BasicBlock.cpp revision dec628eead87b20773c98a00830580df211acc98 
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
25namespace {
26  /// DummyInst - An instance of this class is used to mark the end of the
27  /// instruction list.  This is not a real instruction.
28  struct VISIBILITY_HIDDEN DummyInst : public Instruction {
29    DummyInst() : Instruction(Type::VoidTy, OtherOpsEnd, 0, 0) {
30      // This should not be garbage monitored.
31      LeakDetector::removeGarbageObject(this);
32    }
33
34    virtual Instruction *clone() const {
35      assert(0 && "Cannot clone EOL");abort();
36      return 0;
37    }
38    virtual const char *getOpcodeName() const { return "*end-of-list-inst*"; }
39
40    // Methods for support type inquiry through isa, cast, and dyn_cast...
41    static inline bool classof(const DummyInst *) { return true; }
42    static inline bool classof(const Instruction *I) {
43      return I->getOpcode() == OtherOpsEnd;
44    }
45    static inline bool classof(const Value *V) {
46      return isa<Instruction>(V) && classof(cast<Instruction>(V));
47    }
48  };
49}
50
51Instruction *ilist_traits<Instruction>::createSentinel() {
52  return new DummyInst();
53}
54iplist<Instruction> &ilist_traits<Instruction>::getList(BasicBlock *BB) {
55  return BB->getInstList();
56}
57
58// Explicit instantiation of SymbolTableListTraits since some of the methods
59// are not in the public header file...
60template class SymbolTableListTraits<Instruction, BasicBlock, Function>;
61
62
63BasicBlock::BasicBlock(const std::string &Name, Function *Parent,
64                       BasicBlock *InsertBefore)
65  : Value(Type::LabelTy, Value::BasicBlockVal) {
66  // Initialize the instlist...
67  InstList.setItemParent(this);
68
69  // Make sure that we get added to a function
70  LeakDetector::addGarbageObject(this);
71
72  if (InsertBefore) {
73    assert(Parent &&
74           "Cannot insert block before another block with no function!");
75    Parent->getBasicBlockList().insert(InsertBefore, this);
76  } else if (Parent) {
77    Parent->getBasicBlockList().push_back(this);
78  }
79
80  setName(Name);
81}
82
83
84BasicBlock::~BasicBlock() {
85  assert(getParent() == 0 && "BasicBlock still linked into the program!");
86  dropAllReferences();
87  InstList.clear();
88}
89
90void BasicBlock::setParent(Function *parent) {
91  if (getParent())
92    LeakDetector::addGarbageObject(this);
93
94  InstList.setParent(parent);
95
96  if (getParent())
97    LeakDetector::removeGarbageObject(this);
98}
99
100void BasicBlock::removeFromParent() {
101  getParent()->getBasicBlockList().remove(this);
102}
103
104void BasicBlock::eraseFromParent() {
105  getParent()->getBasicBlockList().erase(this);
106}
107
108/// moveBefore - Unlink this basic block from its current function and
109/// insert it into the function that MovePos lives in, right before MovePos.
110void BasicBlock::moveBefore(BasicBlock *MovePos) {
111  MovePos->getParent()->getBasicBlockList().splice(MovePos,
112                       getParent()->getBasicBlockList(), this);
113}
114
115/// moveAfter - Unlink this basic block from its current function and
116/// insert it into the function that MovePos lives in, right after MovePos.
117void BasicBlock::moveAfter(BasicBlock *MovePos) {
118  Function::iterator I = MovePos;
119  MovePos->getParent()->getBasicBlockList().splice(++I,
120                                       getParent()->getBasicBlockList(), this);
121}
122
123
124TerminatorInst *BasicBlock::getTerminator() {
125  if (InstList.empty()) return 0;
126  return dyn_cast<TerminatorInst>(&InstList.back());
127}
128
129const TerminatorInst *const BasicBlock::getTerminator() const {
130  if (InstList.empty()) return 0;
131  return dyn_cast<TerminatorInst>(&InstList.back());
132}
133
134Instruction* BasicBlock::getFirstNonPHI()
135{
136    BasicBlock::iterator i = begin();
137    // All valid basic blocks should have a terminator,
138    // which is not a PHINode. If we have invalid basic
139    // block we'll get assert when dereferencing past-the-end
140    // iterator.
141    while (isa<PHINode>(i)) ++i;
142    return &*i;
143}
144
145void BasicBlock::dropAllReferences() {
146  for(iterator I = begin(), E = end(); I != E; ++I)
147    I->dropAllReferences();
148}
149
150/// getSinglePredecessor - If this basic block has a single predecessor block,
151/// return the block, otherwise return a null pointer.
152BasicBlock *BasicBlock::getSinglePredecessor() {
153  pred_iterator PI = pred_begin(this), E = pred_end(this);
154  if (PI == E) return 0;         // No preds.
155  BasicBlock *ThePred = *PI;
156  ++PI;
157  return (PI == E) ? ThePred : 0 /*multiple preds*/;
158}
159
160/// removePredecessor - This method is used to notify a BasicBlock that the
161/// specified Predecessor of the block is no longer able to reach it.  This is
162/// actually not used to update the Predecessor list, but is actually used to
163/// update the PHI nodes that reside in the block.  Note that this should be
164/// called while the predecessor still refers to this block.
