Re: gcc-2.7 creates faster code than pgcc-1.1.1

[craig@jcb-sc.com]

>Most of the papers I've read have indicated only trivial gains from value
>range propagation.  I've also had discussions with folks that have implemented
>this opt in a commercial compiler -- it's so minor of a win that they didn't
>consider it worth the effort.

If that we really true *generally*, i.e. for floating-point as well,
then we could happily make -mieee the default for egcs/gcc on Alphas,
for example.  (This would make Alpha-generated floating-point code
much slower, of course, which is why I mention it: if it was possible
to use value-range propagation to determine when the special code-
generation normally needed for full IEEE range wasn't needed after all,
then some of that performance could be regained.)

But, I have to admit I can't document any really clear performance
wins from my own hand-tuned assembly code (written over the past few
decades) deriving solely from value-range propagation, especially
for *integer* (versus floating-point) values.

Generally, I think being able to avoid even checking for exceptional
values might be one source of performance wins from this: e.g. if
we knew it was "impossible" to divide by zero here, square-root a
negative number there, and so on, we could save generated extra
code, thus making Icache misses theoretically less frequent.

        tq vm, (burley)