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/IntrinsicInst.h"
  18 #include "llvm/LLVMContext.h"
  19 #include "llvm/Type.h"
  20 #include "llvm/ADT/STLExtras.h"
  21 #include "llvm/Support/CFG.h"
  22 #include "llvm/Support/LeakDetector.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 llvm::SymbolTableListTraits<Instruction, BasicBlock>;
  40
  41
  42 BasicBlock::BasicBlock(LLVMContext &C, const Twine &Name, Function *NewParent,
  43                        BasicBlock *InsertBefore)
  44   : Value(Type::getLabelTy(C), 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   // If the address of the block is taken and it is being deleted (e.g. because
  63   // it is dead), this means that there is either a dangling constant expr
  64   // hanging off the block, or an undefined use of the block (source code
  65   // expecting the address of a label to keep the block alive even though there
  66   // is no indirect branch).  Handle these cases by zapping the BlockAddress
  67   // nodes.  There are no other possible uses at this point.
  68   if (hasAddressTaken()) {
  69     assert(!use_empty() && "There should be at least one blockaddress!");
  70     Constant *Replacement =
  71       ConstantInt::get(llvm::Type::getInt32Ty(getContext()), 1);
  72     while (!use_empty()) {
  73       BlockAddress *BA = cast<BlockAddress>(use_back());
  74       BA->replaceAllUsesWith(ConstantExpr::getIntToPtr(Replacement,
  75                                                        BA->getType()));
  76       BA->destroyConstant();
  77     }
  78   }
  79
  80   assert(getParent() == 0 && "BasicBlock still linked into the program!");
  81   dropAllReferences();
  82   InstList.clear();
  83 }
  84
  85 void BasicBlock::setParent(Function *parent) {
  86   if (getParent())
  87     LeakDetector::addGarbageObject(this);
  88
  89   // Set Parent=parent, updating instruction symtab entries as appropriate.
  90   InstList.setSymTabObject(&Parent, parent);
  91
  92   if (getParent())
  93     LeakDetector::removeGarbageObject(this);
  94 }
  95
  96 void BasicBlock::removeFromParent() {
  97   getParent()->getBasicBlockList().remove(this);
  98 }
  99
 100 void BasicBlock::eraseFromParent() {
 101   getParent()->getBasicBlockList().erase(this);
 102 }
 103
 104 /// moveBefore - Unlink this basic block from its current function and
 105 /// insert it into the function that MovePos lives in, right before MovePos.
 106 void BasicBlock::moveBefore(BasicBlock *MovePos) {
 107   MovePos->getParent()->getBasicBlockList().splice(MovePos,
 108                        getParent()->getBasicBlockList(), this);
 109 }
 110
 111 /// moveAfter - Unlink this basic block from its current function and
 112 /// insert it into the function that MovePos lives in, right after MovePos.
 113 void BasicBlock::moveAfter(BasicBlock *MovePos) {
 114   Function::iterator I = MovePos;
 115   MovePos->getParent()->getBasicBlockList().splice(++I,
 116                                        getParent()->getBasicBlockList(), this);
 117 }
 118
 119
 120 TerminatorInst *BasicBlock::getTerminator() {
 121   if (InstList.empty()) return 0;
 122   return dyn_cast<TerminatorInst>(&InstList.back());
 123 }
 124
 125 const TerminatorInst *BasicBlock::getTerminator() const {
 126   if (InstList.empty()) return 0;
 127   return dyn_cast<TerminatorInst>(&InstList.back());
 128 }
 129
 130 Instruction* BasicBlock::getFirstNonPHI() {
 131   BasicBlock::iterator i = begin();
 132   // All valid basic blocks should have a terminator,
 133   // which is not a PHINode. If we have an invalid basic
 134   // block we'll get an assertion failure when dereferencing
 135   // a past-the-end iterator.
