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authorDenys Vlasenko2017-04-10 18:30:35 +0200
committerDenys Vlasenko2017-04-10 18:30:35 +0200
commitcc1f8ba489b809d2c859276ef10094df4bcc74ea (patch)
tree2781afa04988916cb29e8e4c310d38c0fdd86d76
parentad5394d591896fc1d025efff2fa6c8c580fb67e9 (diff)
downloadbusybox-cc1f8ba489b809d2c859276ef10094df4bcc74ea.zip
busybox-cc1f8ba489b809d2c859276ef10094df4bcc74ea.tar.gz
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>
-rw-r--r--coreutils/factor.c22
1 files changed, 12 insertions, 10 deletions
diff --git a/coreutils/factor.c b/coreutils/factor.c
index 7400174..11097c1 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 (;;) {