we know can't be looking at a value of the
447 // label type (for the invoke instruction).
448 unsigned ArgNo = CB->getArgOperandNo(U);
449
450 if (ArgNo >= F->getFunctionType()->getNumParams())
451 // The value is passed in through a vararg! Must be live.
452 return Live;
453
454 assert(CB->getArgOperand(ArgNo) == CB->getOperand(U->getOperandNo()) &&
455 "Argument is not where we expected it");
456
457 // Value passed to a normal call. It's only live when the corresponding
458 // argument to the called function turns out live.
459 RetOrArg Use = createArg(F, Idx: ArgNo);
460 return markIfNotLive(Use, MaybeLiveUses);
461 }
462 }
463 // Used in any other way? Value must be live.
464 return Live;
465}
466
467/// Looks at all the uses of the given value
468/// Returns the Liveness deduced from the uses of this value.
469///
470/// Adds all uses that cause the result to be MaybeLive to MaybeLiveRetUses. If
471/// the result is Live, MaybeLiveUses might be modified but its content should
472/// be ignored (since it might not be complete).
473DeadArgumentEliminationPass::Liveness
474DeadArgumentEliminationPass::surveyUses(const Value *V,
475 UseVector &MaybeLiveUses) {
476 // Assume it's dead (which will only hold if there are no uses at all..).
477 Liveness Result = MaybeLive;
478 // Check each use.
479 for (const Use &U : V->uses()) {
480 Result = surveyUse(U: &U, MaybeLiveUses);
481 if (Result == Live)
482 break;
483 }
484 return Result;
485}
486
487/// Performs the initial survey of the specified function, checking out whether
488/// it uses any of its incoming arguments or whether any callers use the return
489/// value. This fills in the LiveValues set and Uses map.
490///
491/// We consider arguments of non-internal functions to be intrinsically alive as
492/// well as arguments to functions which have their "address taken".
493void DeadArgumentEliminationPass::surveyFunction(const Function &F) {
494 // Functions with inalloca/preallocated parameters are expecting args in a
495 // particular register and memory layout.
496 if (F.getAttributes().hasAttrSomewhere(Kind: Attribute::InAlloca) ||
497 F.getAttributes().hasAttrSomewhere(Kind: Attribute::Preallocated)) {
498 markLive(F);
499 return;
500 }
501
502 // Don't touch naked functions. The assembly might be using an argument, or
503 // otherwise rely on the frame layout in a way that this analysis will not
504 // see.
505 if (F.hasFnAttribute(Kind: Attribute::Naked)) {
506 markLive(F);
507 return;
508 }
509
510 unsigned RetCount = numRetVals(F: &F);
511
512 // Assume all return values are dead
513 using RetVals = SmallVector<Liveness, 5>;
514
515 RetVals RetValLiveness(RetCount, MaybeLive);
516
517 using RetUses = SmallVector<UseVector, 5>;
518
519 // These vectors map each return value to the uses that make it MaybeLive, so
520 // we can add those to the Uses map if the return value really turns out to be
521 // MaybeLive. Initialized to a list of RetCount empty lists.
522 RetUses MaybeLiveRetUses(RetCount);
523
524 bool HasMustTailCalls = false;
525 for (const BasicBlock &BB : F) {
526 // If we have any returns of `musttail` results - the signature can't
527 // change
528 if (const auto *TC = BB.getTerminatingMustTailCall()) {
529 HasMustTailCalls = true;
530 // In addition, if the called function is not locally defined (or unknown,
531 // if this is an indirect call), we can't change the callsite and thus
532 // can't change this function's signature either.
533 if (!isMustTailCalleeAnalyzable(CB: *TC)) {
534 markLive(F);
535 return;
536 }
537 }
538 }
539
540 if (HasMustTailCalls) {
541 LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - " << F.getName()
542 << " has musttail calls\n");
543 }
544
545 if (!F.hasLocalLinkage() && (!ShouldHackArguments || F.isIntrinsic())) {
546 markLive(F);
547 return;
548 }
549
550 LLVM_DEBUG(
551 dbgs() << "DeadArgumentEliminationPass - Inspecting callers for fn: "
552 << F.getName() << "\n");
553 // Keep track of the number of live retvals, so we can skip checks once all
554 // of them turn out to be live.
