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use super::unwindregs::Reg;
use arrayvec::ArrayVec;
const ENCODE_REGISTERS: [Reg; 8] = [
Reg::RBX,
Reg::RBP,
Reg::RDI,
Reg::RSI,
Reg::R12,
Reg::R13,
Reg::R14,
Reg::R15,
];
pub fn decode(count: u8, encoded_ordering: u16) -> ArrayVec<Reg, 8> {
let mut regs: ArrayVec<Reg, 8> = ENCODE_REGISTERS.into();
let mut r = encoded_ordering;
let mut n: u16 = 8;
while r != 0 {
let index = r % n;
if index != 0 {
regs[(8 - n as usize)..].swap(index as usize, 0);
}
r /= n;
n -= 1;
}
regs.truncate(count as usize);
regs
}
pub fn encode(registers: &[Reg]) -> Option<(u8, u16)> {
if registers.len() > ENCODE_REGISTERS.len() {
return None;
}
let count = registers.len() as u8;
let mut r: u16 = 0;
let mut reg_order: ArrayVec<Reg, 8> = ENCODE_REGISTERS.into();
let mut scale: u16 = 1;
for (i, reg) in registers.iter().enumerate() {
let index = reg_order[i..].iter().position(|r| r == reg)?;
if index as u16 != 0 {
reg_order[i..].swap(index, 0);
}
r += index as u16 * scale;
scale *= 8 - i as u16;
}
Some((count, r))
}
#[cfg(test)]
mod test {
use super::*;
#[test]
fn unhandled_orderings() {
use super::Reg::*;
assert_eq!(encode(&[RAX]), None, "RAX is a volatile register, i.e. not a callee-save register, so it does not need to be restored during epilogs and is not covered by the encoding.");
assert_eq!(encode(&[RSI, RSI]), None, "Valid register orderings only contain each register (at most) once, so there is no encoding for a sequence with repeated registers.");
}
#[test]
fn roundtrip_all() {
// Test all possible register orderings.
// That is, for all permutations of length 0 to 8 of the ENCODE_REGISTERS array, check that
// the register ordering rountrips successfully through encoding and decoding.
use itertools::Itertools;
for permutation in (0..=8).flat_map(|k| ENCODE_REGISTERS.iter().cloned().permutations(k)) {
let permutation = permutation.as_slice();
let encoding = encode(permutation);
if let Some((count, encoded)) = encoding {
assert_eq!(
decode(count, encoded).as_slice(),
permutation,
"Register permutation should roundtrip correctly",
);
} else {
panic!("Register permutation failed to encode: {permutation:?}");
}
}
}
}