68 lines
1.9 KiB
C++
68 lines
1.9 KiB
C++
/* This file is part of the dynarmic project.
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* Copyright (c) 2018 MerryMage
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* This software may be used and distributed according to the terms of the GNU
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* General Public License version 2 or any later version.
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*/
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#include <array>
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#include "common/math_util.h"
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namespace Dynarmic::Common {
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u8 RecipEstimate(u64 a) {
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using LUT = std::array<u8, 256>;
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static constexpr u64 lut_offset = 256;
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static const LUT lut = [] {
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LUT result{};
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for (u64 i = 0; i < result.size(); i++) {
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u64 a = i + lut_offset;
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a = a * 2 + 1;
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u64 b = (1u << 19) / a;
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result[i] = static_cast<u8>((b + 1) / 2);
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}
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return result;
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}();
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return lut[a - lut_offset];
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}
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/// Input is a u0.9 fixed point number. Only values in [0.25, 1.0) are valid.
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/// Output is a u0.8 fixed point number, with an implied 1 prefixed.
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/// i.e.: The output is a value in [1.0, 2.0).
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u8 RecipSqrtEstimate(u64 a) {
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using LUT = std::array<u8, 512>;
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static const LUT lut = [] {
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LUT result{};
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for (u64 i = 128; i < result.size(); i++) {
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u64 a = i;
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// Convert to u.10 (with 8 significant bits), force to odd
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if (a < 256) {
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// [0.25, 0.5)
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a = a * 2 + 1;
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} else {
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// [0.5, 1.0)
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a = (a | 1) * 2;
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}
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// Calculate largest b which for which b < 1.0 / sqrt(a).
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// Start from b = 1.0 (in u.9) since b cannot be smaller.
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u64 b = 512;
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// u.10 * u.9 * u.9 -> u.28
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while (a * (b + 1) * (b + 1) < (1u << 28)) {
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b++;
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}
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// Round to nearest u0.8 (with implied set integer bit).
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result[i] = static_cast<u8>((b + 1) / 2);
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}
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return result;
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}();
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return lut[a & 0x1FF];
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}
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} // namespace Dynarmic::Common
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