4 ms·

I'm not sure encodability can usefully be considered to define x86's type system. rax can be used in places r8 can't.

• saagarjha 8y ago
  As in, there are certain instructions that operate on rax but not r8? Maybe a more complex type system in this case where rax and r8 are specialized quadwords and certain instructions accept more specific arguments?
  • firethief 8y ago
    Every register besides r8-r15 has special functions, and different registers support different addressing operations. There are at most two pairs of equivalent registers: r8/r10 and r9/r11 (i.e. I don't know of any operation in which r8 couldn't be swapped with r10, and likewise for r9/r11). All other registers are unique (i.e., for any pair of them, there exist valid assembly programs that would become invalid if you swapped usage of the registers in that pair throughout the program). So you can define a type system isomorphic to a lot of things, but this seems to me a degenerate case. What's the point of considering it a type system when no two values can be of the same type, almost all operations are polymorphic, almost all operations are on multiple types, and almost every instruction involves a conversion between types?