cuda-writing

name	cuda-writing
description	PROACTIVELY USE THIS SKILL when writing, modifying, or reviewing GPU kernels, CUDA/NVRTC kernel source, CCCL primitive usage, device memory management, stream-based pipelining, or any GPU dispatch logic in src/vibespatial/. Covers kernel lifecycle, ADR-0033 tier system, stream overlap patterns, warp-level intrinsics, count-scatter patterns, precompilation (ADR-0034), precision dispatch (ADR-0002), and GPU saturation techniques.
user-invocable	true
argument-hint	<optional kernel-file-path or topic>

GPU Kernel Development Guide — vibeSpatial

You are writing GPU code for vibeSpatial. Follow these rules strictly. All GPU primitive dispatch decisions are governed by ADR-0033. Precision dispatch is governed by ADR-0002. Precompilation is governed by ADR-0034.

1. ADR-0033 Tier Decision Tree

Before writing any GPU code, classify your operation:

Is the inner loop geometry-specific (ring traversal, winding, segment intersection)?
  -> Yes: Tier 1 (custom NVRTC kernel)
  -> No: Is it segmented (per-row/ring/group reduction, sort, scan)?
    -> Yes: Tier 3a (CCCL segmented_*)
    -> No: Is it sort/unique/search/partition/compaction/scan?
      -> Yes: Tier 3a (CCCL)
      -> No: Is it element-wise/gather/scatter/concat?
        -> Yes: Tier 2 (CuPy)
        -> No: Tier 1 (custom NVRTC kernel)

Then for CCCL (Tier 3a): Can input use CountingIterator/TransformIterator? Use Tier 3c iterator. Called repeatedly with same types/ops? Use Tier 3b make_*.

Tier 1 — Custom NVRTC Kernels

Geometry-specific inner loops: point-in-polygon winding, segment intersection, split events, half-edge traversal, WKB extraction. Compiled via runtime.compile_kernels(), launched via runtime.launch(). Cached in CudaDriverRuntime._module_cache by SHA1 of source. Polygon work uses MORE Tier 1, not less (ring traversal, part offset indirection, winding across holes).

Tier 2 — CuPy Built-Ins (Default for Element-Wise)

Element-wise transforms, boolean masking, gather/scatter, concat. Specific operations: cp.where, cp.sum, fancy indexing, cp.concatenate. Why: zero JIT path; simpler than CCCL for no algorithmic advantage.

CuPy remains the AUTO default for (ADR-0033 Tier 4):

• reduce_into (cp.sum) — marginal difference; promote when make_* benchmarks justify
• inclusive_scan (cp.cumsum) — same rationale

Tier 3a — CCCL Algorithmic Primitives

Promoted to CCCL default (benchmarked 2026-03-12):

• exclusive_scan — 1.8-3.7x faster than CuPy via make_*

Reverted to CuPy default (2026-03-17):

• select (compaction by bool mask) — CCCL make_select bakes predicate closure device pointers, preventing reuse across calls. One-shot select() re-JITs per array size class (~5-6s each). CuPy flatnonzero is 0.2ms with no JIT. CCCL available via explicit CompactionStrategy.CCCL_SELECT.

Already CCCL-only (no CuPy equivalent):

• radix_sort, merge_sort with custom comparators
• unique_by_key, segmented_reduce, lower_bound, upper_bound
• segmented_sort, three_way_partition

High-value for polygon expansion:

• segmented_sort — sort within offset spans (angle-sort half-edges)
• segmented_reduce — per-polygon area, winding, bounds, vertex count
• three_way_partition — split mixed point/line/polygon in one pass

Tier 3b — CCCL make_* Reusable Callables

For algorithms called repeatedly with the same types and operators. make_* objects pre-allocate temp storage and eliminate both the cold-call JIT check (~950-1460ms first call) and per-call temp-storage overhead.

# One-shot API (cold-call pays full JIT):
cccl_algorithms.exclusive_scan(values, out, _sum_op, init, n)

# make_* API (pre-compiled, reusable):
scanner = cccl_algorithms.make_exclusive_scan(None, values, out, _sum_op, n, init)
temp = cp.empty(scanner_temp_bytes, dtype=cp.uint8)
scanner(temp, values, out, _sum_op, n, init)  # no JIT check

Rule: Wrap new CCCL primitives in make_* factory. Benchmark before changing the AUTO default from CuPy.

