warp-types 0.3.2

//! Inter-Block Sessions: Scaling Beyond Single Warp
//!
//! **STATUS: Research** — Exploratory prototype, not promoted to main API.
//!
//! This module explores how session types can model communication between
//! blocks in a GPU grid. This is fundamentally different from warp-level
//! sessions because:
//!
//! 1. **No lockstep execution**: Blocks execute independently
//! 2. **Memory-based communication**: No shuffles, use global memory
//! 3. **Explicit synchronization**: Barriers, atomics, cooperative groups
//! 4. **Scale**: Thousands of blocks vs 32 lanes
//!
//! # Key Insight
//!
//! Inter-block is closer to traditional MPST (Multiparty Session Types)
//! than our warp-level SIMT types. The "divergence" model doesn't apply
//! because blocks don't share control flow.
//!
//! # Hierarchy of Parallelism
//!
//! ```text
//! Grid (N blocks)
//!   └── Block (M warps)
//!         └── Warp (32/64 lanes)  <- Current work
//! ```
//!
//! Each level has different communication primitives and type constraints.

use std::marker::PhantomData;

// ============================================================================
// GPU HIERARCHY TYPES
// ============================================================================

/// A lane within a warp (0..31 for NVIDIA, 0..63 for AMD)
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub struct LaneId(pub u32);

/// A warp within a block
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub struct WarpId(pub u32);

/// A block within a grid
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub struct BlockId(pub u32);

/// A thread's full identity in the grid
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub struct ThreadId {
    pub block: BlockId,
    pub warp: WarpId,
    pub lane: LaneId,
}

impl ThreadId {
    pub fn global_id(&self, warps_per_block: u32, lanes_per_warp: u32) -> u32 {
        self.block.0 * warps_per_block * lanes_per_warp + self.warp.0 * lanes_per_warp + self.lane.0
    }
}

// ============================================================================
// COMMUNICATION PRIMITIVES BY LEVEL
// ============================================================================

/// Communication within a warp (our existing work).
/// Warp-level communication: shuffles, votes, ballots.
/// All 32 lanes execute in lockstep. No memory needed.
pub mod intra_warp {

    /// Shuffle: direct register exchange
    pub trait ShuffleOp {
        fn shuffle_xor<T: Copy>(val: T, mask: u32) -> T;
        fn shuffle_down<T: Copy>(val: T, delta: u32) -> T;
        fn shuffle_up<T: Copy>(val: T, delta: u32) -> T;
    }

    /// Vote: warp-wide predicate operations
    pub trait VoteOp {
        fn all(pred: bool) -> bool;
        fn any(pred: bool) -> bool;
        fn ballot(pred: bool) -> u32;
    }
}

/// Communication within a block (between warps).
/// Block-level communication: shared memory, barriers.
/// Warps in same block share SMEM. Block-wide `__syncthreads()`.
pub mod intra_block {

    use super::*;

    /// Shared memory region owned by a block
    pub struct SharedMem<T, const SIZE: usize> {
        _marker: PhantomData<T>,
    }

    impl<T: Copy, const SIZE: usize> SharedMem<T, SIZE> {
        /// All threads in block can read (after barrier)
        pub fn read(&self, _idx: usize) -> T
        where
            T: Default,
        {
            // Placeholder: CPU has no shared memory; return default value
            T::default()
        }

        /// Write requires knowing no conflicts
        pub fn write(&mut self, _idx: usize, _val: T) {
            // Placeholder: CPU has no shared memory; write is a no-op
        }
    }

    /// Block-wide barrier
    pub fn sync_threads() {
        // __syncthreads() - all threads in block must reach
    }

    /// Typed barrier: ensures all threads reach before proceeding
    pub struct BlockBarrier<State>(PhantomData<State>);

    pub struct BeforeSync;
    pub struct AfterSync;

    impl BlockBarrier<BeforeSync> {
        pub fn sync(self) -> BlockBarrier<AfterSync> {
            sync_threads();
            BlockBarrier(PhantomData)
        }
    }
}

/// Communication between blocks (the new frontier).
/// Grid-level communication: global memory, atomics, cooperative groups.
/// Blocks execute independently. No implicit synchronization.
pub mod inter_block {

    use super::*;

