Merge pull request #560 from cppalliance/fix_basic

Improve some basic 128-bit operations

Author: mborland

include/boost/decimal/decimal128.hpp

@@ -1439,7 +1439,7 @@ constexpr auto d128_add_impl(T1 lhs_sig, std::int32_t lhs_exp, bool lhs_sign,
 
         BOOST_DECIMAL_IF_CONSTEXPR (std::numeric_limits<T2>::digits10 > std::numeric_limits<std::uint64_t>::digits10)
         {
-            if (rhs_sig >= detail::uint128 {500'000'000'000'000, 0})
+            if (rhs_sig >= detail::uint128 {UINT64_C(0xF684DF56C3E0), UINT64_C(0x1BC6C73200000000)})
             {
                 ++lhs_sig;
             }
@@ -1459,12 +1459,9 @@ constexpr auto d128_add_impl(T1 lhs_sig, std::int32_t lhs_exp, bool lhs_sign,
 
     if (delta_exp <= 3)
     {
-        while (delta_exp > 0)
-        {
-            lhs_sig *= 10;
-            --delta_exp;
-            --lhs_exp;
-        }
+        lhs_sig *= detail::pow10(static_cast<detail::uint128>(delta_exp));
+        lhs_exp -= delta_exp;
+        delta_exp = 0;
     }
     else
     {
@@ -1475,8 +1472,8 @@ constexpr auto d128_add_impl(T1 lhs_sig, std::int32_t lhs_exp, bool lhs_sign,
 
     while (delta_exp > 1)
     {
-        rhs_sig /= 10;
-        --delta_exp;
+        rhs_sig /= detail::pow10(static_cast<detail::uint128>(delta_exp - 1));
+        delta_exp = 1;
     }
 
     if (delta_exp == 1)
@@ -1539,8 +1536,8 @@ constexpr auto d128_sub_impl(T1 lhs_sig, std::int32_t lhs_exp, bool lhs_sign,
 
     while (delta_exp > 1)
     {
-        sig_smaller /= 10;
-        --delta_exp;
+        sig_smaller /= detail::pow10(static_cast<detail::uint128>(delta_exp - 1));
+        delta_exp = 1;
     }
 
     if (delta_exp == 1)

include/boost/decimal/detail/integer_search_trees.hpp

@@ -166,6 +166,53 @@ constexpr auto num_digits(std::uint64_t x) noexcept -> int
 # pragma warning(disable: 4307) // MSVC 14.1 warns of intergral constant overflow
 #endif
 
+#if defined(__cpp_lib_array_constexpr) && __cpp_lib_array_constexpr >= 201603L
+
+template <typename T, std::size_t N>
+constexpr auto generate_array() noexcept -> std::array<T, N>
+{
+    std::array<T, N> values {};
+
+    values[0] = 1;
+    for (std::size_t i {1}; i < N; ++i)
+    {
+        values[i] = values[i - 1] * 10;
+    }
+
+    return values;
+}
+
+constexpr int num_digits(uint128 x) noexcept
+{
+    constexpr auto big_powers_of_10 = generate_array<boost::decimal::detail::uint128, 39>();
+
+    if (x == 0)
+    {
+        return 1;
+    }
+
+    std::uint32_t left = 0U;
+    std::uint32_t right = 38U;
+
+    while (left < right)
+    {
+        std::uint32_t mid = (left + right + 1U) / 2U;
+
+        if (x >= big_powers_of_10[mid])
+        {
+            left = mid;
+        }
+        else
+        {
+            right = mid - 1;
+        }
+    }
+
+    return static_cast<int>(left + 1);
+}
+
+#else
+
 constexpr int num_digits(uint128 x) noexcept
 {
     if (x.high == 0)
@@ -190,6 +237,8 @@ constexpr int num_digits(uint128 x) noexcept
     return 1;
 }
 
+#endif // Constexpr array
+
 constexpr int num_digits(const uint256_t& x) noexcept
 {
     if (x.high == 0)
@@ -222,6 +271,40 @@ constexpr int num_digits(const uint256_t& x) noexcept
 #endif
 
 #ifdef BOOST_DECIMAL_HAS_INT128
+
+#if defined(__cpp_lib_array_constexpr) && __cpp_lib_array_constexpr >= 201603L
+
+constexpr auto num_digits(boost::decimal::detail::uint128_t x) noexcept -> int
+{
+    constexpr auto big_powers_of_10 = generate_array<boost::decimal::detail::uint128_t, 39>();
+
+    if (x == 0)
+    {
+        return 1;
+    }
+
+    std::uint32_t left = 0U;
+    std::uint32_t right = 38U;
+
+    while (left < right)
+    {
+        std::uint32_t mid = (left + right + 1U) / 2U;
+
+        if (x >= big_powers_of_10[mid])
+        {
+            left = mid;
+        }
+        else
+        {
+            right = mid - 1;
+        }
+    }
+
+    return static_cast<int>(left + 1);
+}
+
+#else
+
 // Assume that if someone is using 128 bit ints they are favoring the top end of the range
 // Max value is 340,282,366,920,938,463,463,374,607,431,768,211,455 (39 digits)
 constexpr auto num_digits(boost::decimal::detail::uint128_t x) noexcept -> int
@@ -290,7 +373,10 @@ constexpr auto num_digits(boost::decimal::detail::uint128_t x) noexcept -> int
            (x >= powers_of_10[1])  ?  2 :
            (x >= powers_of_10[0])  ?  1 : 0;
 }
-#endif
+
+#endif // constexpr arrays
+
+#endif // Has int128
 
 } // namespace detail
 } // namespace decimal

test/test_to_chars.cpp

@@ -19,7 +19,8 @@ static constexpr auto N = static_cast<std::size_t>(1024U); // Number of trials
 static constexpr auto N = static_cast<std::size_t>(1024U >> 4U); // Number of trials
 #endif
 
-#if !defined(BOOST_DECIMAL_DISABLE_CLIB)
+// stringop overflow errors from gcc-13 and on with x86
+#if !defined(BOOST_DECIMAL_DISABLE_CLIB) && !(defined(__GNUC__) && __GNUC__ >= 13 && !defined(__aarch64__))
 
 template <typename T>
 void test_value(T val, const char* result, chars_format fmt = boost::decimal::chars_format::general, int precision = -1)