Messages in this thread | ![/](/images/icornerl.gif) | | Date | 21 Sep 2005 07:49:31 -0400 | From | linux@horizon ... | Subject | Re: [PATCH] NTP shift_right cleanup (v. A3) |
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- For fixed shifts, you can just write it as a divide; GCC will DTRT. For interest's sake, GCC 4.0 generates, for x /= 64: testl %eax, %eax jns .L2 addl $63, %eax .L2: sarl $6, %eax
- If you want to be more verbose with the explanation, try something like: (Public domain, copyright abandoned, use freely, yadda yadda.) /* * NTP uses power-of-two divides a lot for speed, but it wants to use * negative numbers. * 1) ANSI C does not guarantee signed right shifts (but GCC does) * 2) Such a shift is like a divide that rounds to -infinity. * NTP wants rounding to zero, i.e. -3/2 = -2, while -3>>1 = -2. */
Interestingly, _Hacker's Delight_ chapter 10 skips over this particular case, signed division by a variable power of two. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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