555 unsigned NumLiveRetVals = 0;
556
557 bool HasMustTailCallers = false;
558
559 // Loop all uses of the function.
560 for (const Use &U : F.uses()) {
561 // If the function is PASSED IN as an argument, its address has been
562 // taken.
563 const auto *CB = dyn_cast<CallBase>(Val: U.getUser());
564 if (!CB || !CB->isCallee(U: &U) ||
565 CB->getFunctionType() != F.getFunctionType()) {
566 markLive(F);
567 return;
568 }
569
570 // The number of arguments for `musttail` call must match the number of
571 // arguments of the caller
572 if (CB->isMustTailCall())
573 HasMustTailCallers = true;
574
575 // If we end up here, we are looking at a direct call to our function.
576
577 // Now, check how our return value(s) is/are used in this caller. Don't
578 // bother checking return values if all of them are live already.
579 if (NumLiveRetVals == RetCount)
580 continue;
581
582 // Check all uses of the return value.
583 for (const Use &UU : CB->uses()) {
584 if (ExtractValueInst *Ext = dyn_cast<ExtractValueInst>(Val: UU.getUser())) {
585 // This use uses a part of our return value, survey the uses of
586 // that part and store the results for this index only.
587 unsigned Idx = *Ext->idx_begin();
588 if (RetValLiveness[Idx] != Live) {
589 RetValLiveness[Idx] = surveyUses(V: Ext, MaybeLiveUses&: MaybeLiveRetUses[Idx]);
590 if (RetValLiveness[Idx] == Live)
591 NumLiveRetVals++;
592 }
593 } else {
594 // Used by something else than extractvalue. Survey, but assume that the
595 // result applies to all sub-values.
596 UseVector MaybeLiveAggregateUses;
597 if (surveyUse(U: &UU, MaybeLiveUses&: MaybeLiveAggregateUses) == Live) {
598 NumLiveRetVals = RetCount;
599 RetValLiveness.assign(NumElts: RetCount, Elt: Live);
600 break;
601 }
602
603 for (unsigned Ri = 0; Ri != RetCount; ++Ri) {
604 if (RetValLiveness[Ri] != Live)
605 MaybeLiveRetUses[Ri].append(in_start: MaybeLiveAggregateUses.begin(),
606 in_end: MaybeLiveAggregateUses.end());
607 }
608 }
609 }
610 }
611
612 if (HasMustTailCallers) {
613 LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - " << F.getName()
614 << " has musttail callers\n");
615 }
616
617 // Now we've inspected all callers, record the liveness of our return values.
618 for (unsigned Ri = 0; Ri != RetCount; ++Ri)
619 markValue(RA: createRet(F: &F, Idx: Ri), L: RetValLiveness[Ri], MaybeLiveUses: MaybeLiveRetUses[Ri]);
620
621 LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - Inspecting args for fn: "
622 << F.getName() << "\n");
623
624 // Now, check all of our arguments.
625 unsigned ArgI = 0;
626 UseVector MaybeLiveArgUses;
627 for (Function::const_arg_iterator AI = F.arg_begin(), E = F.arg_end();
628 AI != E; ++AI, ++ArgI) {
629 Liveness Result;
630 if (F.getFunctionType()->isVarArg() || HasMustTailCallers ||
631 HasMustTailCalls) {
632 // Variadic functions will already have a va_arg function expanded inside
633 // them, making them potentially very sensitive to ABI changes resulting
634 // from removing arguments entirely, so don't. For example AArch64 handles
635 // register and stack HFAs very differently, and this is reflected in the
636 // IR which has already been generated.
637 //
638 // `musttail` calls to this function restrict argument removal attempts.
639 // The signature of the caller must match the signature of the function.
