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From: CBFalconer <cbfalconer AT yahoo DOT com>
Organization: Ched Research
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Subject: Re: gcc 2.953 output - is this a bug?
References: <3C61E6BB DOT 7F9AD526 AT yahoo DOT com> <87d6zim2vy DOT fsf AT pfaff DOT stanford DOT edu>
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Date: Thu, 07 Feb 2002 04:51:47 GMT
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Ben Pfaff wrote:
> 
> CBFalconer <cbfalconer AT yahoo DOT com> writes:
> 
> > Please look at the following disassembly:
> >
> >       h = h * 37UL + (unsigned char) *string++;
> >  585:   8d 04 d2                lea    (%edx,%edx,8),%eax <<***
> >  588:   8d 04 82                lea    (%edx,%eax,4),%eax <<***
> >
> > look at the lines marked "<<***".  I fail to see how this
> > implements h * 37UL.
> 
> Why, you mean it's not perfectly obvious? :-)
> Here's a translation back to C of these lines:
>         eax = edx + 8 * edx
>         eax = edx + 4 * eax
> Here's an equivalent for the first line:
>         eax = 9 * edx
> which, performing a substitution on eax, means that the second
> line becomes equivalent to
>         eax = edx + 4 * (9 * edx)
> and from there it should be evident.
> 
> (GCC is more clever than I thought.)

Thanks.  If I had a handy x86 manual I *might* have eventually
figured it out. Incidentally, I found that gcc generated better
code for:

              h = h * 31UL + *string++;
than for:
              h = (h << 5) - h;
              h += *string++;

the latter used an additional temporary.  Which proves something
or other.

-- 
Chuck F (cbfalconer AT yahoo DOT com) (cbfalconer AT XXXXworldnet DOT att DOT net)
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