It's Log, Log, Log

Reader Bernie submits for our approval this wonderful C# log base 2 implementation. He says: “We can distinguish two halves in that code. It looks like it was originally written for an unsigned 16-bit int, and later on extended for signed 32-bit integers.”
This code reminds me of the change-sorting machine I have in my closet, which sorts change based on weight. That thing has no error handling if you try to sort, say, a huge boulder through it; it'd just fall apart. At least here, we have the assurance that the universe's biggest numbers all have a log base 2 of 31.
public static int Log2(int x)
                {
                        if (x <= 65536)
                        {
                                if (x <= 256)
                                {
                                        if (x <= 16)
                                        {
                                                if (x <= 4)
                                                {
                                                        if (x <= 2)
                                                        {
                                                                if (x <= 1)
                                                                        return 0;
                                                                return 1;
                                                        }
 
                                                        return 2;
                                                }
 
                                                if (x <= 8)
                                                        return 3;
                                                return 4;
                                        }
 
                                        if (x <= 64)
                                        {
                                                if (x <= 32)
                                                        return 5;
                                                return 6;
                                        }
 
                                        if (x <= 128)
                                                return 7;
                                        return 8;
                                }
 
                                if (x <= 4096)
                                {
                                        if (x <= 1024)
                                        {
                                                if (x <= 512)
                                                        return 9;
                                                return 10;
                                        }
 
                                        if (x <= 2048)
                                                return 11;
                                        return 12;
                                }
 
                                if (x <= 16384)
                                {
                                        if (x <= 8192)
                                                return 13;
                                        return 14;
                                }
 
                                if (x <= 32768)
                                        return 15;
                                return 16;
                        }
 
                        if (x <= 16777216)
                        {
                                if (x <= 1048576)
                                {
                                        if (x <= 262144)
                                        {
                                                if (x <= 131072)
                                                        return 17;
                                                return 18;
                                        }
 
                                        if (x <= 524288)
                                                return 19;
                                        return 20;
                                }
 
                                if (x <= 4194304)
                                {
                                        if (x <= 2097152)
                                                return 21;
                                        return 22;
                                }
 
                                if (x <= 8388608)
                                        return 23;
                                return 24;
                        }
 
                        if (x <= 268435456)
                        {
                                if (x <= 67108864)
                                {
                                        if (x <= 33554432)
                                                return 25;
                                        return 26;
                                }
 
                                if (x <= 134217728)
                                        return 27;
                                return 28;
                        }
 
                        if (x <= 1073741824)
                        {
                                if (x <= 536870912)
                                        return 29;
                                return 30;
                        }
 
                        return 31;
                }
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