    /// Global memory region visible to all blocks
    pub struct GlobalMem<T> {
        _marker: PhantomData<T>,
    }

    impl<T: Copy + Default> GlobalMem<T> {
        /// Read from global memory (may be stale without fence)
        pub fn read(&self, _idx: usize) -> T {
            T::default() // CPU placeholder
        }

        /// Write to global memory (may not be visible without fence)
        pub fn write(&mut self, _idx: usize, _val: T) {
            // CPU placeholder — no-op
        }

        /// Atomic add - returns old value
        pub fn atomic_add(&self, _idx: usize, _val: T) -> T
        where
            T: std::ops::Add<Output = T>,
        {
            T::default() // CPU placeholder — returns "old value" of zero
        }
    }

    /// Memory fence - ensures visibility
    pub fn thread_fence_system() {
        // __threadfence_system()
    }

    /// Grid-wide barrier (requires cooperative launch)
    pub fn grid_sync() {
        // cooperative_groups::this_grid().sync()
    }
}

// ============================================================================
// INTER-BLOCK SESSION TYPES
// ============================================================================

/// A role in an inter-block protocol
///
/// Unlike warp lanes (fixed 0..31), block roles are logical.
/// Example: "leader block" vs "worker blocks"
pub trait BlockRole {
    const NAME: &'static str;
}

/// The single leader block (block 0)
pub struct Leader;
impl BlockRole for Leader {
    const NAME: &'static str = "Leader";
}

/// Worker blocks (blocks 1..N-1)
pub struct Worker;
impl BlockRole for Worker {
    const NAME: &'static str = "Worker";
}

/// Protocol states for inter-block sessions
pub trait ProtocolState {}

/// Initial state before any communication
pub struct Initial;
impl ProtocolState for Initial {}

/// After leader has broadcast work
pub struct WorkDistributed;
impl ProtocolState for WorkDistributed {}

/// After workers have signaled completion
pub struct WorkComplete;
impl ProtocolState for WorkComplete {}

/// After results have been collected
pub struct ResultsCollected;
impl ProtocolState for ResultsCollected {}

// ============================================================================
// INTER-BLOCK SESSION TYPES (MPST-like)
// ============================================================================

/// A session between blocks, parameterized by:
/// - Role: which block role we are
/// - State: current protocol state
/// - N: number of blocks (compile-time for typed guarantees)
pub struct BlockSession<Role: BlockRole, State: ProtocolState, const N: usize> {
    block_id: BlockId,
    _role: PhantomData<Role>,
    _state: PhantomData<State>,
}

impl<Role: BlockRole, State: ProtocolState, const N: usize> BlockSession<Role, State, N> {
    /// Create a new session for this block
    pub fn new(block_id: BlockId) -> Self {
        BlockSession {
            block_id,
            _role: PhantomData,
            _state: PhantomData,
        }
    }

    /// Get our block ID
    pub fn block_id(&self) -> BlockId {
        self.block_id
    }
}

// Leader operations
impl<State: ProtocolState, const N: usize> BlockSession<Leader, State, N> {
    /// Broadcast data to all worker blocks
    /// State transition: Initial -> WorkDistributed
    pub fn broadcast<T: Copy>(
        self,
        _data: T,
        _global: &mut inter_block::GlobalMem<T>,
    ) -> BlockSession<Leader, WorkDistributed, N>
    where
        State: Into<Initial>, // Only from Initial state
    {
        // Write to global memory locations for each worker
        // In real code: for i in 1..N { global.write(i, data); }
        BlockSession::new(self.block_id)
    }

    /// Wait for all workers to signal completion
    /// State transition: WorkDistributed -> WorkComplete
    pub fn wait_for_workers(
        self,
        _signal: &inter_block::GlobalMem<u32>,
    ) -> BlockSession<Leader, WorkComplete, N>
    where
        State: Into<WorkDistributed>,
    {
        // Spin on atomic counter until it reaches N-1
        // In real code: while signal.read(0) < N-1 { }
        BlockSession::new(self.block_id)
    }
}

