aboutsummaryrefslogtreecommitdiffstats
path: root/contrib/libs/llvm14/lib/Transforms/Scalar/SCCP.cpp
blob: fa1cfc84e4fdaeaf7550b8256ee1249ddc2e05fc (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
//===- SCCP.cpp - Sparse Conditional Constant Propagation -----------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements sparse conditional constant propagation and merging:
//
// Specifically, this:
//   * Assumes values are constant unless proven otherwise
//   * Assumes BasicBlocks are dead unless proven otherwise
//   * Proves values to be constant, and replaces them with constants
//   * Proves conditional branches to be unconditional
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Scalar/SCCP.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/PointerIntPair.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/ValueLattice.h"
#include "llvm/Analysis/ValueLatticeUtils.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/InstVisitor.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/PredicateInfo.h"
#include <cassert>
#include <utility>
#include <vector>

using namespace llvm;

#define DEBUG_TYPE "sccp"

STATISTIC(NumInstRemoved, "Number of instructions removed");
STATISTIC(NumDeadBlocks , "Number of basic blocks unreachable");
STATISTIC(NumInstReplaced,
          "Number of instructions replaced with (simpler) instruction");

STATISTIC(IPNumInstRemoved, "Number of instructions removed by IPSCCP");
STATISTIC(IPNumArgsElimed ,"Number of arguments constant propagated by IPSCCP");
STATISTIC(IPNumGlobalConst, "Number of globals found to be constant by IPSCCP");
STATISTIC(
    IPNumInstReplaced,
    "Number of instructions replaced with (simpler) instruction by IPSCCP");

// Helper to check if \p LV is either a constant or a constant
// range with a single element. This should cover exactly the same cases as the
// old ValueLatticeElement::isConstant() and is intended to be used in the
// transition to ValueLatticeElement.
static bool isConstant(const ValueLatticeElement &LV) {
  return LV.isConstant() ||
         (LV.isConstantRange() && LV.getConstantRange().isSingleElement());
}

// Helper to check if \p LV is either overdefined or a constant range with more
// than a single element. This should cover exactly the same cases as the old
// ValueLatticeElement::isOverdefined() and is intended to be used in the
// transition to ValueLatticeElement.
static bool isOverdefined(const ValueLatticeElement &LV) {
  return !LV.isUnknownOrUndef() && !isConstant(LV);
}

static bool tryToReplaceWithConstant(SCCPSolver &Solver, Value *V) {
  Constant *Const = nullptr;
  if (V->getType()->isStructTy()) {
    std::vector<ValueLatticeElement> IVs = Solver.getStructLatticeValueFor(V);
    if (llvm::any_of(IVs, isOverdefined))
      return false;
    std::vector<Constant *> ConstVals;
    auto *ST = cast<StructType>(V->getType());
    for (unsigned i = 0, e = ST->getNumElements(); i != e; ++i) {
      ValueLatticeElement V = IVs[i];
      ConstVals.push_back(isConstant(V)
                              ? Solver.getConstant(V)
                              : UndefValue::get(ST->getElementType(i)));
    }
    Const = ConstantStruct::get(ST, ConstVals);
  } else {
    const ValueLatticeElement &IV = Solver.getLatticeValueFor(V);
    if (isOverdefined(IV))
      return false;

    Const =
        isConstant(IV) ? Solver.getConstant(IV) : UndefValue::get(V->getType());
  }
  assert(Const && "Constant is nullptr here!");

  // Replacing `musttail` instructions with constant breaks `musttail` invariant
  // unless the call itself can be removed.
  // Calls with "clang.arc.attachedcall" implicitly use the return value and
  // those uses cannot be updated with a constant.
  CallBase *CB = dyn_cast<CallBase>(V);
  if (CB && ((CB->isMustTailCall() && !CB->isSafeToRemove()) ||
             CB->getOperandBundle(LLVMContext::OB_clang_arc_attachedcall))) {
    Function *F = CB->getCalledFunction();

    // Don't zap returns of the callee
    if (F)
      Solver.addToMustPreserveReturnsInFunctions(F);

    LLVM_DEBUG(dbgs() << "  Can\'t treat the result of call " << *CB
                      << " as a constant\n");
    return false;
  }