640 //
641 // `musttail` calls in this function prevents us from changing its
642 // signature
643 Result = Live;
644 } else {
645 // See what the effect of this use is (recording any uses that cause
646 // MaybeLive in MaybeLiveArgUses).
647 Result = surveyUses(V: &*AI, MaybeLiveUses&: MaybeLiveArgUses);
648 }
649
650 // Mark the result.
651 markValue(RA: createArg(F: &F, Idx: ArgI), L: Result, MaybeLiveUses: MaybeLiveArgUses);
652 // Clear the vector again for the next iteration.
653 MaybeLiveArgUses.clear();
654 }
655}
656
657/// Marks the liveness of RA depending on L. If L is MaybeLive, it also takes
658/// all uses in MaybeLiveUses and records them in Uses, such that RA will be
659/// marked live if any use in MaybeLiveUses gets marked live later on.
660void DeadArgumentEliminationPass::markValue(const RetOrArg &RA, Liveness L,
661 const UseVector &MaybeLiveUses) {
662 switch (L) {
663 case Live:
664 markLive(RA);
665 break;
666 case MaybeLive:
667 assert(!isLive(RA) && "Use is already live!");
668 for (const auto &MaybeLiveUse : MaybeLiveUses) {
669 if (isLive(RA: MaybeLiveUse)) {
670 // A use is live, so this value is live.
671 markLive(RA);
672 break;
673 }
674 // Note any uses of this value, so this value can be
675 // marked live whenever one of the uses becomes live.
676 Uses.emplace(args: MaybeLiveUse, args: RA);
677 }
678 break;
679 }
680}
681
682/// Mark the given Function as alive, meaning that it cannot be changed in any
683/// way. Additionally, mark any values that are used as this function's
684/// parameters or by its return values (according to Uses) live as well.
685void DeadArgumentEliminationPass::markLive(const Function &F) {
686 LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - Intrinsically live fn: "
687 << F.getName() << "\n");
688 // Mark the function as live.
689 LiveFunctions.insert(x: &F);
690 // Mark all arguments as live.
691 for (unsigned ArgI = 0, E = F.arg_size(); ArgI != E; ++ArgI)
692 propagateLiveness(RA: createArg(F: &F, Idx: ArgI));
693 // Mark all return values as live.
694 for (unsigned Ri = 0, E = numRetVals(F: &F); Ri != E; ++Ri)
695 propagateLiveness(RA: createRet(F: &F, Idx: Ri));
696}
697
698/// Mark the given return value or argument as live. Additionally, mark any
699/// values that are used by this value (according to Uses) live as well.
700void DeadArgumentEliminationPass::markLive(const RetOrArg &RA) {
701 if (isLive(RA))
702 return; // Already marked Live.
703
704 LiveValues.insert(x: RA);
705
706 LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - Marking "
707 << RA.getDescription() << " live\n");
708 propagateLiveness(RA);
709}
710
711bool DeadArgumentEliminationPass::isLive(const RetOrArg &RA) {
712 return LiveFunctions.count(x: RA.F) || LiveValues.count(x: RA);
713}
714
715/// Given that RA is a live value, propagate it's liveness to any other values
716/// it uses (according to Uses).
717void DeadArgumentEliminationPass::propagateLiveness(const RetOrArg &RA) {
718 // We don't use upper_bound (or equal_range) here, because our recursive call
719 // to ourselves is likely to cause the upper_bound (which is the first value
720 // not belonging to RA) to become erased and the iterator invalidated.
721 UseMap::iterator Begin = Uses.lower_bound(x: RA);
722 UseMap::iterator E = Uses.end();
723 UseMap::iterator I;
724 for (I = Begin; I != E && I->first == RA; ++I)
725 markLive(RA: I->second);
726
727 // Erase RA from the Uses map (from the lower bound to wherever we ended up
728 // after the loop).
729 Uses.erase(first: Begin, last: I);
730}
731
732/// Remove any arguments and return values from F that are not in LiveValues.
733/// Transform the function and all the callees of the function to not have these
734/// arguments and return values.
