CUDA Academy · レッスン

device関数とhost関数

それぞれの関数を実行できる場所を学びます。

「__device__関数と__host__関数」はCoddyKit上の無料CUDA Academyレッスンです。これはレッスン2/4です。下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。これはCUDA Academy学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 CUDA Academyコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

Beyond the Kernel

Kernels are entry points, but most code lives in helpers. CUDA gives two more qualifiers to say where a plain function may run. Let us meet them.

The device Qualifier

A __device__ function runs on the GPU and is callable only from other GPU code, like a kernel or another device function.

__device__ int square(int x) {
    return x * x;
}

Helpers for Kernels

Think of __device__ functions as your GPU toolbox. They keep kernels tidy by holding logic the device threads reuse.

The host Qualifier

The __host__ qualifier means ordinary CPU code. It is the default, so plain C++ functions are already host functions.

__host__ void setup() {
    // runs on the CPU
}

Default Is Host

Leave off all qualifiers and your function is host-only by default. That is why normal C++ still compiles inside a .cu file.

Both at Once

Combine the two and a function compiles for both sides. nvcc builds one version for the CPU and one for the GPU.

__host__ __device__ int dbl(int x) {
    return x + x;
}

Why Dual-Compile?

Marking a helper __host__ __device__ lets the same math run in CPU reference code and inside kernels, with zero duplication. ✨

Calling Rules

Host code may not call a __device__ function, and device code may not call a host-only one. The compiler enforces the boundary.

No global from Device

A __device__ function is not launchable. Only __global__ kernels accept the triple-angle-bracket launch from the host.

Inlining for Speed

The compiler often inlines small __device__ functions, so wrapping logic in helpers usually costs nothing at runtime.

Pick the Right Tag

Choose by where it runs: __device__ for GPU helpers, __host__ for CPU code, and both when the logic is shared.

Quick Check

Time to test the device and host qualifiers.

Recap

You learned __device__ runs on the GPU, __host__ runs on the CPU, the default is host, and combining both shares one function across worlds. 🎯

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