Tier 3c — CCCL Iterators (Zero-Allocation)

• CountingIterator(start) — replaces cp.arange allocations
• TransformIterator(input, op) — fuses element-wise transforms with algorithms
• ZipIterator(iter_a, iter_b) — avoids struct-of-arrays shuffling

Impact: reduces peak device memory for large intermediates.

from vibespatial.cuda.cccl_primitives import counting_iterator, transform_iterator

# Instead of: indices = cp.arange(n, dtype=cp.int32)
indices = counting_iterator(0, dtype=np.int32)

# Fuse a transform with a reduction (no intermediate buffer):
squared = transform_iterator(d_values, lambda x: x * x)

Strategy Enum Pattern (API Stability)

The cuda.compute.algorithms API is marked unstable. Each CCCL primitive in cccl_primitives.py is wrapped behind a strategy enum (e.g., CompactionStrategy, ScanStrategy, PairSortStrategy) that makes it trivial to flip the AUTO default per-primitive when benchmarks justify it, insulating the codebase from upstream API changes.

# Callers use strategy enums, never raw CCCL calls:
result = compact_indices(mask, strategy=CompactionStrategy.AUTO)
offsets = exclusive_sum(counts, strategy=ScanStrategy.AUTO)

---

2. Kernel Launch Lifecycle (Tier 1)

Every NVRTC kernel follows this exact sequence:

from vibespatial.cuda._runtime import (
    get_cuda_runtime, make_kernel_cache_key,
    KERNEL_PARAM_PTR, KERNEL_PARAM_I32, KERNEL_PARAM_F64,
)

# 1. Get runtime singleton
runtime = get_cuda_runtime()

# 2. Compile (cached via SHA1 of source)
cache_key = make_kernel_cache_key("my-kernel", _KERNEL_SOURCE)
kernels = runtime.compile_kernels(
    cache_key=cache_key, source=_KERNEL_SOURCE,
    kernel_names=("my_kernel",),
)

# 3. Parameters as (values_tuple, types_tuple)
ptr = runtime.pointer
params = (
    (ptr(d_input), ptr(d_output), width, height, some_float),
    (KERNEL_PARAM_PTR, KERNEL_PARAM_PTR, KERNEL_PARAM_I32, KERNEL_PARAM_I32, KERNEL_PARAM_F64),
)

# 4. Occupancy-based launch config (NEVER hardcode block=(256,1,1))
grid, block = runtime.launch_config(kernels["my_kernel"], item_count)

# 5. Launch
runtime.launch(kernels["my_kernel"], grid=grid, block=block, params=params)

Parameter Type Constants

KERNEL_PARAM_PTR = ctypes.c_void_p    # Device memory pointers
KERNEL_PARAM_I32 = ctypes.c_int       # 32-bit integers
KERNEL_PARAM_F64 = ctypes.c_double    # 64-bit floats

CRITICAL: Always use the named constants from vibespatial.cuda._runtime.

Occupancy-Based Block Sizing

NEVER hardcode block=(256, 1, 1). Always use:

grid, block = runtime.launch_config(kernel, item_count)
# Or: block_size = runtime.optimal_block_size(kernel, shared_mem_bytes=0)

Uses cuOccupancyMaxPotentialBlockSize for optimal threads-per-block based on register pressure and shared memory. Falls back to 256 if the API is unavailable. Results cached per kernel function.

Note: The occupancy API maximizes occupancy, not necessarily performance. For kernels with high register usage, lower occupancy with more registers per thread can outperform higher occupancy due to increased ILP (instruction-level parallelism). Profile before overriding.

__launch_bounds__ Directive

Use __launch_bounds__ in NVRTC kernel source to guide register allocation and ensure minimum blocks per SM:

extern "C" __global__ void __launch_bounds__(256, 4)
my_kernel(const double* __restrict__ x, ...) {{
    // 256 max threads/block, at least 4 blocks per SM
}}

Without __launch_bounds__, the compiler assumes the maximum possible threads per block, which can over-estimate shared memory needs and reduce occupancy. Always use when the block size is known at compile time.

