- 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 [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
|
|