Avoid an allocation in BigInteger.Parse("-2147483648") (#104666)

Co-authored-by: Dan Moseley <[email protected]>

Author: kzrnm

src/libraries/System.Runtime.Numerics/src/System/Numerics/BigInteger.cs

@@ -511,16 +511,23 @@ internal BigInteger(ReadOnlySpan<uint> value, bool negative)
             {
                 this = default;
             }
-            else if (value.Length == 1 && value[0] < kuMaskHighBit)
+            else if (value.Length == 1)
             {
-                // Values like (Int32.MaxValue+1) are stored as "0x80000000" and as such cannot be packed into _sign
-                _sign = negative ? -(int)value[0] : (int)value[0];
-                _bits = null;
-                if (_sign == int.MinValue)
+                if (value[0] < kuMaskHighBit)
+                {
+                    _sign = negative ? -(int)value[0] : (int)value[0];
+                    _bits = null;
+                }
+                else if (negative && value[0] == kuMaskHighBit)
                 {
                     // Although Int32.MinValue fits in _sign, we represent this case differently for negate
                     this = s_bnMinInt;
                 }
+                else
+                {
+                    _sign = negative ? -1 : +1;
+                    _bits = [value[0]];
+                }
             }
             else
             {

src/libraries/System.Runtime.Numerics/tests/BigInteger/parse.cs

@@ -233,10 +233,23 @@ private static void VerifyDefaultParse(Random random)
             // BasicTests
             VerifyFailParseToString(null, typeof(ArgumentNullException));
             VerifyFailParseToString(string.Empty, typeof(FormatException));
-            VerifyParseToString("0");
-            VerifyParseToString("000");
-            VerifyParseToString("1");
-            VerifyParseToString("001");
+
+            foreach (var value in new string[]
+            {
+                "0",
+                "000",
+                "1",
+                "001",
+                int.MaxValue.ToString(),
+                int.MinValue.ToString(),
+                long.MaxValue.ToString(),
+                long.MinValue.ToString(),
+                Int128.MaxValue.ToString(),
+                Int128.MinValue.ToString(),
+            })
+            {
+                VerifyParseToString(value);
+            }
 
             // SimpleNumbers - Small
             for (int i = 0; i < s_samples; i++)