swift-mirror/include/swift/SIL/BasicBlockBits.h

//===--- BasicBlockBits.h - SILBasicBlock bit utilities ---------*- C++ -*-===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2021 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
//
// This file defines utilities for BasicBlock bit fields and sets.
//
//===----------------------------------------------------------------------===//

#ifndef SWIFT_SIL_BASICBLOCKBITS_H
#define SWIFT_SIL_BASICBLOCKBITS_H

#include "swift/SIL/SILFunction.h"
#include "llvm/ADT/SmallVector.h"

namespace swift {

/// Utility to add a custom bitfield to a function's basic blocks.
///
/// This can be used by transforms to store temporary flags or tiny values per
/// basic block.
/// The memory managed is a 32 bit field within each basic block (\see
/// BasicBlock::customBits) and thus is very efficient: no memory allocation is
/// needed, no hash set or map is needed for lookup and there is no
/// initialization cost (in contrast to BasicBlockData which needs to iterate
/// over all blocks at initialization).
///
/// Invariants:
/// * BasicBlockBitfield instances must be allocated and deallocated
///   following a strict stack discipline, because bit-positions in
///   BasicBlock::customBits are "allocated" and "freed" with a stack-allocation
///   algorithm. This means, it's fine to use a BasicBlockBitfield as (or in)
///   local variables, e.g. in transformations. But it's not possible to store
///   a BasicBlockBitfield in an Analysis.
/// * The total number of bits which are alive at the same time must not exceed
///   32 (the size of BasicBlock::customBits).
class BasicBlockBitfield {
  /// The bitfield is "added" to the blocks of this function.
  SILFunction *function;

  /// A single linked list of currently alive BasicBlockBitfields (oldest is
  /// last, newest is first).
  /// The head of the list is function->lastAllocatedBitfield.
  BasicBlockBitfield *parent;
  
  /// Initialized with the monotonically increasing currentBitfieldID of the
  /// function.
  /// Used to check if the bitfield in a block is initialized.
  /// If a block's lastInitializedBitfieldID is less than this ID, it means
  /// that the bits of that block are not initialized yet.
  /// See also: SILBasicBlock::lastInitializedBitfieldID,
  ///           SILFunction::currentBitfieldID
  unsigned bitfieldID;

  short startBit;
  short endBit;
  uint32_t mask;

public:
  BasicBlockBitfield(SILFunction *function, int size) :
      function(function),
      parent(function->newestAliveBitfield),
      bitfieldID(function->currentBitfieldID),
      startBit(parent ? parent->endBit : 0),
      endBit(startBit + size),
      mask(0xffffffffu >> (32 - size) << startBit) {
    assert(size > 0 && "bit field size must be > 0");
    assert(endBit <= 32 && "too many/large bit fields allocated in function");
    assert((!parent || bitfieldID > parent->bitfieldID) &&
           "BasicBlockBitfield indices are not in order");
    function->newestAliveBitfield = this;
    ++function->currentBitfieldID;
    assert(function->currentBitfieldID != 0 && "currentBitfieldID overflow");
  }

  ~BasicBlockBitfield() {
    assert(function->newestAliveBitfield == this &&
           "BasicBlockBitfield destructed too early");
    function->newestAliveBitfield = parent;
  }

  BasicBlockBitfield(const BasicBlockBitfield &) = delete;
  BasicBlockBitfield(BasicBlockBitfield &&) = delete;
  BasicBlockBitfield &operator=(const BasicBlockBitfield &) = delete;
  BasicBlockBitfield &operator=(BasicBlockBitfield &&) = delete;

  SILFunction *getFunction() const { return function; }

  unsigned get(SILBasicBlock *block) const {
    assert(block->getParent() == function);
    if (bitfieldID > block->lastInitializedBitfieldID) {
      // The bitfield is not initialized yet in this block.
      return 0;
    }
    return (block->customBits & mask) >> startBit;
  }

  void set(SILBasicBlock *block, unsigned value) {
    assert(block->getParent() == function);
    assert(((value << startBit) & ~mask) == 0 &&
           "value too large for BasicBlockBitfield");
    unsigned clearMask = mask;
    if (bitfieldID > block->lastInitializedBitfieldID) {

      // The bitfield is not initialized yet in this block.
      // Initialize the bitfield, and also initialize all parent bitfields,
      // which are not initialized, yet. Example:
      //
      //  This field   Last initialized field
      //    |            |
      //    V            V
      //   EE DDD C BB AAA
      //
      // block->lastInitializedBitfieldID == AAA.bitfieldID
      // -> we have to initialize the fields: BB, C, DDD and EE
      //
      BasicBlockBitfield *bf = parent;
      while (bf && bf->bitfieldID > block->lastInitializedBitfieldID) {
        clearMask |= bf->mask;
        bf = bf->parent;
      }
      block->lastInitializedBitfieldID = bitfieldID;
    }
    block->customBits = (block->customBits & ~clearMask) | (value << startBit);
  }
};

/// A BasicBlockBitfield containing a single bit - a flag.
class BasicBlockFlag {
  BasicBlockBitfield bit;

public:
  BasicBlockFlag(SILFunction *function) : bit(function, 1) {}

  SILFunction *getFunction() const { return bit.getFunction(); }
  
  bool get(SILBasicBlock *block) const { return (bool)bit.get(block); }

  void set(SILBasicBlock *block, bool value = true) {
    bit.set(block, (unsigned)value);
  }
  void reset(SILBasicBlock *block) { bit.set(block, 0); }

  /// Sets the flag and returns the old value.
  bool testAndSet(SILBasicBlock *block) {
    bool oldValue = get(block);
    set(block);
    return oldValue;
  }
};

/// A BasicBlockFlag with a set-like API.
class BasicBlockSet {
  BasicBlockFlag flag;

public:
  BasicBlockSet(SILFunction *function) : flag(function) {}

  SILFunction *getFunction() const { return flag.getFunction(); }

  bool contains(SILBasicBlock *block) const { return flag.get(block); }

  /// Returns true if \p block was not contained in the set before inserting.
  bool insert(SILBasicBlock *block) { return !flag.testAndSet(block); }

  void erase(SILBasicBlock *block) { flag.reset(block); }
};

} // namespace swift

#endif