1//===-- BasicBlock.cpp - Implement BasicBlock related methods -------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file implements the BasicBlock class for the IR library.
11//
12//===----------------------------------------------------------------------===//
13
14#include "llvm/IR/BasicBlock.h"
15#include "SymbolTableListTraitsImpl.h"
16#include "llvm/ADT/STLExtras.h"
17#include "llvm/IR/CFG.h"
18#include "llvm/IR/Constants.h"
19#include "llvm/IR/Instructions.h"
20#include "llvm/IR/IntrinsicInst.h"
21#include "llvm/IR/LLVMContext.h"
22#include "llvm/IR/Type.h"
23#include <algorithm>
24using namespace llvm;
25
26ValueSymbolTable *BasicBlock::getValueSymbolTable() {
27  if (Function *F = getParent())
28    return &F->getValueSymbolTable();
29  return nullptr;
30}
31
32LLVMContext &BasicBlock::getContext() const {
33  return getType()->getContext();
34}
35
36// Explicit instantiation of SymbolTableListTraits since some of the methods
37// are not in the public header file...
38template class llvm::SymbolTableListTraits<Instruction, BasicBlock>;
39
40
41BasicBlock::BasicBlock(LLVMContext &C, const Twine &Name, Function *NewParent,
42                       BasicBlock *InsertBefore)
43  : Value(Type::getLabelTy(C), Value::BasicBlockVal), Parent(nullptr) {
44
45  if (NewParent)
46    insertInto(NewParent, InsertBefore);
47  else
48    assert(!InsertBefore &&
49           "Cannot insert block before another block with no function!");
50
51  setName(Name);
52}
53
54void BasicBlock::insertInto(Function *NewParent, BasicBlock *InsertBefore) {
55  assert(NewParent && "Expected a parent");
56  assert(!Parent && "Already has a parent");
57
58  if (InsertBefore)
59    NewParent->getBasicBlockList().insert(InsertBefore, this);
60  else
61    NewParent->getBasicBlockList().push_back(this);
62}
63
64BasicBlock::~BasicBlock() {
65  // If the address of the block is taken and it is being deleted (e.g. because
66  // it is dead), this means that there is either a dangling constant expr
67  // hanging off the block, or an undefined use of the block (source code
68  // expecting the address of a label to keep the block alive even though there
69  // is no indirect branch).  Handle these cases by zapping the BlockAddress
70  // nodes.  There are no other possible uses at this point.
71  if (hasAddressTaken()) {
72    assert(!use_empty() && "There should be at least one blockaddress!");
73    Constant *Replacement =
74      ConstantInt::get(llvm::Type::getInt32Ty(getContext()), 1);
75    while (!use_empty()) {
76      BlockAddress *BA = cast<BlockAddress>(user_back());
77      BA->replaceAllUsesWith(ConstantExpr::getIntToPtr(Replacement,
78                                                       BA->getType()));
79      BA->destroyConstant();
80    }
81  }
82
83  assert(getParent() == nullptr && "BasicBlock still linked into the program!");
84  dropAllReferences();
85  InstList.clear();
86}
87
88void BasicBlock::setParent(Function *parent) {
89  // Set Parent=parent, updating instruction symtab entries as appropriate.
90  InstList.setSymTabObject(&Parent, parent);
91}
92
93void BasicBlock::removeFromParent() {
94  getParent()->getBasicBlockList().remove(this);
95}
96
97iplist<BasicBlock>::iterator BasicBlock::eraseFromParent() {
98  return getParent()->getBasicBlockList().erase(this);
99}
100
101/// Unlink this basic block from its current function and
102/// insert it into the function that MovePos lives in, right before MovePos.
103void BasicBlock::moveBefore(BasicBlock *MovePos) {
104  MovePos->getParent()->getBasicBlockList().splice(MovePos,
105                       getParent()->getBasicBlockList(), this);
106}
107
108/// Unlink this basic block from its current function and
109/// insert it into the function that MovePos lives in, right after MovePos.
