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

[Vincent Lefevre]

On 2006-12-31 09:22:22 -0500, Robert Dewar wrote:
> Vincent Lefevre wrote:
> >>My point was that if you see this in a source program, it is in
> >>fact a possible candidiate for code that can be destroyed by
> >>the optimization.
> >
> >Well, only for non-portable code (i.e. code based on wrap). I also
> >suppose that this kind of code is used only to check for overflows.
> 
> No, you suppose wrong, this is an idiom for a normal range test. If
> you have
> 
>    if (a > x && a < y)
> 
> you can often replace this by a single test with a wrapping subtraction.
> As Richard said, you should do this unsigned, but I can easily imagine
> those who are accustomed to signed arithmetic wrapping not bothering

Doing that in unsigned arithmetic is much more readable anyway. So,
I doubt that programmers would do that in signed arithmetic. Or do
you have any real example?

> >>And that's the trouble, this is an optimization which does improve
> >>performance, but may destroy existing code, and the very example
> >>you gave to talk about improved performance is also a nice case
> >>of showing why it may destroy performance. In fact the wrap
> >>around range test is a standard idiom for "hand optimization"
> >>of range tests.
> >
> >Yes, and the lack of optimization would be even worse.
> 
> Well that's a claim without substantiation. No one has any data that
> I know of that shows that the optimization of copmarisons like this
> is important. Sure you can concoct an example, but that says nothing
> about real world code.

I meant that the lack of optimization would be even worse in the above
case only. The point is that one can write "a - 10" in order to say
that a >= INT_MIN + 10 and allow some optimizations. But if, instead
of doing an optimization, the compiler really generates a - 10 because
of wrap, then in this case, writing "a - 10" is worse than writing
code without trying to reduce the range, i.e. instead of having a
faster code, one has a slower code.

I'm not saying that this kind of code is common, but it is quite bad
to penalize a code based on the standard (that tries to give
information to the compiler in order to run faster), just because of
existing non-conforming code.

> This is a long thread, but it is on an important subject. I find
> that compiler writers (and hardware manufacturers too) tend to
> be far too focused on performance, when what is important to the
> majority of users is reliability.

IMHO, GCC should first focus on reliability of code based on the
standard. If reliability is important, why not assuming -ffloat-store
by default when need be? Without it on some platforms, it cannot be a
conforming compiler (even with it, it isn't, but that's much better
with it).

-- 
Vincent Lefèvre <address@hidden> - Web: <http://www.vinc17.org/>
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Work: CR INRIA - computer arithmetic / Arenaire project (LIP, ENS-Lyon)