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Roll back ipow changes due to register pressure. #15242

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Mar 11, 2024
Merged

Roll back ipow changes due to register pressure. #15242

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60 changes: 16 additions & 44 deletions cpp/include/cudf/fixed_point/fixed_point.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -85,41 +85,7 @@ constexpr inline auto is_supported_construction_value_type()
namespace detail {

/**
* @brief Recursively computes integer exponentiation
*
* @note This is intended to be run at compile time
*
* @tparam Rep Representation type for return type
* @tparam Base The base to be exponentiated
* @param exp The exponent to be used for exponentiation
* @return Result of `Base` to the power of `exponent` of type `Rep`
*/
template <typename Rep, int32_t Base>
CUDF_HOST_DEVICE inline constexpr Rep get_power(int32_t exp)
{
// Compute power recursively
return (exp > 0) ? Rep(Base) * get_power<Rep, Base>(exp - 1) : 1;
}

/**
* @brief Implementation of integer exponentiation by array lookup
*
* @tparam Rep Representation type for return type
* @tparam Base The base to be exponentiated
* @tparam Exponents The exponents for the array entries
* @param exponent The exponent to be used for exponentiation
* @return Result of `Base` to the power of `exponent` of type `Rep`
*/
template <typename Rep, int32_t Base, std::size_t... Exponents>
CUDF_HOST_DEVICE inline Rep ipow_impl(int32_t exponent, cuda::std::index_sequence<Exponents...>)
{
// Compute powers at compile time, storing into array
static constexpr Rep powers[] = {get_power<Rep, Base>(Exponents)...};
return powers[exponent];
}

/**
* @brief A function for integer exponentiation by array lookup
* @brief A function for integer exponentiation by squaring.
*
* @tparam Rep Representation type for return type
* @tparam Base The base to be exponentiated
Expand All @@ -134,16 +100,22 @@ template <typename Rep,
CUDF_HOST_DEVICE inline Rep ipow(T exponent)
{
cudf_assert(exponent >= 0 && "integer exponentiation with negative exponent is not possible.");
if constexpr (Base == numeric::Radix::BASE_2) {
return static_cast<Rep>(1) << exponent;
} else { // BASE_10
// Build index sequence for building power array at compile time
static constexpr auto max_exp = cuda::std::numeric_limits<Rep>::digits10;
static constexpr auto exponents = cuda::std::make_index_sequence<max_exp + 1>{};

// Get compile-time result
return ipow_impl<Rep, static_cast<int32_t>(Base)>(exponent, exponents);

if constexpr (Base == numeric::Radix::BASE_2) { return static_cast<Rep>(1) << exponent; }

// Note: Including an array here introduces too much register pressure
// https://simple.wikipedia.org/wiki/Exponentiation_by_squaring
// This is the iterative equivalent of the recursive definition (faster)
// Quick-bench for squaring: http://quick-bench.com/Wg7o7HYQC9FW5M0CO0wQAjSwP_Y
if (exponent == 0) { return static_cast<Rep>(1); }
auto extra = static_cast<Rep>(1);
auto square = static_cast<Rep>(Base);
while (exponent > 1) {
if (exponent & 1) { extra *= square; }
exponent >>= 1;
square *= square;
}
return square * extra;
}

/** @brief Function that performs a `right shift` scale "times" on the `val`
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