110void BasicBlock::moveAfter(BasicBlock *MovePos) {
111  Function::iterator I = MovePos;
112  MovePos->getParent()->getBasicBlockList().splice(++I,
113                                       getParent()->getBasicBlockList(), this);
114}
115
116const Module *BasicBlock::getModule() const {
117  return getParent()->getParent();
118}
119
120TerminatorInst *BasicBlock::getTerminator() {
121  if (InstList.empty()) return nullptr;
122  return dyn_cast<TerminatorInst>(&InstList.back());
123}
124
125const TerminatorInst *BasicBlock::getTerminator() const {
126  if (InstList.empty()) return nullptr;
127  return dyn_cast<TerminatorInst>(&InstList.back());
128}
129
130CallInst *BasicBlock::getTerminatingMustTailCall() {
131  if (InstList.empty())
132    return nullptr;
133  ReturnInst *RI = dyn_cast<ReturnInst>(&InstList.back());
134  if (!RI || RI == &InstList.front())
135    return nullptr;
136
137  Instruction *Prev = RI->getPrevNode();
138  if (!Prev)
139    return nullptr;
140
141  if (Value *RV = RI->getReturnValue()) {
142    if (RV != Prev)
143      return nullptr;
144
145    // Look through the optional bitcast.
146    if (auto *BI = dyn_cast<BitCastInst>(Prev)) {
147      RV = BI->getOperand(0);
148      Prev = BI->getPrevNode();
149      if (!Prev || RV != Prev)
150        return nullptr;
151    }
152  }
153
154  if (auto *CI = dyn_cast<CallInst>(Prev)) {
155    if (CI->isMustTailCall())
156      return CI;
157  }
158  return nullptr;
159}
160
161Instruction* BasicBlock::getFirstNonPHI() {
162  BasicBlock::iterator i = begin();
163  // All valid basic blocks should have a terminator,
164  // which is not a PHINode. If we have an invalid basic
165  // block we'll get an assertion failure when dereferencing
166  // a past-the-end iterator.
167  while (isa<PHINode>(i)) ++i;
168  return &*i;
169}
170
171Instruction* BasicBlock::getFirstNonPHIOrDbg() {
172  BasicBlock::iterator i = begin();
173  // All valid basic blocks should have a terminator,
174  // which is not a PHINode. If we have an invalid basic
175  // block we'll get an assertion failure when dereferencing
176  // a past-the-end iterator.
177  while (isa<PHINode>(i) || isa<DbgInfoIntrinsic>(i)) ++i;
178  return &*i;
179}
180
181Instruction* BasicBlock::getFirstNonPHIOrDbgOrLifetime() {
182  // All valid basic blocks should have a terminator,
183  // which is not a PHINode. If we have an invalid basic
184  // block we'll get an assertion failure when dereferencing
185  // a past-the-end iterator.
186  BasicBlock::iterator i = begin();
187  for (;; ++i) {
188    if (isa<PHINode>(i) || isa<DbgInfoIntrinsic>(i))
189      continue;
190
191    const IntrinsicInst *II = dyn_cast<IntrinsicInst>(i);
192    if (!II)
193      break;
194    if (II->getIntrinsicID() != Intrinsic::lifetime_start &&
195        II->getIntrinsicID() != Intrinsic::lifetime_end)
196      break;
197  }
198  return &*i;
199}
200
201BasicBlock::iterator BasicBlock::getFirstInsertionPt() {
202  iterator InsertPt = getFirstNonPHI();
203  if (isa<LandingPadInst>(InsertPt)) ++InsertPt;
204  return InsertPt;
205}
206
207void BasicBlock::dropAllReferences() {
208  for(iterator I = begin(), E = end(); I != E; ++I)
209    I->dropAllReferences();
210}
211
212/// If this basic block has a single predecessor block,
213/// return the block, otherwise return a null pointer.
214BasicBlock *BasicBlock::getSinglePredecessor() {
215  pred_iterator PI = pred_begin(this), E = pred_end(this);
216  if (PI == E) return nullptr;         // No preds.
217  BasicBlock *ThePred = *PI;
218  ++PI;
219  return (PI == E) ? ThePred : nullptr /*multiple preds*/;
220}
221
222/// If this basic block has a unique predecessor block,
223/// return the block, otherwise return a null pointer.
