From:	Shawn Hargreaves <Shawn AT talula DOT demon DOT co DOT uk>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: fixed point math: NEED HELP QUICK!
Date:	Sat, 1 Feb 1997 11:36:26 +0000
Organization:	None
Distribution:	world
Message-ID:	<0MfOUIA6qy8yEw2n@talula.demon.co.uk>
References:	<19970128 DOT 181621 DOT 14527 DOT 2 DOT chambersb AT juno DOT com>
	<Pine DOT LNX DOT 3 DOT 95 DOT 970131164737 DOT 140G-100000 AT capslock DOT com>
NNTP-Posting-Host:	talula.demon.co.uk
MIME-Version:	1.0
Lines:	33
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

>If *ALL* integer mults/divs are converted to shifts, then what would
>be the equivalent for:
>
>other_stuff = stuff * 57

Ask gcc. Write a little function:

    int times57(int stuff)
    {
        return stuff * 57;
    }

Compile it with 'gcc -m486 -O3 test.c -S', and you get:

    _times57:
        pushl %ebp
        movl %esp,%ebp
        movl 8(%ebp),%edx               # edx = stuff
        leal (%edx,%edx,2),%eax         # eax = stuff * 3
        leal (%edx,%eax,2),%eax         # eax = stuff * 7
        leal (%edx,%eax,8),%eax         # eax = stuff * 57
        movl %ebp,%esp
        popl %ebp
        ret

Rather faster than a multiply! This doesn't work for divides, though:
they will produce a genuine division instruction unless they are by
powers of two.

/*
 *  Shawn Hargreaves - shawn AT talula DOT demon DOT co DOT uk - http://www.talula.demon.co.uk/
 *  Ghoti: 'gh' as in 'enough', 'o' as in 'women', and 'ti' as in 'nation'.
 */

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