Improve performance of integer formatting (#76726)

* Improve performance of integer formatting

1. The DivRem(..., 10) for each digit in the number ends up being the most expensive aspect of formatting.  This employs a trick other formatting libraries use, which is to have a table for all the formatted values between 00 and 99, and to then DivRem(..., 100) to cut the number of operations in half, which for longer values is meaningful.
2. Avoids going through the digit counting path when we know at the call site it won't be needed.
3. Employs a branch-free, table-based lookup for CountDigits(ulong) to go with a similar approach added for uint.

* Address PR feedback (move license notice)

diff --git a/THIRD-PARTY-NOTICES.TXT b/THIRD-PARTY-NOTICES.TXT
index c682d59..5359da5 100644 (file)
--- a/THIRD-PARTY-NOTICES.TXT
+++ b/THIRD-PARTY-NOTICES.TXT
@@ -1143,3 +1143,35 @@ OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABIL
 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 POSSIBILITY OF SUCH DAMAGE.
+
+License notice for fmtlib/fmt
+-------------------------------
+
+Formatting library for C++
+
+Copyright (c) 2012 - present, Victor Zverovich
+
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of this software and associated documentation files (the
+"Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+--- Optional exception to the license ---
+
+As an exception, if, as a result of your compiling your source code, portions
+of this Software are embedded into a machine-executable object form of such
+source code, you may redistribute such embedded portions in such object form
+without including the above copyright and permission notices.

diff --git a/src/libraries/System.Private.CoreLib/src/System/Buffers/Text/FormattingHelpers.CountDigits.cs b/src/libraries/System.Private.CoreLib/src/System/Buffers/Text/FormattingHelpers.CountDigits.cs
index cb9f536..1bb918a 100644 (file)
--- a/src/libraries/System.Private.CoreLib/src/System/Buffers/Text/FormattingHelpers.CountDigits.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Buffers/Text/FormattingHelpers.CountDigits.cs
@@ -52,60 +52,58 @@ namespace System.Buffers.Text
             return digits;
         }
 
+        // Based on do_count_digits from https://github.com/fmtlib/fmt/blob/662adf4f33346ba9aba8b072194e319869ede54a/include/fmt/format.h#L1124
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static int CountDigits(ulong value)
         {
-            int digits = 1;
-            uint part;
-            if (value >= 10000000)
+            // Map the log2(value) to a power of 10.
+            ReadOnlySpan<byte> log2ToPow10 = new byte[]
             {
-                if (value >= 100000000000000)
-                {
-                    part = (uint)(value / 100000000000000);
-                    digits += 14;
-                }
-                else
-                {
-                    part = (uint)(value / 10000000);
-                    digits += 7;
-                }
-            }
-            else
-            {
-                part = (uint)value;
-            }
+                1,  1,  1,  2,  2,  2,  3,  3,  3,  4,  4,  4,  4,  5,  5,  5,
+                6,  6,  6,  7,  7,  7,  7,  8,  8,  8,  9,  9,  9,  10, 10, 10,
+                10, 11, 11, 11, 12, 12, 12, 13, 13, 13, 13, 14, 14, 14, 15, 15,
+                15, 16, 16, 16, 16, 17, 17, 17, 18, 18, 18, 19, 19, 19, 19, 20
+            };
+            Debug.Assert(log2ToPow10.Length == 64);
+            uint index = Unsafe.Add(ref MemoryMarshal.GetReference(log2ToPow10), BitOperations.Log2(value));
 