735bool DeadArgumentEliminationPass::removeDeadStuffFromFunction(Function *F) {
736 // Don't modify fully live functions
737 if (LiveFunctions.count(x: F))
738 return false;
739
740 // Start by computing a new prototype for the function, which is the same as
741 // the old function, but has fewer arguments and a different return type.
742 FunctionType *FTy = F->getFunctionType();
743 std::vector<Type *> Params;
744
745 // Keep track of if we have a live 'returned' argument
746 bool HasLiveReturnedArg = false;
747
748 // Set up to build a new list of parameter attributes.
749 SmallVector<AttributeSet, 8> ArgAttrVec;
750 const AttributeList &PAL = F->getAttributes();
751
752 // Remember which arguments are still alive.
753 SmallVector<bool, 10> ArgAlive(FTy->getNumParams(), false);
754 // Construct the new parameter list from non-dead arguments. Also construct
755 // a new set of parameter attributes to correspond. Skip the first parameter
756 // attribute, since that belongs to the return value.
757 unsigned ArgI = 0;
758 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E;
759 ++I, ++ArgI) {
760 RetOrArg Arg = createArg(F, Idx: ArgI);
761 if (LiveValues.erase(x: Arg)) {
762 Params.push_back(x: I->getType());
763 ArgAlive[ArgI] = true;
764 ArgAttrVec.push_back(Elt: PAL.getParamAttrs(ArgNo: ArgI));
765 HasLiveReturnedArg |= PAL.hasParamAttr(ArgNo: ArgI, Kind: Attribute::Returned);
766 } else {
767 ++NumArgumentsEliminated;
768 LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - Removing argument "
769 << ArgI << " (" << I->getName() << ") from "
770 << F->getName() << "\n");
771 }
772 }
773
774 // Find out the new return value.
775 Type *RetTy = FTy->getReturnType();
776 Type *NRetTy = nullptr;
777 unsigned RetCount = numRetVals(F);
778
779 // -1 means unused, other numbers are the new index
780 SmallVector<int, 5> NewRetIdxs(RetCount, -1);
781 std::vector<Type *> RetTypes;
782
783 // If there is a function with a live 'returned' argument but a dead return
784 // value, then there are two possible actions:
785 // 1) Eliminate the return value and take off the 'returned' attribute on the
786 // argument.
787 // 2) Retain the 'returned' attribute and treat the return value (but not the
788 // entire function) as live so that it is not eliminated.
789 //
790 // It's not clear in the general case which option is more profitable because,
791 // even in the absence of explicit uses of the return value, code generation
792 // is free to use the 'returned' attribute to do things like eliding
793 // save/restores of registers across calls. Whether this happens is target and
794 // ABI-specific as well as depending on the amount of register pressure, so
795 // there's no good way for an IR-level pass to figure this out.
796 //
797 // Fortunately, the only places where 'returned' is currently generated by
798 // the FE are places where 'returned' is basically free and almost always a
799 // performance win, so the second option can just be used always for now.
800 //
801 // This should be revisited if 'returned' is ever applied more liberally.
802 if (RetTy->isVoidTy() || HasLiveReturnedArg) {
803 NRetTy = RetTy;
804 } else {
805 // Look at each of the original return values individually.
806 for (unsigned Ri = 0; Ri != RetCount; ++Ri) {
807 RetOrArg Ret = createRet(F, Idx: Ri);
808 if (LiveValues.erase(x: Ret)) {
809 RetTypes.push_back(x: getRetComponentType(F, Idx: Ri));
810 NewRetIdxs[Ri] = RetTypes.size() - 1;
811 } else {
812 ++NumRetValsEliminated;
813 LLVM_DEBUG(
814 dbgs() << "DeadArgumentEliminationPass - Removing return value "
815 << Ri << " from " << F->getName() << "\n");
816 }
817 }
818 if (RetTypes.size() > 1) {
819 // More than one return type? Reduce it down to size.
820 if (StructType *STy = dyn_cast<StructType>(Val: RetTy)) {
821 // Make the new struct packed if we used to return a packed struct
822 // already.