  LLVM_DEBUG(dbgs() << "  Constant: " << *Const << " = " << *V << '\n');

  // Replaces all of the uses of a variable with uses of the constant.
  V->replaceAllUsesWith(Const);
  return true;
}

static bool simplifyInstsInBlock(SCCPSolver &Solver, BasicBlock &BB,
                                 SmallPtrSetImpl<Value *> &InsertedValues,
                                 Statistic &InstRemovedStat,
                                 Statistic &InstReplacedStat) {
  bool MadeChanges = false;
  for (Instruction &Inst : make_early_inc_range(BB)) {
    if (Inst.getType()->isVoidTy())
      continue;
    if (tryToReplaceWithConstant(Solver, &Inst)) {
      if (Inst.isSafeToRemove())
        Inst.eraseFromParent();

      MadeChanges = true;
      ++InstRemovedStat;
    } else if (isa<SExtInst>(&Inst)) {
      Value *ExtOp = Inst.getOperand(0);
      if (isa<Constant>(ExtOp) || InsertedValues.count(ExtOp))
        continue;
      const ValueLatticeElement &IV = Solver.getLatticeValueFor(ExtOp);
      if (!IV.isConstantRange(/*UndefAllowed=*/false))
        continue;
      if (IV.getConstantRange().isAllNonNegative()) {
        auto *ZExt = new ZExtInst(ExtOp, Inst.getType(), "", &Inst);
        InsertedValues.insert(ZExt);
        Inst.replaceAllUsesWith(ZExt);
        Solver.removeLatticeValueFor(&Inst);
        Inst.eraseFromParent();
        InstReplacedStat++;
        MadeChanges = true;
      }
    }
  }
  return MadeChanges;
}

// runSCCP() - Run the Sparse Conditional Constant Propagation algorithm,
// and return true if the function was modified.
static bool runSCCP(Function &F, const DataLayout &DL,
                    const TargetLibraryInfo *TLI) {
  LLVM_DEBUG(dbgs() << "SCCP on function '" << F.getName() << "'\n");
  SCCPSolver Solver(
      DL, [TLI](Function &F) -> const TargetLibraryInfo & { return *TLI; },
      F.getContext());

  // Mark the first block of the function as being executable.
  Solver.markBlockExecutable(&F.front());

  // Mark all arguments to the function as being overdefined.
  for (Argument &AI : F.args())
    Solver.markOverdefined(&AI);

  // Solve for constants.
  bool ResolvedUndefs = true;
  while (ResolvedUndefs) {
    Solver.solve();
    LLVM_DEBUG(dbgs() << "RESOLVING UNDEFs\n");
    ResolvedUndefs = Solver.resolvedUndefsIn(F);
  }

  bool MadeChanges = false;

  // If we decided that there are basic blocks that are dead in this function,
  // delete their contents now.  Note that we cannot actually delete the blocks,
  // as we cannot modify the CFG of the function.

  SmallPtrSet<Value *, 32> InsertedValues;
  for (BasicBlock &BB : F) {
    if (!Solver.isBlockExecutable(&BB)) {
      LLVM_DEBUG(dbgs() << "  BasicBlock Dead:" << BB);

      ++NumDeadBlocks;
      NumInstRemoved += removeAllNonTerminatorAndEHPadInstructions(&BB).first;

      MadeChanges = true;
      continue;
    }

    MadeChanges |= simplifyInstsInBlock(Solver, BB, InsertedValues,
                                        NumInstRemoved, NumInstReplaced);
  }

  return MadeChanges;
}

PreservedAnalyses SCCPPass::run(Function &F, FunctionAnalysisManager &AM) {
  const DataLayout &DL = F.getParent()->getDataLayout();
  auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
  if (!runSCCP(F, DL, &TLI))
    return PreservedAnalyses::all();

  auto PA = PreservedAnalyses();
  PA.preserveSet<CFGAnalyses>();
  return PA;
}

namespace {

//===--------------------------------------------------------------------===//
//
/// SCCP Class - This class uses the SCCPSolver to implement a per-function
/// Sparse Conditional Constant Propagator.
///
class SCCPLegacyPass : public FunctionPass {
public:
  // Pass identification, replacement for typeid
  static char ID;

