[Python-Dev] String hash function multiplier

[Jeff Epler]

On Tue, Apr 13, 2004 at 10:16:16PM -0400, Raymond Hettinger wrote:
> Yes, the loop runs 20% faster on my Pentium III.  The speedup ought to
> be much more dramatic on the Pentium IV (where the integer ALU
> instructions speedup from 1 clock to 0.5 clocks while the latency on
> integer multiplication slows down from 4 clocks to 14-18 clocks).

I got different results here, also on a Pentium III (but in a laptop)
I'm using gcc, I don't know what you're using.

I get the best results from -O3 -mcpu=pentium3 for both MUL values and
do worse with CLEVER in most cases.

In my test, PyStringObject is not quite the same as in Python, so this
could explain different results.  Use of a different compiler, or gcc
with the implied default -mcpu=i386 (?), might explain why you
benchmarked the shifts as better, too. (-O{2,3} -mcpu=i386)

[* = best alternative for these optimizer flags]
-O -mcpu=i386        -DMUL=100003         1.78
-O -mcpu=i386        -DMUL=65599          1.37    *
-O -mcpu=i386        -DCLEVER             1.40

-O -mcpu=pentium3    -DMUL=100003         1.27    *
-O -mcpu=pentium3    -DMUL=65599          1.27    *
-O -mcpu=pentium3    -DCLEVER             1.35

-O2 -mcpu=i386       -DMUL=100003         1.93
-O2 -mcpu=i386       -DMUL=65599          1.54
-O2 -mcpu=i386       -DCLEVER             1.28    *

-O2 -mcpu=pentium3   -DMUL=100003         1.11    *
-O2 -mcpu=pentium3   -DMUL=65599          1.12
-O2 -mcpu=pentium3   -DCLEVER             1.29

-O3 -mcpu=i386       -DMUL=100003         1.69
-O3 -mcpu=i386       -DMUL=65599          1.28
-O3 -mcpu=i386       -DCLEVER             1.10    *

-O3 -mcpu=pentium3   -DMUL=100003         0.90    *
-O3 -mcpu=pentium3   -DMUL=65599          0.90    *
-O3 -mcpu=pentium3   -DCLEVER             1.05

-Os -mcpu=i386       -DMUL=100003         1.16    *
-Os -mcpu=i386       -DMUL=65599          1.16    *
-Os -mcpu=i386       -DCLEVER             1.45

-Os -mcpu=pentium3   -DMUL=100003         1.05    *
-Os -mcpu=pentium3   -DMUL=65599          1.05    *
-Os -mcpu=pentium3   -DCLEVER             1.30

# alternatives.py
OPT = [
    '-O -mcpu=i386', '-O -mcpu=pentium3',
    '-O2 -mcpu=i386', '-O2 -mcpu=pentium3',
    '-O3 -mcpu=i386', '-O3 -mcpu=pentium3',
    '-Os -mcpu=i386', '-Os -mcpu=pentium3',
]

HASH = ['-DMUL=100003', '-DMUL=65599', '-DCLEVER']

import sys, os
for o in OPT:
    for h in HASH:
        sys.stdout.write("%-20s %-20s " % (o, h))
        sys.stdout.flush()
        os.system("gcc %s %s hashtest.c && TIME=%%U /usr/bin/time ./a.out"
                     % (o, h))
    print


// hashtest.c
#ifndef MUL
#define MUL 100003
#endif

typedef struct {
    long ob_shash;
    unsigned long ob_size;
    unsigned char ob_sval[16];
} PyStringObject;

static long
string_hash(PyStringObject *a)
{
        register int len;
        register unsigned char *p;
        register long x;

        if (a->ob_shash != -1)
                return a->ob_shash;
        len = a->ob_size;
        p = (unsigned char *) a->ob_sval;
        x = *p << 7;
        while (--len >= 0)
#ifdef CLEVER
                x = (x << 6) + (x << 16) - x + (long)*p++;
#else
                x = (MUL*x) ^ *p++;
#endif
        x ^= a->ob_size;
        if (x == -1)
                x = -2;
        a->ob_shash = x;
        return x;
}

int main(void) {
    PyStringObject s = {-1, 13, "hello raymond"};
    int i;
    for(i=0; i < 1<<23; i++) {
        string_hash(&s);
        s.ob_shash = -1;
    }
    return 0;
}