823 NRetTy = StructType::get(Context&: STy->getContext(), Elements: RetTypes, isPacked: STy->isPacked());
824 } else {
825 assert(isa<ArrayType>(RetTy) && "unexpected multi-value return");
826 NRetTy = ArrayType::get(ElementType: RetTypes[0], NumElements: RetTypes.size());
827 }
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.empty())
833 // No return types? Make it void, but only if we didn't use to return {}.
834 NRetTy = Type::getVoidTy(C&: F->getContext());
835 }
836
837 assert(NRetTy && "No new return type found?");
838
839 // The existing function return attributes.
840 AttrBuilder RAttrs(F->getContext(), PAL.getRetAttrs());
841
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())
847 RAttrs.remove(AM: AttributeFuncs::typeIncompatible(Ty: NRetTy));
848 else
849 assert(!RAttrs.overlaps(AttributeFuncs::typeIncompatible(NRetTy)) &&
850 "Return attributes no longer compatible?");
851
852 AttributeSet RetAttrs = AttributeSet::get(C&: F->getContext(), B: RAttrs);
853
854 // Strip allocsize attributes. They might refer to the deleted arguments.
855 AttributeSet FnAttrs =
856 PAL.getFnAttrs().removeAttribute(C&: F->getContext(), Kind: Attribute::AllocSize);
857
858 // Reconstruct the AttributesList based on the vector we constructed.
859 assert(ArgAttrVec.size() == Params.size());
860 AttributeList NewPAL =
861 AttributeList::get(C&: F->getContext(), FnAttrs, RetAttrs, ArgAttrs: ArgAttrVec);
862
863 // Create the new function type based on the recomputed parameters.
864 FunctionType *NFTy = FunctionType::get(Result: NRetTy, Params, isVarArg: FTy->isVarArg());
865
866 // No change?
867 if (NFTy == FTy)
868 return false;
869
870 // Create the new function body and insert it into the module...
871 Function *NF = Function::Create(Ty: NFTy, Linkage: F->getLinkage(), AddrSpace: F->getAddressSpace());
872 NF->copyAttributesFrom(Src: F);
873 NF->setComdat(F->getComdat());
874 NF->setAttributes(NewPAL);
875 // Insert the new function before the old function, so we won't be processing
876 // it again.
877 F->getParent()->getFunctionList().insert(where: F->getIterator(), New: NF);
878 NF->takeName(V: F);
879 NF->IsNewDbgInfoFormat = F->IsNewDbgInfoFormat;
880
881 // Loop over all the callers of the function, transforming the call sites to
882 // pass in a smaller number of arguments into the new function.
883 std::vector<Value *> Args;
884 while (!F->use_empty()) {
885 CallBase &CB = cast<CallBase>(Val&: *F->user_back());
886
887 ArgAttrVec.clear();
888 const AttributeList &CallPAL = CB.getAttributes();
889
890 // Adjust the call return attributes in case the function was changed to
891 // return void.
892 AttrBuilder RAttrs(F->getContext(), CallPAL.getRetAttrs());
893 RAttrs.remove(AM: AttributeFuncs::typeIncompatible(Ty: NRetTy));
894 AttributeSet RetAttrs = AttributeSet::get(C&: F->getContext(), B: RAttrs);
895
896 // Declare these outside of the loops, so we can reuse them for the second
897 // loop, which loops the varargs.
898 auto *I = CB.arg_begin();
899 unsigned Pi = 0;
900 // Loop over those operands, corresponding to the normal arguments to the
901 // original function, and add those that are still alive.
902 for (unsigned E = FTy->getNumParams(); Pi != E; ++I, ++Pi)
903 if (ArgAlive[Pi]) {
904 Args.push_back(x: *I);
905 // Get original parameter attributes, but skip return attributes.
