[release/7.0-rc1] Fix for Random failures in System.Numerics.Tests.modpowTest.ModPowAxiom test (#74181)

* Fixed #70330

* Removed pessimistic buffer cleanup

Co-authored-by: sakno <roman.sakno@gmail.com>

Author: github-actions[bot]

src/libraries/System.Runtime.Numerics/src/System/Numerics/BigIntegerCalculator.PowMod.cs

@@ -217,7 +217,11 @@ public static void Pow(ReadOnlySpan<uint> value, uint power,
             Span<uint> valueCopy = (size <= StackAllocThreshold ?
                                    stackalloc uint[StackAllocThreshold]
                                    : valueCopyFromPool = ArrayPool<uint>.Shared.Rent(size)).Slice(0, size);
-            valueCopy.Clear();
+
+            // smallish optimization here:
+            // subsequent operations will copy the elements to the beginning of the buffer,
+            // no need to clear everything
+            valueCopy.Slice(value.Length).Clear();
 
             if (value.Length > modulus.Length)
             {
@@ -262,7 +266,11 @@ public static void Pow(ReadOnlySpan<uint> value, ReadOnlySpan<uint> power,
             Span<uint> valueCopy = (size <= StackAllocThreshold ?
                                    stackalloc uint[StackAllocThreshold]
                                    : valueCopyFromPool = ArrayPool<uint>.Shared.Rent(size)).Slice(0, size);
-            valueCopy.Clear();
+
+            // smallish optimization here:
+            // subsequent operations will copy the elements to the beginning of the buffer,
+            // no need to clear everything
+            valueCopy.Slice(value.Length).Clear();
 
             if (value.Length > modulus.Length)
             {
@@ -464,7 +472,7 @@ private static Span<uint> PowCore(Span<uint> value, int valueLength,
                 power >>= 1;
             }
 
-            return result.Slice(0, resultLength);
+            return result;
         }
 
         private static Span<uint> PowCore(Span<uint> value, int valueLength,

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

@@ -274,6 +274,18 @@ public static void ModPowAxiom()
             }
         }
 
+        [Fact]
+        public static void RegressionIssue70330()
+        {
+            byte[] tempByteArray1 = { 226, 32 };
+            byte[] tempByteArray2 = { 113 };
+            byte[] tempByteArray3 = { 15, 8, 201, 158, 96, 200, 233, 243, 184, 0, 33, 203, 210, 80, 174, 198, 244, 177, 223, 221, 168, 243, 233, 133, 103, 252, 219, 195, 187, 227, 215, 54, 66, 248, 37, 186, 232, 45, 227, 147, 100, 14, 121, 244, 56, 89, 181, 120, 205, 4, 59, 48, 65, 239, 221, 28, 30, 68, 55, 99, 237, 38, 56, 213, 40, 234, 136, 218, 42, 244, 222, 198, 205 };
+            VerifyIdentityString(
+                Print(tempByteArray3) + Print(tempByteArray2) + Print(tempByteArray1) + "tModPow",
+                Print(tempByteArray3) + Print(tempByteArray2) + Print(tempByteArray1) + "bPow" + " bRemainder"
+            );
+        }
+
         [Fact]
         public static void ModPowBoundary()
         {