  SCCPLegacyPass() : FunctionPass(ID) {
    initializeSCCPLegacyPassPass(*PassRegistry::getPassRegistry());
  }

  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.addRequired<TargetLibraryInfoWrapperPass>();
    AU.addPreserved<GlobalsAAWrapperPass>();
    AU.setPreservesCFG();
  }

  // runOnFunction - Run the Sparse Conditional Constant Propagation
  // algorithm, and return true if the function was modified.
  bool runOnFunction(Function &F) override {
    if (skipFunction(F))
      return false;
    const DataLayout &DL = F.getParent()->getDataLayout();
    const TargetLibraryInfo *TLI =
        &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
    return runSCCP(F, DL, TLI);
  }
};

} // end anonymous namespace

char SCCPLegacyPass::ID = 0;

INITIALIZE_PASS_BEGIN(SCCPLegacyPass, "sccp",
                      "Sparse Conditional Constant Propagation", false, false)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
INITIALIZE_PASS_END(SCCPLegacyPass, "sccp",
                    "Sparse Conditional Constant Propagation", false, false)

// createSCCPPass - This is the public interface to this file.
FunctionPass *llvm::createSCCPPass() { return new SCCPLegacyPass(); }

static void findReturnsToZap(Function &F,
                             SmallVector<ReturnInst *, 8> &ReturnsToZap,
                             SCCPSolver &Solver) {
  // We can only do this if we know that nothing else can call the function.
  if (!Solver.isArgumentTrackedFunction(&F))
    return;

  if (Solver.mustPreserveReturn(&F)) {
    LLVM_DEBUG(
        dbgs()
        << "Can't zap returns of the function : " << F.getName()
        << " due to present musttail or \"clang.arc.attachedcall\" call of "
           "it\n");
    return;
  }

  assert(
      all_of(F.users(),
             [&Solver](User *U) {
               if (isa<Instruction>(U) &&
                   !Solver.isBlockExecutable(cast<Instruction>(U)->getParent()))
                 return true;
               // Non-callsite uses are not impacted by zapping. Also, constant
               // uses (like blockaddresses) could stuck around, without being
               // used in the underlying IR, meaning we do not have lattice
               // values for them.
               if (!isa<CallBase>(U))
                 return true;
               if (U->getType()->isStructTy()) {
                 return all_of(Solver.getStructLatticeValueFor(U),
                               [](const ValueLatticeElement &LV) {
                                 return !isOverdefined(LV);
                               });
               }
               return !isOverdefined(Solver.getLatticeValueFor(U));
             }) &&
      "We can only zap functions where all live users have a concrete value");

  for (BasicBlock &BB : F) {
    if (CallInst *CI = BB.getTerminatingMustTailCall()) {
      LLVM_DEBUG(dbgs() << "Can't zap return of the block due to present "
                        << "musttail call : " << *CI << "\n");
      (void)CI;
      return;
    }

    if (auto *RI = dyn_cast<ReturnInst>(BB.getTerminator()))
      if (!isa<UndefValue>(RI->getOperand(0)))
        ReturnsToZap.push_back(RI);
  }
}

static bool removeNonFeasibleEdges(const SCCPSolver &Solver, BasicBlock *BB,
                                   DomTreeUpdater &DTU,
                                   BasicBlock *&NewUnreachableBB) {
  SmallPtrSet<BasicBlock *, 8> FeasibleSuccessors;
  bool HasNonFeasibleEdges = false;
  for (BasicBlock *Succ : successors(BB)) {
    if (Solver.isEdgeFeasible(BB, Succ))
      FeasibleSuccessors.insert(Succ);
    else
      HasNonFeasibleEdges = true;
  }

  // All edges feasible, nothing to do.
  if (!HasNonFeasibleEdges)
    return false;

