BasicBlock.cpp revision cdfc940912d56a63b6f12eaa7f3faf79cf74c693 
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 VMCore library.
11//
12//===----------------------------------------------------------------------===//
13
14#include "llvm/BasicBlock.h"
15#include "llvm/Constants.h"
16#include "llvm/Instructions.h"
17#include "llvm/LLVMContext.h"
18#include "llvm/Type.h"
19#include "llvm/ADT/STLExtras.h"
20#include "llvm/Support/CFG.h"
21#include "llvm/Support/LeakDetector.h"
22#include "SymbolTableListTraitsImpl.h"
23#include <algorithm>
24using namespace llvm;
25
26ValueSymbolTable *BasicBlock::getValueSymbolTable() {
27  if (Function *F = getParent())
28    return &F->getValueSymbolTable();
29  return 0;
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 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(0) {
44
45  // Make sure that we get added to a function
46  LeakDetector::addGarbageObject(this);
47
48  if (InsertBefore) {
49    assert(NewParent &&
50           "Cannot insert block before another block with no function!");
51    NewParent->getBasicBlockList().insert(InsertBefore, this);
52  } else if (NewParent) {
53    NewParent->getBasicBlockList().push_back(this);
54  }
55
56  setName(Name);
57}
58
59
60BasicBlock::~BasicBlock() {
61  // If the address of the block is taken and it is being deleted (e.g. because
62  // it is dead), this means that there is either a dangling constant expr
63  // hanging off the block, or an undefined use of the block (source code
64  // expecting the address of a label to keep the block alive even though there
65  // is no indirect branch).  Handle these cases by zapping the BlockAddress
66  // nodes.  There are no other possible uses at this point.
67  if (hasAddressTaken()) {
68    assert(!use_empty() && "There should be at least one blockaddress!");
69    Constant *Replacement =
70      ConstantInt::get(llvm::Type::getInt32Ty(getContext()), 1);
71    while (!use_empty()) {
72      BlockAddress *BA = cast<BlockAddress>(use_back());
73      BA->replaceAllUsesWith(ConstantExpr::getIntToPtr(Replacement,
74                                                       BA->getType()));
75      BA->destroyConstant();
76    }
77  }
78
79  assert(getParent() == 0 && "BasicBlock still linked into the program!");
80  dropAllReferences();
81  InstList.clear();
82}
83
84void BasicBlock::setParent(Function *parent) {
85  if (getParent())
86    LeakDetector::addGarbageObject(this);
87
88  // Set Parent=parent, updating instruction symtab entries as appropriate.
89  InstList.setSymTabObject(&Parent, parent);
90
91  if (getParent())
92    LeakDetector::removeGarbageObject(this);
93}
94
95void BasicBlock::removeFromParent() {
96  getParent()->getBasicBlockList().remove(this);
97}
98
99void BasicBlock::eraseFromParent() {
100  getParent()->getBasicBlockList().erase(this);
101}
102
103/// moveBefore - Unlink this basic block from its current function and
104/// insert it into the function that MovePos lives in, right before MovePos.
105void BasicBlock::moveBefore(BasicBlock *MovePos) {
106  MovePos->getParent()->getBasicBlockList().splice(MovePos,
107                       getParent()->getBasicBlockList(), this);
108}
109
110/// moveAfter - Unlink this basic block from its current function and
111/// insert it into the function that MovePos lives in, right after MovePos.
112void BasicBlock::moveAfter(BasicBlock *MovePos) {
113  Function::iterator I = MovePos;
114  MovePos->getParent()->getBasicBlockList().splice(++I,
115                                       getParent()->getBasicBlockList(), this);
116}
117
118
119TerminatorInst *BasicBlock::getTerminator() {
120  if (InstList.empty()) return 0;
121  return dyn_cast<TerminatorInst>(&InstList.back());
122}
123
124const TerminatorInst *BasicBlock::getTerminator() const {
125  if (InstList.empty()) return 0;
126  return dyn_cast<TerminatorInst>(&InstList.back());
127}
128
129Instruction* BasicBlock::getFirstNonPHI() {
130  BasicBlock::iterator i = begin();
131  // All valid basic blocks should have a terminator,
132  // which is not a PHINode. If we have an invalid basic
133  // block we'll get an assertion failure when dereferencing
134  // a past-the-end iterator.
135  while (isa<PHINode>(i)) ++i;
136  return &*i;
137}
138
139void BasicBlock::dropAllReferences() {
140  for(iterator I = begin(), E = end(); I != E; ++I)
141    I->dropAllReferences();
142}
143
144/// getSinglePredecessor - If this basic block has a single predecessor block,
145/// return the block, otherwise return a null pointer.
146BasicBlock *BasicBlock::getSinglePredecessor() {
147  pred_iterator PI = pred_begin(this), E = pred_end(this);
148  if (PI == E) return 0;         // No preds.
149  BasicBlock *ThePred = *PI;
150  ++PI;
151  return (PI == E) ? ThePred : 0 /*multiple preds*/;
152}
153
154/// getUniquePredecessor - If this basic block has a unique predecessor block,
155/// return the block, otherwise return a null pointer.
156/// Note that unique predecessor doesn't mean single edge, there can be
157/// multiple edges from the unique predecessor to this block (for example
158/// a switch statement with multiple cases having the same destination).
159BasicBlock *BasicBlock::getUniquePredecessor() {
160  pred_iterator PI = pred_begin(this), E = pred_end(this);
161  if (PI == E) return 0; // No preds.
162  BasicBlock *PredBB = *PI;
163  ++PI;
164  for (;PI != E; ++PI) {
165    if (*PI != PredBB)
166      return 0;
167    // The same predecessor appears multiple times in the predecessor list.