906 AttributeSet Attrs = CallPAL.getParamAttrs(ArgNo: Pi);
907 if (NRetTy != RetTy && Attrs.hasAttribute(Kind: Attribute::Returned)) {
908 // If the return type has changed, then get rid of 'returned' on the
909 // call site. The alternative is to make all 'returned' attributes on
910 // call sites keep the return value alive just like 'returned'
911 // attributes on function declaration, but it's less clearly a win and
912 // this is not an expected case anyway
913 ArgAttrVec.push_back(Elt: AttributeSet::get(
914 C&: F->getContext(), B: AttrBuilder(F->getContext(), Attrs)
915 .removeAttribute(Val: Attribute::Returned)));
916 } else {
917 // Otherwise, use the original attributes.
918 ArgAttrVec.push_back(Elt: Attrs);
919 }
920 }
921
922 // Push any varargs arguments on the list. Don't forget their attributes.
923 for (auto *E = CB.arg_end(); I != E; ++I, ++Pi) {
924 Args.push_back(x: *I);
925 ArgAttrVec.push_back(Elt: CallPAL.getParamAttrs(ArgNo: Pi));
926 }
927
928 // Reconstruct the AttributesList based on the vector we constructed.
929 assert(ArgAttrVec.size() == Args.size());
930
931 // Again, be sure to remove any allocsize attributes, since their indices
932 // may now be incorrect.
933 AttributeSet FnAttrs = CallPAL.getFnAttrs().removeAttribute(
934 C&: F->getContext(), Kind: Attribute::AllocSize);
935
936 AttributeList NewCallPAL =
937 AttributeList::get(C&: F->getContext(), FnAttrs, RetAttrs, ArgAttrs: ArgAttrVec);
938
939 SmallVector<OperandBundleDef, 1> OpBundles;
940 CB.getOperandBundlesAsDefs(Defs&: OpBundles);
941
942 CallBase *NewCB = nullptr;
943 if (InvokeInst *II = dyn_cast<InvokeInst>(Val: &CB)) {
944 NewCB = InvokeInst::Create(Func: NF, IfNormal: II->getNormalDest(), IfException: II->getUnwindDest(),
945 Args, Bundles: OpBundles, NameStr: "", InsertBefore: CB.getParent());
946 } else {
947 NewCB = CallInst::Create(Ty: NFTy, Func: NF, Args, Bundles: OpBundles, NameStr: "", InsertBefore: CB.getIterator());
948 cast<CallInst>(Val: NewCB)->setTailCallKind(
949 cast<CallInst>(Val: &CB)->getTailCallKind());
950 }
951 NewCB->setCallingConv(CB.getCallingConv());
952 NewCB->setAttributes(NewCallPAL);
953 NewCB->copyMetadata(SrcInst: CB, WL: {LLVMContext::MD_prof, LLVMContext::MD_dbg});
954 Args.clear();
955 ArgAttrVec.clear();
956
957 if (!CB.use_empty() || CB.isUsedByMetadata()) {
958 if (NewCB->getType() == CB.getType()) {
959 // Return type not changed? Just replace users then.
960 CB.replaceAllUsesWith(V: NewCB);
961 NewCB->takeName(V: &CB);
962 } else if (NewCB->getType()->isVoidTy()) {
963 // If the return value is dead, replace any uses of it with poison
964 // (any non-debug value uses will get removed later on).
965 if (!CB.getType()->isX86_MMXTy())
966 CB.replaceAllUsesWith(V: PoisonValue::get(T: CB.getType()));
967 } else {
968 assert((RetTy->isStructTy() || RetTy->isArrayTy()) &&
969 "Return type changed, but not into a void. The old return type"
970 " must have been a struct or an array!");
971 Instruction *InsertPt = &CB;
972 if (InvokeInst *II = dyn_cast<InvokeInst>(Val: &CB)) {
973 BasicBlock *NewEdge =
974 SplitEdge(From: NewCB->getParent(), To: II->getNormalDest());
975 InsertPt = &*NewEdge->getFirstInsertionPt();
976 }
977
978 // We used to return a struct or array. Instead of doing smart stuff
979 // with all the uses, we will just rebuild it using extract/insertvalue
980 // chaining and let instcombine clean that up.
