Demo that RyuJIT eliminates unreachable "typeof(T)==typeof(…)" branches in generic instantiations of a method

README.md

What's this?

Found an interesting comment in SqlDataReader's source code:

// this block of type specific shortcuts uses RyuJIT jit behaviours to achieve fast implementations of the primitive types
// RyuJIT will be able to determine at compilation time that the typeof(T)==typeof(<primitive>) options are constant
// and be able to remove all implementations which cannot be reached. this will eliminate non-specialized code for 

This test carried out in SharpLab proves it to be true!

test.asm

; Core CLR v4.700.19.57202 (coreclr.dll) on x86.

C..ctor()
    L0000: ret

C.M[[System.Int32, System.Private.CoreLib]](Int32, Int32, Int32)
    L0000: mov eax, edx
    L0002: ret 0x8

C.M[[System.Int16, System.Private.CoreLib]](Int16, Int16, Int16)
    L0000: mov eax, [esp+0x8]
    L0004: movsx eax, ax
    L0007: ret 0x8

C.M[[System.Byte, System.Private.CoreLib]](Byte, Byte, Byte)
    L0000: mov eax, [esp+0x4]
    L0004: movzx eax, al
    L0007: ret 0x8

test.cs

using SharpLab.Runtime;
public class C {
    [JitGeneric(typeof(int)),
     JitGeneric(typeof(short)),
     JitGeneric(typeof(byte))]
    public T M<T>(T a, T b, T c) {
        if (typeof(T) == typeof(int)) {
            return (T)(object)a;
        }
        if (typeof(T) == typeof(short)) {
            return (T)(object)b;
        }
        return c;
    }
}