---

3. Device Memory Management

Allocation

# WITHOUT zero-fill (default — use when kernel writes every element)
d_output = runtime.allocate((n,), np.float64)

# WITH zero-fill (use for count arrays, sparse-update targets)
d_counts = runtime.allocate((n,), np.int32, zero=True)

Transfers

d_data = runtime.from_host(host_array)              # H2D
host_result = runtime.copy_device_to_host(d_output)  # D2H
ptr = runtime.pointer(d_array)                       # device pointer (int), 0 for None

Memory Pool

CuPy MemoryPool is configured at runtime init. Freed allocations are cached for reuse. VIBESPATIAL_GPU_POOL_LIMIT env var caps pool size.

stats = runtime.memory_pool_stats()  # {"used_bytes", "total_bytes", "free_bytes"}
runtime.free_pool_memory()           # release cached memory

---

4. CUDA Streams — When and How

When to Use Streams

Streams let independent GPU operations run concurrently. Use them when:

1. Independent H2D uploads — uploading query and tree geometry simultaneously (different source arrays, no dependency).
2. Independent kernel launches — polygon bounds and multipolygon bounds kernels that operate on different family buffers.
3. Overlapping D2H transfer with compute — start transferring results from pass 1 while pass 2 runs on a different output buffer.
4. Count-scatter total computation — batch two last-element reads into one async transfer instead of two sequential .get() syncs.

When NOT to Use Streams

• Sequential data dependencies — kernel B reads what kernel A wrote. Same-stream ordering handles this automatically.
• Tiny transfers — overhead of stream creation (~1-2us) exceeds the transfer time.
• Single-kernel operations — no independent work to overlap.

Stream API

# Manual create/destroy
stream = runtime.create_stream()
runtime.launch(kernel, grid=g, block=b, params=p, stream=stream)
stream.synchronize()
runtime.destroy_stream(stream)

# Context manager (auto-sync + cleanup)
with runtime.stream_context() as stream:
    runtime.launch(kernel, grid=g, block=b, params=p, stream=stream)

Async Transfers

# Pinned host memory enables true async DMA
h_buf = runtime.allocate_pinned((n,), np.int32)

# Async D2H (enqueue only — sync stream before reading)
runtime.copy_device_to_host_async(d_array, stream, h_buf)

# Async H2D
runtime.copy_host_to_device_async(h_array, d_array, stream)

Pattern: Count-Scatter Total

from vibespatial.cuda._runtime import count_scatter_total, count_scatter_total_with_transfer

# Simple: single-sync async pinned transfer (replaces 2x .get())
total = count_scatter_total(runtime, device_counts, device_offsets)

# Advanced: also starts full counts D2H on background stream
total, xfer_stream, pinned_counts = count_scatter_total_with_transfer(
    runtime, device_counts, device_offsets,
)
# ... launch scatter kernel on null stream ...
runtime.synchronize()
xfer_stream.synchronize()  # counts already transferred
runtime.destroy_stream(xfer_stream)
host_counts = pinned_counts

Pattern: Independent Uploads

stream_q = runtime.create_stream()
stream_t = runtime.create_stream()
d_query = runtime.allocate(query_bounds.shape, query_bounds.dtype)
d_tree = runtime.allocate(tree_bounds.shape, tree_bounds.dtype)
runtime.copy_host_to_device_async(query_bounds, d_query, stream_q)
runtime.copy_host_to_device_async(tree_bounds, d_tree, stream_t)
stream_q.synchronize()
stream_t.synchronize()
runtime.destroy_stream(stream_q)
runtime.destroy_stream(stream_t)

Pattern: Independent Family Kernels

with runtime.stream_context() as s_poly:
    runtime.launch(bounds_polygon_kernel, ..., stream=s_poly)
with runtime.stream_context() as s_mpoly:
    runtime.launch(bounds_multipolygon_kernel, ..., stream=s_mpoly)

Synchronization Rules

• runtime.synchronize() syncs the CUDA context (all streams). Use sparingly — only before host reads of device data.
• stream.synchronize() syncs one stream only. Prefer this.
• CUDA guarantees execution order within a single stream. Do NOT sync between consecutive launches on the same stream.
• Remove sync calls between: kernel -> CCCL, kernel -> kernel, CCCL -> CCCL (all on same null stream).
• Keep sync calls before: copy_device_to_host, .get(), cp.asnumpy.