224/// Note that unique predecessor doesn't mean single edge, there can be
225/// multiple edges from the unique predecessor to this block (for example
226/// a switch statement with multiple cases having the same destination).
227BasicBlock *BasicBlock::getUniquePredecessor() {
228  pred_iterator PI = pred_begin(this), E = pred_end(this);
229  if (PI == E) return nullptr; // No preds.
230  BasicBlock *PredBB = *PI;
231  ++PI;
232  for (;PI != E; ++PI) {
233    if (*PI != PredBB)
234      return nullptr;
235    // The same predecessor appears multiple times in the predecessor list.
236    // This is OK.
237  }
238  return PredBB;
239}
240
241BasicBlock *BasicBlock::getUniqueSuccessor() {
242  succ_iterator SI = succ_begin(this), E = succ_end(this);
243  if (SI == E) return NULL; // No successors
244  BasicBlock *SuccBB = *SI;
245  ++SI;
246  for (;SI != E; ++SI) {
247    if (*SI != SuccBB)
248      return NULL;
249    // The same successor appears multiple times in the successor list.
250    // This is OK.
251  }
252  return SuccBB;
253}
254
255/// This method is used to notify a BasicBlock that the
256/// specified Predecessor of the block is no longer able to reach it.  This is
257/// actually not used to update the Predecessor list, but is actually used to
258/// update the PHI nodes that reside in the block.  Note that this should be
259/// called while the predecessor still refers to this block.
260///
261void BasicBlock::removePredecessor(BasicBlock *Pred,
262                                   bool DontDeleteUselessPHIs) {
263  assert((hasNUsesOrMore(16)||// Reduce cost of this assertion for complex CFGs.
264          find(pred_begin(this), pred_end(this), Pred) != pred_end(this)) &&
265         "removePredecessor: BB is not a predecessor!");
266
267  if (InstList.empty()) return;
268  PHINode *APN = dyn_cast<PHINode>(&front());
269  if (!APN) return;   // Quick exit.
270
271  // If there are exactly two predecessors, then we want to nuke the PHI nodes
272  // altogether.  However, we cannot do this, if this in this case:
273  //
274  //  Loop:
275  //    %x = phi [X, Loop]
276  //    %x2 = add %x, 1         ;; This would become %x2 = add %x2, 1
277  //    br Loop                 ;; %x2 does not dominate all uses
278  //
279  // This is because the PHI node input is actually taken from the predecessor
280  // basic block.  The only case this can happen is with a self loop, so we
281  // check for this case explicitly now.
282  //
283  unsigned max_idx = APN->getNumIncomingValues();
284  assert(max_idx != 0 && "PHI Node in block with 0 predecessors!?!?!");
285  if (max_idx == 2) {
286    BasicBlock *Other = APN->getIncomingBlock(APN->getIncomingBlock(0) == Pred);
287
288    // Disable PHI elimination!
289    if (this == Other) max_idx = 3;
290  }
291
292  // <= Two predecessors BEFORE I remove one?
293  if (max_idx <= 2 && !DontDeleteUselessPHIs) {
294    // Yup, loop through and nuke the PHI nodes
295    while (PHINode *PN = dyn_cast<PHINode>(&front())) {
296      // Remove the predecessor first.
297      PN->removeIncomingValue(Pred, !DontDeleteUselessPHIs);
298
299      // If the PHI _HAD_ two uses, replace PHI node with its now *single* value
300      if (max_idx == 2) {
301        if (PN->getIncomingValue(0) != PN)
302          PN->replaceAllUsesWith(PN->getIncomingValue(0));
303        else
304          // We are left with an infinite loop with no entries: kill the PHI.
305          PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
306        getInstList().pop_front();    // Remove the PHI node
307      }
308
309      // If the PHI node already only had one entry, it got deleted by
310      // removeIncomingValue.
311    }
312  } else {
313    // Okay, now we know that we need to remove predecessor #pred_idx from all
314    // PHI nodes.  Iterate over each PHI node fixing them up
315    PHINode *PN;
316    for (iterator II = begin(); (PN = dyn_cast<PHINode>(II)); ) {
317      ++II;
318      PN->removeIncomingValue(Pred, false);
319      // If all incoming values to the Phi are the same, we can replace the Phi
320      // with that value.