981 //
982 // Start out building up our return value from poison
983 Value *RetVal = PoisonValue::get(T: RetTy);
984 for (unsigned Ri = 0; Ri != RetCount; ++Ri)
985 if (NewRetIdxs[Ri] != -1) {
986 Value *V;
987 IRBuilder<NoFolder> IRB(InsertPt);
988 if (RetTypes.size() > 1)
989 // We are still returning a struct, so extract the value from our
990 // return value
991 V = IRB.CreateExtractValue(Agg: NewCB, Idxs: NewRetIdxs[Ri], Name: "newret");
992 else
993 // We are now returning a single element, so just insert that
994 V = NewCB;
995 // Insert the value at the old position
996 RetVal = IRB.CreateInsertValue(Agg: RetVal, Val: V, Idxs: Ri, Name: "oldret");
997 }
998 // Now, replace all uses of the old call instruction with the return
999 // struct we built
1000 CB.replaceAllUsesWith(V: RetVal);
1001 NewCB->takeName(V: &CB);
1002 }
1003 }
1004
1005 // Finally, remove the old call from the program, reducing the use-count of
1006 // F.
1007 CB.eraseFromParent();
1008 }
1009
1010 // Since we have now created the new function, splice the body of the old
1011 // function right into the new function, leaving the old rotting hulk of the
1012 // function empty.
1013 NF->splice(ToIt: NF->begin(), FromF: F);
1014
1015 // Loop over the argument list, transferring uses of the old arguments over to
1016 // the new arguments, also transferring over the names as well.
1017 ArgI = 0;
1018 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(),
1019 I2 = NF->arg_begin();
1020 I != E; ++I, ++ArgI)
1021 if (ArgAlive[ArgI]) {
1022 // If this is a live argument, move the name and users over to the new
1023 // version.
1024 I->replaceAllUsesWith(V: &*I2);
1025 I2->takeName(V: &*I);
1026 ++I2;
1027 } else {
1028 // If this argument is dead, replace any uses of it with poison
1029 // (any non-debug value uses will get removed later on).
1030 if (!I->getType()->isX86_MMXTy())
1031 I->replaceAllUsesWith(V: PoisonValue::get(T: I->getType()));
1032 }
1033
1034 // If we change the return value of the function we must rewrite any return
1035 // instructions. Check this now.
1036 if (F->getReturnType() != NF->getReturnType())
1037 for (BasicBlock &BB : *NF)
1038 if (ReturnInst *RI = dyn_cast<ReturnInst>(Val: BB.getTerminator())) {
1039 IRBuilder<NoFolder> IRB(RI);
1040 Value *RetVal = nullptr;
1041
1042 if (!NFTy->getReturnType()->isVoidTy()) {
1043 assert(RetTy->isStructTy() || RetTy->isArrayTy());
1044 // The original return value was a struct or array, insert
1045 // extractvalue/insertvalue chains to extract only the values we need
1046 // to return and insert them into our new result.
1047 // This does generate messy code, but we'll let it to instcombine to
1048 // clean that up.
1049 Value *OldRet = RI->getOperand(i_nocapture: 0);
1050 // Start out building up our return value from poison
1051 RetVal = PoisonValue::get(T: NRetTy);
1052 for (unsigned RetI = 0; RetI != RetCount; ++RetI)
1053 if (NewRetIdxs[RetI] != -1) {
1054 Value *EV = IRB.CreateExtractValue(Agg: OldRet, Idxs: RetI, Name: "oldret");
1055
1056 if (RetTypes.size() > 1) {
1057 // We're still returning a struct, so reinsert the value into
1058 // our new return value at the new index
1059
1060 RetVal = IRB.CreateInsertValue(Agg: RetVal, Val: EV, Idxs: NewRetIdxs[RetI],
1061 Name: "newret");
1062 } else {
1063 // We are now only returning a simple value, so just return the
1064 // extracted value.
1065 RetVal = EV;
1066 }
1067 }
1068 }
1069 // Replace the return instruction with one returning the new return
1070 // value (possibly 0 if we became void).