  // SCCP can only determine non-feasible edges for br, switch and indirectbr.
  Instruction *TI = BB->getTerminator();
  assert((isa<BranchInst>(TI) || isa<SwitchInst>(TI) ||
          isa<IndirectBrInst>(TI)) &&
         "Terminator must be a br, switch or indirectbr");

  if (FeasibleSuccessors.size() == 1) {
    // Replace with an unconditional branch to the only feasible successor.
    BasicBlock *OnlyFeasibleSuccessor = *FeasibleSuccessors.begin();
    SmallVector<DominatorTree::UpdateType, 8> Updates;
    bool HaveSeenOnlyFeasibleSuccessor = false;
    for (BasicBlock *Succ : successors(BB)) {
      if (Succ == OnlyFeasibleSuccessor && !HaveSeenOnlyFeasibleSuccessor) {
        // Don't remove the edge to the only feasible successor the first time
        // we see it. We still do need to remove any multi-edges to it though.
        HaveSeenOnlyFeasibleSuccessor = true;
        continue;
      }

      Succ->removePredecessor(BB);
      Updates.push_back({DominatorTree::Delete, BB, Succ});
    }

    BranchInst::Create(OnlyFeasibleSuccessor, BB);
    TI->eraseFromParent();
    DTU.applyUpdatesPermissive(Updates);
  } else if (FeasibleSuccessors.size() > 1) {
    SwitchInstProfUpdateWrapper SI(*cast<SwitchInst>(TI));
    SmallVector<DominatorTree::UpdateType, 8> Updates;

    // If the default destination is unfeasible it will never be taken. Replace
    // it with a new block with a single Unreachable instruction.
    BasicBlock *DefaultDest = SI->getDefaultDest();
    if (!FeasibleSuccessors.contains(DefaultDest)) {
      if (!NewUnreachableBB) {
        NewUnreachableBB =
            BasicBlock::Create(DefaultDest->getContext(), "default.unreachable",
                               DefaultDest->getParent(), DefaultDest);
        new UnreachableInst(DefaultDest->getContext(), NewUnreachableBB);
      }

      SI->setDefaultDest(NewUnreachableBB);
      Updates.push_back({DominatorTree::Delete, BB, DefaultDest});
      Updates.push_back({DominatorTree::Insert, BB, NewUnreachableBB});
    }

    for (auto CI = SI->case_begin(); CI != SI->case_end();) {
      if (FeasibleSuccessors.contains(CI->getCaseSuccessor())) {
        ++CI;
        continue;
      }

      BasicBlock *Succ = CI->getCaseSuccessor();
      Succ->removePredecessor(BB);
      Updates.push_back({DominatorTree::Delete, BB, Succ});
      SI.removeCase(CI);
      // Don't increment CI, as we removed a case.
    }

    DTU.applyUpdatesPermissive(Updates);
  } else {
    llvm_unreachable("Must have at least one feasible successor");
  }
  return true;
}

bool llvm::runIPSCCP(
    Module &M, const DataLayout &DL,
    std::function<const TargetLibraryInfo &(Function &)> GetTLI,
    function_ref<AnalysisResultsForFn(Function &)> getAnalysis) {
  SCCPSolver Solver(DL, GetTLI, M.getContext());

  // Loop over all functions, marking arguments to those with their addresses
  // taken or that are external as overdefined.
  for (Function &F : M) {
    if (F.isDeclaration())
      continue;

    Solver.addAnalysis(F, getAnalysis(F));

    // Determine if we can track the function's return values. If so, add the
    // function to the solver's set of return-tracked functions.
    if (canTrackReturnsInterprocedurally(&F))
      Solver.addTrackedFunction(&F);

    // Determine if we can track the function's arguments. If so, add the
    // function to the solver's set of argument-tracked functions.
    if (canTrackArgumentsInterprocedurally(&F)) {
      Solver.addArgumentTrackedFunction(&F);
      continue;
    }

    // Assume the function is called.
    Solver.markBlockExecutable(&F.front());

    // Assume nothing about the incoming arguments.
    for (Argument &AI : F.args())
      Solver.markOverdefined(&AI);
  }

  // Determine if we can track any of the module's global variables. If so, add
  // the global variables we can track to the solver's set of tracked global
  // variables.
  for (GlobalVariable &G : M.globals()) {
    G.removeDeadConstantUsers();
    if (canTrackGlobalVariableInterprocedurally(&G))
      Solver.trackValueOfGlobalVariable(&G);
  }

