BasicBlock.cpp revision bca81448ac8e19c588c9a4ad16fc70732b76327c 
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/Constant.h"
16#include "llvm/Instructions.h"
17#include "llvm/Type.h"
18#include "llvm/Support/CFG.h"
19#include "llvm/SymbolTable.h"
20#include "llvm/Support/LeakDetector.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 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, Name) {
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
81
82BasicBlock::~BasicBlock() {
83  assert(getParent() == 0 && "BasicBlock still linked into the program!");
84  dropAllReferences();
85  InstList.clear();
86}
87
88void BasicBlock::setParent(Function *parent) {
89  if (getParent())
90    LeakDetector::addGarbageObject(this);
91
92  InstList.setParent(parent);
93
94  if (getParent())
95    LeakDetector::removeGarbageObject(this);
96}
97
98// Specialize setName to take care of symbol table majik
99void BasicBlock::setName(const std::string &name, SymbolTable *ST) {
100  Function *P;
101  assert((ST == 0 || (!getParent() || ST == &getParent()->getSymbolTable())) &&
102	 "Invalid symtab argument!");
103  if ((P = getParent()) && hasName()) P->getSymbolTable().remove(this);
104  Value::setName(name);
105  if (P && hasName()) P->getSymbolTable().insert(this);
106}
107
108void BasicBlock::removeFromParent() {
109  getParent()->getBasicBlockList().remove(this);
110}
111
112void BasicBlock::eraseFromParent() {
113  getParent()->getBasicBlockList().erase(this);
114}
115
116
117TerminatorInst *BasicBlock::getTerminator() {
118  if (InstList.empty()) return 0;
119  return dyn_cast<TerminatorInst>(&InstList.back());
120}
121
122const TerminatorInst *const BasicBlock::getTerminator() const {
123  if (InstList.empty()) return 0;
124  return dyn_cast<TerminatorInst>(&InstList.back());
125}
126
127void BasicBlock::dropAllReferences() {
128  for(iterator I = begin(), E = end(); I != E; ++I)
129    I->dropAllReferences();
130}
131
132// removePredecessor - This method is used to notify a BasicBlock that the
133// specified Predecessor of the block is no longer able to reach it.  This is
134// actually not used to update the Predecessor list, but is actually used to
135// update the PHI nodes that reside in the block.  Note that this should be
136// called while the predecessor still refers to this block.
137//
138void BasicBlock::removePredecessor(BasicBlock *Pred) {
139  assert(find(pred_begin(this), pred_end(this), Pred) != pred_end(this) &&
140	 "removePredecessor: BB is not a predecessor!");
141  if (InstList.empty()) return;
142  PHINode *APN = dyn_cast<PHINode>(&front());
143  if (!APN) return;   // Quick exit.
144
145  // If there are exactly two predecessors, then we want to nuke the PHI nodes
146  // altogether.  However, we cannot do this, if this in this case:
147  //
148  //  Loop:
149  //    %x = phi [X, Loop]
150  //    %x2 = add %x, 1         ;; This would become %x2 = add %x2, 1
151  //    br Loop                 ;; %x2 does not dominate all uses
152  //
153  // This is because the PHI node input is actually taken from the predecessor
154  // basic block.  The only case this can happen is with a self loop, so we
155  // check for this case explicitly now.
156  //
157  unsigned max_idx = APN->getNumIncomingValues();
158  assert(max_idx != 0 && "PHI Node in block with 0 predecessors!?!?!");
159  if (max_idx == 2) {
160    BasicBlock *Other = APN->getIncomingBlock(APN->getIncomingBlock(0) == Pred);
161
162    // Disable PHI elimination!
163    if (this == Other) max_idx = 3;
164  }
165
166  if (max_idx <= 2) {                // <= Two predecessors BEFORE I remove one?
167    // Yup, loop through and nuke the PHI nodes
168    while (PHINode *PN = dyn_cast<PHINode>(&front())) {
169      PN->removeIncomingValue(Pred); // Remove the predecessor first...
170
171      // If the PHI _HAD_ two uses, replace PHI node with its now *single* value
172      if (max_idx == 2) {
173        if (PN->getOperand(0) != PN)
174          PN->replaceAllUsesWith(PN->getOperand(0));
175        else
176          // We are left with an infinite loop with no entries: kill the PHI.
177          PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
178        getInstList().pop_front();    // Remove the PHI node
179      }
180
181      // If the PHI node already only had one entry, it got deleted by
182      // removeIncomingValue.
183    }
184  } else {
185    // Okay, now we know that we need to remove predecessor #pred_idx from all
186    // PHI nodes.  Iterate over each PHI node fixing them up
187    PHINode *PN;
188    for (iterator II = begin(); (PN = dyn_cast<PHINode>(II)); ++II)
189      PN->removeIncomingValue(Pred);
190  }
191}
192
193
194// splitBasicBlock - This splits a basic block into two at the specified
195// instruction.  Note that all instructions BEFORE the specified iterator stay
196// as part of the original basic block, an unconditional branch is added to
197// the new BB, and the rest of the instructions in the BB are moved to the new
198// BB, including the old terminator.  This invalidates the iterator.
199//
200// Note that this only works on well formed basic blocks (must have a
201// terminator), and 'I' must not be the end of instruction list (which would
202// cause a degenerate basic block to be formed, having a terminator inside of
203// the basic block).
204//
205BasicBlock *BasicBlock::splitBasicBlock(iterator I, const std::string &BBName) {
206  assert(getTerminator() && "Can't use splitBasicBlock on degenerate BB!");
207  assert(I != InstList.end() &&
208	 "Trying to get me to create degenerate basic block!");
209
210  BasicBlock *New = new BasicBlock(BBName, getParent(), getNext());
211
212  // Move all of the specified instructions from the original basic block into
213  // the new basic block.
214  New->getInstList().splice(New->end(), this->getInstList(), I, end());
215
216  // Add a branch instruction to the newly formed basic block.
217  new BranchInst(New, this);
218
219  // Now we must loop through all of the successors of the New block (which
220  // _were_ the successors of the 'this' block), and update any PHI nodes in
221  // successors.  If there were PHI nodes in the successors, then they need to
222  // know that incoming branches will be from New, not from Old.
223  //
224  for (succ_iterator I = succ_begin(New), E = succ_end(New); I != E; ++I) {
225    // Loop over any phi nodes in the basic block, updating the BB field of
226    // incoming values...
227    BasicBlock *Successor = *I;
228    PHINode *PN;
229    for (BasicBlock::iterator II = Successor->begin();
230         (PN = dyn_cast<PHINode>(II)); ++II) {
231      int IDX = PN->getBasicBlockIndex(this);
232      while (IDX != -1) {
233        PN->setIncomingBlock((unsigned)IDX, New);
234        IDX = PN->getBasicBlockIndex(this);
235      }
236    }
237  }
238  return New;
239}
240