C++ BranchInst::replaceUsesOfWith方法代码示例

/// Check whether \param BB is the merge block of a if-region.  If yes, check
/// whether there exists an adjacent if-region upstream, the two if-regions
/// contain identical instuctions and can be legally merged.  \returns true if
/// the two if-regions are merged.
///
/// From:
/// if (a)
///   statement;
/// if (b)
///   statement;
///
/// To:
/// if (a || b)
///   statement;
///
bool FlattenCFGOpt::MergeIfRegion(BasicBlock *BB, IRBuilder<> &Builder,
                                  Pass *P) {
  BasicBlock *IfTrue2, *IfFalse2;
  Value *IfCond2 = GetIfCondition(BB, IfTrue2, IfFalse2);
  Instruction *CInst2 = dyn_cast_or_null<Instruction>(IfCond2);
  if (!CInst2)
    return false;

  BasicBlock *SecondEntryBlock = CInst2->getParent();
  if (SecondEntryBlock->hasAddressTaken())
    return false;

  BasicBlock *IfTrue1, *IfFalse1;
  Value *IfCond1 = GetIfCondition(SecondEntryBlock, IfTrue1, IfFalse1);
  Instruction *CInst1 = dyn_cast_or_null<Instruction>(IfCond1);
  if (!CInst1)
    return false;

  BasicBlock *FirstEntryBlock = CInst1->getParent();

  // Either then-path or else-path should be empty.
  if ((IfTrue1 != FirstEntryBlock) && (IfFalse1 != FirstEntryBlock))
    return false;
  if ((IfTrue2 != SecondEntryBlock) && (IfFalse2 != SecondEntryBlock))
    return false;

  TerminatorInst *PTI2 = SecondEntryBlock->getTerminator();
  Instruction *PBI2 = SecondEntryBlock->begin();

  if (!CompareIfRegionBlock(FirstEntryBlock, SecondEntryBlock, IfTrue1,
                            IfTrue2))
    return false;

  if (!CompareIfRegionBlock(FirstEntryBlock, SecondEntryBlock, IfFalse1,
                            IfFalse2))
    return false;

  // Check whether \param SecondEntryBlock has side-effect and is safe to
  // speculate.
  for (BasicBlock::iterator BI = PBI2, BE = PTI2; BI != BE; ++BI) {
    Instruction *CI = BI;
    if (isa<PHINode>(CI) || CI->mayHaveSideEffects() ||
        !isSafeToSpeculativelyExecute(CI))
      return false;
  }

  // Merge \param SecondEntryBlock into \param FirstEntryBlock.
  FirstEntryBlock->getInstList().pop_back();
  FirstEntryBlock->getInstList()
      .splice(FirstEntryBlock->end(), SecondEntryBlock->getInstList());
  BranchInst *PBI = dyn_cast<BranchInst>(FirstEntryBlock->getTerminator());
  Value *CC = PBI->getCondition();
  BasicBlock *SaveInsertBB = Builder.GetInsertBlock();
  BasicBlock::iterator SaveInsertPt = Builder.GetInsertPoint();
  Builder.SetInsertPoint(PBI);
  Value *NC = Builder.CreateOr(CInst1, CC);
  PBI->replaceUsesOfWith(CC, NC);
  Builder.SetInsertPoint(SaveInsertBB, SaveInsertPt);

  // Remove IfTrue1
  if (IfTrue1 != FirstEntryBlock) {
    IfTrue1->dropAllReferences();
    IfTrue1->eraseFromParent();
  }

  // Remove IfFalse1
  if (IfFalse1 != FirstEntryBlock) {
    IfFalse1->dropAllReferences();
    IfFalse1->eraseFromParent();
  }

  // Remove \param SecondEntryBlock
  SecondEntryBlock->dropAllReferences();
  SecondEntryBlock->eraseFromParent();
  DEBUG(dbgs() << "If conditions merged into:\n" << *FirstEntryBlock);
  return true;
}