165///
166void BasicBlock::removePredecessor(BasicBlock *Pred,
167                                   bool DontDeleteUselessPHIs) {
168  assert((hasNUsesOrMore(16)||// Reduce cost of this assertion for complex CFGs.
169          find(pred_begin(this), pred_end(this), Pred) != pred_end(this)) &&
170         "removePredecessor: BB is not a predecessor!");
171
172  if (InstList.empty()) return;
173  PHINode *APN = dyn_cast<PHINode>(&front());
174  if (!APN) return;   // Quick exit.
175
176  // If there are exactly two predecessors, then we want to nuke the PHI nodes
177  // altogether.  However, we cannot do this, if this in this case:
178  //
179  //  Loop:
180  //    %x = phi [X, Loop]
181  //    %x2 = add %x, 1         ;; This would become %x2 = add %x2, 1
182  //    br Loop                 ;; %x2 does not dominate all uses
183  //
184  // This is because the PHI node input is actually taken from the predecessor
185  // basic block.  The only case this can happen is with a self loop, so we
186  // check for this case explicitly now.
187  //
188  unsigned max_idx = APN->getNumIncomingValues();
189  assert(max_idx != 0 && "PHI Node in block with 0 predecessors!?!?!");
190  if (max_idx == 2) {
191    BasicBlock *Other = APN->getIncomingBlock(APN->getIncomingBlock(0) == Pred);
192
193    // Disable PHI elimination!
194    if (this == Other) max_idx = 3;
195  }
196
197  // <= Two predecessors BEFORE I remove one?
198  if (max_idx <= 2 && !DontDeleteUselessPHIs) {
199    // Yup, loop through and nuke the PHI nodes
200    while (PHINode *PN = dyn_cast<PHINode>(&front())) {
201      // Remove the predecessor first.
202      PN->removeIncomingValue(Pred, !DontDeleteUselessPHIs);
203
204      // If the PHI _HAD_ two uses, replace PHI node with its now *single* value
205      if (max_idx == 2) {
206        if (PN->getOperand(0) != PN)
207          PN->replaceAllUsesWith(PN->getOperand(0));
208        else
209          // We are left with an infinite loop with no entries: kill the PHI.
210          PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
211        getInstList().pop_front();    // Remove the PHI node
212      }
213
214      // If the PHI node already only had one entry, it got deleted by
215      // removeIncomingValue.
216    }
217  } else {
218    // Okay, now we know that we need to remove predecessor #pred_idx from all
219    // PHI nodes.  Iterate over each PHI node fixing them up
220    PHINode *PN;
221    for (iterator II = begin(); (PN = dyn_cast<PHINode>(II)); ) {
222      ++II;
223      PN->removeIncomingValue(Pred, false);
224      // If all incoming values to the Phi are the same, we can replace the Phi
225      // with that value.
226      Value* PNV = 0;
227      if (!DontDeleteUselessPHIs && (PNV = PN->hasConstantValue())) {
228        PN->replaceAllUsesWith(PNV);
229        PN->eraseFromParent();
230      }
231    }
232  }
233}
234
235
236/// splitBasicBlock - This splits a basic block into two at the specified
237/// instruction.  Note that all instructions BEFORE the specified iterator stay
238/// as part of the original basic block, an unconditional branch is added to
239/// the new BB, and the rest of the instructions in the BB are moved to the new
240/// BB, including the old terminator.  This invalidates the iterator.
241///
242/// Note that this only works on well formed basic blocks (must have a
243/// terminator), and 'I' must not be the end of instruction list (which would
244/// cause a degenerate basic block to be formed, having a terminator inside of
245/// the basic block).
246///
247BasicBlock *BasicBlock::splitBasicBlock(iterator I, const std::string &BBName) {
248  assert(getTerminator() && "Can't use splitBasicBlock on degenerate BB!");
249  assert(I != InstList.end() &&
250         "Trying to get me to create degenerate basic block!");
251
252  BasicBlock *New = new BasicBlock(BBName, getParent(), getNext());
253
254  // Move all of the specified instructions from the original basic block into
255  // the new basic block.
256  New->getInstList().splice(New->end(), this->getInstList(), I, end());
257
258  // Add a branch instruction to the newly formed basic block.
259  new BranchInst(New, this);
260
261  // Now we must loop through all of the successors of the New block (which
262  // _were_ the successors of the 'this' block), and update any PHI nodes in
263  // successors.  If there were PHI nodes in the successors, then they need to
264  // know that incoming branches will be from New, not from Old.
265  //
266  for (succ_iterator I = succ_begin(New), E = succ_end(New); I != E; ++I) {
267    // Loop over any phi nodes in the basic block, updating the BB field of
268    // incoming values...
269    BasicBlock *Successor = *I;
270    PHINode *PN;
271    for (BasicBlock::iterator II = Successor->begin();
272         (PN = dyn_cast<PHINode>(II)); ++II) {
273      int IDX = PN->getBasicBlockIndex(this);
274      while (IDX != -1) {
275        PN->setIncomingBlock((unsigned)IDX, New);
276        IDX = PN->getBasicBlockIndex(this);
277      }
278    }
279  }
280  return New;
281}
282