// Worker operations
impl<State: ProtocolState, const N: usize> BlockSession<Worker, State, N> {
    /// Receive broadcast data from leader
    /// State transition: Initial -> WorkDistributed
    pub fn receive<T: Copy + Default>(
        self,
        _global: &inter_block::GlobalMem<T>,
    ) -> (T, BlockSession<Worker, WorkDistributed, N>)
    where
        State: Into<Initial>,
    {
        // Read from our global memory slot
        // In real code: let data = global.read(self.block_id.0);
        let data: T = T::default(); // Placeholder
        (data, BlockSession::new(self.block_id))
    }

    /// Signal completion to leader
    /// State transition: WorkDistributed -> WorkComplete
    pub fn signal_done(
        self,
        _signal: &inter_block::GlobalMem<u32>,
    ) -> BlockSession<Worker, WorkComplete, N>
    where
        State: Into<WorkDistributed>,
    {
        // Atomically increment completion counter
        // In real code: signal.atomic_add(0, 1);
        BlockSession::new(self.block_id)
    }
}

// ============================================================================
// COOPERATIVE GROUPS: BRIDGING THE GAP
// ============================================================================

/// Cooperative groups blur the boundary between warp, block, and grid.
/// They provide a uniform interface for different granularities.
pub mod cooperative {
    use super::*;

    /// A cooperative group - a subset of threads that can synchronize
    pub trait CooperativeGroup {
        /// Number of threads in this group
        fn size(&self) -> u32;

        /// This thread's rank within the group (0..size-1)
        fn thread_rank(&self) -> u32;

        /// Synchronize all threads in the group
        fn sync(&self);
    }

    /// Thread block group (all threads in a block)
    pub struct ThreadBlockGroup {
        block_id: BlockId,
        num_threads: u32,
    }

    impl CooperativeGroup for ThreadBlockGroup {
        fn size(&self) -> u32 {
            self.num_threads
        }
        fn thread_rank(&self) -> u32 {
            0
        } // CPU single-thread placeholder
        fn sync(&self) {
            intra_block::sync_threads();
        }
    }

    /// Grid group (all threads in the grid) - requires cooperative launch
    pub struct GridGroup {
        num_blocks: u32,
        threads_per_block: u32,
    }

    impl CooperativeGroup for GridGroup {
        fn size(&self) -> u32 {
            self.num_blocks * self.threads_per_block
        }
        fn thread_rank(&self) -> u32 {
            0
        } // CPU single-thread placeholder
        fn sync(&self) {
            inter_block::grid_sync();
        }
    }

    /// Coalesced group (subset of a warp with active lanes)
    /// This connects to our warp divergence work!
    pub struct CoalescedGroup {
        mask: u32, // Active lane mask
    }

    impl CooperativeGroup for CoalescedGroup {
        fn size(&self) -> u32 {
            self.mask.count_ones()
        }
        fn thread_rank(&self) -> u32 {
            0
        } // CPU single-thread placeholder
        fn sync(&self) { /* Implicit in warp execution */
        }
    }

    /// Tiled partition (fixed-size subset of a warp)
    pub struct TiledPartition<const SIZE: u32> {
        _marker: PhantomData<()>,
    }

    impl<const SIZE: u32> CooperativeGroup for TiledPartition<SIZE> {
        fn size(&self) -> u32 {
            SIZE
        }
        fn thread_rank(&self) -> u32 {
            0
        } // CPU single-thread placeholder
        fn sync(&self) { /* Implicit in warp execution for SIZE <= 32 */
        }
    }
}

// ============================================================================
// UNIFIED SESSION HIERARCHY
// ============================================================================

/// A unified view of sessions at any level of the GPU hierarchy.
///
/// The key insight: each level has similar operations but different
/// implementations and constraints.
pub trait SessionLevel {
    /// The identity type at this level
    type Id: Copy + Eq;

    /// The communication primitive
    type CommPrimitive;

    /// The synchronization primitive
    type SyncPrimitive;

    /// Maximum participants at this level
    const MAX_PARTICIPANTS: u32;
}

/// Warp level: 32 lanes, shuffle, implicit sync
pub struct WarpLevel;
impl SessionLevel for WarpLevel {
    type Id = LaneId;
    type CommPrimitive = (); // Shuffles
    type SyncPrimitive = (); // Implicit (SIMT)
    const MAX_PARTICIPANTS: u32 = 32;
}

