LLVM API Documentation

BasicBlock.cpp

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00001 //===-- BasicBlock.cpp - Implement BasicBlock related methods -------------===//
00002 //
00003 //                     The LLVM Compiler Infrastructure
00004 //
00005 // This file was developed by the LLVM research group and is distributed under
00006 // the University of Illinois Open Source License. See LICENSE.TXT for details.
00007 //
00008 //===----------------------------------------------------------------------===//
00009 //
00010 // This file implements the BasicBlock class for the VMCore library.
00011 //
00012 //===----------------------------------------------------------------------===//
00013 
00014 #include "llvm/BasicBlock.h"
00015 #include "llvm/Constants.h"
00016 #include "llvm/Instructions.h"
00017 #include "llvm/Type.h"
00018 #include "llvm/Support/CFG.h"
00019 #include "llvm/Support/LeakDetector.h"
00020 #include "SymbolTableListTraitsImpl.h"
00021 #include <algorithm>
00022 using namespace llvm;
00023 
00024 namespace {
00025   /// DummyInst - An instance of this class is used to mark the end of the
00026   /// instruction list.  This is not a real instruction.
00027   struct DummyInst : public Instruction {
00028     DummyInst() : Instruction(Type::VoidTy, OtherOpsEnd, 0, 0) {
00029       // This should not be garbage monitored.
00030       LeakDetector::removeGarbageObject(this);
00031     }
00032 
00033     virtual Instruction *clone() const {
00034       assert(0 && "Cannot clone EOL");abort();
00035       return 0;
00036     }
00037     virtual const char *getOpcodeName() const { return "*end-of-list-inst*"; }
00038 
00039     // Methods for support type inquiry through isa, cast, and dyn_cast...
00040     static inline bool classof(const DummyInst *) { return true; }
00041     static inline bool classof(const Instruction *I) {
00042       return I->getOpcode() == OtherOpsEnd;
00043     }
00044     static inline bool classof(const Value *V) {
00045       return isa<Instruction>(V) && classof(cast<Instruction>(V));
00046     }
00047   };
00048 }
00049 
00050 Instruction *ilist_traits<Instruction>::createSentinel() {
00051   return new DummyInst();
00052 }
00053 iplist<Instruction> &ilist_traits<Instruction>::getList(BasicBlock *BB) {
00054   return BB->getInstList();
00055 }
00056 
00057 // Explicit instantiation of SymbolTableListTraits since some of the methods
00058 // are not in the public header file...
00059 template class SymbolTableListTraits<Instruction, BasicBlock, Function>;
00060 
00061 
00062 BasicBlock::BasicBlock(const std::string &Name, Function *Parent,
00063                        BasicBlock *InsertBefore)
00064   : Value(Type::LabelTy, Value::BasicBlockVal, Name) {
00065   // Initialize the instlist...
00066   InstList.setItemParent(this);
00067 
00068   // Make sure that we get added to a function
00069   LeakDetector::addGarbageObject(this);
00070 
00071   if (InsertBefore) {
00072     assert(Parent &&
00073            "Cannot insert block before another block with no function!");
00074     Parent->getBasicBlockList().insert(InsertBefore, this);
00075   } else if (Parent) {
00076     Parent->getBasicBlockList().push_back(this);
00077   }
00078 }
00079 
00080 
00081 BasicBlock::~BasicBlock() {
00082   assert(getParent() == 0 && "BasicBlock still linked into the program!");
00083   dropAllReferences();
00084   InstList.clear();
00085 }
00086 
00087 void BasicBlock::setParent(Function *parent) {
00088   if (getParent())
00089     LeakDetector::addGarbageObject(this);
00090 
00091   InstList.setParent(parent);
00092 
00093   if (getParent())
00094     LeakDetector::removeGarbageObject(this);
00095 }
00096 
00097 void BasicBlock::removeFromParent() {
00098   getParent()->getBasicBlockList().remove(this);
00099 }
00100 
00101 void BasicBlock::eraseFromParent() {
00102   getParent()->getBasicBlockList().erase(this);
00103 }
00104 
00105 /// moveBefore - Unlink this instruction from its current function and
00106 /// insert it into the function that MovePos lives in, right before
00107 /// MovePos.
00108 void BasicBlock::moveBefore(BasicBlock *MovePos) {
00109   MovePos->getParent()->getBasicBlockList().splice(MovePos,
00110                        getParent()->getBasicBlockList(), this);
00111 }
00112 
00113 
00114 TerminatorInst *BasicBlock::getTerminator() {
00115   if (InstList.empty()) return 0;
00116   return dyn_cast<TerminatorInst>(&InstList.back());
00117 }
00118 
00119 const TerminatorInst *const BasicBlock::getTerminator() const {
00120   if (InstList.empty()) return 0;
00121   return dyn_cast<TerminatorInst>(&InstList.back());
00122 }
00123 
00124 void BasicBlock::dropAllReferences() {
00125   for(iterator I = begin(), E = end(); I != E; ++I)
00126     I->dropAllReferences();
00127 }
00128 
00129 /// getSinglePredecessor - If this basic block has a single predecessor block,
00130 /// return the block, otherwise return a null pointer.
00131 BasicBlock *BasicBlock::getSinglePredecessor() {
00132   pred_iterator PI = pred_begin(this), E = pred_end(this);
00133   if (PI == E) return 0;         // No preds.
00134   BasicBlock *ThePred = *PI;
00135   ++PI;
00136   return (PI == E) ? ThePred : 0 /*multiple preds*/;
00137 }
00138 
00139 /// removePredecessor - This method is used to notify a BasicBlock that the
00140 /// specified Predecessor of the block is no longer able to reach it.  This is
00141 /// actually not used to update the Predecessor list, but is actually used to
00142 /// update the PHI nodes that reside in the block.  Note that this should be
00143 /// called while the predecessor still refers to this block.
