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SubjectRe: [RFC][PATCH] Faster generic_fls
On Wed, 30 Apr 2003 13:55:12 -0700
Andrew Morton <akpm@digeo.com> wrote:

> Daniel Phillips <dphillips@sistina.com> wrote:
> >
> > +static inline unsigned fls8(unsigned n)
> > +{
> > + return n & 0xf0?
> > + n & 0xc0? (n >> 7) + 7: (n >> 5) + 5:
> > + n & 0x0c? (n >> 3) + 3: n - ((n + 1) >> 2);
> > +}
>
> return fls_table[n];
>
>
> That'll be faster in benchmarks, possibly slower in real world. As usual.
>
Here is table version of the fls. Yes it fast than other.
typedef struct fls_table_t {
unsigned min;
unsigned max;
unsigned value;
} fls_table_t;
static fls_table_t fls_table[] = {
{0, 1, 1},
{1, 2, 2},
{2, 4, 3},
{4, 8, 4},
{8, 16, 5},
{16, 32, 6},
{32, 64, 7},
{64, 128, 8},
{128, 256, 9},
{256, 512, 10},
{512, 1024, 11},
{1024, 2048, 12},
{2048, 4096, 13},
{4096, 8192, 14},
{8192, 16384, 15},
{16384, 32768, 16},
{32768, 65536, 17},
{65536, 131072, 18},
{131072, 262144, 19},
{262144, 524288, 20},
{524288, 1048576, 21},
{1048576, 2097152, 22},
{2097152, 4194304, 23},
{4194304, 8388608, 24},
{8388608, 16777216, 25},
{16777216, 33554432, 26},
{33554432, 67108864, 27},
{67108864, 134217728, 28},
{134217728, 268435456, 29},
{268435456, 536870912, 30},
{536870912, 1073741824, 31},
{1073741824, 2147483648, 32},
{0, -1, 32},
};
static inline int fls_table_fls(unsigned n)
{
unsigned i = 0;
do {
i++;
} while (n <= fls_table[i].max && n > fls_table[i].min);
return fls_table[i].value;
}
--test log is here----
model name : Intel(R) Pentium(R) 4 Mobile CPU 1.60GHz
Reading specs from /usr/lib/gcc-lib/i386-linux/2.95.4/specs
gcc version 2.95.4 20011002 (Debian prerelease)
==== -march=i686 -O3 ====
test_fls.c:110: warning: decimal constant is so large that it is unsigned
4294967295 iterations of nil... checksum = 4294967295
real 0m22.311s
user 0m21.850s
sys 0m0.010s
4294967295 iterations of old... checksum = 4294967265

real 0m34.673s
user 0m33.960s
sys 0m0.000s
4294967295 iterations of new... checksum = 4294967265

real 0m57.702s
user 0m56.530s
sys 0m0.040s
4294967295 iterations of new2... checksum = 4294967265

real 0m41.709s
user 0m40.810s
sys 0m0.000s
4294967295 iterations of table... checksum = 4294967295

real 0m22.274s
user 0m21.640s
sys 0m0.010s
------------------
==== -march=i686 -O2 ====
test_fls.c:110: warning: decimal constant is so large that it is unsigned
4294967295 iterations of nil... checksum = 4294967295
real 0m29.501s
user 0m28.880s
sys 0m0.040s
4294967295 iterations of old... checksum = 4294967265

real 0m49.054s
user 0m47.840s
sys 0m0.040s
4294967295 iterations of new... checksum = 4294967265

real 0m52.331s
user 0m51.270s
sys 0m0.010s
4294967295 iterations of new2... checksum = 4294967265

real 1m7.043s
user 1m5.660s
sys 0m0.010s
4294967295 iterations of table... checksum = 4294967295

real 0m45.783s
user 0m44.860s
sys 0m0.010s
------------------
==== -O2 ====
test_fls.c:110: warning: decimal constant is so large that it is unsigned
4294967295 iterations of nil... checksum = 4294967295
real 0m34.395s
user 0m33.670s
sys 0m0.000s
4294967295 iterations of old... checksum = 4294967265

real 0m52.277s
user 0m51.230s
sys 0m0.000s
4294967295 iterations of new... checksum = 4294967265

real 0m54.750s
user 0m53.640s
sys 0m0.010s
4294967295 iterations of new2... checksum = 4294967265

real 0m57.728s
user 0m56.590s
sys 0m0.000s
4294967295 iterations of table... checksum = 4294967295

real 0m44.540s
user 0m43.600s
sys 0m0.020s
------------------
==== -O3 ====
test_fls.c:110: warning: decimal constant is so large that it is unsigned
4294967295 iterations of nil... checksum = 4294967295
real 0m16.196s
user 0m15.880s
sys 0m0.000s
4294967295 iterations of old... checksum = 4294967265

real 0m34.342s
user 0m33.630s
sys 0m0.010s
4294967295 iterations of new... checksum = 4294967265

real 0m58.554s
user 0m57.380s
sys 0m0.000s
4294967295 iterations of new2... checksum = 4294967265

real 0m37.140s
user 0m36.320s
sys 0m0.040s
4294967295 iterations of table... checksum = 4294967295

real 0m21.723s
user 0m21.270s
sys 0m0.000s
------------------
--
Hu Gang / Steve
Email : huagng@soulinfo.com, steve@soulinfo.com
GPG FinePrint: 4099 3F1D AE01 1817 68F7 D499 A6C2 C418 86C8 610E
ICQ# : 205800361
Registered Linux User : 204016
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