 136   while (isa<PHINode>(i)) ++i;
 137   return &*i;
 138 }
 139
 140 Instruction* BasicBlock::getFirstNonPHIOrDbg() {
 141   BasicBlock::iterator i = begin();
 142   // All valid basic blocks should have a terminator,
 143   // which is not a PHINode. If we have an invalid basic
 144   // block we'll get an assertion failure when dereferencing
 145   // a past-the-end iterator.
 146   while (isa<PHINode>(i) || isa<DbgInfoIntrinsic>(i)) ++i;
 147   return &*i;
 148 }
 149
 150 void BasicBlock::dropAllReferences() {
 151   for(iterator I = begin(), E = end(); I != E; ++I)
 152     I->dropAllReferences();
 153 }
 154
 155 /// getSinglePredecessor - If this basic block has a single predecessor block,
 156 /// return the block, otherwise return a null pointer.
 157 BasicBlock *BasicBlock::getSinglePredecessor() {
 158   pred_iterator PI = pred_begin(this), E = pred_end(this);
 159   if (PI == E) return 0;         // No preds.
 160   BasicBlock *ThePred = *PI;
 161   ++PI;
 162   return (PI == E) ? ThePred : 0 /*multiple preds*/;
 163 }
 164
 165 /// getUniquePredecessor - If this basic block has a unique predecessor block,
 166 /// return the block, otherwise return a null pointer.
 167 /// Note that unique predecessor doesn't mean single edge, there can be
 168 /// multiple edges from the unique predecessor to this block (for example
 169 /// a switch statement with multiple cases having the same destination).
 170 BasicBlock *BasicBlock::getUniquePredecessor() {
 171   pred_iterator PI = pred_begin(this), E = pred_end(this);
 172   if (PI == E) return 0; // No preds.
 173   BasicBlock *PredBB = *PI;
 174   ++PI;
 175   for (;PI != E; ++PI) {
 176     if (*PI != PredBB)
 177       return 0;
 178     // The same predecessor appears multiple times in the predecessor list.
 179     // This is OK.
 180   }
 181   return PredBB;
 182 }
 183
 184 /// removePredecessor - This method is used to notify a BasicBlock that the
 185 /// specified Predecessor of the block is no longer able to reach it.  This is
 186 /// actually not used to update the Predecessor list, but is actually used to
 187 /// update the PHI nodes that reside in the block.  Note that this should be
 188 /// called while the predecessor still refers to this block.
 189 ///
 190 void BasicBlock::removePredecessor(BasicBlock *Pred,
 191                                    bool DontDeleteUselessPHIs) {
 192   assert((hasNUsesOrMore(16)||// Reduce cost of this assertion for complex CFGs.
 193           find(pred_begin(this), pred_end(this), Pred) != pred_end(this)) &&
 194          "removePredecessor: BB is not a predecessor!");
 195
 196   if (InstList.empty()) return;
 197   PHINode *APN = dyn_cast<PHINode>(&front());
 198   if (!APN) return;   // Quick exit.
 199
 200   // If there are exactly two predecessors, then we want to nuke the PHI nodes
 201   // altogether.  However, we cannot do this, if this in this case:
 202   //
 203   //  Loop:
 204   //    %x = phi [X, Loop]
 205   //    %x2 = add %x, 1         ;; This would become %x2 = add %x2, 1
 206   //    br Loop                 ;; %x2 does not dominate all uses
 207   //
 208   // This is because the PHI node input is actually taken from the predecessor
 209   // basic block.  The only case this can happen is with a self loop, so we
 210   // check for this case explicitly now.
 211   //
 212   unsigned max_idx = APN->getNumIncomingValues();
 213   assert(max_idx != 0 && "PHI Node in block with 0 predecessors!?!?!");
 214   if (max_idx == 2) {
 215     BasicBlock *Other = APN->getIncomingBlock(APN->getIncomingBlock(0) == Pred);
 216
 217     // Disable PHI elimination!
 218     if (this == Other) max_idx = 3;
 219   }
 220
 221   // <= Two predecessors BEFORE I remove one?