321      Value* PNV = nullptr;
322      if (!DontDeleteUselessPHIs && (PNV = PN->hasConstantValue()))
323        if (PNV != PN) {
324          PN->replaceAllUsesWith(PNV);
325          PN->eraseFromParent();
326        }
327    }
328  }
329}
330
331
332/// This splits a basic block into two at the specified
333/// instruction.  Note that all instructions BEFORE the specified iterator stay
334/// as part of the original basic block, an unconditional branch is added to
335/// the new BB, and the rest of the instructions in the BB are moved to the new
336/// BB, including the old terminator.  This invalidates the iterator.
337///
338/// Note that this only works on well formed basic blocks (must have a
339/// terminator), and 'I' must not be the end of instruction list (which would
340/// cause a degenerate basic block to be formed, having a terminator inside of
341/// the basic block).
342///
343BasicBlock *BasicBlock::splitBasicBlock(iterator I, const Twine &BBName) {
344  assert(getTerminator() && "Can't use splitBasicBlock on degenerate BB!");
345  assert(I != InstList.end() &&
346         "Trying to get me to create degenerate basic block!");
347
348  BasicBlock *InsertBefore = std::next(Function::iterator(this))
349                               .getNodePtrUnchecked();
350  BasicBlock *New = BasicBlock::Create(getContext(), BBName,
351                                       getParent(), InsertBefore);
352
353  // Move all of the specified instructions from the original basic block into
354  // the new basic block.
355  New->getInstList().splice(New->end(), this->getInstList(), I, end());
356
357  // Add a branch instruction to the newly formed basic block.
358  BranchInst::Create(New, this);
359
360  // Now we must loop through all of the successors of the New block (which
361  // _were_ the successors of the 'this' block), and update any PHI nodes in
362  // successors.  If there were PHI nodes in the successors, then they need to
363  // know that incoming branches will be from New, not from Old.
364  //
365  for (succ_iterator I = succ_begin(New), E = succ_end(New); I != E; ++I) {
366    // Loop over any phi nodes in the basic block, updating the BB field of
367    // incoming values...
368    BasicBlock *Successor = *I;
369    PHINode *PN;
370    for (BasicBlock::iterator II = Successor->begin();
371         (PN = dyn_cast<PHINode>(II)); ++II) {
372      int IDX = PN->getBasicBlockIndex(this);
373      while (IDX != -1) {
374        PN->setIncomingBlock((unsigned)IDX, New);
375        IDX = PN->getBasicBlockIndex(this);
376      }
377    }
378  }
379  return New;
380}
381
382void BasicBlock::replaceSuccessorsPhiUsesWith(BasicBlock *New) {
383  TerminatorInst *TI = getTerminator();
384  if (!TI)
385    // Cope with being called on a BasicBlock that doesn't have a terminator
386    // yet. Clang's CodeGenFunction::EmitReturnBlock() likes to do this.
387    return;
388  for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
389    BasicBlock *Succ = TI->getSuccessor(i);
390    // N.B. Succ might not be a complete BasicBlock, so don't assume
391    // that it ends with a non-phi instruction.
392    for (iterator II = Succ->begin(), IE = Succ->end(); II != IE; ++II) {
393      PHINode *PN = dyn_cast<PHINode>(II);
394      if (!PN)
395        break;
396      int i;
397      while ((i = PN->getBasicBlockIndex(this)) >= 0)
398        PN->setIncomingBlock(i, New);
399    }
400  }
401}
402
403/// Return true if this basic block is a landing pad. I.e., it's
404/// the destination of the 'unwind' edge of an invoke instruction.
405bool BasicBlock::isLandingPad() const {
406  return isa<LandingPadInst>(getFirstNonPHI());
407}
408
409/// Return the landingpad instruction associated with the landing pad.
410LandingPadInst *BasicBlock::getLandingPadInst() {
411  return dyn_cast<LandingPadInst>(getFirstNonPHI());
412}
413const LandingPadInst *BasicBlock::getLandingPadInst() const {
414  return dyn_cast<LandingPadInst>(getFirstNonPHI());
415}
416