-            if (part < 10)
-            {
-                // no-op
-            }
-            else if (part < 100)
+            // TODO https://github.com/dotnet/runtime/issues/60948: Use ReadOnlySpan<ulong> instead of ReadOnlySpan<byte>.
+            // Read the associated power of 10.
+            ReadOnlySpan<byte> powersOf10 = new byte[]
             {
-                digits++;
-            }
-            else if (part < 1000)
-            {
-                digits += 2;
-            }
-            else if (part < 10000)
-            {
-                digits += 3;
-            }
-            else if (part < 100000)
-            {
-                digits += 4;
-            }
-            else if (part < 1000000)
-            {
-                digits += 5;
-            }
-            else
+                0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // unused entry to avoid needing to subtract
+                0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // 0
+                0x0A, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // 10
+                0x64, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // 100
+                0xE8, 0x03, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // 1000
+                0x10, 0x27, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, // 10000
+                0xA0, 0x86, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, // 100000
+                0x40, 0x42, 0x0F, 0x00, 0x00, 0x00, 0x00, 0x00, // 1000000
+                0x80, 0x96, 0x98, 0x00, 0x00, 0x00, 0x00, 0x00, // 10000000
+                0x00, 0xE1, 0xF5, 0x05, 0x00, 0x00, 0x00, 0x00, // 100000000
+                0x00, 0xCA, 0x9A, 0x3B, 0x00, 0x00, 0x00, 0x00, // 1000000000
+                0x00, 0xE4, 0x0B, 0x54, 0x02, 0x00, 0x00, 0x00, // 10000000000
+                0x00, 0xE8, 0x76, 0x48, 0x17, 0x00, 0x00, 0x00, // 100000000000
+                0x00, 0x10, 0xA5, 0xD4, 0xE8, 0x00, 0x00, 0x00, // 1000000000000
+                0x00, 0xA0, 0x72, 0x4E, 0x18, 0x09, 0x00, 0x00, // 10000000000000
+                0x00, 0x40, 0x7A, 0x10, 0xF3, 0x5A, 0x00, 0x00, // 100000000000000
+                0x00, 0x80, 0xC6, 0xA4, 0x7E, 0x8D, 0x03, 0x00, // 1000000000000000
+                0x00, 0x00, 0xC1, 0x6F, 0xF2, 0x86, 0x23, 0x00, // 10000000000000000
+                0x00, 0x00, 0x8A, 0x5D, 0x78, 0x45, 0x63, 0x01, // 100000000000000000
+                0x00, 0x00, 0x64, 0xA7, 0xB3, 0xB6, 0xE0, 0x0D, // 1000000000000000000
+                0x00, 0x00, 0xE8, 0x89, 0x04, 0x23, 0xC7, 0x8A, // 10000000000000000000
+            };
+            Debug.Assert((index + 1) * sizeof(ulong) <= powersOf10.Length);
+            ulong powerOf10 = Unsafe.ReadUnaligned<ulong>(ref Unsafe.Add(ref MemoryMarshal.GetReference(powersOf10), index * sizeof(ulong)));
+            if (!BitConverter.IsLittleEndian)
             {
-                Debug.Assert(part < 10000000);
-                digits += 6;
+                powerOf10 = BinaryPrimitives.ReverseEndianness(powerOf10);
             }
 
-            return digits;
+            // Return the number of digits based on the power of 10, shifted by 1
+            // if it falls below the threshold.
+            bool lessThan = value < powerOf10;
+            return (int)(index - Unsafe.As<bool, byte>(ref lessThan)); // while arbitrary bools may be non-0/1, comparison operators are expected to return 0/1
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]

diff --git a/src/libraries/System.Private.CoreLib/src/System/Number.Formatting.cs b/src/libraries/System.Private.CoreLib/src/System/Number.Formatting.cs
index 2faf570..c545115 100644 (file)
--- a/src/libraries/System.Private.CoreLib/src/System/Number.Formatting.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Number.Formatting.cs
@@ -301,6 +301,31 @@ namespace System
             "(#)", "-#", "- #", "#-", "# -",
         };
 
+        // Optimizations using "TwoDigits" inspired by:
+        // https://engineering.fb.com/2013/03/15/developer-tools/three-optimization-tips-for-c/
+        private const string TwoDigitsChars =
+            "00010203040506070809" +
+            "10111213141516171819" +
+            "20212223242526272829" +
+            "30313233343536373839" +
+            "40414243444546474849" +
+            "50515253545556575859" +
+            "60616263646566676869" +
+            "70717273747576777879" +
+            "80818283848586878889" +
+            "90919293949596979899";
+        private static ReadOnlySpan<byte> TwoDigitsBytes =>
+            "00010203040506070809"u8 +
+            "10111213141516171819"u8 +
+            "20212223242526272829"u8 +
+            "30313233343536373839"u8 +
+            "40414243444546474849"u8 +
+            "50515253545556575859"u8 +
+            "60616263646566676869"u8 +
+            "70717273747576777879"u8 +
+            "80818283848586878889"u8 +
+            "90919293949596979899"u8;
+
         public static unsafe string FormatDecimal(decimal value, ReadOnlySpan<char> format, NumberFormatInfo info)
         {
             char fmt = ParseFormatSpecifier(format, out int digits);
@@ -833,7 +858,7 @@ namespace System
             if (format.Length == 0)
             {
                 return value >= 0 ?
-                    TryUInt32ToDecStr((uint)value, digits: -1, destination, out charsWritten) :
+                    TryUInt32ToDecStr((uint)value, destination, out charsWritten) :
                     TryNegativeInt32ToDecStr(value, digits: -1, NumberFormatInfo.GetInstance(provider).NegativeSign, destination, out charsWritten);
             }
 
