Unnecessary extra and instruction in uint % 256

[stephentoub]

Consider:

using System.Runtime.CompilerServices;

class Program
{
    static void Main()
    {
        Mod(123);
        Cast(123);
        And(123);
    }

    [MethodImpl(MethodImplOptions.NoInlining)] static byte Cast(uint i) => (byte)(i);
    [MethodImpl(MethodImplOptions.NoInlining)] static byte And(uint i) => (byte)(i & 0xFF);
    [MethodImpl(MethodImplOptions.NoInlining)] static byte Mod(uint i) => (byte)(i % 256);
}

For Cast and And I get:

       0FB6C1               movzx    rax, cl
       C3                   ret

but for Mod I get:

       81E1FF000000         and      ecx, 255
       0FB6C1               movzx    rax, cl
       C3                   ret

Is there a reason that extra and instruction needs to exist?

category:cq
theme:basic-cq
skill-level:intermediate
cost:medium

[mikedn]

Is there a reason that extra and instruction needs to exist?

Nope.

The JIT likely detects the and+cast combination early but not mod+cast. And later mod is transformed into and but it's too late to detect the and+cast combination now.