/// Block level: variable warps, shared memory, __syncthreads
pub struct BlockLevel;
impl SessionLevel for BlockLevel {
    type Id = WarpId;
    type CommPrimitive = (); // Shared memory
    type SyncPrimitive = (); // __syncthreads
    const MAX_PARTICIPANTS: u32 = 32; // Typical max warps per block
}

/// Grid level: variable blocks, global memory, cooperative groups
pub struct GridLevel;
impl SessionLevel for GridLevel {
    type Id = BlockId;
    type CommPrimitive = (); // Global memory
    type SyncPrimitive = (); // grid.sync()
    const MAX_PARTICIPANTS: u32 = 65535; // Max blocks
}

// ============================================================================
// COMPARISON: WARP vs INTER-BLOCK
// ============================================================================

// Key differences between warp-level and inter-block sessions:
//
// | Aspect           | Warp-Level              | Inter-Block            |
// |------------------|-------------------------|------------------------|
// | Execution model  | SIMT (lockstep)         | Independent            |
// | Communication    | Shuffles (registers)    | Memory (global)        |
// | Synchronization  | Implicit                | Explicit barriers      |
// | Divergence       | Yes (some lanes idle)   | N/A (blocks run fully) |
// | Session model    | Quiescent participants  | Traditional MPST       |
// | Participants     | Fixed (32/64)           | Variable (1..65535)    |
// | Failure modes    | Deadlock-free (SIMT)    | Deadlock possible      |
//
// The warp-level "session-typed divergence" model is novel because:
// 1. Divergence = some participants go quiescent (not in traditional MPST)
// 2. Reconvergence = quiescent participants rejoin
// 3. Implicit sync means no deadlock (if divergence matches)
//
// Inter-block is closer to traditional MPST:
// 1. All blocks always "active" (no quiescence)
// 2. Explicit message passing via memory
// 3. Deadlock possible if barriers misaligned

// ============================================================================
// CROSS-LEVEL PROTOCOLS
// ============================================================================

/// A protocol that spans multiple levels of the hierarchy.
///
/// Example: Hierarchical reduction
///
/// 1. Each warp reduces its 32 values (warp-level)
/// 2. Warp 0 collects results from all warps (block-level)
/// 3. Block 0 collects results from all blocks (grid-level)
pub trait HierarchicalProtocol {
    /// Phase 1: Intra-warp (32 values -> 1)
    fn warp_reduce(&self) -> u32;

    /// Phase 2: Intra-block (N warps -> 1)
    fn block_reduce(&self) -> u32;

    /// Phase 3: Inter-block (M blocks -> 1)
    fn grid_reduce(&self) -> u32;
}

/// Type-safe hierarchical reduction
///
/// Each phase transitions the protocol state, preventing out-of-order execution.
pub mod hierarchical_reduce {
    use super::*;

    /// Phase states
    pub struct WarpPhase;
    pub struct BlockPhase;
    pub struct GridPhase;
    pub struct Complete;

    /// Reduction session
    pub struct ReductionSession<Phase> {
        value: u32,
        _phase: PhantomData<Phase>,
    }

    impl ReductionSession<WarpPhase> {
        pub fn new(value: u32) -> Self {
            ReductionSession {
                value,
                _phase: PhantomData,
            }
        }

        /// Warp reduction using shuffles
        /// Returns the result (in lane 0) and advances to block phase
        pub fn warp_reduce(self) -> (u32, ReductionSession<BlockPhase>) {
            // In real code: use shuffle_down reduction
            let result = self.value; // Placeholder
            (
                result,
                ReductionSession {
                    value: result,
                    _phase: PhantomData,
                },
            )
        }
    }

    impl ReductionSession<BlockPhase> {
        /// Block reduction using shared memory
        /// Only warp 0 continues to grid phase
        pub fn block_reduce(self) -> (u32, ReductionSession<GridPhase>) {
            // In real code: write to shared mem, sync, reduce
            let result = self.value;
            (
                result,
                ReductionSession {
                    value: result,
                    _phase: PhantomData,
                },
            )
        }
    }

    impl ReductionSession<GridPhase> {
        /// Grid reduction using global memory
        /// Only block 0 gets final result
        pub fn grid_reduce(self) -> (u32, ReductionSession<Complete>) {
            // In real code: write to global mem, grid sync, reduce
            let result = self.value;
            (
                result,
                ReductionSession {
                    value: result,
                    _phase: PhantomData,
                },
            )
        }
    }

    impl ReductionSession<Complete> {
        /// Get the final result (only meaningful in block 0, warp 0, lane 0)
        pub fn result(self) -> u32 {
            self.value
        }
    }
}

