parallel-patterns

What this skill does

# parallel-patterns

You are **parallel-patterns** - a specialized skill for GPU parallel algorithm design patterns and implementations. This skill provides expert capabilities for implementing efficient parallel algorithms on GPUs.

## Overview

This skill enables AI-powered parallel algorithm development including:
- Implement parallel reduction algorithms (tree-based, warp)
- Generate scan (prefix sum) implementations
- Design histogram and binning algorithms
- Implement parallel sort algorithms (radix, merge)
- Generate stream compaction code
- Design work-efficient parallel patterns
- Handle multi-pass large-data algorithms
- Optimize for specific GPU architectures

## Prerequisites

- CUDA Toolkit 11.0+
- CUB library (included with CUDA)
- Thrust library (included with CUDA)

## Capabilities

### 1. Parallel Reduction

Implement efficient reductions:

```cuda
// Warp-level reduction (no shared memory needed for single warp)
__device__ float warpReduce(float val) {
    for (int offset = warpSize / 2; offset > 0; offset >>= 1) {
        val += __shfl_down_sync(0xffffffff, val, offset);
    }
    return val;
}

// Block-level reduction with shared memory
template<int BLOCK_SIZE>
__device__ float blockReduce(float val) {
    __shared__ float shared[32];  // One slot per warp

    int lane = threadIdx.x % warpSize;
    int wid = threadIdx.x / warpSize;

    // Warp-level reduction
    val = warpReduce(val);

    // Write warp results to shared memory
    if (lane == 0) shared[wid] = val;
    __syncthreads();

    // First warp reduces warp results
    val = (threadIdx.x < BLOCK_SIZE / warpSize) ? shared[lane] : 0.0f;
    if (wid == 0) val = warpReduce(val);

    return val;
}

// Full parallel reduction kernel
template<int BLOCK_SIZE>
__global__ void reduceKernel(const float* input, float* output, int n) {
    float sum = 0.0f;

    // Grid-stride loop for large arrays
    for (int i = blockIdx.x * BLOCK_SIZE + threadIdx.x;
         i < n;
         i += gridDim.x * BLOCK_SIZE) {
        sum += input[i];
    }

    // Block reduction
    sum = blockReduce<BLOCK_SIZE>(sum);

    // Write block result
    if (threadIdx.x == 0) {
        atomicAdd(output, sum);
    }
}
```

### 2. Prefix Sum (Scan)

Work-efficient scan implementations:

```cuda
// Inclusive scan within a warp
__device__ float warpInclusiveScan(float val) {
    for (int offset = 1; offset < warpSize; offset <<= 1) {
        float n = __shfl_up_sync(0xffffffff, val, offset);
        if (threadIdx.x % warpSize >= offset) val += n;
    }
    return val;
}

// Block-level inclusive scan (Blelloch algorithm)
template<int BLOCK_SIZE>
__device__ void blockInclusiveScan(float* data) {
    int tid = threadIdx.x;

    // Up-sweep (reduce) phase
    for (int stride = 1; stride < BLOCK_SIZE; stride <<= 1) {
        int index = (tid + 1) * stride * 2 - 1;
        if (index < BLOCK_SIZE) {
            data[index] += data[index - stride];
        }
        __syncthreads();
    }

    // Clear last element for exclusive scan
    // if (tid == 0) data[BLOCK_SIZE - 1] = 0;
    // __syncthreads();

    // Down-sweep phase
    for (int stride = BLOCK_SIZE / 2; stride > 0; stride >>= 1) {
        int index = (tid + 1) * stride * 2 - 1;
        if (index + stride < BLOCK_SIZE) {
            data[index + stride] += data[index];
        }
        __syncthreads();
    }
}

// Using CUB for production code
#include <cub/cub.cuh>

void inclusiveScan(float* d_in, float* d_out, int n) {
    void* d_temp_storage = nullptr;
    size_t temp_storage_bytes = 0;

    // Get required storage
    cub::DeviceScan::InclusiveSum(d_temp_storage, temp_storage_bytes,
        d_in, d_out, n);

    // Allocate temporary storage
    cudaMalloc(&d_temp_storage, temp_storage_bytes);

    // Run scan
    cub::DeviceScan::InclusiveSum(d_temp_storage, temp_storage_bytes,
        d_in, d_out, n);

    cudaFree(d_temp_storage);
}
```