@@ -931,7 +956,7 @@ namespace System
             // Fast path for default format
             if (format.Length == 0)
             {
-                return TryUInt32ToDecStr(value, digits: -1, destination, out charsWritten);
+                return TryUInt32ToDecStr(value, destination, out charsWritten);
             }
 
             return TryFormatUInt32Slow(value, format, provider, destination, out charsWritten);
@@ -979,7 +1004,7 @@ namespace System
             if (string.IsNullOrEmpty(format))
             {
                 return value >= 0 ?
-                    UInt64ToDecStr((ulong)value, digits: -1) :
+                    UInt64ToDecStr((ulong)value) :
                     NegativeInt64ToDecStr(value, digits: -1, NumberFormatInfo.GetInstance(provider).NegativeSign);
             }
 
@@ -1031,7 +1056,7 @@ namespace System
             if (format.Length == 0)
             {
                 return value >= 0 ?
-                    TryUInt64ToDecStr((ulong)value, digits: -1, destination, out charsWritten) :
+                    TryUInt64ToDecStr((ulong)value, destination, out charsWritten) :
                     TryNegativeInt64ToDecStr(value, digits: -1, NumberFormatInfo.GetInstance(provider).NegativeSign, destination, out charsWritten);
             }
 
@@ -1081,7 +1106,7 @@ namespace System
             // Fast path for default format
             if (string.IsNullOrEmpty(format))
             {
-                return UInt64ToDecStr(value, digits: -1);
+                return UInt64ToDecStr(value);
             }
 
             return FormatUInt64Slow(value, format, provider);
@@ -1129,7 +1154,7 @@ namespace System
             // Fast path for default format
             if (format.Length == 0)
             {
-                return TryUInt64ToDecStr(value, digits: -1, destination, out charsWritten);
+                return TryUInt64ToDecStr(value, destination, out charsWritten);
             }
 
             return TryFormatUInt64Slow(value, format, provider, destination, out charsWritten);
@@ -1540,54 +1565,121 @@ namespace System
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static unsafe void WriteTwoDigits(char* ptr, uint value)
+        {
+            Debug.Assert(value <= 99);
+            Unsafe.WriteUnaligned(ptr,
+                Unsafe.ReadUnaligned<uint>(
+                    ref Unsafe.As<char, byte>(
+                        ref Unsafe.Add(ref TwoDigitsChars.GetRawStringData(), (int)value * 2))));
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static unsafe void WriteTwoDigits(byte* ptr, uint value)
+        {
+            Debug.Assert(value <= 99);
+            Unsafe.WriteUnaligned(ptr,
+                Unsafe.ReadUnaligned<ushort>(
+                    ref Unsafe.Add(ref MemoryMarshal.GetReference(TwoDigitsBytes), (int)value * 2)));
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         internal static unsafe byte* UInt32ToDecChars(byte* bufferEnd, uint value)
         {
-            do
+            if (value >= 10)
             {
-                uint remainder;
-                (value, remainder) = Math.DivRem(value, 10);
-                *(--bufferEnd) = (byte)(remainder + '0');
+                // Handle all values >= 100 two-digits at a time so as to avoid expensive integer division operations.
+                while (value >= 100)
+                {
+                    bufferEnd -= 2;
+                    (value, uint remainder) = Math.DivRem(value, 100);
+                    WriteTwoDigits(bufferEnd, remainder);
+                }
+
+                // If there are two digits remaining, store them.
+                if (value >= 10)
+                {
+                    bufferEnd -= 2;
+                    WriteTwoDigits(bufferEnd, value);
+                    return bufferEnd;
+                }
             }
-            while (value != 0);
 
