`static_for`

Defined in <cuda/utility> header.

namespace cuda {

template <auto Size, typename Operator, typename... TArgs>
__host__ __device__ constexpr
void static_for(Operator op, TArgs&&... args) noexcept(/*see-below*/); // (1)

template <auto Start, decltype(Start) End, decltype(Start) Step = 1, typename Operator, typename... TArgs>
__host__ __device__ constexpr
void static_for(Operator op, TArgs&&... args) noexcept(/*see-below*/); // (2)

template <typename T, T Size, typename Operator, typename... TArgs>
__host__ __device__ constexpr
void static_for(Operator op, TArgs&&... args) noexcept(/*see-below*/); // (3)

template <typename T, T Start, T End, T Step = 1, typename Operator, typename... TArgs>
__host__ __device__ constexpr
void static_for(Operator op, TArgs&&... args) noexcept(/*see-below*/); // (4)

} // namespace cuda

The functionality provides a for loop with compile-time indices.

static_for is available in two forms:

Executes op for each value in the range [0, Size) (1, 3).
Executes op for each value in the range [Start, End) with step Step (2, 4).

The function is noexcept if all invocations of op with integral_constant</*index-type*/, /*index-value*/> and the args... are non-throwing. Only visited indices participate in the noexcept evaluation.

Parameters

Size: the number of iterations (1, 3).
Start, End, Step: the start, end, and step of the range. Note that End and Step are converted to the type of Start (2, 4).
T: type of the loop index (3, 4).
op: the function to execute.
args: additional arguments to pass to op.

op is a callable object that accepts an integral_constant of the same type of Size or Start.

Performance considerations

The functions are useful as metaprogramming utility and when a loop requires full unrolling, independently of the compiler’s constrains, optimization level, and heuristics. In addition, the index is a compile-time constant, which can be used in a constant expression and further optimize the code.
Conversely, static_for is more expensive to compile compared to #pragma unroll. Additionally, the preprocessor directive interacts with the compiler, which tunes the loop unrolling based on register usage, binary size, and instruction cache.

Example

#include <cuda/utility>
#include <cstdio>

__global__ void kernel() {
    cuda::static_for<5>([](auto i){ static_assert(i >= 0 && i < 5); });

    cuda::static_for<5>([](auto i){ printf("%d, ", i()); }); // 0, 1, 2, 3, 4,
    printf("\n");

    cuda::static_for<short, 5>([](auto i){ printf("%d, ", i()); }); // 0, 1, 2, 3, 4,
    printf("\n");

    cuda::static_for<-5, 7, 3>([](auto i){ printf("%d, ", i()); }); // -5, -2, 1, 4,
    printf("\n");

    cuda::static_for<5>([](auto i){
        if constexpr (i > 0) {
            cuda::static_for<i()>([](auto j){ printf("%d, ", j()); });
            printf("\n");
        }
    });
    // 0,
    // 0, 1,
    // 0, 1, 2,
    // 0, 1, 2, 3,
    cuda::static_for<5>([](auto i, int a, int b, int c){}, 1, 2, 3); // 1, 2, 3 optional arguments
}

int main() {
    kernel<<<1, 1>>>();
    cudaDeviceSynchronize();
}

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static_for

Example

`static_for`