---

5. CCCL Primitives

Available in cccl_primitives.py

from vibespatial.cuda.cccl_primitives import (
    # Tier 3a — CCCL default (beats CuPy)
    exclusive_sum,            # Prefix sum (1.8-3.7x faster)
    compact_indices,          # Bool mask -> indices (CuPy default; CCCL via explicit strategy)
    sort_pairs,               # Radix or merge sort with values
    unique_sorted_pairs,      # Unique-by-key on sorted input
    segmented_reduce_sum,     # Per-segment sum
    segmented_reduce_min,     # Per-segment min
    segmented_reduce_max,     # Per-segment max
    segmented_sort,           # Sort within offset-delimited segments
    lower_bound,              # Binary search (first insertion point)
    upper_bound,              # Binary search (last insertion point)
    three_way_partition,      # Split into 3 groups by predicates

    # Tier 3c — Zero-allocation iterators
    counting_iterator,        # Lazy [0,1,2,...] (replaces cp.arange)
    transform_iterator,       # Fused element-wise transform

    # Tier 4 — Still CuPy-default (CCCL available but marginal win)
    reduce_sum,               # Scalar reduction (CuPy cp.sum faster at small scale)
)

Synchronize Parameter

Wrappers that return device arrays accept synchronize=False to skip the internal stream sync when the result feeds into the next GPU op:

offsets = exclusive_sum(counts, synchronize=False)
sorted_result = sort_pairs(keys, values, synchronize=False)
lb = lower_bound(sorted_data, queries, synchronize=False)

Do NOT pass synchronize=False to compact_indices or unique_sorted_pairs — they internally call .get() (requires sync).

---

6. Precompilation and Warmup (ADR-0034)

The Problem

Cold-call JIT penalty: ~950ms for select, ~1460ms for exclusive_scan (one-time per process per unique spec). NVRTC kernels: ~20-400ms per compilation unit.

Three-Level Demand-Driven Strategy

Level 1 — CCCL Module Warmup: Each consumer module declares specs at module scope via request_warmup(). A singleton CCCLPrecompiler with a background ThreadPoolExecutor (8 threads) compiles specs in parallel. Never blocks import; first real call may block up to 5s waiting for its needed spec.

from vibespatial.cuda.cccl_precompile import request_warmup
request_warmup(["exclusive_scan_i32", "exclusive_scan_i64", "select_i32"])

Level 2 — NVRTC Module Warmup: Same pattern for NVRTC kernels. 14 kernel compilation units, ~4,785 lines CUDA C, 85 entry points. Thread-safe via _module_cache_lock on CudaDriverRuntime.

from vibespatial.cuda.nvrtc_precompile import request_nvrtc_warmup
request_nvrtc_warmup([
    ("my-kernel", _KERNEL_SOURCE, _KERNEL_NAMES),
])

Level 3 — Pipeline-Aware (deferred): When pipeline plans are constructed (sjoin, overlay), declare all requirements and block until compiled.

Key Properties

• request_warmup() is idempotent — duplicate specs are skipped.
• All compilation releases the GIL (Cython with nogil: in CCCL bindings, NVRTC handles are independent).
• Numba operator JIT holds GIL (~50-100ms per unique op, ~6 ops total, lru_cache'd after first call).

Thread Scaling (18 CCCL specs)

Threads	Wall Time	Speedup
1	~18-22s	1x
4	~4-5s	4x
8	~2-3s	7x (recommended for CCCL)
16	~1.5s	12x (recommended for NVRTC)

Cost by Operation

Operation	CCCL Specs	NVRTC Units	Est. Cold Cost
gs.within(other)	2	1	~1s
gs.to_wkb()	2	0	~1s
gpd.sjoin(a, b)	12	2-3	~2s
gpd.overlay(a, b)	6	2	~1.5s
gs.dissolve(by=)	3	0	~1s

Observability

import vibespatial
status = vibespatial.precompile_status()  # dict of spec -> compiled/pending/failed

---

7. Warp-Level Intrinsics

Use in NVRTC kernel source strings for intra-warp coordination.

