factor: don't be too clever in isqrt - be small instead

function                                             old     new   delta
isqrt_odd                                            111      70     -41

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 12 insertions, 10 deletions

diff --git a/coreutils/factor.c b/coreutils/factor.c
index 7400174a7..11097c12d 100644
--- a/coreutils/factor.c
+++ b/coreutils/factor.c
@@ -47,25 +47,27 @@ typedef unsigned long half_t;
 /* Returns such x that x+1 > sqrt(N) */
 static inline half_t isqrt(wide_t N)
 {
-        wide_t mask_2bits;
         half_t x;
 
-// Never called with N < 1
-//      if (N == 0)
-//              return 0;
+        // Never called with N < 1
+        //if (N == 0)
+        //      return 0;
 
+        x = HALF_MAX;
         /* First approximation of x+1 > sqrt(N) - all-ones, half as many bits:
          * 1xxxxx -> 111 (six bits to three)
          * 01xxxx -> 111
          * 001xxx -> 011
          * 0001xx -> 011 and so on.
          */
-        x = HALF_MAX;
-        mask_2bits = TOPMOST_WIDE_BIT | (TOPMOST_WIDE_BIT >> 1);
-        while (!(N & mask_2bits)) {
-                x >>= 1;
-                mask_2bits >>= 2;
-        }
+        // It is actually not performance-critical at all.
+        // Can simply start Newton loop with very conservative x=0xffffffff.
+        //wide_t mask_2bits;
+        //mask_2bits = TOPMOST_WIDE_BIT | (TOPMOST_WIDE_BIT >> 1);
+        //while (!(N & mask_2bits)) {
+        //      x >>= 1;
+        //      mask_2bits >>= 2;
+        //}
         dbg("x:%"HALF_FMT"x", x);
 
         for (;;) {