llAbs(-2147483648) raises this:
System.OverflowException: Value is too small.
at System.Math.Abs (Int32 value) [0x00000] in <filename unknown>:0
at LindenLab.SecondLife.Library.llAbs (Int32 i) [0x00000] in <filename unknown>:0
So it's actually not computable. In LSO it returns -2147483648, though.