What is the meaning of -2 in this IL instruction?

Question

I found a simple IL code :

 long x = 0; for(long i = 0;i< int.MaxValue * 2L; i++) { x = i; } Console.WriteLine(x); 

I create this code in Release mode and the IL code generated:

 .method private hidebysig static void Main(string[] args) cil managed { .entrypoint // Code size 28 (0x1c) .maxstack 2 .locals init ([0] int64 x, [1] int64 i) IL_0000: ldc.i4.0 IL_0001: conv.i8 IL_0002: stloc.0 IL_0003: ldc.i4.0 IL_0004: conv.i8 IL_0005: stloc.1 IL_0006: br.s IL_000f IL_0008: ldloc.1 IL_0009: stloc.0 IL_000a: ldloc.1 IL_000b: ldc.i4.1 IL_000c: conv.i8 IL_000d: add IL_000e: stloc.1 IL_000f: ldloc.1 IL_0010: ldc.i4.s -2 IL_0012: conv.u8 IL_0013: blt.s IL_0008 IL_0015: ldloc.0 IL_0016: call void [mscorlib]System.Console::WriteLine(int64) IL_001b: ret } // end of method Program::Main 

I almost completely understand all the inscriptions, except for this:

  IL_0010: ldc.i4.s -2 

Now this add-in should push int.MaxValue * 2L onto the stack, and then blt.s will compare it with i , if i less than the value, go back to IL_0008 . But, t find out why it loads -2 ? If I changed the loop as shown below:

 for(long i = 0;i < int.MaxValue * 3L; i++) { x = i; } 

It loads the expected value:

 IL_0010: ldc.i8 0x17ffffffd 

What is the meaning of -2 in this code?

Answer 1

int.MaxValue * 2L is a 64-bit number that still fits in 32-bit ( 4,294,967,294 or 0xFFFFFFFE ). So, the compiler loads 0xFFFFFFFE (which is -2 when interpreting Int32 ), and then expands it to an unsigned 64-bit value.

The reason she used the signed form is because the number, when interpreted as the sign value of -2 , fits into one signed byte ( -128 to 127 ), which means that the compiler was able to emit the short form ldc.i4.s opcode to load a 32-bit value from one byte. It took only 2 bytes and 1 more byte to load a 32-bit signed integer to convert it to a 64-bit value - this is much better than using a 64-bit load command followed by a full 8-byte unsigned integer.

Answer 2

It seems that the compiler uses bitwise math to its advantage. It so happened that the value of Two Complement is equal to the value of unsigned integers (int.MaxValue * 2L)

In a beaten view:

 - 1111 1111 1111 1111 1111 1111 1111 1110 (int) - 1111 1111 1111 1111 1111 1111 1111 1110 (uint) - 0000 0000 0000 0000 0000 0000 0000 0000 1111 1111 1111 1111 1111 1111 1111 1110 (long