3 ms·

Limiting the registers to be specific pairs is awful if you're implementing a register allocator.

• pjc50 6y ago
  Is it? These are stack management instructions; you know in advance from the platform ABI which registers are "scratch" and which are "save", so if you allocate any of the "save" registers in your function you emit the corresponding push/pop pairs in the start and end of the function. At worst you push/pop a register you don't need to - but in ARM64, the stack has to be 128-bit aligned, so you have to push/pop pairs.
  • pm215 6y ago
    Then your allocator would need to know that if it's already decided to use one register of a pair that the other half of the pair gets the save/restore for 'free' and is now better than using a different callee-saves register. I suspect that unless your allocator was designed from the start to be able to deal with that kind of "my choice of register here affects costs and thus my decisions about allocation for a completely different value over there" it's not going to be able to do a great job under that kind of constraint.
    • brandmeyer 6y ago
      Callee-saved registers are saved on function entry, all at once. There is no interaction other than the register allocation step choosing how many values need to be preserved across function calls.
  • roca 6y ago
    OK, but that only works for prologue/epilogue --- it's better if you can use the instructions elsewhere too.