mantissa_util: Implement ResidualErrorOnRightShift
Accurately calculate residual error that is shifted out
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4 changed files with 113 additions and 0 deletions
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@ -18,6 +18,7 @@ add_library(dynarmic
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common/crc32.h
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common/fp/fpsr.h
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common/fp/info.h
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common/fp/mantissa_util.h
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common/fp/process_exception.cpp
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common/fp/process_exception.h
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common/fp/rounding_mode.h
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48
src/common/fp/mantissa_util.h
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48
src/common/fp/mantissa_util.h
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@ -0,0 +1,48 @@
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/* 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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#pragma once
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#include "common/bit_util.h"
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#include "common/common_types.h"
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namespace Dynarmic::FP {
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enum class ResidualError {
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Zero,
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LessThanHalf,
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Half,
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GreaterThanHalf,
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};
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template<typename MantissaT>
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ResidualError ResidualErrorOnRightShift(MantissaT mantissa, int shift_amount) {
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if (shift_amount <= 0 || mantissa == 0) {
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return ResidualError::Zero;
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}
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if (shift_amount > static_cast<int>(Common::BitSize<MantissaT>())) {
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return Common::MostSignificantBit(mantissa) ? ResidualError::GreaterThanHalf : ResidualError::LessThanHalf;
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}
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const size_t half_bit_position = static_cast<size_t>(shift_amount - 1);
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const MantissaT half = static_cast<MantissaT>(1) << half_bit_position;
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const MantissaT error_mask = Common::Ones<MantissaT>(static_cast<size_t>(shift_amount));
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const MantissaT error = mantissa & error_mask;
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if (error == 0) {
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return ResidualError::Zero;
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}
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if (error < half) {
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return ResidualError::LessThanHalf;
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}
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if (error == half) {
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return ResidualError::Half;
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}
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return ResidualError::GreaterThanHalf;
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}
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} // namespace Dynarmic::FP
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@ -29,6 +29,7 @@ add_executable(dynarmic_tests
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A64/inst_gen.cpp
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A64/inst_gen.h
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A64/testenv.h
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fp/mantissa_util_tests.cpp
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fp/unpacked_tests.cpp
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main.cpp
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mp.cpp
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63
tests/fp/mantissa_util_tests.cpp
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63
tests/fp/mantissa_util_tests.cpp
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@ -0,0 +1,63 @@
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/* 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 <tuple>
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#include <vector>
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#include <catch.hpp>
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#include "common/fp/mantissa_util.h"
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#include "common/safe_ops.h"
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#include "rand_int.h"
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using namespace Dynarmic;
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using namespace Dynarmic::FP;
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TEST_CASE("ResidualErrorOnRightShift", "[fp]") {
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const std::vector<std::tuple<u32, int, ResidualError>> test_cases {
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{0x00000001, 1, ResidualError::Half},
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{0x00000002, 1, ResidualError::Zero},
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{0x00000001, 2, ResidualError::LessThanHalf},
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{0x00000002, 2, ResidualError::Half},
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{0x00000003, 2, ResidualError::GreaterThanHalf},
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{0x00000004, 2, ResidualError::Zero},
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{0x00000005, 2, ResidualError::LessThanHalf},
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{0x00000006, 2, ResidualError::Half},
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{0x00000007, 2, ResidualError::GreaterThanHalf},
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};
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for (auto [mantissa, shift, expected_result] : test_cases) {
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const ResidualError result = ResidualErrorOnRightShift(mantissa, shift);
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REQUIRE(result == expected_result);
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}
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}
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TEST_CASE("ResidualErrorOnRightShift Randomized", "[fp]") {
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for (size_t test = 0; test < 100000; test++) {
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const u32 mantissa = RandInt<u32>(0, 0xFFFFFFFF);
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const int shift = RandInt<int>(-60, 60);
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const ResidualError result = ResidualErrorOnRightShift(mantissa, shift);
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const u64 calculated_error = Safe::ArithmeticShiftRightDouble(Common::SignExtend<32, u64>(mantissa), u64(0), shift);
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const ResidualError expected_result = [&]{
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constexpr u64 half_error = 0x8000'0000'0000'0000ull;
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if (calculated_error == 0) {
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return ResidualError::Zero;
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}
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if (calculated_error < half_error) {
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return ResidualError::LessThanHalf;
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}
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if (calculated_error == half_error) {
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return ResidualError::Half;
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}
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return ResidualError::GreaterThanHalf;
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}();
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INFO(std::hex << "mantissa " << mantissa << " shift " << shift << " calculated_error " << calculated_error);
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REQUIRE(result == expected_result);
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}
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}
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