1071 auto *NewRet =
1072 ReturnInst::Create(C&: F->getContext(), retVal: RetVal, InsertBefore: RI->getIterator());
1073 NewRet->setDebugLoc(RI->getDebugLoc());
1074 RI->eraseFromParent();
1075 }
1076
1077 // Clone metadata from the old function, including debug info descriptor.
1078 SmallVector<std::pair<unsigned, MDNode *>, 1> MDs;
1079 F->getAllMetadata(MDs);
1080 for (auto [KindID, Node] : MDs)
1081 NF->addMetadata(KindID, MD&: *Node);
1082
1083 // If either the return value(s) or argument(s) are removed, then probably the
1084 // function does not follow standard calling conventions anymore. Hence, add
1085 // DW_CC_nocall to DISubroutineType to inform debugger that it may not be safe
1086 // to call this function or try to interpret the return value.
1087 if (NFTy != FTy && NF->getSubprogram()) {
1088 DISubprogram *SP = NF->getSubprogram();
1089 auto Temp = SP->getType()->cloneWithCC(CC: llvm::dwarf::DW_CC_nocall);
1090 SP->replaceType(Ty: MDNode::replaceWithPermanent(N: std::move(Temp)));
1091 }
1092
1093 // Now that the old function is dead, delete it.
1094 F->eraseFromParent();
1095
1096 return true;
1097}
1098
1099void DeadArgumentEliminationPass::propagateVirtMustcallLiveness(
1100 const Module &M) {
1101 // If a function was marked "live", and it has musttail callers, they in turn
1102 // can't change either.
1103 LiveFuncSet NewLiveFuncs(LiveFunctions);
1104 while (!NewLiveFuncs.empty()) {
1105 LiveFuncSet Temp;
1106 for (const auto *F : NewLiveFuncs)
1107 for (const auto *U : F->users())
1108 if (const auto *CB = dyn_cast<CallBase>(Val: U))
1109 if (CB->isMustTailCall())
1110 if (!LiveFunctions.count(x: CB->getParent()->getParent()))
1111 Temp.insert(x: CB->getParent()->getParent());
1112 NewLiveFuncs.clear();
1113 NewLiveFuncs.insert(first: Temp.begin(), last: Temp.end());
1114 for (const auto *F : Temp)
1115 markLive(F: *F);
1116 }
1117}
1118
1119PreservedAnalyses DeadArgumentEliminationPass::run(Module &M,
1120 ModuleAnalysisManager &) {
1121 bool Changed = false;
1122
1123 // First pass: Do a simple check to see if any functions can have their "..."
1124 // removed. We can do this if they never call va_start. This loop cannot be
1125 // fused with the next loop, because deleting a function invalidates
1126 // information computed while surveying other functions.
1127 LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - Deleting dead varargs\n");
1128 for (Function &F : llvm::make_early_inc_range(Range&: M))
1129 if (F.getFunctionType()->isVarArg())
1130 Changed |= deleteDeadVarargs(F);
1131
1132 // Second phase: Loop through the module, determining which arguments are
1133 // live. We assume all arguments are dead unless proven otherwise (allowing us
1134 // to determine that dead arguments passed into recursive functions are dead).
1135 LLVM_DEBUG(dbgs() << "DeadArgumentEliminationPass - Determining liveness\n");
1136 for (auto &F : M)
1137 surveyFunction(F);
1138
1139 propagateVirtMustcallLiveness(M);
1140
1141 // Now, remove all dead arguments and return values from each function in
1142 // turn. We use make_early_inc_range here because functions will probably get
1143 // removed (i.e. replaced by new ones).
1144 for (Function &F : llvm::make_early_inc_range(Range&: M))
1145 Changed |= removeDeadStuffFromFunction(F: &F);
1146
1147 // Finally, look for any unused parameters in functions with non-local
1148 // linkage and replace the passed in parameters with poison.
1149 for (auto &F : M)
1150 Changed |= removeDeadArgumentsFromCallers(F);
1151
1152 if (!Changed)
1153 return PreservedAnalyses::all();
1154 return PreservedAnalyses::none();
1155}
1156