lib/VMCore/BasicBlock.cpp

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