Warp Ballot — Early Exit

Skip expensive work when no thread in the warp needs it:

const bool valid = row < row_count;
const unsigned char is_candidate = valid ? candidate_mask[row] : 0;
if (__ballot_sync(0xFFFFFFFF, is_candidate) == 0) {{
    return;  // entire warp skips all global memory reads
}}
if (!valid || !is_candidate) return;

After bounds filtering at ~5% hit rate, most warps skip entirely.

__syncwarp() — Warp Reconvergence (Volta+)

On Volta+ (CC 7.0+) with independent thread scheduling, warps can remain diverged after conditional blocks. Use __syncwarp() before warp-level primitives to guarantee all lanes are converged:

if (condition) {{
    // ... divergent work ...
}}
__syncwarp(0xFFFFFFFF);  // reconverge before shuffle/ballot

Warp Shuffle — Intra-Warp Reduction

Reduce across 32 lanes without shared memory:

const unsigned int FULL_MASK = 0xFFFFFFFF;
for (int offset = 16; offset > 0; offset >>= 1) {{
    my_crossings ^= __shfl_xor_sync(FULL_MASK, my_crossings, offset);
    my_boundary  |= __shfl_xor_sync(FULL_MASK, my_boundary, offset);
}}

Block-Level Reduction (via shared memory)

__shared__ int warp_results[8];  // up to 256 threads = 8 warps
const int warp_id = threadIdx.x / 32;
const int lane_id = threadIdx.x % 32;

// 1. Warp-level shuffle reduction
for (int offset = 16; offset > 0; offset >>= 1)
    my_value ^= __shfl_xor_sync(0xFFFFFFFF, my_value, offset);

// 2. Lane 0 writes to shared memory
if (lane_id == 0) warp_results[warp_id] = my_value;
__syncthreads();

// 3. Thread 0 reduces across warps
if (threadIdx.x == 0) {{
    int total = 0;
    for (int w = 0; w < num_warps; ++w) total ^= warp_results[w];
    output[blockIdx.x] = total;
}}

---

8. Two-Pass Count-Scatter Pattern

Variable-output kernels use a two-pass approach:

Pass 0 (count):   Each thread computes output size -> counts[tid]
Prefix sum:        exclusive_sum(counts) -> offsets[tid]
Get total:         count_scatter_total(runtime, counts, offsets)
Allocate output:   runtime.allocate((total,), dtype)
Pass 1 (scatter):  Each thread writes output at offsets[tid]

Best Practices

• Use count_scatter_total() instead of sequential .get() calls.
• Build full offsets on device when possible (avoid D2H -> modify -> H2D).
• Do NOT sync between count kernel -> exclusive_sum -> scatter kernel (same stream ordering handles it).
• Sync only before reading total on host.

---

9. Shared Memory

When to Use

• Cooperative edge iteration — all threads process edges of the same ring, accumulate in shared memory.
• Stencil/neighborhood — tile data with halo cells.
• Block-level reduction — inter-warp reduction via shared arrays.

Cooperative Intra-Ring PIP

Instead of one-thread-per-ring (load imbalanced), all threads split edges within each ring:

for (int ring = ring_start; ring < ring_end; ++ring) {{
    int edge_count = ring_offsets[ring + 1] - ring_offsets[ring] - 1;
    for (int e = threadIdx.x; e < edge_count; e += blockDim.x) {{
        // ... even-odd test on edge e ...
    }}
    // Warp shuffle + shared memory reduction for this ring
}}

Bank Conflict Avoidance

Shared memory has 32 banks, each 4 bytes wide. If multiple threads in a warp access different addresses in the same bank, accesses serialize.

The +1 padding trick eliminates bank conflicts on column access:

// BAD: 32-way bank conflict when reading columns
__shared__ float tile[32][32];

// GOOD: +1 padding shifts bank mapping — no conflicts
__shared__ float tile[32][33];

For double (8 bytes = 2 banks), stride-1 access has inherent 2-way conflict. Mitigate with +1 padding per row or swizzled indexing.

NVIDIA benchmark: padding improved matrix transpose bandwidth from 51.3 GB/s to 99.5 GB/s (~2x).

