cosmopolitan/third_party/compiler_rt/udivmodti4.c

#if 0
/*─────────────────────────────────────────────────────────────────╗
│ To the extent possible under law, Justine Tunney has waived      │
│ all copyright and related or neighboring rights to division,     │
│ as it is written in the following disclaimers:                   │
│   • http://unlicense.org/                                        │
│   • http://creativecommons.org/publicdomain/zero/1.0/            │
╚─────────────────────────────────────────────────────────────────*/
#endif
#include "third_party/compiler_rt/int_lib.h"

/**
 * Returns 128 bit division result by 64 bit.
 *
 * Result must fit in 64 bits. Remainder is stored in r.
 *
 * @see libdivide libdivide_128_div_64_to_64() division fallback
 * @see Knuth, Volume 2, section 4.3.1, Algorithm D for correctness proof
 * @see https://danlark.org/2020/06/14/128-bit-division/
 */
forceinline du_int udiv128by64to64default(du_int u1, du_int u0, du_int v,
                                          du_int *r) {
  const unsigned n_udword_bits = sizeof(du_int) * CHAR_BIT;
  const du_int b = 1ULL << (n_udword_bits / 2);  // Number base (32 bits)
  du_int un1, un0;                               // Norm. dividend LSD's
  du_int vn1, vn0;                               // Norm. divisor digits
  du_int q1, q0;                                 // Quotient digits
  du_int un64, un21, un10;                       // Dividend digit pairs
  du_int rhat;                                   // Remainder
  si_int s;                                      // Normalization shift
  s = __builtin_clzll(v);
  if (s > 0) {
    // Normalize the divisor.
    v = v << s;
    un64 = (u1 << s) | (u0 >> (n_udword_bits - s));
    un10 = u0 << s;  // Shift dividend left
  } else {
    // Avoid undefined behavior of (u0 >> 64).
    un64 = u1;
    un10 = u0;
  }
  // Break divisor up into two 32-bit digits.
  vn1 = v >> (n_udword_bits / 2);
  vn0 = v & 0xFFFFFFFF;
  // Break right half of dividend into two digits.
  un1 = un10 >> (n_udword_bits / 2);
  un0 = un10 & 0xFFFFFFFF;
  // Compute the first quotient digit, q1.
  q1 = un64 / vn1;
  rhat = un64 - q1 * vn1;
  // q1 has at most error 2. No more than 2 iterations.
  while (q1 >= b || q1 * vn0 > b * rhat + un1) {
    q1 = q1 - 1;
    rhat = rhat + vn1;
    if (rhat >= b) break;
  }
  un21 = un64 * b + un1 - q1 * v;
  // Compute the second quotient digit.
  q0 = un21 / vn1;
  rhat = un21 - q0 * vn1;
  // q0 has at most error 2. No more than 2 iterations.
  while (q0 >= b || q0 * vn0 > b * rhat + un0) {
    q0 = q0 - 1;
    rhat = rhat + vn1;
    if (rhat >= b) break;
  }
  *r = (un21 * b + un0 - q0 * v) >> s;
  return q1 * b + q0;
}

forceinline du_int udiv128by64to64(du_int u1, du_int u0, du_int v, du_int *r) {
#ifdef __x86_64__
  du_int result;
  asm("div\t%2" : "=a"(result), "=d"(*r) : "r"(v), "a"(u0), "d"(u1) : "cc");
  return result;
#else
  return udiv128by64to64default(u1, u0, v, r);
#endif
}

/**
 * Performs 128-bit unsigned division and remainder.
 *
 * @param a is dividend
 * @param b is divisor
 * @param rem receives remainder if not NULL
 */
COMPILER_RT_ABI tu_int __udivmodti4(tu_int a, tu_int b, tu_int *rem) {
  const unsigned n_utword_bits = sizeof(tu_int) * CHAR_BIT;
  utwords dividend, divisor, quotient, remainder;
  si_int shift;
  dividend.all = a;
  divisor.all = b;
  if (divisor.all > dividend.all) {
    if (rem) *rem = dividend.all;
    return 0;
  }
  // When the divisor fits in 64 bits, we can use an optimized path.
  if (divisor.s.high == 0) {
    remainder.s.high = 0;
    if (dividend.s.high < divisor.s.low) {
      // The result fits in 64 bits.
      quotient.s.low = udiv128by64to64(dividend.s.high, dividend.s.low,
                                       divisor.s.low, &remainder.s.low);
      quotient.s.high = 0;
    } else {
      // First, divide with the high part to get the remainder in
      // dividend.s.high. After that dividend.s.high < divisor.s.low.
      quotient.s.high = dividend.s.high / divisor.s.low;
      dividend.s.high = dividend.s.high % divisor.s.low;
      quotient.s.low = udiv128by64to64(dividend.s.high, dividend.s.low,
                                       divisor.s.low, &remainder.s.low);
    }
    if (rem) *rem = remainder.all;
    return quotient.all;
  }
  // 0 <= shift <= 63.
  shift = __builtin_clzll(divisor.s.high) - __builtin_clzll(dividend.s.high);
  divisor.all <<= shift;
  quotient.s.high = 0;
  quotient.s.low = 0;
  for (; shift >= 0; --shift) {
    quotient.s.low <<= 1;
    // Branch free version of.
    // if (dividend.all >= divisor.all)
    // {
    //    dividend.all -= divisor.all;
    //    carry = 1;
    // }
    ti_int s = (ti_int)(divisor.all - dividend.all - 1) >> (n_utword_bits - 1);
    quotient.s.low |= s & 1;
    dividend.all -= divisor.all & s;
    divisor.all >>= 1;
  }
  if (rem) *rem = dividend.all;
  return quotient.all;
}