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| author | Denys Vlasenko | 2017-04-10 18:30:35 +0200 |
|---|---|---|
| committer | Denys Vlasenko | 2017-04-10 18:30:35 +0200 |
| commit | cc1f8ba489b809d2c859276ef10094df4bcc74ea (patch) | |
| tree | 2781afa04988916cb29e8e4c310d38c0fdd86d76 | |
| parent | ad5394d591896fc1d025efff2fa6c8c580fb67e9 (diff) | |
| download | busybox-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.c | 22 |
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 (;;) { |