src/tint/utils/math.h

// Copyright 2021 The Tint Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#ifndef SRC_TINT_UTILS_MATH_H_
#define SRC_TINT_UTILS_MATH_H_

#include <string>
#include <type_traits>

namespace tint::utils {

/// @param alignment the next multiple to round `value` to
/// @param value the value to round to the next multiple of `alignment`
/// @return `value` rounded to the next multiple of `alignment`
/// @note `alignment` must be positive. An alignment of zero will cause a DBZ.
template <typename T>
inline constexpr T RoundUp(T alignment, T value) {
    return ((value + alignment - 1) / alignment) * alignment;
}

/// @param value the value to check whether it is a power-of-two
/// @returns true if `value` is a power-of-two
/// @note `value` must be positive if `T` is signed
template <typename T>
inline constexpr bool IsPowerOfTwo(T value) {
    return (value & (value - 1)) == 0;
}

/// @param value the input value
/// @returns the base-2 logarithm of @p value
inline constexpr uint32_t Log2(uint64_t value) {
#if defined(__clang__) || defined(__GNUC__)
    return 63 - static_cast<uint32_t>(__builtin_clzll(value));
#elif defined(_MSC_VER) && !defined(__clang__) && __cplusplus >= 202002L  // MSVC and C++20+
    // note: std::is_constant_evaluated() added in C++20
    //       required here as _BitScanReverse64 is not constexpr
    if (!std::is_constant_evaluated()) {
        // NOLINTNEXTLINE(runtime/int)
        if constexpr (sizeof(unsigned long) == 8) {  // 64-bit
            // NOLINTNEXTLINE(runtime/int)
            unsigned long first_bit_index = 0;
            _BitScanReverse64(&first_bit_index, value);
            return first_bit_index;
        } else {  // 32-bit
            // NOLINTNEXTLINE(runtime/int)
            unsigned long first_bit_index = 0;
            if (_BitScanReverse(&first_bit_index, value >> 32)) {
                return first_bit_index + 32;
            }
            _BitScanReverse(&first_bit_index, value & 0xffffffff);
            return first_bit_index;
        }
    }
#endif

    // Non intrinsic (slow) path. Supports constexpr evaluation.
    for (uint64_t clz = 0; clz < 64; clz++) {
        uint64_t bit = 63 - clz;
        if (value & (static_cast<uint64_t>(1u) << bit)) {
            return static_cast<uint32_t>(bit);
        }
    }
    return 64;
}

/// @param value the input value
/// @returns the next power of two number greater or equal to @p value
inline constexpr uint64_t NextPowerOfTwo(uint64_t value) {
    if (value <= 1) {
        return 1;
    } else {
        return static_cast<uint64_t>(1) << (Log2(value - 1) + 1);
    }
}

/// @param value the input value
/// @returns the largest power of two that `value` is a multiple of
template <typename T>
inline std::enable_if_t<std::is_unsigned<T>::value, T> MaxAlignOf(T value) {
    T pot = 1;
    while (value && ((value & 1u) == 0)) {
        pot <<= 1;
        value >>= 1;
    }
    return pot;
}

}  // namespace tint::utils

#endif  // SRC_TINT_UTILS_MATH_H_