+            // Otherwise, store the single digit remaining.
+            *(--bufferEnd) = (byte)(value + '0');
             return bufferEnd;
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         internal static unsafe byte* UInt32ToDecChars(byte* bufferEnd, uint value, int digits)
         {
-            while (--digits >= 0 || value != 0)
+            uint remainder;
+            while (value >= 100)
             {
-                uint remainder;
+                bufferEnd -= 2;
+                digits -= 2;
+                (value, remainder) = Math.DivRem(value, 100);
+                WriteTwoDigits(bufferEnd, remainder);
+            }
+
+            while (value != 0 || digits > 0)
+            {
+                digits--;
                 (value, remainder) = Math.DivRem(value, 10);
                 *(--bufferEnd) = (byte)(remainder + '0');
             }
+
             return bufferEnd;
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         internal static unsafe char* UInt32ToDecChars(char* bufferEnd, uint value)
         {
-            do
+            if (value >= 10)
             {
-                uint remainder;
-                (value, remainder) = Math.DivRem(value, 10);
-                *(--bufferEnd) = (char)(remainder + '0');
+                // Handle all values >= 100 two-digits at a time so as to avoid expensive integer division operations.
+                while (value >= 100)
+                {
+                    bufferEnd -= 2;
+                    (value, uint remainder) = Math.DivRem(value, 100);
+                    WriteTwoDigits(bufferEnd, remainder);
+                }
+
+                // If there are two digits remaining, store them.
+                if (value >= 10)
+                {
+                    bufferEnd -= 2;
+                    WriteTwoDigits(bufferEnd, value);
+                    return bufferEnd;
+                }
             }
-            while (value != 0);
 
+            // Otherwise, store the single digit remaining.
+            *(--bufferEnd) = (char)(value + '0');
             return bufferEnd;
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         internal static unsafe char* UInt32ToDecChars(char* bufferEnd, uint value, int digits)
         {
-            while (--digits >= 0 || value != 0)
+            // Handle all values >= 100 two-digits at a time so as to avoid expensive integer division operations.
+            uint remainder;
+            while (value >= 100)
             {
-                uint remainder;
+                bufferEnd -= 2;
+                digits -= 2;
+                (value, remainder) = Math.DivRem(value, 100);
+                WriteTwoDigits(bufferEnd, remainder);
+            }
+
+            // Continue writing single digits until we've exhausted both the value and the requested number of digits.
+            while (value != 0 || digits > 0)
+            {
+                digits--;
                 (value, remainder) = Math.DivRem(value, 10);
                 *(--bufferEnd) = (char)(remainder + '0');
             }
+
             return bufferEnd;
         }
 
@@ -1627,30 +1719,44 @@ namespace System
             return result;
         }
 
-        private static unsafe bool TryUInt32ToDecStr(uint value, int digits, Span<char> destination, out int charsWritten)
+        private static unsafe bool TryUInt32ToDecStr(uint value, Span<char> destination, out int charsWritten)
         {
-            int bufferLength = Math.Max(digits, FormattingHelpers.CountDigits(value));
-            if (bufferLength > destination.Length)
+            int bufferLength = FormattingHelpers.CountDigits(value);
+            if (bufferLength <= destination.Length)
             {
-                charsWritten = 0;
-                return false;
+                charsWritten = bufferLength;
+                fixed (char* buffer = &MemoryMarshal.GetReference(destination))
+                {
+                    char* p = UInt32ToDecChars(buffer + bufferLength, value);
+                    Debug.Assert(p == buffer);
+                }
+                return true;
             }
 
-            charsWritten = bufferLength;
-            fixed (char* buffer = &MemoryMarshal.GetReference(destination))
+            charsWritten = 0;
+            return false;
+        }
+
+        private static unsafe bool TryUInt32ToDecStr(uint value, int digits, Span<char> destination, out int charsWritten)
+        {
+            int countedDigits = FormattingHelpers.CountDigits(value);
+            int bufferLength = Math.Max(digits, countedDigits);
+            if (bufferLength <= destination.Length)
             {
-                char* p = buffer + bufferLength;
-                if (digits <= 1)
+                charsWritten = bufferLength;
+                fixed (char* buffer = &MemoryMarshal.GetReference(destination))
                 {
-                    p = UInt32ToDecChars(p, value);
+                    char* p = buffer + bufferLength;
+                    p = digits > countedDigits ?
+                        UInt32ToDecChars(p, value, digits) :
+                        UInt32ToDecChars(p, value);
+                    Debug.Assert(p == buffer);
                 }
-                else
-                {
-                    p = UInt32ToDecChars(p, value, digits);
-                }
-                Debug.Assert(p == buffer);
+                return true;
             }
-            return true;
+
+            charsWritten = 0;
+            return false;
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
@@ -1850,14 +1956,27 @@ namespace System
             }
             return UInt32ToDecChars(bufferEnd, (uint)value);
 #else
-            do
+            if (value >= 10)
             {
-                ulong remainder;
-                (value, remainder) = Math.DivRem(value, 10);
-                *(--bufferEnd) = (byte)(remainder + '0');
+                // Handle all values >= 100 two-digits at a time so as to avoid expensive integer division operations.
+                while (value >= 100)
+                {
+                    bufferEnd -= 2;
+                    (value, ulong remainder) = Math.DivRem(value, 100);
+                    WriteTwoDigits(bufferEnd, (uint)remainder);
+                }
+
+                // If there are two digits remaining, store them.
+                if (value >= 10)
+                {
+                    bufferEnd -= 2;
+                    WriteTwoDigits(bufferEnd, (uint)value);
+                    return bufferEnd;
+                }
             }
-            while (value != 0);
 
