On Thu, May 01, 2003 at 01:16:05PM +0800, hugang wrote:
> Here is table version of the fls. Yes it fast than other.
Sorry, but this code returns wrong results. Test it with less
iterations, it doesn't return the same result.
> unsigned i = 0;
>
> do {
> i++;
> } while (n <= fls_table[i].max && n > fls_table[i].min);
You never even compare with table[0] !
> --test log is here----
I recoded a table based on your idea, and it's clearly faster than others, and
even faster than yours :
============
static unsigned tbl[33] = { 2147483648, 1073741824, 536870912, 268435456,
134217728, 67108864, 33554432, 16777216,
8388608, 4194304, 2097152, 1048576,
524288, 262144, 131072, 65536,
32768, 16384, 8192, 4096,
2048, 1024, 512, 256,
128, 64, 32, 16,
8, 4, 2, 1, 0 };
static inline int fls_tbl_fls(unsigned n)
{
unsigned i = 0;
while (n < tbl[i])
i++;
return 32 - i;
}
===========
it returns the right result. Compiled with gcc 2.95.3 -march=i686 -O3, athlon
1.533 GHz (1800+) :
4294967295 iterations of new... checksum = 4294967265
real 1m6.182s
user 1m6.060s
sys 0m0.133s
4294967295 iterations of new2... checksum = 4294967265
real 0m36.504s
user 0m36.294s
sys 0m0.202s
4294967295 iterations of fls_table... checksum = 4294967295
real 0m21.962s
user 0m21.833s
sys 0m0.124s
4294967295 iterations of fls_tbl... checksum = 4294967265
real 0m19.268s
user 0m19.102s
sys 0m0.168s
The same compiled with gcc-3.2.3 :
4294967295 iterations of fls_table... checksum = 4294967295
real 0m14.211s
user 0m14.210s
sys 0m0.000s
Cheers,
Willy
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This archive was generated by hypermail 2b29 : Wed May 07 2003 - 22:00:12 EST