MachineBlockPlacement.cpp

  // backedges that were introduced purely because of the loop rotations done
  // during this layout pass.
  // FIXME: This isn't quite right, we shouldn't align backedges that result
  // from blocks being sunken below the exit block for the function.
  if (F.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
    return;
  unsigned Align = TLI->getPrefLoopAlignment();
  if (!Align)
    return;  // Don't care about loop alignment.

  SmallPtrSet<MachineBasicBlock *, 16> PreviousBlocks;
  for (BlockChain::iterator BI = FunctionChain.begin(),
                            BE = FunctionChain.end();
       BI != BE; ++BI) {
    PreviousBlocks.insert(*BI);
    // Set alignment on the destination of all the back edges in the new
    // ordering.
    for (MachineBasicBlock::succ_iterator SI = (*BI)->succ_begin(),
                                          SE = (*BI)->succ_end();
         SI != SE; ++SI)
      if (PreviousBlocks.count(*SI))
        (*SI)->setAlignment(Align);
  }
}

bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &F) {
  // Check for single-block functions and skip them.
  if (llvm::next(F.begin()) == F.end())
    return false;

  MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
  MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
  MLI = &getAnalysis<MachineLoopInfo>();
  TII = F.getTarget().getInstrInfo();
  TLI = F.getTarget().getTargetLowering();
  assert(BlockToChain.empty());

  buildCFGChains(F);

  BlockToChain.clear();
  ChainAllocator.DestroyAll();

  // We always return true as we have no way to track whether the final order
  // differs from the original order.
  return true;
}

namespace {
/// \brief A pass to compute block placement statistics.
///
/// A separate pass to compute interesting statistics for evaluating block
/// placement. This is separate from the actual placement pass so that they can
/// be computed in the absence of any placement transformations or when using
/// alternative placement strategies.
class MachineBlockPlacementStats : public MachineFunctionPass {
  /// \brief A handle to the branch probability pass.
  const MachineBranchProbabilityInfo *MBPI;

  /// \brief A handle to the function-wide block frequency pass.
  const MachineBlockFrequencyInfo *MBFI;

public:
  static char ID; // Pass identification, replacement for typeid
  MachineBlockPlacementStats() : MachineFunctionPass(ID) {
    initializeMachineBlockPlacementStatsPass(*PassRegistry::getPassRegistry());
  }

  bool runOnMachineFunction(MachineFunction &F);

  void getAnalysisUsage(AnalysisUsage &AU) const {
    AU.addRequired<MachineBranchProbabilityInfo>();
    AU.addRequired<MachineBlockFrequencyInfo>();
    AU.setPreservesAll();
    MachineFunctionPass::getAnalysisUsage(AU);
  }
};
}

char MachineBlockPlacementStats::ID = 0;
char &llvm::MachineBlockPlacementStatsID = MachineBlockPlacementStats::ID;
INITIALIZE_PASS_BEGIN(MachineBlockPlacementStats, "block-placement-stats",
                      "Basic Block Placement Stats", false, false)
INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
INITIALIZE_PASS_END(MachineBlockPlacementStats, "block-placement-stats",
                    "Basic Block Placement Stats", false, false)

bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) {
  // Check for single-block functions and skip them.
  if (llvm::next(F.begin()) == F.end())
    return false;

  MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
  MBFI = &getAnalysis<MachineBlockFrequencyInfo>();

  for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
    BlockFrequency BlockFreq = MBFI->getBlockFreq(I);
    Statistic &NumBranches = (I->succ_size() > 1) ? NumCondBranches
                                                  : NumUncondBranches;
    Statistic &BranchTakenFreq = (I->succ_size() > 1) ? CondBranchTakenFreq
                                                      : UncondBranchTakenFreq;
    for (MachineBasicBlock::succ_iterator SI = I->succ_begin(),
                                          SE = I->succ_end();
         SI != SE; ++SI) {
      // Skip if this successor is a fallthrough.
      if (I->isLayoutSuccessor(*SI))
        continue;

      BlockFrequency EdgeFreq = BlockFreq * MBPI->getEdgeProbability(I, *SI);
      ++NumBranches;
      BranchTakenFreq += EdgeFreq.getFrequency();
    }
  }

  return false;
}