+            // Otherwise, store the single digit remaining.
+            *(--bufferEnd) = (byte)(value + '0');
             return bufferEnd;
 #endif
         }
@@ -1875,12 +1994,22 @@ namespace System
             }
             return UInt32ToDecChars(bufferEnd, (uint)value, digits);
 #else
-            while (--digits >= 0 || value != 0)
+            ulong remainder;
+            while (value >= 100)
+            {
+                bufferEnd -= 2;
+                digits -= 2;
+                (value, remainder) = Math.DivRem(value, 100);
+                WriteTwoDigits(bufferEnd, (uint)remainder);
+            }
+
+            while (value != 0 || digits > 0)
             {
-                ulong remainder;
+                digits--;
                 (value, remainder) = Math.DivRem(value, 10);
                 *(--bufferEnd) = (byte)(remainder + '0');
             }
+
             return bufferEnd;
 #endif
         }
@@ -1897,14 +2026,27 @@ namespace System
             }
             return UInt32ToDecChars(bufferEnd, (uint)value);
 #else
-            do
+            if (value >= 10)
             {
-                ulong remainder;
-                (value, remainder) = Math.DivRem(value, 10);
-                *(--bufferEnd) = (char)(remainder + '0');
+                // Handle all values >= 100 two-digits at a time so as to avoid expensive integer division operations.
+                while (value >= 100)
+                {
+                    bufferEnd -= 2;
+                    (value, ulong remainder) = Math.DivRem(value, 100);
+                    WriteTwoDigits(bufferEnd, (uint)remainder);
+                }
+
+                // If there are two digits remaining, store them.
+                if (value >= 10)
+                {
+                    bufferEnd -= 2;
+                    WriteTwoDigits(bufferEnd, (uint)value);
+                    return bufferEnd;
+                }
             }
-            while (value != 0);
 
+            // Otherwise, store the single digit remaining.
+            *(--bufferEnd) = (char)(value + '0');
             return bufferEnd;
 #endif
         }
@@ -1922,12 +2064,22 @@ namespace System
             }
             return UInt32ToDecChars(bufferEnd, (uint)value, digits);
 #else
-            while (--digits >= 0 || value != 0)
+            ulong remainder;
+            while (value >= 100)
             {
-                ulong remainder;
+                bufferEnd -= 2;
+                digits -= 2;
+                (value, remainder) = Math.DivRem(value, 100);
+                WriteTwoDigits(bufferEnd, (uint)remainder);
+            }
+
+            while (value != 0 || digits > 0)
+            {
+                digits--;
                 (value, remainder) = Math.DivRem(value, 10);
                 *(--bufferEnd) = (char)(remainder + '0');
             }
+
             return bufferEnd;
 #endif
         }
@@ -1968,30 +2120,45 @@ namespace System
             return result;
         }
 
