84 lines
2.2 KiB
ReStructuredText
84 lines
2.2 KiB
ReStructuredText
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.. _libcudacxx-extended-api-math-ilog:
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``cuda::ilog2``, ``cuda::ceil_ilog2``, ``cuda::ilog10``, and ``cuda::ceil_ilog10``
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=================================================================================
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Defined in the ``<cuda/cmath>`` header.
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.. code:: cuda
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namespace cuda {
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template <typename T>
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[[nodiscard]] __host__ __device__ __tile__ constexpr
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int ilog2(T value) noexcept;
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template <typename T>
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[[nodiscard]] __host__ __device__ __tile__ constexpr
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int ceil_ilog2(T value) noexcept;
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template <typename T>
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[[nodiscard]] __host__ __device__ __tile__ constexpr
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int ilog10(T value) noexcept;
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template <typename T>
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[[nodiscard]] __host__ __device__ __tile__ constexpr
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int ceil_ilog10(T value) noexcept;
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} // namespace cuda
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The functions compute the logarithm to the base 2 and 10 of an integer value.
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**Parameters**
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- ``value``: The input value.
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**Return value**
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- ``ilog2``, ``ceil_ilog2``: The logarithm to the base 2, rounded down and up to the nearest integer respectively.
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- ``ilog10``, ``ceil_ilog10``: The logarithm to the base 10, rounded down and up to the nearest integer respectively.
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**Constraints**
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- ``T`` is an integer type.
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**Preconditions**
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- ``value > 0``
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**Performance considerations**
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The functions perform the following operations in device code:
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- ``ilog2``: ``FLO``
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- ``ceil_ilog2``: ``FLO``, ``POPC``, ``ADD``, comparison
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- ``ilog10``: ``FLO``, ``FMUL``, ``F2I``, constant memory lookup, ``SEL`` + ``IADD`` only if ``T == uint32_t`` or ``T == __uint128_t``
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- ``ceil_ilog10``: ``ilog10`` with an additional comparison, subtraction, and ``IADD``
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Example
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-------
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.. code:: cuda
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#include <cuda/cmath>
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#include <cuda/std/cassert>
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__global__ void ilog_kernel() {
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assert(cuda::ilog2(20) == 4);
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assert(cuda::ceil_ilog2(20) == 5);
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assert(cuda::ilog2(32) == 5);
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assert(cuda::ceil_ilog2(32) == 5);
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assert(cuda::ilog10(100) == 2);
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assert(cuda::ilog10(2000) == 3);
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assert(cuda::ceil_ilog10(100) == 2);
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assert(cuda::ceil_ilog10(2000) == 4);
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}
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int main() {
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ilog_kernel<<<1, 1>>>();
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cudaDeviceSynchronize();
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return 0;
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}
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`See it on Godbolt 🔗 <https://godbolt.org/z/7W3WaGd3c>`__
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