BasicBlock.cpp revision 17fcdd5e1b78b829068ca657c97357a39d6e768b
1//===-- BasicBlock.cpp - Implement BasicBlock related methods -------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file was developed by the LLVM research group and is distributed under
6// the University of Illinois Open Source License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file implements the BasicBlock class for the VMCore library.
11//
12//===----------------------------------------------------------------------===//
13
14#include "llvm/BasicBlock.h"
15#include "llvm/Constants.h"
16#include "llvm/Instructions.h"
17#include "llvm/Type.h"
18#include "llvm/Support/CFG.h"
19#include "llvm/Support/LeakDetector.h"
20#include "llvm/Support/Compiler.h"
21#include "SymbolTableListTraitsImpl.h"
22#include <algorithm>
23using namespace llvm;
24
25inline ValueSymbolTable *
26ilist_traits<Instruction>::getSymTab(BasicBlock *BB) {
27  if (BB)
28    if (Function *F = BB->getParent())
29      return &F->getValueSymbolTable();
30  return 0;
31}
32
33
34namespace {
35  /// DummyInst - An instance of this class is used to mark the end of the
36  /// instruction list.  This is not a real instruction.
37  struct VISIBILITY_HIDDEN DummyInst : public Instruction {
38    DummyInst() : Instruction(Type::VoidTy, OtherOpsEnd, 0, 0) {
39      // This should not be garbage monitored.
40      LeakDetector::removeGarbageObject(this);
41    }
42
43    virtual Instruction *clone() const {
44      assert(0 && "Cannot clone EOL");abort();
45      return 0;
46    }
47    virtual const char *getOpcodeName() const { return "*end-of-list-inst*"; }
48
49    // Methods for support type inquiry through isa, cast, and dyn_cast...
50    static inline bool classof(const DummyInst *) { return true; }
51    static inline bool classof(const Instruction *I) {
52      return I->getOpcode() == OtherOpsEnd;
53    }
54    static inline bool classof(const Value *V) {
55      return isa<Instruction>(V) && classof(cast<Instruction>(V));
56    }
57  };
58}
59
60Instruction *ilist_traits<Instruction>::createSentinel() {
61  return new DummyInst();
62}
63iplist<Instruction> &ilist_traits<Instruction>::getList(BasicBlock *BB) {
64  return BB->getInstList();
65}
66
67// Explicit instantiation of SymbolTableListTraits since some of the methods
68// are not in the public header file...
69template class SymbolTableListTraits<Instruction, BasicBlock>;
70
71
72BasicBlock::BasicBlock(const std::string &Name, Function *NewParent,
73                       BasicBlock *InsertBefore)
74  : Value(Type::LabelTy, Value::BasicBlockVal), Parent(0) {
75  // Initialize the instlist.
76  InstList.setItemParent(this);
77
78  // Make sure that we get added to a function
79  LeakDetector::addGarbageObject(this);
80
81  if (InsertBefore) {
82    assert(NewParent &&
83           "Cannot insert block before another block with no function!");
84    NewParent->getBasicBlockList().insert(InsertBefore, this);
85  } else if (NewParent) {
86    NewParent->getBasicBlockList().push_back(this);
87  }
88
89  setName(Name);
90}
91
92
93BasicBlock::~BasicBlock() {
94  assert(getParent() == 0 && "BasicBlock still linked into the program!");
95  dropAllReferences();
96  InstList.clear();
97}
98
99void BasicBlock::setParent(Function *parent) {
100  if (getParent())
101    LeakDetector::addGarbageObject(this);
102
103  // Set Parent=parent, updating instruction symtab entries as appropriate.
104  InstList.setSymTabObject(&Parent, parent);
105
106  if (getParent())
107    LeakDetector::removeGarbageObject(this);
108}
109
110void BasicBlock::removeFromParent() {
111  getParent()->getBasicBlockList().remove(this);
112}
113
114void BasicBlock::eraseFromParent() {
115  getParent()->getBasicBlockList().erase(this);
116}
117
118/// moveBefore - Unlink this basic block from its current function and
119/// insert it into the function that MovePos lives in, right before MovePos.
120void BasicBlock::moveBefore(BasicBlock *MovePos) {
121  MovePos->getParent()->getBasicBlockList().splice(MovePos,
122                       getParent()->getBasicBlockList(), this);
123}
124
125/// moveAfter - Unlink this basic block from its current function and
126/// insert it into the function that MovePos lives in, right after MovePos.
127void BasicBlock::moveAfter(BasicBlock *MovePos) {
128  Function::iterator I = MovePos;
129  MovePos->getParent()->getBasicBlockList().splice(++I,
130                                       getParent()->getBasicBlockList(), this);
131}
132
133
134TerminatorInst *BasicBlock::getTerminator() {
135  if (InstList.empty()) return 0;
136  return dyn_cast<TerminatorInst>(&InstList.back());
137}
138
139const TerminatorInst *const BasicBlock::getTerminator() const {
140  if (InstList.empty()) return 0;
141  return dyn_cast<TerminatorInst>(&InstList.back());
142}
143
144Instruction* BasicBlock::getFirstNonPHI()
145{
146    BasicBlock::iterator i = begin();
147    // All valid basic blocks should have a terminator,
148    // which is not a PHINode. If we have invalid basic
149    // block we'll get assert when dereferencing past-the-end
150    // iterator.
151    while (isa<PHINode>(i)) ++i;
152    return &*i;
153}
154
155void BasicBlock::dropAllReferences() {
156  for(iterator I = begin(), E = end(); I != E; ++I)
157    I->dropAllReferences();
158}
159
160/// getSinglePredecessor - If this basic block has a single predecessor block,
161/// return the block, otherwise return a null pointer.
