Messages in this thread | | | Date | Thu, 1 May 2003 12:22:30 +0200 | From | Willy TARREAU <> | Subject | Re: [RFC][PATCH] Faster generic_fls |
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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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