compute median without subtract

Author: nietras

src/System.Memory/src/System/SpanHelpers.BinarySearch.cs

@@ -34,8 +34,9 @@ internal static int BinarySearch<T, TComparable>(
                 // TODO: Test/investigate if below is faster (if it gives better asm), perhaps via Unsafe.As to avoid unnecessary conversions
                 //       This is safe since we know span.Length < int.MaxValue, and indeces are >= 0
                 //       and thus cannot overflow an uint. Saves on subtraction per loop.
-                // int i = (int)(((uint)hi + (uint)lo) >> 1)
-                int i = lo + ((hi - lo) >> 1);
+                int i = (int)(((uint)hi + (uint)lo) >> 1);
+                // Below was intended to avoid overflows, but this cannot happen if we do the computation in uint
+                //int i = lo + ((hi - lo) >> 1);
 
                 // TODO: We probably need to add `ref readonly`/`in` overloads or `AddReadOnly`to unsafe, 
                 //       if this will be available in language

src/System.Memory/tests/ReadOnlySpan/BinarySearch.cs

@@ -74,8 +74,7 @@ public unsafe static void BinarySearch_MaxLength_NoOverflow()
             {
                 // Allocate max size span memory
                 var length = int.MaxValue;
-                IntPtr memory;
-                if (!AllocationHelper.TryAllocNative(new IntPtr(length), out memory))
+                if (!AllocationHelper.TryAllocNative(new IntPtr(length), out IntPtr memory))
                 {
                     Console.WriteLine($"Span.BinarySearch test {nameof(BinarySearch_MaxLength_NoOverflow)} skipped (could not alloc memory).");
                     return;