162BasicBlock *BasicBlock::getSinglePredecessor() {
163  pred_iterator PI = pred_begin(this), E = pred_end(this);
164  if (PI == E) return 0;         // No preds.
165  BasicBlock *ThePred = *PI;
166  ++PI;
167  return (PI == E) ? ThePred : 0 /*multiple preds*/;
168}
169
170/// removePredecessor - This method is used to notify a BasicBlock that the
171/// specified Predecessor of the block is no longer able to reach it.  This is
172/// actually not used to update the Predecessor list, but is actually used to
173/// update the PHI nodes that reside in the block.  Note that this should be
174/// called while the predecessor still refers to this block.
175///
176void BasicBlock::removePredecessor(BasicBlock *Pred,
177                                   bool DontDeleteUselessPHIs) {
178  assert((hasNUsesOrMore(16)||// Reduce cost of this assertion for complex CFGs.
179          find(pred_begin(this), pred_end(this), Pred) != pred_end(this)) &&
180         "removePredecessor: BB is not a predecessor!");
181
182  if (InstList.empty()) return;
183  PHINode *APN = dyn_cast<PHINode>(&front());
184  if (!APN) return;   // Quick exit.
185
186  // If there are exactly two predecessors, then we want to nuke the PHI nodes
187  // altogether.  However, we cannot do this, if this in this case:
188  //
189  //  Loop:
190  //    %x = phi [X, Loop]
191  //    %x2 = add %x, 1         ;; This would become %x2 = add %x2, 1
192  //    br Loop                 ;; %x2 does not dominate all uses
193  //
194  // This is because the PHI node input is actually taken from the predecessor
195  // basic block.  The only case this can happen is with a self loop, so we
196  // check for this case explicitly now.
197  //
198  unsigned max_idx = APN->getNumIncomingValues();
199  assert(max_idx != 0 && "PHI Node in block with 0 predecessors!?!?!");
200  if (max_idx == 2) {
201    BasicBlock *Other = APN->getIncomingBlock(APN->getIncomingBlock(0) == Pred);
202
203    // Disable PHI elimination!
204    if (this == Other) max_idx = 3;
205  }
206
207  // <= Two predecessors BEFORE I remove one?
208  if (max_idx <= 2 && !DontDeleteUselessPHIs) {
209    // Yup, loop through and nuke the PHI nodes
210    while (PHINode *PN = dyn_cast<PHINode>(&front())) {
211      // Remove the predecessor first.
212      PN->removeIncomingValue(Pred, !DontDeleteUselessPHIs);
213
214      // If the PHI _HAD_ two uses, replace PHI node with its now *single* value
215      if (max_idx == 2) {
216        if (PN->getOperand(0) != PN)
217          PN->replaceAllUsesWith(PN->getOperand(0));
218        else
219          // We are left with an infinite loop with no entries: kill the PHI.
220          PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
221        getInstList().pop_front();    // Remove the PHI node
222      }
223
224      // If the PHI node already only had one entry, it got deleted by
225      // removeIncomingValue.
226    }
227  } else {
228    // Okay, now we know that we need to remove predecessor #pred_idx from all
229    // PHI nodes.  Iterate over each PHI node fixing them up
230    PHINode *PN;
231    for (iterator II = begin(); (PN = dyn_cast<PHINode>(II)); ) {
232      ++II;
233      PN->removeIncomingValue(Pred, false);
234      // If all incoming values to the Phi are the same, we can replace the Phi
235      // with that value.
236      Value* PNV = 0;
237      if (!DontDeleteUselessPHIs && (PNV = PN->hasConstantValue())) {
238        PN->replaceAllUsesWith(PNV);
239        PN->eraseFromParent();
240      }
241    }
242  }
243}
244
245
246/// splitBasicBlock - This splits a basic block into two at the specified
247/// instruction.  Note that all instructions BEFORE the specified iterator stay
248/// as part of the original basic block, an unconditional branch is added to
249/// the new BB, and the rest of the instructions in the BB are moved to the new
250/// BB, including the old terminator.  This invalidates the iterator.
251///
252/// Note that this only works on well formed basic blocks (must have a
253/// terminator), and 'I' must not be the end of instruction list (which would
254/// cause a degenerate basic block to be formed, having a terminator inside of
255/// the basic block).
256///
257BasicBlock *BasicBlock::splitBasicBlock(iterator I, const std::string &BBName) {
258  assert(getTerminator() && "Can't use splitBasicBlock on degenerate BB!");
259  assert(I != InstList.end() &&
260         "Trying to get me to create degenerate basic block!");
261
262  BasicBlock *New = new BasicBlock(BBName, getParent(), getNext());
263
264  // Move all of the specified instructions from the original basic block into
265  // the new basic block.
266  New->getInstList().splice(New->end(), this->getInstList(), I, end());
267
268  // Add a branch instruction to the newly formed basic block.
269  new BranchInst(New, this);
270
271  // Now we must loop through all of the successors of the New block (which
272  // _were_ the successors of the 'this' block), and update any PHI nodes in
273  // successors.  If there were PHI nodes in the successors, then they need to
274  // know that incoming branches will be from New, not from Old.
275  //
276  for (succ_iterator I = succ_begin(New), E = succ_end(New); I != E; ++I) {
277    // Loop over any phi nodes in the basic block, updating the BB field of
278    // incoming values...
279    BasicBlock *Successor = *I;
280    PHINode *PN;
281    for (BasicBlock::iterator II = Successor->begin();
282         (PN = dyn_cast<PHINode>(II)); ++II) {
283      int IDX = PN->getBasicBlockIndex(this);
284      while (IDX != -1) {
285        PN->setIncomingBlock((unsigned)IDX, New);
286        IDX = PN->getBasicBlockIndex(this);
287      }
288    }
289  }
290  return New;
291}
292