  // Solve for constants.
  bool ResolvedUndefs = true;
  Solver.solve();
  while (ResolvedUndefs) {
    LLVM_DEBUG(dbgs() << "RESOLVING UNDEFS\n");
    ResolvedUndefs = false;
    for (Function &F : M) {
      if (Solver.resolvedUndefsIn(F))
        ResolvedUndefs = true;
    }
    if (ResolvedUndefs)
      Solver.solve();
  }

  bool MadeChanges = false;

  // Iterate over all of the instructions in the module, replacing them with
  // constants if we have found them to be of constant values.

  for (Function &F : M) {
    if (F.isDeclaration())
      continue;

    SmallVector<BasicBlock *, 512> BlocksToErase;

    if (Solver.isBlockExecutable(&F.front())) {
      bool ReplacedPointerArg = false;
      for (Argument &Arg : F.args()) {
        if (!Arg.use_empty() && tryToReplaceWithConstant(Solver, &Arg)) {
          ReplacedPointerArg |= Arg.getType()->isPointerTy();
          ++IPNumArgsElimed;
        }
      }

      // If we replaced an argument, the argmemonly and
      // inaccessiblemem_or_argmemonly attributes do not hold any longer. Remove
      // them from both the function and callsites.
      if (ReplacedPointerArg) {
        AttributeMask AttributesToRemove;
        AttributesToRemove.addAttribute(Attribute::ArgMemOnly);
        AttributesToRemove.addAttribute(Attribute::InaccessibleMemOrArgMemOnly);
        F.removeFnAttrs(AttributesToRemove);

        for (User *U : F.users()) {
          auto *CB = dyn_cast<CallBase>(U);
          if (!CB || CB->getCalledFunction() != &F)
            continue;

          CB->removeFnAttrs(AttributesToRemove);
        }
      }
      MadeChanges |= ReplacedPointerArg;
    }

    SmallPtrSet<Value *, 32> InsertedValues;
    for (BasicBlock &BB : F) {
      if (!Solver.isBlockExecutable(&BB)) {
        LLVM_DEBUG(dbgs() << "  BasicBlock Dead:" << BB);
        ++NumDeadBlocks;

        MadeChanges = true;

        if (&BB != &F.front())
          BlocksToErase.push_back(&BB);
        continue;
      }

      MadeChanges |= simplifyInstsInBlock(Solver, BB, InsertedValues,
                                          IPNumInstRemoved, IPNumInstReplaced);
    }

    DomTreeUpdater DTU = Solver.getDTU(F);
    // Change dead blocks to unreachable. We do it after replacing constants
    // in all executable blocks, because changeToUnreachable may remove PHI
    // nodes in executable blocks we found values for. The function's entry
    // block is not part of BlocksToErase, so we have to handle it separately.
    for (BasicBlock *BB : BlocksToErase) {
      NumInstRemoved += changeToUnreachable(BB->getFirstNonPHI(),
                                            /*PreserveLCSSA=*/false, &DTU);
    }
    if (!Solver.isBlockExecutable(&F.front()))
      NumInstRemoved += changeToUnreachable(F.front().getFirstNonPHI(),
                                            /*PreserveLCSSA=*/false, &DTU);

    BasicBlock *NewUnreachableBB = nullptr;
    for (BasicBlock &BB : F)
      MadeChanges |= removeNonFeasibleEdges(Solver, &BB, DTU, NewUnreachableBB);

    for (BasicBlock *DeadBB : BlocksToErase)
      DTU.deleteBB(DeadBB);

    for (BasicBlock &BB : F) {
      for (Instruction &Inst : llvm::make_early_inc_range(BB)) {
        if (Solver.getPredicateInfoFor(&Inst)) {
          if (auto *II = dyn_cast<IntrinsicInst>(&Inst)) {
            if (II->getIntrinsicID() == Intrinsic::ssa_copy) {
              Value *Op = II->getOperand(0);
              Inst.replaceAllUsesWith(Op);
              Inst.eraseFromParent();
            }
          }
        }
      }
    }
  }

