Re: [PATCH 00/10] simplify and canonicalize signed compares

[Luc Van Oostenryck <luc.vanoostenryck@gmail.com>]

On Sat, Apr 17, 2021 at 10:16:31AM -0700, Linus Torvalds wrote:
> On Tue, Jan 26, 2021 at 7:45 PM Luc Van Oostenryck
> <luc.vanoostenryck@gmail.com> wrote:
> >
> > This series fixes and improves the simplification and the
> > canonicalization of signed compares.
> 
> Hmm. Sorry for not replying earlier, but I just checked the most
> common simplification of signed compares, and it didn't work.
> 
> This:
> 
>     _Bool test(int a)
>     {
>         return a >=0 && a < 16;
>     }
> 
> should simplify to be the same as
> 
>     _Bool test(int a)
>     {
>         return (unsigned)a < 16;
>     }
> 
> but it doesn't. It generates the silly - but straightforward - "two
> comparisons and a 'and' of the result".

Yes, I've a draft for this but I still needs to add some tests and such.

> In fact, the recent canonicalizations means that the compare against
> zero is actually pessimised, and ">= 0" becomes "> 0xffffffff", which
> is often a much more expensive operation.

Yes, I'm aware of the problem, more or less.
When I did the canonicalization, I wondered what was better:
* canonicalize toward 0
* canonicalize toward the smallest

I choose the later because at the moment it was somehow advantageous
because it reduced by 2 some patterns (it allows to eliminate all >=
and all <; when doing it toward 0 you can eliminate one set for positive
and the other one for negatives values so you need both sets). The
compare with -1 instead of with 0 didn't looked much problematic and
relatively rare. But in the following days I realized it was in fact
quite annoying.

So, yes, it's something I'll change very soon.

-- Luc