是否存在像编译器那样优化整数除法的CPU？

优化编译器通常会把非2的幂次常量除法转换为倒数乘法，以此规避耗时的div指令。有人好奇：有没有CPU能在运行时做这类优化——把常用除数对应的「魔法常数」缓存到内部表中，要是后续碰到相同除数，就用缓存的常数通过乘法来计算除法？

为了验证这点，有人分别在x64（基于C++）和M1（基于Rust）平台开展实验：对比重复除以固定除数与除数动态变化两种场景下的除法性能。结果显示二者性能相近，由此推测这些CPU并不支持该运行时优化，但目前未找到相关公开官方资料佐证。

Rust实验代码 #

type div32 = i32;

#[inline(never)]
fn div_bench_fn(dividend: div32, mut divisor: div32, mut step: div32) -> f64 {
    step = std::hint::black_box(step);
    const LOOP_COUNT: u32 = 100_000_000;
    let start = std::time::Instant::now();
    let mut tmp = 0;
    for _ in 0..LOOP_COUNT {
        let res = dividend % divisor;
        divisor += step;
        tmp ^= res;
        std::hint::black_box(tmp);
    }
    let elapsed = start.elapsed().as_secs_f64();
    let div_sec = (LOOP_COUNT as f64) / elapsed;
    div_sec / 1_000_000.
}

fn main() {
    for _ in 0..10 {
        println!("--");
        println!("  static {:.2}m div/sec", div_bench_fn(0x12345678, 127, 0));
        println!("  dynamic {:.2}m div/sec", div_bench_fn(0x12345678, 127, 1));
    }
}

ARMv8.5-A循环汇编代码 #

divbench[0x1000017e8] <+64>:  cbz    w9, 0x10000185c           ; <+180> // panic if divisor == 0
divbench[0x1000017ec] <+68>:  sdiv   w13, w11, w9
divbench[0x1000017f0] <+72>:  msub   w13, w13, w9, w11
divbench[0x1000017f4] <+76>:  add    w9, w9, w19
divbench[0x1000017f8] <+80>:  eor    w8, w13, w8
divbench[0x1000017fc] <+84>:  str    w8, [sp, #0xc]
divbench[0x100001800] <+88>:  subs   w10, w10, #0x1
divbench[0x100001804] <+92>:  b.ne   0x1000017e8               ; <+64>

实验结果（数值越高性能越好） #

--
  static 1018.66m div/sec
  dynamic 1568.52m div/sec
--
  static 1574.76m div/sec
  dynamic 1574.63m div/sec
--
  static 1575.77m div/sec
  dynamic 1574.38m div/sec
--
  static 1577.74m div/sec
  dynamic 1581.09m div/sec
--
  static 1585.35m div/sec
  dynamic 1591.46m div/sec
--
  static 1572.47m div/sec
  dynamic 1585.13m div/sec
--
  static 1560.05m div/sec
  dynamic 1571.18m div/sec
--
  static 1575.05m div/sec
  dynamic 1550.98m div/sec
--
  static 1556.27m div/sec
  dynamic 1571.85m div/sec
--
  static 1581.93m div/sec
  dynamic 1579.43m div/sec

内容的提问来源于stack exchange，提问作者Iwa