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util: Add integer left shift helpers
The helpers are used in the following commits to increase the safety of conversions during cache size calculations. Co-authored-by: Ryan Ofsky <ryan@ofsky.org> Co-authored-by: stickies-v <stickies-v@protonmail.com>
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TheCharlatan
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3 changed files with 155 additions and 0 deletions
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@ -13,6 +13,7 @@
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#include <test/util/setup_common.h>
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#include <uint256.h>
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#include <util/bitdeque.h>
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#include <util/byte_units.h>
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#include <util/fs.h>
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#include <util/fs_helpers.h>
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#include <util/moneystr.h>
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@ -1877,4 +1878,100 @@ BOOST_AUTO_TEST_CASE(clearshrink_test)
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}
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}
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template <typename T>
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void TestCheckedLeftShift()
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{
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constexpr auto MAX{std::numeric_limits<T>::max()};
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// Basic operations
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BOOST_CHECK_EQUAL(CheckedLeftShift<T>(0, 1), 0);
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BOOST_CHECK_EQUAL(CheckedLeftShift<T>(0, 127), 0);
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BOOST_CHECK_EQUAL(CheckedLeftShift<T>(1, 1), 2);
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BOOST_CHECK_EQUAL(CheckedLeftShift<T>(2, 2), 8);
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BOOST_CHECK_EQUAL(CheckedLeftShift<T>(MAX >> 1, 1), MAX - 1);
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// Max left shift
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BOOST_CHECK_EQUAL(CheckedLeftShift<T>(1, std::numeric_limits<T>::digits - 1), MAX / 2 + 1);
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// Overflow cases
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BOOST_CHECK(!CheckedLeftShift<T>((MAX >> 1) + 1, 1));
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BOOST_CHECK(!CheckedLeftShift<T>(MAX, 1));
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BOOST_CHECK(!CheckedLeftShift<T>(1, std::numeric_limits<T>::digits));
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BOOST_CHECK(!CheckedLeftShift<T>(1, std::numeric_limits<T>::digits + 1));
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if constexpr (std::is_signed_v<T>) {
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constexpr auto MIN{std::numeric_limits<T>::min()};
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// Negative input
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BOOST_CHECK_EQUAL(CheckedLeftShift<T>(-1, 1), -2);
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BOOST_CHECK_EQUAL(CheckedLeftShift<T>((MIN >> 2), 1), MIN / 2);
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BOOST_CHECK_EQUAL(CheckedLeftShift<T>((MIN >> 1) + 1, 1), MIN + 2);
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BOOST_CHECK_EQUAL(CheckedLeftShift<T>(MIN >> 1, 1), MIN);
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// Overflow negative
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BOOST_CHECK(!CheckedLeftShift<T>((MIN >> 1) - 1, 1));
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BOOST_CHECK(!CheckedLeftShift<T>(MIN >> 1, 2));
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BOOST_CHECK(!CheckedLeftShift<T>(-1, 100));
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}
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}
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template <typename T>
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void TestSaturatingLeftShift()
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{
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constexpr auto MAX{std::numeric_limits<T>::max()};
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// Basic operations
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BOOST_CHECK_EQUAL(SaturatingLeftShift<T>(0, 1), 0);
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BOOST_CHECK_EQUAL(SaturatingLeftShift<T>(0, 127), 0);
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BOOST_CHECK_EQUAL(SaturatingLeftShift<T>(1, 1), 2);
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BOOST_CHECK_EQUAL(SaturatingLeftShift<T>(2, 2), 8);
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BOOST_CHECK_EQUAL(SaturatingLeftShift<T>(MAX >> 1, 1), MAX - 1);
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// Max left shift
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BOOST_CHECK_EQUAL(SaturatingLeftShift<T>(1, std::numeric_limits<T>::digits - 1), MAX / 2 + 1);
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// Saturation cases
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BOOST_CHECK_EQUAL(SaturatingLeftShift<T>((MAX >> 1) + 1, 1), MAX);
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BOOST_CHECK_EQUAL(SaturatingLeftShift<T>(MAX, 1), MAX);
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BOOST_CHECK_EQUAL(SaturatingLeftShift<T>(1, std::numeric_limits<T>::digits), MAX);
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BOOST_CHECK_EQUAL(SaturatingLeftShift<T>(1, std::numeric_limits<T>::digits + 1), MAX);
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if constexpr (std::is_signed_v<T>) {
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constexpr auto MIN{std::numeric_limits<T>::min()};
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// Negative input
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BOOST_CHECK_EQUAL(SaturatingLeftShift<T>(-1, 1), -2);
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BOOST_CHECK_EQUAL(SaturatingLeftShift<T>((MIN >> 2), 1), MIN / 2);
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BOOST_CHECK_EQUAL(SaturatingLeftShift<T>((MIN >> 1) + 1, 1), MIN + 2);
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BOOST_CHECK_EQUAL(SaturatingLeftShift<T>(MIN >> 1, 1), MIN);
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// Saturation negative
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BOOST_CHECK_EQUAL(SaturatingLeftShift<T>((MIN >> 1) - 1, 1), MIN);
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BOOST_CHECK_EQUAL(SaturatingLeftShift<T>(MIN >> 1, 2), MIN);
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BOOST_CHECK_EQUAL(SaturatingLeftShift<T>(-1, 100), MIN);
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}
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}
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BOOST_AUTO_TEST_CASE(checked_left_shift_test)
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{
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TestCheckedLeftShift<uint8_t>();
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TestCheckedLeftShift<int8_t>();
