Re: kernel + gcc 4.1 = several problems

[Geert Bosch <bosch@adacore.com>]

On Jan 4, 2007, at 13:34, Segher Boessenkool wrote:

> The "signed wrap is undefined" thing doesn't fit in this category
> though:
>
> -- It is an important optimisation for loops with a signed
>    induction variable;

It certainly isn't that important. Even SpecINT compiled with
-O3 and top-of-tree GCC *improves* 1% by adding -fwrapv.
If the compiler itself can rely on wrap-around semantics and
doesn't have to worry about introducing overflows between
optimization passes, it can reorder simple chains of additions.
This is more important for many real-world applications than
being able to perform some complex loop-interchange.
Compiler developers always make the mistake of overrating
their optimizations.

If GCC does really poorly on a few important loops that matter,
that issue is easily addressed. If GCC generates unreliable
code for millions of boring lines of important real-world C,
the compiler is worthless.

   -Geert