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