  // If we inferred constant or undef return values for a function, we replaced
  // all call uses with the inferred value.  This means we don't need to bother
  // actually returning anything from the function.  Replace all return
  // instructions with return undef.
  //
  // Do this in two stages: first identify the functions we should process, then
  // actually zap their returns.  This is important because we can only do this
  // if the address of the function isn't taken.  In cases where a return is the
  // last use of a function, the order of processing functions would affect
  // whether other functions are optimizable.
  SmallVector<ReturnInst*, 8> ReturnsToZap;

  for (const auto &I : Solver.getTrackedRetVals()) {
    Function *F = I.first;
    const ValueLatticeElement &ReturnValue = I.second;

    // If there is a known constant range for the return value, add !range
    // metadata to the function's call sites.
    if (ReturnValue.isConstantRange() &&
        !ReturnValue.getConstantRange().isSingleElement()) {
      // Do not add range metadata if the return value may include undef.
      if (ReturnValue.isConstantRangeIncludingUndef())
        continue;

      auto &CR = ReturnValue.getConstantRange();
      for (User *User : F->users()) {
        auto *CB = dyn_cast<CallBase>(User);
        if (!CB || CB->getCalledFunction() != F)
          continue;

        // Limit to cases where the return value is guaranteed to be neither
        // poison nor undef. Poison will be outside any range and currently
        // values outside of the specified range cause immediate undefined
        // behavior.
        if (!isGuaranteedNotToBeUndefOrPoison(CB, nullptr, CB))
          continue;

        // Do not touch existing metadata for now.
        // TODO: We should be able to take the intersection of the existing
        // metadata and the inferred range.
        if (CB->getMetadata(LLVMContext::MD_range))
          continue;

        LLVMContext &Context = CB->getParent()->getContext();
        Metadata *RangeMD[] = {
            ConstantAsMetadata::get(ConstantInt::get(Context, CR.getLower())),
            ConstantAsMetadata::get(ConstantInt::get(Context, CR.getUpper()))};
        CB->setMetadata(LLVMContext::MD_range, MDNode::get(Context, RangeMD));
      }
      continue;
    }
    if (F->getReturnType()->isVoidTy())
      continue;
    if (isConstant(ReturnValue) || ReturnValue.isUnknownOrUndef())
      findReturnsToZap(*F, ReturnsToZap, Solver);
  }

  for (auto F : Solver.getMRVFunctionsTracked()) {
    assert(F->getReturnType()->isStructTy() &&
           "The return type should be a struct");
    StructType *STy = cast<StructType>(F->getReturnType());
    if (Solver.isStructLatticeConstant(F, STy))
      findReturnsToZap(*F, ReturnsToZap, Solver);
  }

  // Zap all returns which we've identified as zap to change.
  SmallSetVector<Function *, 8> FuncZappedReturn;
  for (unsigned i = 0, e = ReturnsToZap.size(); i != e; ++i) {
    Function *F = ReturnsToZap[i]->getParent()->getParent();
    ReturnsToZap[i]->setOperand(0, UndefValue::get(F->getReturnType()));
    // Record all functions that are zapped.
    FuncZappedReturn.insert(F);
  }

  // Remove the returned attribute for zapped functions and the
  // corresponding call sites.
  for (Function *F : FuncZappedReturn) {
    for (Argument &A : F->args())
      F->removeParamAttr(A.getArgNo(), Attribute::Returned);
    for (Use &U : F->uses()) {
      // Skip over blockaddr users.
      if (isa<BlockAddress>(U.getUser()))
        continue;
      CallBase *CB = cast<CallBase>(U.getUser());
      for (Use &Arg : CB->args())
        CB->removeParamAttr(CB->getArgOperandNo(&Arg), Attribute::Returned);
    }
  }

  // If we inferred constant or undef values for globals variables, we can
  // delete the global and any stores that remain to it.
  for (auto &I : make_early_inc_range(Solver.getTrackedGlobals())) {
    GlobalVariable *GV = I.first;
    if (isOverdefined(I.second))
      continue;
    LLVM_DEBUG(dbgs() << "Found that GV '" << GV->getName()
                      << "' is constant!\n");
    while (!GV->use_empty()) {
      StoreInst *SI = cast<StoreInst>(GV->user_back());
      SI->eraseFromParent();
      MadeChanges = true;
    }
    M.getGlobalList().erase(GV);
    ++IPNumGlobalConst;
  }

  return MadeChanges;
}