policy.hpp

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/// @file policy.hpp
/// @brief Backend enum for linear algebra operations.
///
/// Each module defines its own backend enum for the choices relevant to it.
/// This enum covers linalg (vector, matrix, solvers, factorization, eigen, svd).
/// Other modules define their own -- e.g. spectral/ uses FFTBackend.
///
///   num::seq      -- naive serial C++              (always available)
///   num::blocked  -- cache-blocked, no intrinsics  (compiler auto-vectorizes)
///   num::simd     -- hand-written SIMD             (AVX2 on x86, NEON on AArch64)
///   num::blas     -- cblas / LAPACKE               (optional, NUMERICS_HAS_BLAS)
///   num::omp      -- OpenMP parallel               (optional, NUMERICS_HAS_OMP)
///   num::gpu      -- CUDA                          (optional, NUMERICS_HAS_CUDA)
#pragma once
 
namespace num {
 
/// @brief Selects which backend handles a linalg operation.
enum class Backend {
    seq,      ///< Naive textbook loops  -- always available
    blocked,  ///< Cache-blocked; compiler auto-vectorizes inner loops
    simd,     ///< Hand-written SIMD intrinsics (AVX2 or NEON)
    blas,     ///< cblas/LAPACKE  -- OpenBLAS, MKL, Apple Accelerate
    omp,      ///< OpenMP parallel blocked loops
    gpu,      ///< CUDA  -- custom kernels or cuBLAS
};
 
// Convenience constants  -- use these at call sites:
//   matmul(A, B, C, num::blas);
inline constexpr Backend seq     = Backend::seq;
inline constexpr Backend blocked = Backend::blocked;
inline constexpr Backend simd    = Backend::simd;
inline constexpr Backend blas    = Backend::blas;
inline constexpr Backend omp     = Backend::omp;
inline constexpr Backend gpu     = Backend::gpu;
 
// -- Compile-time capability flags --------------------------------------------
/// True when a BLAS/cblas library was found at configure time.
inline constexpr bool has_blas =
#if defined(NUMERICS_HAS_BLAS)
    true;
#else
    false;
#endif
 
/// True when OpenMP was found at configure time.
inline constexpr bool has_omp =
#if defined(NUMERICS_HAS_OMP)
    true;
#else
    false;
#endif
 
// -- Default and best backends -------------------------------------------------
/// Automatically selected at configure time: blas > blocked.
/// Used as the default parameter for all operations.
inline constexpr Backend default_backend =
#if defined(NUMERICS_HAS_BLAS)
    Backend::blas;
#else
    Backend::blocked;
#endif
 
/// Best backend for memory-bound vector ops: blas > omp > blocked.
inline constexpr Backend best_backend =
#if defined(NUMERICS_HAS_BLAS)
    Backend::blas;
#elif defined(NUMERICS_HAS_OMP)
    Backend::omp;
#else
    Backend::blocked;
#endif
 
} // namespace num