Merge bitcoin/bitcoin#24224: util: Add SaturatingAdd helper

faa7d8a util: Add SaturatingAdd helper (MarcoFalke)

Pull request description:

  Seems good to have this in the repo, as it might be needed to write cleaner code. For example:

  * bitcoin/bitcoin#24090 (comment)
  * bitcoin/bitcoin#23418 (comment)
  * ...

ACKs for top commit:
  MarcoFalke:
    Added a test. Should be trivial to re-ACK with `git range-diff bitcoin-core/master fa90189cbf faa7d8a`
  klementtan:
    reACK faa7d8a
  vasild:
    ACK faa7d8a

Tree-SHA512: d0e6efdba7dfcbdd16ab4539a7f5e45a97d17792e42586c3c52caaae3fc70612dc9e364359658de5de5718fb8c2a765a59ceb2230098355394fa067a9732bc2a

Author: MarcoFalke

src/test/fuzz/addition_overflow.cpp

@@ -26,18 +26,27 @@ void TestAdditionOverflow(FuzzedDataProvider& fuzzed_data_provider)
     const T i = fuzzed_data_provider.ConsumeIntegral<T>();
     const T j = fuzzed_data_provider.ConsumeIntegral<T>();
     const bool is_addition_overflow_custom = AdditionOverflow(i, j);
+    const auto maybe_add{CheckedAdd(i, j)};
+    const auto sat_add{SaturatingAdd(i, j)};
+    assert(is_addition_overflow_custom == !maybe_add.has_value());
+    assert(is_addition_overflow_custom == AdditionOverflow(j, i));
+    assert(maybe_add == CheckedAdd(j, i));
+    assert(sat_add == SaturatingAdd(j, i));
 #if defined(HAVE_BUILTIN_ADD_OVERFLOW)
     T result_builtin;
     const bool is_addition_overflow_builtin = __builtin_add_overflow(i, j, &result_builtin);
     assert(is_addition_overflow_custom == is_addition_overflow_builtin);
     if (!is_addition_overflow_custom) {
         assert(i + j == result_builtin);
     }
-#else
-    if (!is_addition_overflow_custom) {
-        (void)(i + j);
-    }
 #endif
+    if (is_addition_overflow_custom) {
+        assert(sat_add == std::numeric_limits<T>::min() || sat_add == std::numeric_limits<T>::max());
+    } else {
+        const auto add{i + j};
+        assert(add == maybe_add.value());
+        assert(add == sat_add);
+    }
 }
 } // namespace
 

src/test/util_tests.cpp

@@ -1474,9 +1474,17 @@ static void TestAddMatrixOverflow()
     constexpr T MAXI{std::numeric_limits<T>::max()};
     BOOST_CHECK(!CheckedAdd(T{1}, MAXI));
     BOOST_CHECK(!CheckedAdd(MAXI, MAXI));
+    BOOST_CHECK_EQUAL(MAXI, SaturatingAdd(T{1}, MAXI));
+    BOOST_CHECK_EQUAL(MAXI, SaturatingAdd(MAXI, MAXI));
+
     BOOST_CHECK_EQUAL(0, CheckedAdd(T{0}, T{0}).value());
     BOOST_CHECK_EQUAL(MAXI, CheckedAdd(T{0}, MAXI).value());
     BOOST_CHECK_EQUAL(MAXI, CheckedAdd(T{1}, MAXI - 1).value());
+    BOOST_CHECK_EQUAL(MAXI - 1, CheckedAdd(T{1}, MAXI - 2).value());
+    BOOST_CHECK_EQUAL(0, SaturatingAdd(T{0}, T{0}));
+    BOOST_CHECK_EQUAL(MAXI, SaturatingAdd(T{0}, MAXI));
+    BOOST_CHECK_EQUAL(MAXI, SaturatingAdd(T{1}, MAXI - 1));
+    BOOST_CHECK_EQUAL(MAXI - 1, SaturatingAdd(T{1}, MAXI - 2));
 }
 
 /* Check for overflow or underflow */
@@ -1488,9 +1496,17 @@ static void TestAddMatrix()
     constexpr T MAXI{std::numeric_limits<T>::max()};
     BOOST_CHECK(!CheckedAdd(T{-1}, MINI));
     BOOST_CHECK(!CheckedAdd(MINI, MINI));
+    BOOST_CHECK_EQUAL(MINI, SaturatingAdd(T{-1}, MINI));
+    BOOST_CHECK_EQUAL(MINI, SaturatingAdd(MINI, MINI));
+
     BOOST_CHECK_EQUAL(MINI, CheckedAdd(T{0}, MINI).value());
     BOOST_CHECK_EQUAL(MINI, CheckedAdd(T{-1}, MINI + 1).value());
     BOOST_CHECK_EQUAL(-1, CheckedAdd(MINI, MAXI).value());
+    BOOST_CHECK_EQUAL(MINI + 1, CheckedAdd(T{-1}, MINI + 2).value());
+    BOOST_CHECK_EQUAL(MINI, SaturatingAdd(T{0}, MINI));
+    BOOST_CHECK_EQUAL(MINI, SaturatingAdd(T{-1}, MINI + 1));
+    BOOST_CHECK_EQUAL(MINI + 1, SaturatingAdd(T{-1}, MINI + 2));
+    BOOST_CHECK_EQUAL(-1, SaturatingAdd(MINI, MAXI));
 }
 
 BOOST_AUTO_TEST_CASE(util_overflow)

src/util/overflow.h

@@ -13,7 +13,7 @@ template <class T>
 [[nodiscard]] bool AdditionOverflow(const T i, const T j) noexcept
 {
     static_assert(std::is_integral<T>::value, "Integral required.");
-    if (std::numeric_limits<T>::is_signed) {
+    if constexpr (std::numeric_limits<T>::is_signed) {
         return (i > 0 && j > std::numeric_limits<T>::max() - i) ||
                (i < 0 && j < std::numeric_limits<T>::min() - i);
     }
@@ -29,4 +29,22 @@ template <class T>
     return i + j;
 }
 
+template <class T>
+[[nodiscard]] T SaturatingAdd(const T i, const T j) noexcept
+{
+    if constexpr (std::numeric_limits<T>::is_signed) {
+        if (i > 0 && j > std::numeric_limits<T>::max() - i) {
+            return std::numeric_limits<T>::max();
+        }
+        if (i < 0 && j < std::numeric_limits<T>::min() - i) {
+            return std::numeric_limits<T>::min();
+        }
+    } else {
+        if (std::numeric_limits<T>::max() - i < j) {
+            return std::numeric_limits<T>::max();
+        }
+    }
+    return i + j;
+}
+
 #endif // BITCOIN_UTIL_OVERFLOW_H