-        private static unsafe bool TryUInt64ToDecStr(ulong value, int digits, Span<char> destination, out int charsWritten)
+        private static unsafe bool TryUInt64ToDecStr(ulong value, Span<char> destination, out int charsWritten)
         {
-            int bufferLength = Math.Max(digits, FormattingHelpers.CountDigits(value));
-            if (bufferLength > destination.Length)
-            {
-                charsWritten = 0;
-                return false;
-            }
-
-            charsWritten = bufferLength;
-            fixed (char* buffer = &MemoryMarshal.GetReference(destination))
+            int bufferLength = FormattingHelpers.CountDigits(value);
+            if (bufferLength <= destination.Length)
             {
-                char* p = buffer + bufferLength;
-                if (digits <= 1)
+                charsWritten = bufferLength;
+                fixed (char* buffer = &MemoryMarshal.GetReference(destination))
                 {
+                    char* p = buffer + bufferLength;
                     p = UInt64ToDecChars(p, value);
+                    Debug.Assert(p == buffer);
                 }
-                else
+                return true;
+            }
+
+            charsWritten = 0;
+            return false;
+        }
+
+        private static unsafe bool TryUInt64ToDecStr(ulong value, int digits, Span<char> destination, out int charsWritten)
+        {
+            int countedDigits = FormattingHelpers.CountDigits(value);
+            int bufferLength = Math.Max(digits, countedDigits);
+            if (bufferLength <= destination.Length)
+            {
+                charsWritten = bufferLength;
+                fixed (char* buffer = &MemoryMarshal.GetReference(destination))
                 {
-                    p = UInt64ToDecChars(p, value, digits);
+                    char* p = buffer + bufferLength;
+                    p = digits > countedDigits ?
+                        UInt64ToDecChars(p, value, digits) :
+                        UInt64ToDecChars(p, value);
+                    Debug.Assert(p == buffer);
                 }
-                Debug.Assert(p == buffer);
+                return true;
             }
-            return true;
+
+            charsWritten = 0;
+            return false;
         }
 
         private static unsafe void Int128ToNumber(Int128 value, ref NumberBuffer number)
@@ -2174,16 +2341,6 @@ namespace System
         }
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        internal static unsafe byte* UInt128ToDecChars(byte* bufferEnd, UInt128 value)
-        {
-            while (value.Upper != 0)
-            {
-                bufferEnd = UInt64ToDecChars(bufferEnd, Int128DivMod1E19(ref value), 19);
-            }
-            return UInt64ToDecChars(bufferEnd, value.Lower);
-        }
-
-        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         internal static unsafe byte* UInt128ToDecChars(byte* bufferEnd, UInt128 value, int digits)
         {
             while (value.Upper != 0)
@@ -2254,28 +2411,24 @@ namespace System
 
         private static unsafe bool TryUInt128ToDecStr(UInt128 value, int digits, Span<char> destination, out int charsWritten)
         {
-            int bufferLength = Math.Max(digits, FormattingHelpers.CountDigits(value));
-            if (bufferLength > destination.Length)
+            int countedDigits = FormattingHelpers.CountDigits(value);
+            int bufferLength = Math.Max(digits, countedDigits);
+            if (bufferLength <= destination.Length)
             {
-                charsWritten = 0;
-                return false;
-            }
-
-            charsWritten = bufferLength;
-            fixed (char* buffer = &MemoryMarshal.GetReference(destination))
-            {
-                char* p = buffer + bufferLength;
-                if (digits <= 1)
+                charsWritten = bufferLength;
+                fixed (char* buffer = &MemoryMarshal.GetReference(destination))
                 {
-                    p = UInt128ToDecChars(p, value);
+                    char* p = buffer + bufferLength;
+                    p = digits > countedDigits ?
+                        UInt128ToDecChars(p, value, digits) :
+                        UInt128ToDecChars(p, value);
+                    Debug.Assert(p == buffer);
                 }
-                else
-                {
-                    p = UInt128ToDecChars(p, value, digits);
-                }
-                Debug.Assert(p == buffer);
+                return true;
             }
-            return true;
+
+            charsWritten = 0;
+            return false;
         }
 
         internal static unsafe char ParseFormatSpecifier(ReadOnlySpan<char> format, out int digits)

diff --git a/src/libraries/System.Private.CoreLib/src/System/UInt64.cs b/src/libraries/System.Private.CoreLib/src/System/UInt64.cs
index 15cff16..9c7467f 100644 (file)
--- a/src/libraries/System.Private.CoreLib/src/System/UInt64.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/UInt64.cs
@@ -99,12 +99,12 @@ namespace System
 
         public override string ToString()
         {
-            return Number.UInt64ToDecStr(m_value, -1);
+            return Number.UInt64ToDecStr(m_value);
         }
 
         public string ToString(IFormatProvider? provider)
         {
-            return Number.UInt64ToDecStr(m_value, -1);
+            return Number.UInt64ToDecStr(m_value);
         }
 
         public string ToString([StringSyntax(StringSyntaxAttribute.NumericFormat)] string? format)