Yukihiro "Matz" Matsumoto 8e91554c6d mruby-bigint: rewrite mpz_gcd main loop as binary Stein algorithm ...

Replace the classical Euclidean main loop (mpz_mod per iteration)
with Stein's binary GCD: subtract + factor out trailing 2s on
odd-maintained operands. Keep an mpz_mod fallback for heavily
unbalanced pairs (the smaller operand has at least two fewer limbs)
where one long division replaces many Stein subtracts.

The old loop allocated a temporary mpz_t every iteration to hold the
mod result; the Stein loop is allocation-free thanks to the in-place
paths in mpz_sub and mpz_div_2exp. This matches the "memory first"
priority and also happens to be faster on typical inputs because
mpz_mod's setup cost dominates when the quotient is small (the
classical Fibonacci-neighbor worst case).

Also add a small mpz_swap helper used by the new loop.

Benchmark (bin/mruby benchmark/bm_bigint_gcd.rb, median of 3):

  case                       before    after    ratio
  single-limb                 42 ms    50 ms    1.19  (fast-path
                                                      unchanged;
                                                      noise)
  fib(200) vs fib(201)        82 ms    23 ms    0.28
  balanced ~700-bit shared    46 ms    32 ms    0.70
  unbalanced big vs small     35 ms    28 ms    0.80
  power-of-2 path             38 ms    42 ms    1.11  (fast-path
                                                      unchanged;
                                                      noise)
  balanced ~2800-bit shared   19 ms    15 ms    0.79

Co-authored-by: Claude <noreply@anthropic.com>

2026-04-23 22:52:05 +09:00

..

bigint.c

mruby-bigint: rewrite mpz_gcd main loop as binary Stein algorithm

2026-04-23 22:52:05 +09:00

bigint.h

mruby-bigint: always use 32-bit limbs by default

2026-01-14 10:12:23 +09:00