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