 222   if (max_idx <= 2 && !DontDeleteUselessPHIs) {
 223     // Yup, loop through and nuke the PHI nodes
 224     while (PHINode *PN = dyn_cast<PHINode>(&front())) {
 225       // Remove the predecessor first.
 226       PN->removeIncomingValue(Pred, !DontDeleteUselessPHIs);
 227
 228       // If the PHI _HAD_ two uses, replace PHI node with its now *single* value
 229       if (max_idx == 2) {
 230         if (PN->getOperand(0) != PN)
 231           PN->replaceAllUsesWith(PN->getOperand(0));
 232         else
 233           // We are left with an infinite loop with no entries: kill the PHI.
 234           PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
 235         getInstList().pop_front();    // Remove the PHI node
 236       }
 237
 238       // If the PHI node already only had one entry, it got deleted by
 239       // removeIncomingValue.
 240     }
 241   } else {
 242     // Okay, now we know that we need to remove predecessor #pred_idx from all
 243     // PHI nodes.  Iterate over each PHI node fixing them up
 244     PHINode *PN;
 245     for (iterator II = begin(); (PN = dyn_cast<PHINode>(II)); ) {
 246       ++II;
 247       PN->removeIncomingValue(Pred, false);
 248       // If all incoming values to the Phi are the same, we can replace the Phi
 249       // with that value.
 250       Value* PNV = 0;
 251       if (!DontDeleteUselessPHIs && (PNV = PN->hasConstantValue())) {
 252         PN->replaceAllUsesWith(PNV);
 253         PN->eraseFromParent();
 254       }
 255     }
 256   }
 257 }
 258
 259
 260 /// splitBasicBlock - This splits a basic block into two at the specified
 261 /// instruction.  Note that all instructions BEFORE the specified iterator stay
 262 /// as part of the original basic block, an unconditional branch is added to
 263 /// the new BB, and the rest of the instructions in the BB are moved to the new
 264 /// BB, including the old terminator.  This invalidates the iterator.
 265 ///
 266 /// Note that this only works on well formed basic blocks (must have a
 267 /// terminator), and 'I' must not be the end of instruction list (which would
 268 /// cause a degenerate basic block to be formed, having a terminator inside of
 269 /// the basic block).
 270 ///
 271 BasicBlock *BasicBlock::splitBasicBlock(iterator I, const Twine &BBName) {
 272   assert(getTerminator() && "Can't use splitBasicBlock on degenerate BB!");
 273   assert(I != InstList.end() &&
 274          "Trying to get me to create degenerate basic block!");
 275
 276   BasicBlock *InsertBefore = llvm::next(Function::iterator(this))
 277                                .getNodePtrUnchecked();
 278   BasicBlock *New = BasicBlock::Create(getContext(), BBName,
 279                                        getParent(), InsertBefore);
 280
 281   // Move all of the specified instructions from the original basic block into
 282   // the new basic block.
 283   New->getInstList().splice(New->end(), this->getInstList(), I, end());
 284
 285   // Add a branch instruction to the newly formed basic block.
 286   BranchInst::Create(New, this);
 287
 288   // Now we must loop through all of the successors of the New block (which
 289   // _were_ the successors of the 'this' block), and update any PHI nodes in
 290   // successors.  If there were PHI nodes in the successors, then they need to
 291   // know that incoming branches will be from New, not from Old.
 292   //
 293   for (succ_iterator I = succ_begin(New), E = succ_end(New); I != E; ++I) {
 294     // Loop over any phi nodes in the basic block, updating the BB field of
 295     // incoming values...
 296     BasicBlock *Successor = *I;
 297     PHINode *PN;
 298     for (BasicBlock::iterator II = Successor->begin();
 299          (PN = dyn_cast<PHINode>(II)); ++II) {
 300       int IDX = PN->getBasicBlockIndex(this);
 301       while (IDX != -1) {
 302         PN->setIncomingBlock((unsigned)IDX, New);
 303         IDX = PN->getBasicBlockIndex(this);
 304       }
 305     }
 306   }
 307   return New;
 308 }
 309