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Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."

From: Ian Lance Taylor
Subject: Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."
Date: 31 Dec 2006 21:37:00 -0800
User-agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.4

address@hidden (Richard Kenner) writes:

> > But didn't this thread get started by a real program that was broken
> > by an optimization of loop invariants?  Certainly I got a real bug
> > report of a real problem, which you can see here:
> > 
> >
> I just thought of something interesting about this case.  We have a loop
> looking like:
>  for (j = 1; 0 < j; j *= 2)
> in this case, the compiler is quite capable of arguing "this end condition
> won't terminate the loop except in the case of overflow, so obviously the
> programmer must be relying on wrapping semantics. So let's use those here"

Sure, but it's not the loop optimizer which is causing this code to

> > Here is a bit more discussion:
> > 
> >
> Indeed that message hinted at exactly that point.  Note that if you do
> argue along those lines, you can convert this to a loop with a new IV
> that increments up to 31 (or is it 32?).

That is in fact precisely what happens if you compile with


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