# Re: Common subexpression analysis

## f88ho@efd.lth.se (Hans Olsson)Tue, 14 Jul 1992 08:53:59 GMT

From comp.compilers

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Re: Common subexpression analysis (summary) Bruce.Hoult@bbs.actrix.gen.nz (1992-07-13)
Re: Common subexpression analysis f88ho@efd.lth.se (1992-07-14)
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 Newsgroups: comp.compilers From: f88ho@efd.lth.se (Hans Olsson) Organization: Lund Institute of Technology, Sweden Date: Tue, 14 Jul 1992 08:53:59 GMT References: 92-06-135 92-07-028 Keywords: optimize

Bruce.Hoult@bbs.actrix.gen.nz writes:
[Original article deleted]
>Why didn't you do strength reduction while you were at it?
>
> r2 = r*r;
> x2 = x*x;
> y2 = y*y;
> while (y >= 0)
> if (x2 + y2 > r2)
> --y, y2 -= y+y-1;
> else
> ++x, x2 += x+x+1;

Or somewhat better:

..
if (x2+y2 > r2)
y2-=y, --y, y2-=y;
else
x2+=x, ++x, x2+=x;

Or even better:

goto L0;

L2:
if (x2+y2>r2) goto L3;
x2+=x;
x++;
x2+=x;
goto L2;

L3:
y2-=y;
--y;
y2-=y;
L0:
if (y>=0) goto L2;

(I hope I got it right. The basic idea is that it's a waste of time
to test if y>=0 if y hasn't changed).

>
>
>... or even go whole hog and do...
>
> t = x*x + y*y - r*r;
> x2 = x+x+1;
> y2 = y+y-1;
> while (y2 >= 0)

while (y2 >= -1)

> if (t > 0)
> y2 -= 2, t -= y2;
> else
> x2 += 2, t += x2;
>
>
>I'll bet if there are any compilers that can do this, they're probably
>for FORTRAN :-)

Probably true.

--

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