168    // This is OK.
169  }
170  return PredBB;
171}
172
173/// removePredecessor - This method is used to notify a BasicBlock that the
174/// specified Predecessor of the block is no longer able to reach it.  This is
175/// actually not used to update the Predecessor list, but is actually used to
176/// update the PHI nodes that reside in the block.  Note that this should be
177/// called while the predecessor still refers to this block.
178///
179void BasicBlock::removePredecessor(BasicBlock *Pred,
180                                   bool DontDeleteUselessPHIs) {
181  assert((hasNUsesOrMore(16)||// Reduce cost of this assertion for complex CFGs.
182          find(pred_begin(this), pred_end(this), Pred) != pred_end(this)) &&
183         "removePredecessor: BB is not a predecessor!");
184
185  if (InstList.empty()) return;
186  PHINode *APN = dyn_cast<PHINode>(&front());
187  if (!APN) return;   // Quick exit.
188
189  // If there are exactly two predecessors, then we want to nuke the PHI nodes
190  // altogether.  However, we cannot do this, if this in this case:
191  //
192  //  Loop:
193  //    %x = phi [X, Loop]
194  //    %x2 = add %x, 1         ;; This would become %x2 = add %x2, 1
195  //    br Loop                 ;; %x2 does not dominate all uses
196  //
197  // This is because the PHI node input is actually taken from the predecessor
198  // basic block.  The only case this can happen is with a self loop, so we
199  // check for this case explicitly now.
200  //
201  unsigned max_idx = APN->getNumIncomingValues();
202  assert(max_idx != 0 && "PHI Node in block with 0 predecessors!?!?!");
203  if (max_idx == 2) {
204    BasicBlock *Other = APN->getIncomingBlock(APN->getIncomingBlock(0) == Pred);
205
206    // Disable PHI elimination!
207    if (this == Other) max_idx = 3;
208  }
209
210  // <= Two predecessors BEFORE I remove one?
211  if (max_idx <= 2 && !DontDeleteUselessPHIs) {
212    // Yup, loop through and nuke the PHI nodes
213    while (PHINode *PN = dyn_cast<PHINode>(&front())) {
214      // Remove the predecessor first.
215      PN->removeIncomingValue(Pred, !DontDeleteUselessPHIs);
216
217      // If the PHI _HAD_ two uses, replace PHI node with its now *single* value
218      if (max_idx == 2) {
219        if (PN->getOperand(0) != PN)
220          PN->replaceAllUsesWith(PN->getOperand(0));
221        else
222          // We are left with an infinite loop with no entries: kill the PHI.
223          PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
224        getInstList().pop_front();    // Remove the PHI node
225      }
226
227      // If the PHI node already only had one entry, it got deleted by
228      // removeIncomingValue.
229    }
230  } else {
231    // Okay, now we know that we need to remove predecessor #pred_idx from all
232    // PHI nodes.  Iterate over each PHI node fixing them up
233    PHINode *PN;
234    for (iterator II = begin(); (PN = dyn_cast<PHINode>(II)); ) {
235      ++II;
236      PN->removeIncomingValue(Pred, false);
237      // If all incoming values to the Phi are the same, we can replace the Phi
238      // with that value.
239      Value* PNV = 0;
240      if (!DontDeleteUselessPHIs && (PNV = PN->hasConstantValue())) {
241        PN->replaceAllUsesWith(PNV);
242        PN->eraseFromParent();
243      }
244    }
245  }
246}
247
248
249/// splitBasicBlock - This splits a basic block into two at the specified
250/// instruction.  Note that all instructions BEFORE the specified iterator stay
251/// as part of the original basic block, an unconditional branch is added to
252/// the new BB, and the rest of the instructions in the BB are moved to the new
253/// BB, including the old terminator.  This invalidates the iterator.
254///
255/// Note that this only works on well formed basic blocks (must have a
256/// terminator), and 'I' must not be the end of instruction list (which would
257/// cause a degenerate basic block to be formed, having a terminator inside of
258/// the basic block).
259///
260BasicBlock *BasicBlock::splitBasicBlock(iterator I, const Twine &BBName) {
261  assert(getTerminator() && "Can't use splitBasicBlock on degenerate BB!");
262  assert(I != InstList.end() &&
263         "Trying to get me to create degenerate basic block!");
264
265  BasicBlock *InsertBefore = next(Function::iterator(this))
266                               .getNodePtrUnchecked();
267  BasicBlock *New = BasicBlock::Create(getContext(), BBName,
268                                       getParent(), InsertBefore);
269
270  // Move all of the specified instructions from the original basic block into
271  // the new basic block.
272  New->getInstList().splice(New->end(), this->getInstList(), I, end());
273
274  // Add a branch instruction to the newly formed basic block.
275  BranchInst::Create(New, this);
276
277  // Now we must loop through all of the successors of the New block (which
278  // _were_ the successors of the 'this' block), and update any PHI nodes in
279  // successors.  If there were PHI nodes in the successors, then they need to
280  // know that incoming branches will be from New, not from Old.
281  //
282  for (succ_iterator I = succ_begin(New), E = succ_end(New); I != E; ++I) {
283    // Loop over any phi nodes in the basic block, updating the BB field of
284    // incoming values...
285    BasicBlock *Successor = *I;
286    PHINode *PN;
287    for (BasicBlock::iterator II = Successor->begin();
288         (PN = dyn_cast<PHINode>(II)); ++II) {
289      int IDX = PN->getBasicBlockIndex(this);
290      while (IDX != -1) {
291        PN->setIncomingBlock((unsigned)IDX, New);
292        IDX = PN->getBasicBlockIndex(this);
293      }
294    }
295  }
296  return New;
297}
298
299