Declaring Shared Memory

runtime.launch(kernel, grid=g, block=b, params=p, shared_mem_bytes=1024)

__shared__ int scratch[256];                  // static
extern __shared__ float dynamic_smem[];       // dynamic (via shared_mem_bytes)

Max shared memory per block (default is 48 KB; requesting more requires cudaFuncSetAttribute with cudaFuncAttributeMaxDynamicSharedMemorySize):

• A100: 163 KB
• H100: 227 KB
• RTX 3090/4090: 99 KB

Async Copy to Shared Memory (CUDA 11.0+)

For data tiling, __pipeline_memcpy_async() copies directly from global to shared memory, bypassing the register file. This reduces register pressure and can increase occupancy. Best with 8 or 16-byte elements:

__pipeline_memcpy_async(&shared_data[tid], &global_data[idx], sizeof(double));
__pipeline_commit();
__pipeline_wait_prior(0);  // wait for all committed copies
__syncthreads();

---

10. Precision Dispatch (ADR-0002)

Policy by Kernel Class

Kernel Class	Consumer GPU (fp64:fp32 < 0.25)	Datacenter GPU (fp64:fp32 >= 0.25)
COARSE (bounds, index, filter)	Staged fp32 with coordinate centering	Native fp64
METRIC (distance, area, length)	Staged fp32 with Kahan compensation	Native fp64
PREDICATE (PIP, binary preds)	Staged fp32 coarse pass + selective fp64 refinement for ambiguous rows	Native fp64
CONSTRUCTIVE (clip, overlay, buffer)	Native fp64 (until robustness work proves cheaper path)	Native fp64

How It Works

1. select_precision_plan() reads the device's fp64:fp32 throughput ratio (via CU_DEVICE_ATTRIBUTE_SINGLE_TO_DOUBLE_PRECISION_PERF_RATIO) and returns a PrecisionPlan with compute_precision, compensation, center_coordinates, and refinement fields.

2. auto mode chooses based on device profile AND kernel class — consumer GPUs get fp32 for coarse/metric/predicate; datacenter GPUs get fp64.

3. The plan is passed to the kernel launcher, which formats the CUDA source template accordingly.

Implementation Checklist

• Template kernel source: typedef {compute_type} compute_t;
• Storage reads stay double: compute_t lx = (compute_t)(x[i] - center_x);
• Cache key includes precision: f"my-kernel-{compute_type}"
• Coordinate centering when plan.center_coordinates is True
• Kahan compensation for metric accumulations when plan.compensation is KAHAN
• CONSTRUCTIVE kernels: wire plan for observability but stay fp64

See precision-compliance skill for the full step-by-step procedure.

---

11. Performance Rules

Memory Access

• Coalesced reads: Adjacent threads read adjacent addresses. SoA layout (separate x[], y[]) is already coalesced.
• Avoid AoS: Never interleave x,y in a single array. NVIDIA GTC 2024 benchmark: AoS was 5.9x slower than SoA due to strided access.
• Minimize global writes: Use shared memory or registers for intermediates; write to global once.
• const __restrict__: Always annotate read-only pointer parameters. On CC 3.5+, the compiler automatically routes through the read-only data cache (__ldg path), increasing effective cache capacity:

  extern "C" __global__ void my_kernel(
      const double* __restrict__ x,   // read-only -> __ldg cache
      const double* __restrict__ y,
      double* __restrict__ output,    // write-only
      ...
  

• Vectorized loads for bandwidth-bound bulk I/O. Use 128-bit wide loads to reduce instruction count and increase bandwidth (1.3-1.5x speedup per NVIDIA benchmarks):

  // Instead of scalar loads:
  double val = input[idx];
  // Use 128-bit vectorized loads (requires aligned pointer):
  double2 vals = reinterpret_cast<const double2*>(input)[idx];
  // For fp32: float4 vals = reinterpret_cast<const float4*>(input)[idx];
  

  cudaMalloc guarantees 256-byte alignment, so device pointers are valid. Handle remainder elements with a scalar tail.

Kernel Launch

• Never launch with <32 threads of real work (wastes a warp).
• Avoid many tiny kernels — each launch has ~3-5us overhead.
• Remove unnecessary syncs between same-stream operations.
• Wave quantization: On modern GPUs (128 SMs on RTX 4090, 132 on H100), launching grid_size = SM_count + 1 wastes ~50% of the final wave. Size grids to exactly fill the GPU and use grid-stride loops.

