From: Paul Eggert <eggert@cs.ucla.edu>
Date: Thu, 26 Sep 2024 00:55:00 +0000 (-0700)
Subject: factor: tweak prime_p zero counting
X-Git-Tag: v9.6~159
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=2da88ed1795201384296b105ee70c3c0f1b3ed55;p=thirdparty%2Fcoreutils.git

factor: tweak prime_p zero counting

* src/factor.c (prime_p): Use stdc_trailing_zero here too.
This doesn’t make much of a performance difference but
we might as well be consistent.
---

diff --git a/src/factor.c b/src/factor.c
index 081bb0d57c..80757c0837 100644
--- a/src/factor.c
+++ b/src/factor.c
@@ -1166,7 +1166,6 @@ mp_millerrabin (mpz_srcptr n, mpz_srcptr nm1, mpz_ptr x, mpz_ptr y,
 static bool ATTRIBUTE_PURE
 prime_p (uintmax_t n)
 {
-  mp_bitcnt_t k;
   bool is_prime;
   uintmax_t a_prim, one, ni;
   struct factors factors;
@@ -1179,9 +1178,8 @@ prime_p (uintmax_t n)
     return true;
 
   /* Precomputation for Miller-Rabin.  */
-  uintmax_t q = n - 1;
-  for (k = 0; (q & 1) == 0; k++)
-    q >>= 1;
+  int k = stdc_trailing_zeros (n - 1);
+  uintmax_t q = (n - 1) >> k;
 
   uintmax_t a = 2;
   binv (ni, n);                 /* ni <- 1/n mod B */