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TestCheckedLeftShift<size_t>();
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TestCheckedLeftShift<uint64_t>();
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TestCheckedLeftShift<int64_t>();
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}
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BOOST_AUTO_TEST_CASE(saturating_left_shift_test)
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{
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TestSaturatingLeftShift<uint8_t>();
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TestSaturatingLeftShift<int8_t>();
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TestSaturatingLeftShift<size_t>();
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TestSaturatingLeftShift<uint64_t>();
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TestSaturatingLeftShift<int64_t>();
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}
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BOOST_AUTO_TEST_CASE(mib_string_literal_test)
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{
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BOOST_CHECK_EQUAL(0_MiB, 0);
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BOOST_CHECK_EQUAL(1_MiB, 1024 * 1024);
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const auto max_mib{std::numeric_limits<size_t>::max() >> 20};
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BOOST_CHECK_EXCEPTION(operator""_MiB(static_cast<unsigned long long>(max_mib) + 1), std::overflow_error, HasReason("MiB value too large for size_t byte conversion"));
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}
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BOOST_AUTO_TEST_SUITE_END()
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22
src/util/byte_units.h
Normal file
22
src/util/byte_units.h
Normal file
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@ -0,0 +1,22 @@
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// Copyright (c) 2025-present The Bitcoin Core developers
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// Distributed under the MIT software license, see the accompanying
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// file COPYING or http://www.opensource.org/licenses/mit-license.php.
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#ifndef BITCOIN_UTIL_BYTE_UNITS_H
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#define BITCOIN_UTIL_BYTE_UNITS_H
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#include <util/overflow.h>
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#include <stdexcept>
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//! Overflow-safe conversion of MiB to bytes.
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constexpr size_t operator"" _MiB(unsigned long long mebibytes)
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{
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auto bytes{CheckedLeftShift(mebibytes, 20)};
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if (!bytes || *bytes > std::numeric_limits<size_t>::max()) {
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throw std::overflow_error("MiB value too large for size_t byte conversion");
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}
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return *bytes;
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}
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#endif // BITCOIN_UTIL_BYTE_UNITS_H
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@ -5,6 +5,8 @@
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#ifndef BITCOIN_UTIL_OVERFLOW_H
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#define BITCOIN_UTIL_OVERFLOW_H
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#include <climits>
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#include <concepts>
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#include <limits>
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#include <optional>
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#include <type_traits>
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@ -47,4 +49,38 @@ template <class T>
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return i + j;
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}
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/**
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* @brief Left bit shift with overflow checking.
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* @param input The input value to be left shifted.
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* @param shift The number of bits to left shift.
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* @return (input * 2^shift) or nullopt if it would not fit in the return type.
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*/
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template <std::integral T>
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constexpr std::optional<T> CheckedLeftShift(T input, unsigned shift) noexcept
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{
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if (shift == 0 || input == 0) return input;
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// Avoid undefined c++ behaviour if shift is >= number of bits in T.
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if (shift >= sizeof(T) * CHAR_BIT) return std::nullopt;
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// If input << shift is too big to fit in T, return nullopt.
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if (input > (std::numeric_limits<T>::max() >> shift)) return std::nullopt;
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if (input < (std::numeric_limits<T>::min() >> shift)) return std::nullopt;
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return input << shift;
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}
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/**
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* @brief Left bit shift with safe minimum and maximum values.
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* @param input The input value to be left shifted.
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* @param shift The number of bits to left shift.
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* @return (input * 2^shift) clamped to fit between the lowest and highest
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* representable values of the type T.
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*/
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template <std::integral T>
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constexpr T SaturatingLeftShift(T input, unsigned shift) noexcept
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{
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if (auto result{CheckedLeftShift(input, shift)}) return *result;
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// If input << shift is too big to fit in T, return biggest positive or negative
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// number that fits.
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return input < 0 ? std::numeric_limits<T>::min() : std::numeric_limits<T>::max();
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}
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#endif // BITCOIN_UTIL_OVERFLOW_H
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