91 lines
3.5 KiB
C++
91 lines
3.5 KiB
C++
/*
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* Copyright (C) 2022 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#pragma once
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#include <atomic>
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#include <cstdint>
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#include <tuple>
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#include <type_traits>
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#include <log/log.h>
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namespace android::mediautils {
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// The goal of this class is to detect and accumulate wraparound occurrences on a
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// lower sized integer.
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// This class assumes that the underlying unsigned type is either incremented or
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// decremented by at most the underlying signed type between any two subsequent
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// polls (or construction). This is well-defined as the modular nature of
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// unsigned arithmetic ensures that every new value maps 1-1 to an
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// increment/decrement over the same sized signed type. It also ensures that our
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// counter will be equivalent mod the size of the integer even if the underlying
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// type is modified outside of this range.
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//
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// For convenience, this class is thread compatible. Additionally, it is safe
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// as long as there is only one writer.
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template <typename Integral = uint32_t, typename AccumulatingType = uint64_t>
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class ExtendedAccumulator {
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static_assert(sizeof(Integral) < sizeof(AccumulatingType),
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"Accumulating type should be larger than underlying type");
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static_assert(std::is_integral_v<Integral> && std::is_unsigned_v<Integral>,
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"Wraparound behavior is only well-defiend for unsigned ints");
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static_assert(std::is_integral_v<AccumulatingType>);
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public:
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enum class Wrap {
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NORMAL = 0,
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UNDERFLOW = 1,
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OVERFLOW = 2,
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};
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using UnsignedInt = Integral;
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using SignedInt = std::make_signed_t<UnsignedInt>;
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explicit ExtendedAccumulator(AccumulatingType initial = 0) : mAccumulated(initial) {}
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// Returns a pair of the calculated change on the accumulating value, and a
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// Wrap type representing the type of wraparound (if any) which occurred.
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std::pair<SignedInt, Wrap> poll(UnsignedInt value) {
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auto acc = mAccumulated.load(std::memory_order_relaxed);
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const auto bottom_bits = static_cast<UnsignedInt>(acc);
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std::pair<SignedInt, Wrap> res = {0, Wrap::NORMAL};
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const bool overflow = __builtin_sub_overflow(value, bottom_bits, &res.first);
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if (overflow) {
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res.second = (res.first > 0) ? Wrap::OVERFLOW : Wrap::UNDERFLOW;
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}
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const bool acc_overflow = __builtin_add_overflow(acc, res.first, &acc);
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// If our *accumulating* type overflows or underflows (depending on its
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// signedness), we should abort.
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if (acc_overflow) LOG_ALWAYS_FATAL("Unexpected overflow/underflow in %s", __func__);
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mAccumulated.store(acc, std::memory_order_relaxed);
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return res;
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}
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AccumulatingType getValue() const { return mAccumulated.load(std::memory_order_relaxed); }
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private:
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// Invariant - the bottom underlying bits of accumulated are the same as the
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// last value provided to poll.
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std::atomic<AccumulatingType> mAccumulated;
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};
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} // namespace android::mediautils
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