Value range tracing (Jeremy Fitzhardinge)
Fri, 1 Dec 1995 08:58:59 GMT

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Newsgroups: comp.compilers
From: (Jeremy Fitzhardinge)
Keywords: analysis, optimize, question
Organization: Softway Pty Limited
Date: Fri, 1 Dec 1995 08:58:59 GMT

Hi all,

I'm wondering if people bother with value range tracing. That is,
keeping track of the possible range of variables at each point in the
program. For example:

bar(int i) { /* "i" has no known properties */
if (i < 10) {
int arry[4];
/* We know i < 10, so we can give a warning that it may be over 4 */
arry[i] = bar();
/* We know i >= 10 here */
if (i < 4) { /* Whole block deleted */
switch(i) {
case 0: ... /* delete */
case 2: ... /* delete */
case 8: ... /* maybe... */
/* i is the constant "8" */

Is this kind of thing useful? Is it useful enough to implement?
This isn't very complex, so I assume people have thought about it,
but I've seen very little literature on it.

I suppose it is, in effect, a more general handling of constant
propagation. Rather than saying "this particular value is here", you
can say "this range is possible". On the other hand, ranges are
somewhat limited as well, since you might want to say "this variable is
a power of 2" or something similar. However, I suspect attaching
general functions expressing constraints to variables in the compiler
would be hard to deal with to get useful information (unless, I
suppose, you wrote the compiler in prolog).

So what do people do in practice? Or does everyone just use



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