MachineBlockPlacement.cpp

//===-- MachineBlockPlacement.cpp - Basic Block Code Layout optimization --===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements basic block placement transformations using the CFG
// structure and branch probability estimates.
//
// The pass strives to preserve the structure of the CFG (that is, retain
// a topological ordering of basic blocks) in the absense of a *strong* signal
// to the contrary from probabilities. However, within the CFG structure, it
// attempts to choose an ordering which favors placing more likely sequences of
// blocks adjacent to each other.
//
// The algorithm works from the inner-most loop within a function outward, and
// at each stage walks through the basic blocks, trying to coalesce them into
// sequential chains where allowed by the CFG (or demanded by heavy
// probabilities). Finally, it walks the blocks in topological order, and the
// first time it reaches a chain of basic blocks, it schedules them in the
// function in-order.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "block-placement2"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/SCCIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetLowering.h"
#include <algorithm>
using namespace llvm;

STATISTIC(NumCondBranches, "Number of conditional branches");
STATISTIC(NumUncondBranches, "Number of uncondittional branches");
STATISTIC(CondBranchTakenFreq,
          "Potential frequency of taking conditional branches");
STATISTIC(UncondBranchTakenFreq,
          "Potential frequency of taking unconditional branches");

namespace {
/// \brief A structure for storing a weighted edge.
///
/// This stores an edge and its weight, computed as the product of the
/// frequency that the starting block is entered with the probability of
/// a particular exit block.
struct WeightedEdge {
  BlockFrequency EdgeFrequency;
  MachineBasicBlock *From, *To;

  bool operator<(const WeightedEdge &RHS) const {
    return EdgeFrequency < RHS.EdgeFrequency;
  }
};
}

namespace {
class BlockChain;
/// \brief Type for our function-wide basic block -> block chain mapping.
typedef DenseMap<MachineBasicBlock *, BlockChain *> BlockToChainMapType;
}

namespace {
/// \brief A chain of blocks which will be laid out contiguously.
///
/// This is the datastructure representing a chain of consecutive blocks that
/// are profitable to layout together in order to maximize fallthrough
/// probabilities. We also can use a block chain to represent a sequence of
/// basic blocks which have some external (correctness) requirement for
/// sequential layout.
///
/// Eventually, the block chains will form a directed graph over the function.
/// We provide an SCC-supporting-iterator in order to quicky build and walk the
/// SCCs of block chains within a function.
///
/// The block chains also have support for calculating and caching probability
/// information related to the chain itself versus other chains. This is used
/// for ranking during the final layout of block chains.
class BlockChain {
  /// \brief The sequence of blocks belonging to this chain.
  ///
  /// This is the sequence of blocks for a particular chain. These will be laid
  /// out in-order within the function.
  SmallVector<MachineBasicBlock *, 4> Blocks;

  /// \brief A handle to the function-wide basic block to block chain mapping.
  ///
  /// This is retained in each block chain to simplify the computation of child
  /// block chains for SCC-formation and iteration. We store the edges to child
  /// basic blocks, and map them back to their associated chains using this
  /// structure.
  BlockToChainMapType &BlockToChain;

public:
  /// \brief Construct a new BlockChain.
  ///
  /// This builds a new block chain representing a single basic block in the
  /// function. It also registers itself as the chain that block participates
  /// in with the BlockToChain mapping.
  BlockChain(BlockToChainMapType &BlockToChain, MachineBasicBlock *BB)
    : Blocks(1, BB), BlockToChain(BlockToChain), LoopPredecessors(0) {
    assert(BB && "Cannot create a chain with a null basic block");
    BlockToChain[BB] = this;
  }

  /// \brief Iterator over blocks within the chain.
  typedef SmallVectorImpl<MachineBasicBlock *>::iterator iterator;
  typedef SmallVectorImpl<MachineBasicBlock *>::reverse_iterator
      reverse_iterator;

  /// \brief Beginning of blocks within the chain.
  iterator begin() { return Blocks.begin(); }
  reverse_iterator rbegin() { return Blocks.rbegin(); }

  /// \brief End of blocks within the chain.
  iterator end() { return Blocks.end(); }
  reverse_iterator rend() { return Blocks.rend(); }

  /// \brief Merge a block chain into this one.
  ///
  /// This routine merges a block chain into this one. It takes care of forming
  /// a contiguous sequence of basic blocks, updating the edge list, and
  /// updating the block -> chain mapping. It does not free or tear down the
  /// old chain, but the old chain's block list is no longer valid.
  void merge(MachineBasicBlock *BB, BlockChain *Chain) {
    assert(BB);
    assert(!Blocks.empty());

    // Fast path in case we don't have a chain already.
    if (!Chain) {
      assert(!BlockToChain[BB]);
      Blocks.push_back(BB);
      BlockToChain[BB] = this;
      return;
    }

    assert(BB == *Chain->begin());
    assert(Chain->begin() != Chain->end());

    // Update the incoming blocks to point to this chain, and add them to the
    // chain structure.
    for (BlockChain::iterator BI = Chain->begin(), BE = Chain->end();
         BI != BE; ++BI) {
      Blocks.push_back(*BI);
      assert(BlockToChain[*BI] == Chain && "Incoming blocks not in chain");
      BlockToChain[*BI] = this;
    }
  }

  /// \brief Count of predecessors within the loop currently being processed.
  ///
  /// This count is updated at each loop we process to represent the number of
  /// in-loop predecessors of this chain.
  unsigned LoopPredecessors;
};
}

namespace {
class MachineBlockPlacement : public MachineFunctionPass {
  /// \brief A typedef for a block filter set.
  typedef SmallPtrSet<MachineBasicBlock *, 16> BlockFilterSet;

  /// \brief A handle to the branch probability pass.
  const MachineBranchProbabilityInfo *MBPI;

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

  /// \brief A handle to the loop info.
  const MachineLoopInfo *MLI;

  /// \brief A handle to the target's instruction info.
  const TargetInstrInfo *TII;

  /// \brief A handle to the target's lowering info.
  const TargetLowering *TLI;

  /// \brief Allocator and owner of BlockChain structures.
  ///
  /// We build BlockChains lazily by merging together high probability BB
  /// sequences acording to the "Algo2" in the paper mentioned at the top of
  /// the file. To reduce malloc traffic, we allocate them using this slab-like
  /// allocator, and destroy them after the pass completes.