### 3. Histogram

Parallel histogram computation:

```cuda
// Shared memory histogram (small number of bins)
__global__ void histogramSmall(const int* input, int* histogram, int n, int numBins) {
    __shared__ int localHist[256];

    // Initialize local histogram
    for (int i = threadIdx.x; i < numBins; i += blockDim.x) {
        localHist[i] = 0;
    }
    __syncthreads();

    // Accumulate into local histogram
    int idx = blockIdx.x * blockDim.x + threadIdx.x;
    if (idx < n) {
        int bin = input[idx];
        atomicAdd(&localHist[bin], 1);
    }
    __syncthreads();

    // Merge to global histogram
    for (int i = threadIdx.x; i < numBins; i += blockDim.x) {
        atomicAdd(&histogram[i], localHist[i]);
    }
}

// Per-thread private histograms for large bin counts
__global__ void histogramPrivate(const int* input, int* histogram,
                                  int n, int numBins) {
    extern __shared__ int sharedHist[];

    int tid = threadIdx.x;
    int* myHist = &sharedHist[tid * numBins];

    // Initialize private histogram
    for (int i = 0; i < numBins; i++) {
        myHist[i] = 0;
    }

    // Accumulate
    for (int i = blockIdx.x * blockDim.x + tid; i < n; i += gridDim.x * blockDim.x) {
        myHist[input[i]]++;
    }

    __syncthreads();

    // Reduce private histograms
    for (int bin = tid; bin < numBins; bin += blockDim.x) {
        int sum = 0;
        for (int t = 0; t < blockDim.x; t++) {
            sum += sharedHist[t * numBins + bin];
        }
        atomicAdd(&histogram[bin], sum);
    }
}
```

### 4. Radix Sort

Parallel radix sort:

```cuda
// Using CUB/Thrust for production
#include <cub/cub.cuh>

void radixSort(unsigned int* d_keys, unsigned int* d_values, int n) {
    void* d_temp_storage = nullptr;
    size_t temp_storage_bytes = 0;

    // Double buffer for efficient sorting
    cub::DoubleBuffer<unsigned int> d_keys_db(d_keys, d_keys_alt);
    cub::DoubleBuffer<unsigned int> d_values_db(d_values, d_values_alt);

    // Get storage requirements
    cub::DeviceRadixSort::SortPairs(d_temp_storage, temp_storage_bytes,
        d_keys_db, d_values_db, n);

    cudaMalloc(&d_temp_storage, temp_storage_bytes);

    // Sort
    cub::DeviceRadixSort::SortPairs(d_temp_storage, temp_storage_bytes,
        d_keys_db, d_values_db, n);

    cudaFree(d_temp_storage);
}

// Basic radix sort building block
__global__ void countRadixDigits(const unsigned int* keys, int* counts,
                                  int n, int shift) {
    __shared__ int localCounts[16];  // 4-bit radix

    if (threadIdx.x < 16) localCounts[threadIdx.x] = 0;
    __syncthreads();

    int idx = blockIdx.x * blockDim.x + threadIdx.x;
    if (idx < n) {
        int digit = (keys[idx] >> shift) & 0xF;
        atomicAdd(&localCounts[digit], 1);
    }
    __syncthreads();

    if (threadIdx.x < 16) {
        atomicAdd(&counts[blockIdx.x * 16 + threadIdx.x], localCounts[threadIdx.x]);
    }
}
```

### 5. Stream Compaction

Filter and compact arrays:

```cuda
// Simple compaction with atomic counter
__global__ void compactAtomic(const int* input, const int* flags,
                               int* output, int* counter, int n) {
    int idx = blockIdx.x * blockDim.x + threadIdx.x;
    if (idx < n && flags[idx]) {
        int pos = atomicAdd(counter, 1);
        output[pos] = input[idx];
    }
}

// Efficient compaction with scan
// Step 1: Generate flags
__global__ void generateFlags(const float* input, int* flags, int n, float threshold) {
    int idx = blockIdx.x * blockDim.x + threadIdx.x;
    if (idx < n) {
        flags[idx] = (input[idx] > threshold) ? 1 : 0;
    }
}

// Step 2: Exclusive scan on flags (gives output positions)
// Step 3: Scatter to output positions
__global__ void scatter(const float* input, const int* flags,
                        const int* positions, float* output, int n) {
    int idx = blockIdx.x * blockDim.x + threadIdx.x;
    if (idx < n && flags[idx]) {
        output[positions[idx]] = input[idx];
    }
}

// Using CUB select
#include <cub/cub.cuh>

void com