Clause Management in Painless: ClauseExchange and ClauseBuffer

Table of Contents

• ClauseExchange
  • Key Features
  • Usage Example
• ClauseBuffer
  • Key Features
  • Usage Example
• toRawPtr and fromRawPtr

ClauseExchange

ClauseExchange is a memory-efficient class for storing and managing SAT clauses of varying sizes. It uses a flexible array member for optimized memory usage and intrusive reference counting for efficient memory management.

Key Features

1. Memory Layout
   • Uses a flexible array member (lits[0]) for storing literals
   • Fixed-size header containing:

     unsigned int lbd;          // Literal Block Distance
     int from;                  // Source identifier
     unsigned int size;         // Number of literals
     std::atomic<unsigned int> refCounter; // Reference count

2. Smart Pointer Management
   • Uses boost::intrusive_ptr for reference counting
   • Type alias: using ClauseExchangePtr = boost::intrusive_ptr<ClauseExchange>
   • Custom reference counting through:

     void intrusive_ptr_add_ref(ClauseExchange* ce);
     void intrusive_ptr_release(ClauseExchange* ce);

3. Iterator Support
   • STL-compatible iterators
   • Supports range-based for loops

     // Begin/end iterators
     int* begin();
     int* end();
     const int* begin() const;
     const int* end() const;

4. Utility Functions

   // Array-style access
   int& operator[](unsigned int index);
   const int& operator[](unsigned int index) const;
    
   // Sorting functions
   void sortLiterals();
   void sortLiteralsDescending();
    
   // Debug support
   std::string toString() const;

5. LBD Handling
   • Forces LBD ≥ 2 for non-unit clauses
   • Unit clauses can have LBD of 0 or 1 (I am thinking of forcing it to 0).

Usage Example

// Create a clause with 3 literals
auto clause = ClauseExchange::create(3, 2, 1);  // size=3, lbd=2, from=1
 
// Fill literals
clause->lits[0] = 1;
clause->lits[1] = -2;
clause->lits[2] = 3;
 
// Or using iterators
for (int& lit : *clause) {
    // Process literals
}

ClauseBuffer

ClauseBuffer is a thread-safe buffer for exchanging clauses between producers and consumers. It's built on top of boost::lockfree::queue for efficient multiple producers-multiple consumers operations.

Key Features

1. Lock-Free Operations
   • Based on boost::lockfree::queue
   • Non-fixed size queue
   • Multiple producer/consumer safe
2. Atomic Size Tracking

   std::atomic<size_t> m_size;  // Current number of elements

3. Clause Addition

   // Add single clause
   bool addClause(ClauseExchangePtr clause);
    
   // Add multiple clauses
   size_t addClauses(const std::vector<ClauseExchangePtr>& clauses);
    
   // Bounded versions (with capacity limit)
   bool tryAddClauseBounded(ClauseExchangePtr clause);
   size_t tryAddClausesBounded(const std::vector<ClauseExchangePtr>& clauses);

4. Clause Retrieval

   // Get single clause
   bool getClause(ClauseExchangePtr& clause);
    
   // Get all available clauses
   void getClauses(std::vector<ClauseExchangePtr>& clauses);

5. Utility Functions

   // Get current size
   size_t size() const;
    
   // Check if empty
   bool empty() const;
    
   // Clear all clauses
   void clear();

Usage Example

// Create buffer with initial capacity
ClauseBuffer buffer(1000);
 
// Producer code
auto clause = ClauseExchange::create(/* ... */);
if (buffer.addClause(clause)) {
    // Successfully added
}
 
// Consumer code
ClauseExchangePtr received;
if (buffer.getClause(received)) {
    // Process received clause
}
 
// Batch operations
std::vector<ClauseExchangePtr> clauses;
buffer.getClauses(clauses);  // Get all available clauses

toRawPtr and fromRawPtr

ClauseBuffer internally uses boost::lockfree::queue which operates on raw pointers, while the external interface uses ClauseExchangePtr (smart pointers). The toRawPtr and fromRawPtr methods of ClauseExchange bridge this gap safely.

// In ClauseExchange
ClauseExchange* toRawPtr() {
    refCounter.fetch_add(1, std::memory_order_relaxed);
    return this;
}
 
// It is static, but we interact with the same ClauseExchange instance, since we have its pointer.
static ClauseExchangePtr fromRawPtr(ClauseExchange* ptr) {
    return ClauseExchangePtr(ptr, false); // false is to not increment the reference counter, since it was incremented at toRawPtr().
}

• Usage in ClauseBuffer :

bool ClauseBuffer::addClause(ClauseExchangePtr clause) {
    // Convert to raw pointer while incrementing ref count
    ClauseExchange* raw = clause->toRawPtr();
    
    if (queue.push(raw)) {
        m_size.fetch_add(1, std::memory_order_release);
        return true;
    } else {
        // Reference count is decremented since the returned ClauseExchangePtr is not stored
        ClauseExchange::fromRawPtr(raw);
        return false;
    }
}
 
bool ClauseBuffer::getClause(ClauseExchangePtr& clause) {
    ClauseExchange* raw;
    if (queue.pop(raw)) {
        // Convert back to smart pointer without incrementing ref count
        clause = ClauseExchange::fromRawPtr(raw);
        m_size.fetch_sub(1, std::memory_order_release);
        return true;
    }
    return false;
}

• toRawPtr(): Increments reference count before storing in queue
• fromRawPtr(): Creates smart pointer without additional increment

Important

A ClauseExchangePtr should not be gotten through .get():

// DON'T: Direct raw pointer usage
ClauseExchange* raw = clause.get();  // Wrong! Bypasses ref counting
// DO: Use provided conversion methods
ClauseExchange* raw = clause->toRawPtr();

Be careful to not leave hanging raw pointers, otherwise the ClauseExchange instance will never be freed.

• Example Flow :

// Life cycle of a clause through the buffer
ClauseBuffer queue(10)
{
    ClauseExchangePtr clause = ClauseExchange::create(3);  // ref_count = 1
    queue.addClause(raw);                                  // ref_count = 2
    // Original clause goes out of scope                   // ref_count = 1
}
ClauseExchangePtr newClause;        
queue.getClause(newClause);                            // Same ref_count = 1
// newClause handles final cleanup                     // ref_count = 0