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Tue Apr 13 22:16:16 EDT 2004 · https://mail.python.org/pipermail/python-dev/2004-April/044239.html

[Jeff Epler]
> > With -O2 -mcpu=i686 or newer, gcc uses "imul" for both 100003 and
> > 65599,
> > rather than shifts and adds.

[Bob Ippolito]
> It's not expected that GCC optimize an integer constant into shifts on
> its own. 

Right, the actual diff is:

*** 1145,1151 ****
        p = (unsigned char *) a->ob_sval;
        x = *p << 7;
        while (--len >= 0)
!               x = (1000003*x) ^ *p++;
        x ^= a->ob_size;
        if (x == -1)
                x = -2;
--- 1152,1158 ----
        p = (unsigned char *) a->ob_sval;
        x = *p << 7;
        while (--len >= 0)
!               x = (x << 6) + (x << 16) - x + (long)*p++;
        x ^= a->ob_size;
        if (x == -1)
                x = -2;

> I guess the real question for Raymond is, does it really make a
> measurable difference?

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).

>  And what effect does it have on pickled dicts
> (or other such hash-using data structures), if any?

The test suite runs fine.  Dicts may display in a different order than
in previous pythons.  That may upset some doctests if the writer didn't
take Tim's documented advice about making tests that do not depend on
display order.

Raymond

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[Python-Dev] String hash function multiplier