CUDA Kernel Development & Optimization

Skill for developing and optimizing custom CUDA kernels in the candle framework for qwen3-tts-rs.

Trigger Words

cuda kernel, custom kernel, fused op, write kernel, ptx, kernel launch, CustomOp, nsys, ncu, profiling, roofline, occupancy, register pressure

Candle Custom Op Patterns

CustomOp1 (single input tensor)

use candle_core::{CustomOp1, Layout, Shape, DType, backend::BackendStorage, CudaStorage};

struct MyFusedOp { /* params */ }

impl CustomOp1 for MyFusedOp {
    fn name(&self) -> &'static str { "my_fused_op" }

    fn cpu_fwd(&self, storage: &CpuStorage, layout: &Layout) -> candle_core::Result<(CpuStorage, Shape)> {
        // CPU fallback (can panic/unimplemented for CUDA-only ops)
        todo!("CPU fallback")
    }

    #[cfg(feature = "cuda")]
    fn cuda_fwd(&self, storage: &CudaStorage, layout: &Layout) -> candle_core::Result<(CudaStorage, Shape)> {
        // Launch PTX kernel here
    }
}

// Usage: tensor.apply_op1(MyFusedOp { ... })?

CustomOp2 (two input tensors)

impl CustomOp2 for FusedSiluMul {
    fn name(&self) -> &'static str { "fused_silu_mul" }

    fn cpu_fwd(&self, s1: &CpuStorage, l1: &Layout, s2: &CpuStorage, l2: &Layout)
        -> candle_core::Result<(CpuStorage, Shape)> { todo!() }

    #[cfg(feature = "cuda")]
    fn cuda_fwd(&self, s1: &CudaStorage, l1: &Layout, s2: &CudaStorage, l2: &Layout)
        -> candle_core::Result<(CudaStorage, Shape)> {
        // Launch PTX kernel
    }
}

// Usage: tensor_a.apply_op2(tensor_b, FusedSiluMul)?

InplaceOp1 (mutate tensor in place)

impl candle_core::InplaceOp1 for MyInplaceOp {
    fn name(&self) -> &'static str { "my_inplace" }

    fn cpu_fwd(&self, storage: &mut CpuStorage, layout: &Layout) -> candle_core::Result<()> { todo!() }

    #[cfg(feature = "cuda")]
    fn cuda_fwd(&self, storage: &mut CudaStorage, layout: &Layout) -> candle_core::Result<()> {
        // Modify storage in place — no allocation
    }
}

// Usage: tensor.inplace_op1(&MyInplaceOp { ... })?

PTX Kernel Embedding

Write .cu files in kernels/, compile to PTX, embed via include_str!:

use candle_core::cuda_backend::cudarc::driver::{LaunchConfig, LaunchAsync};
use candle_core::cuda_backend::WrapErr;

const PTX_SRC: &str = include_str!("../../kernels/my_kernel.ptx");

// Inside cuda_fwd():
let dev = storage.device();
let func = dev.get_or_load_func("my_kernel_name", PTX_SRC)?;
let cfg = LaunchConfig::for_num_elems(num_elements as u32);
unsafe { func.launch(cfg, (input_ptr, output_ptr, num_elements)) }.w()?;

Compiling .cu → .ptx

nvcc -ptx -arch=sm_80 -o kernels/my_kernel.ptx kernels/my_kernel.cu

Or in build.rs for automatic compilation.

Docker Dev Loop

All CUDA compilation and testing happens in Docker (NGC PyTorch base image):

# Quick kernel unit test
./scripts/test-kernel.sh fused_rmsnorm

# Full e2e benchmark
make profile-chrome MODEL_DIR=test_data/models/1.7B-CustomVoice

# Count kernel launches per frame
./scripts/count-kernels.sh

Testing Pattern

Unit tests use synthetic tensors (no model weights needed):

#[test]
fn test_fused_vs_sequential() {
    let device = candle_core::Device::cuda_if_available(0).unwrap();
    let a = Tensor::randn(0., 1., (1, 2048), &device).unwrap();

    let sequential = a.silu().unwrap().mul(&b).unwrap();
    let fused = a.apply_op2(&b, FusedSiluMul).unwrap();

    let diff = (sequential - fused).unwrap().abs().unwrap().max(0).unwrap().to_scalar::<f32>().unwrap();
    assert!(diff < 1e-5, "fused vs sequential diff: {diff}");
}

Run with: cargo test --lib -- fused_silu_mul

Project-Specific Notes

• Compute dtype: BF16 on CUDA, F32 on CPU
• Decoder and speaker encoder always F32
• Profiling spans gated behind #[cfg(feature = "profiling")]
• GPU sync audit: make audit-gpu-syncs
• Kernel plan: docs/CUSTOM_CUDA_KERNELS_PLAN.md
• 33 layers total per frame (28 talker + 5 code predictor)
• Target: reduce ~700 kernel launches/frame to ~545

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Optimization & Profiling

One change per cycle: baseline → profile → classify → optimize → verify → compare → loop. Multiple simultaneous changes make it impossible to attribute improvement. Revert on regression.

See REFERENCE.md for the full profiling reference (nsys, ncu, SASS inspection, bottleneck classification, optimization strategies).