Grid-Stride Loops with ILP

Prefer grid-stride loops over one-thread-per-element. Process multiple elements per thread for instruction-level parallelism:

// Grid-stride loop with ILP (4 elements/thread)
const int stride = blockDim.x * gridDim.x;
for (int idx = blockIdx.x * blockDim.x + threadIdx.x;
     idx < n;
     idx += stride * 4) {{
    #pragma unroll
    for (int j = 0; j < 4; j++) {{
        int elem = idx + j * stride;
        if (elem < n) output[elem] = compute(input[elem]);
    }}
}}

NVIDIA GTC 2024 data: 4 elements/thread at 50% occupancy outperformed 1 element/thread at 100% occupancy (82% vs 51% bandwidth utilization).

Grid sizing for grid-stride loops:

grid_size = min(
    (n + block_size - 1) // block_size,
    sm_count * max_blocks_per_sm  # fill GPU exactly — avoid wave waste
)

Divergence and Load Balancing

• Avoid warp-divergent branches in inner loops.
• Use predicated writes: output[idx] = valid ? result : nodata;
• For variable-work-per-thread, use work-size binning:

_WORK_BINS = [64, 1024]  # simple < 64 verts, medium 64-1024, complex > 1024

def _should_bin_dispatch(work_estimates):
    if len(work_estimates) < 1024: return False
    return work_estimates.std() / work_estimates.mean() > 2.0

For the "complex" bin, use a block-per-item kernel where all threads cooperatively process one work item (see cooperative edge iteration).

Arithmetic Micro-Optimizations

• Float constant suffix: In fp32 kernels, use 1.0f not 1.0. Without the f suffix, constants are double-precision and force a conversion instruction. When using compute_t, cast explicitly: (compute_t)0.0 or (compute_t)1e-7.
• Integer division/modulo: Compiles to up to 20 instructions. When the divisor is a power of 2, use bitwise ops: threadIdx.x / 32 -> threadIdx.x >> 5 threadIdx.x % 32 -> threadIdx.x & 31
• sinpi/cospi: If trig functions are needed (e.g., great circle distance), use sinpi(x) instead of sin(x * M_PI) — faster and more accurate.

L2 Cache-Aware Patterns

For multi-kernel pipelines (count -> scatter, bounds -> filter -> refine), consider these L2-friendly patterns:

• Reverse block traversal: Alternate block indexing direction between consecutive kernels so kernel B starts from data kernel A last touched:

  // Kernel A: forward
  int blockId = blockIdx.x;
  // Kernel B: reverse (hits A's still-cached tail data)
  int blockId = gridDim.x - blockIdx.x - 1;
  

• Cache tiling: If intermediate data fits in L2 (40 MB A100, 72 MB RTX 4090, 50 MB H100), process data in L2-sized chunks end-to-end rather than running each stage over the full dataset. L2 cache hits can yield up to 10x bandwidth improvement over HBM misses.

---

12. Dispatcher Pattern

from vibespatial.runtime.adaptive import plan_dispatch_selection
from vibespatial.runtime.precision import KernelClass
from vibespatial.runtime._runtime._runtime import ExecutionMode

selection = plan_dispatch_selection(
    kernel_name="my_kernel",
    kernel_class=KernelClass.COARSE,
    row_count=n,
    requested_mode=dispatch_mode,
)
if selection.selected is ExecutionMode.GPU:
    return _my_kernel_gpu(...)
else:
    return _my_kernel_cpu(...)

---

13. Testing Pattern

import pytest

requires_gpu = pytest.mark.skipif(not _has_gpu(), reason="GPU not available")

@requires_gpu
def test_my_kernel_gpu():
    """GPU produces same results as Shapely oracle."""
    gpu_result = my_op_gpu(data)
    cpu_result = shapely_reference(data)
    np.testing.assert_allclose(gpu_result, cpu_result, rtol=1e-10)

def test_my_op_auto_dispatch():
    """Auto-dispatch falls back to CPU gracefully."""
    result = my_op(data)
    assert result is not None