00144 ///
00145 void BasicBlock::removePredecessor(BasicBlock *Pred,
00146                                    bool DontDeleteUselessPHIs) {
00147   assert((hasNUsesOrMore(16)||// Reduce cost of this assertion for complex CFGs.
00148           find(pred_begin(this), pred_end(this), Pred) != pred_end(this)) &&
00149          "removePredecessor: BB is not a predecessor!");
00150 
00151   if (InstList.empty()) return;
00152   PHINode *APN = dyn_cast<PHINode>(&front());
00153   if (!APN) return;   // Quick exit.
00154 
00155   // If there are exactly two predecessors, then we want to nuke the PHI nodes
00156   // altogether.  However, we cannot do this, if this in this case:
00157   //
00158   //  Loop:
00159   //    %x = phi [X, Loop]
00160   //    %x2 = add %x, 1         ;; This would become %x2 = add %x2, 1
00161   //    br Loop                 ;; %x2 does not dominate all uses
00162   //
00163   // This is because the PHI node input is actually taken from the predecessor
00164   // basic block.  The only case this can happen is with a self loop, so we
00165   // check for this case explicitly now.
00166   //
00167   unsigned max_idx = APN->getNumIncomingValues();
00168   assert(max_idx != 0 && "PHI Node in block with 0 predecessors!?!?!");
00169   if (max_idx == 2) {
00170     BasicBlock *Other = APN->getIncomingBlock(APN->getIncomingBlock(0) == Pred);
00171 
00172     // Disable PHI elimination!
00173     if (this == Other) max_idx = 3;
00174   }
00175 
00176   // <= Two predecessors BEFORE I remove one?
00177   if (max_idx <= 2 && !DontDeleteUselessPHIs) {
00178     // Yup, loop through and nuke the PHI nodes
00179     while (PHINode *PN = dyn_cast<PHINode>(&front())) {
00180       // Remove the predecessor first.
00181       PN->removeIncomingValue(Pred, !DontDeleteUselessPHIs);
00182 
00183       // If the PHI _HAD_ two uses, replace PHI node with its now *single* value
00184       if (max_idx == 2) {
00185         if (PN->getOperand(0) != PN)
00186           PN->replaceAllUsesWith(PN->getOperand(0));
00187         else
00188           // We are left with an infinite loop with no entries: kill the PHI.
00189           PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
00190         getInstList().pop_front();    // Remove the PHI node
00191       }
00192 
00193       // If the PHI node already only had one entry, it got deleted by
00194       // removeIncomingValue.
00195     }
00196   } else {
00197     // Okay, now we know that we need to remove predecessor #pred_idx from all
00198     // PHI nodes.  Iterate over each PHI node fixing them up
00199     PHINode *PN;
00200     for (iterator II = begin(); (PN = dyn_cast<PHINode>(II)); ) {
00201       ++II;
00202       PN->removeIncomingValue(Pred, false);
00203       // If all incoming values to the Phi are the same, we can replace the Phi
00204       // with that value.
00205       if (Value *PNV = PN->hasConstantValue()) {
00206         PN->replaceAllUsesWith(PNV);
00207         PN->eraseFromParent();
00208       }
00209     }
00210   }
00211 }
00212 
00213 
00214 /// splitBasicBlock - This splits a basic block into two at the specified
00215 /// instruction.  Note that all instructions BEFORE the specified iterator stay
00216 /// as part of the original basic block, an unconditional branch is added to
00217 /// the new BB, and the rest of the instructions in the BB are moved to the new
00218 /// BB, including the old terminator.  This invalidates the iterator.
00219 ///
00220 /// Note that this only works on well formed basic blocks (must have a
00221 /// terminator), and 'I' must not be the end of instruction list (which would
00222 /// cause a degenerate basic block to be formed, having a terminator inside of
00223 /// the basic block).
00224 ///
00225 BasicBlock *BasicBlock::splitBasicBlock(iterator I, const std::string &BBName) {
00226   assert(getTerminator() && "Can't use splitBasicBlock on degenerate BB!");
00227   assert(I != InstList.end() &&
00228          "Trying to get me to create degenerate basic block!");
00229 
00230   BasicBlock *New = new BasicBlock(BBName, getParent(), getNext());
00231 
00232   // Move all of the specified instructions from the original basic block into
00233   // the new basic block.
00234   New->getInstList().splice(New->end(), this->getInstList(), I, end());
00235 
00236   // Add a branch instruction to the newly formed basic block.
00237   new BranchInst(New, this);
00238 
00239   // Now we must loop through all of the successors of the New block (which
00240   // _were_ the successors of the 'this' block), and update any PHI nodes in
00241   // successors.  If there were PHI nodes in the successors, then they need to
00242   // know that incoming branches will be from New, not from Old.
00243   //
00244   for (succ_iterator I = succ_begin(New), E = succ_end(New); I != E; ++I) {
00245     // Loop over any phi nodes in the basic block, updating the BB field of
00246     // incoming values...
00247     BasicBlock *Successor = *I;
00248     PHINode *PN;
00249     for (BasicBlock::iterator II = Successor->begin();
00250          (PN = dyn_cast<PHINode>(II)); ++II) {
00251       int IDX = PN->getBasicBlockIndex(this);
00252       while (IDX != -1) {
00253         PN->setIncomingBlock((unsigned)IDX, New);
00254         IDX = PN->getBasicBlockIndex(this);
00255       }
00256     }
00257   }
00258   return New;
00259 }