//.........这里部分代码省略.........
    if (Preds.count(PS) == 0) {
      // Case 2.
      LastCondBlock = Pred;
    } else {
      // Case 1
      BranchInst *BPS = dyn_cast<BranchInst>(PS->getTerminator());
      if (BPS && BPS->isUnconditional()) {
        // Case 1: PS(BB3) should be an unconditional branch.
        LastCondBlock = Pred;
      }
    }
  }

  if (!FirstCondBlock || !LastCondBlock || (FirstCondBlock == LastCondBlock))
    return false;

  TerminatorInst *TBB = LastCondBlock->getTerminator();
  BasicBlock *PS1 = TBB->getSuccessor(0);
  BasicBlock *PS2 = TBB->getSuccessor(1);
  BranchInst *PBI1 = dyn_cast<BranchInst>(PS1->getTerminator());
  BranchInst *PBI2 = dyn_cast<BranchInst>(PS2->getTerminator());

  // If PS1 does not jump into PS2, but PS2 jumps into PS1,
  // attempt branch inversion.
  if (!PBI1 || !PBI1->isUnconditional() ||
      (PS1->getTerminator()->getSuccessor(0) != PS2)) {
    // Check whether PS2 jumps into PS1.
    if (!PBI2 || !PBI2->isUnconditional() ||
        (PS2->getTerminator()->getSuccessor(0) != PS1))
      return false;

    // Do branch inversion.
    BasicBlock *CurrBlock = LastCondBlock;
    bool EverChanged = false;
    while (1) {
      BranchInst *BI = dyn_cast<BranchInst>(CurrBlock->getTerminator());
      CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
      CmpInst::Predicate Predicate = CI->getPredicate();
      // Cannonicalize icmp_ne -> icmp_eq, fcmp_one -> fcmp_oeq
      if ((Predicate == CmpInst::ICMP_NE) || (Predicate == CmpInst::FCMP_ONE)) {
        CI->setPredicate(ICmpInst::getInversePredicate(Predicate));
        BI->swapSuccessors();
        EverChanged = true;
      }
      if (CurrBlock == FirstCondBlock)
        break;
      CurrBlock = CurrBlock->getSinglePredecessor();
    }
    return EverChanged;
  }

  // PS1 must have a conditional branch.
  if (!PBI1 || !PBI1->isUnconditional())
    return false;

  // PS2 should not contain PHI node.
  PHI = dyn_cast<PHINode>(PS2->begin());
  if (PHI)
    return false;

  // Do the transformation.
  BasicBlock *CB;
  BranchInst *PBI = dyn_cast<BranchInst>(FirstCondBlock->getTerminator());
  bool Iteration = true;
  BasicBlock *SaveInsertBB = Builder.GetInsertBlock();
  BasicBlock::iterator SaveInsertPt = Builder.GetInsertPoint();
  Value *PC = PBI->getCondition();

  do {
    CB = PBI->getSuccessor(1 - Idx);
    // Delete the conditional branch.
    FirstCondBlock->getInstList().pop_back();
    FirstCondBlock->getInstList()
        .splice(FirstCondBlock->end(), CB->getInstList());
    PBI = cast<BranchInst>(FirstCondBlock->getTerminator());
    Value *CC = PBI->getCondition();
    // Merge conditions.
    Builder.SetInsertPoint(PBI);
    Value *NC;
    if (Idx == 0)
      // Case 2, use parallel or.
      NC = Builder.CreateOr(PC, CC);
    else
      // Case 1, use parallel and.
      NC = Builder.CreateAnd(PC, CC);

    PBI->replaceUsesOfWith(CC, NC);
    PC = NC;
    if (CB == LastCondBlock)
      Iteration = false;
    // Remove internal conditional branches.
    CB->dropAllReferences();
    // make CB unreachable and let downstream to delete the block.
    new UnreachableInst(CB->getContext(), CB);
  } while (Iteration);

  Builder.SetInsertPoint(SaveInsertBB, SaveInsertPt);
  DEBUG(dbgs() << "Use parallel and/or in:\n" << *FirstCondBlock);
  return true;
}