// ============================================================================
// RESEARCH QUESTIONS
// ============================================================================

/// Open questions for inter-block session types:
///
/// 1. **Deadlock freedom**: How to statically verify no deadlock?
///    - Warp-level: trivial (SIMT guarantees convergence)
///    - Inter-block: need progress analysis (all blocks reach barriers)
///
/// 2. **Partial participation**: What if some blocks don't participate?
///    - Unlike warp divergence, blocks can't "go quiescent"
///    - Must exit entirely or participate fully
///
/// 3. **Dynamic block count**: Grid size often runtime-determined
///    - Type-level N requires const generics
///    - Runtime N needs existential types or runtime checks
///
/// 4. **Failure modes**: What if a block crashes?
///    - Warp: other lanes continue (masked out)
///    - Block: undefined behavior, likely hang
///
/// 5. **Composability**: How to compose warp/block/grid sessions?
///    - Hierarchical protocols (reduce) are straightforward
///    - General composition needs more thought
///
/// 6. **Performance**: Do types add overhead?
///    - State machine approach should compile away
///    - But global memory communication has inherent cost

// ============================================================================
// TESTS
// ============================================================================

#[cfg(test)]
mod tests {
    use super::hierarchical_reduce::*;
    use super::*;

    #[test]
    fn test_thread_id() {
        let tid = ThreadId {
            block: BlockId(2),
            warp: WarpId(3),
            lane: LaneId(7),
        };

        // 2 blocks * 4 warps * 32 lanes + 3 * 32 + 7
        let global = tid.global_id(4, 32);
        assert_eq!(global, 2 * 4 * 32 + 3 * 32 + 7);
    }

    #[test]
    fn test_hierarchical_reduction_types() {
        // This test verifies the type-state transitions compile correctly
        let session = ReductionSession::<WarpPhase>::new(42);

        // Phase 1: Warp reduce
        let (warp_result, session) = session.warp_reduce();
        assert_eq!(warp_result, 42);

        // Phase 2: Block reduce
        let (block_result, session) = session.block_reduce();
        assert_eq!(block_result, 42);

        // Phase 3: Grid reduce
        let (grid_result, session) = session.grid_reduce();
        assert_eq!(grid_result, 42);

        // Complete
        let final_result = session.result();
        assert_eq!(final_result, 42);
    }

    #[test]
    fn test_block_session_state_transitions() {
        // Leader session
        let leader: BlockSession<Leader, Initial, 4> = BlockSession::new(BlockId(0));
        assert_eq!(leader.block_id().0, 0);

        // Worker session
        let worker: BlockSession<Worker, Initial, 4> = BlockSession::new(BlockId(1));
        assert_eq!(worker.block_id().0, 1);
    }

    // Compile-time test: this should NOT compile if uncommented
    // #[test]
    // fn test_wrong_state_transition() {
    //     let session = ReductionSession::<WarpPhase>::new(42);
    //     // Try to skip to grid phase - should fail!
    //     let (_, session) = session.grid_reduce();  // ERROR: no method grid_reduce on WarpPhase
    // }
}

// ============================================================================
// SUMMARY: KEY INSIGHTS
// ============================================================================

// 1. **Inter-block ≠ warp divergence**: Blocks don't go quiescent, they
//    either run or don't exist. The "session-typed divergence" model
//    is specific to SIMT warp execution.
//
// 2. **Cooperative groups bridge levels**: They provide a uniform API
//    across warp/block/grid, but types still differ per level.
//
// 3. **Traditional MPST applies**: Inter-block is closer to distributed
//    systems. Existing MPST research (Honda/Yoshida/Carbone) applies.
//
// 4. **Hierarchical protocols are natural**: Warp → Block → Grid
//    reduction is a clean type-state machine.
//
// 5. **Deadlock is the main concern**: Unlike warp-level (where SIMT
//    guarantees convergence), inter-block needs explicit deadlock analysis.
//
// 6. **Research opportunity**: Unify warp-level divergence typing with
//    inter-block MPST. Cooperative groups might be the bridge.