Re: [PATCH] Convert manual unsigned +/- overflow checking into {ADD,SUB}_OVERFLOW (PR target/67089)

[Richard Biener <rguenther@suse.de>]

On Wed, 25 Nov 2015, Marc Glisse wrote:

> On Wed, 25 Nov 2015, Jakub Jelinek wrote:
> 
> > > The same is true whether we write it b > a or (a - b) > a (I don't think
> > > PRE
> > > + SCCVN avoid increasing register pressure).
> > > 
> > > > So, I'd really prefer doing x-y>x to y>x only for single use.
> > > 
> > > Ok (for now).
> > 
> > Do you plan to work on that (my match.pd experience is smaller than yours),
> > or should I add to my todo list?
> 
> Are we talking stage 3 or next stage 1? If you want something for stage 3, I
> think you'll have to do it, it shouldn't be much longer than
> 
> (for cmp (gt le)
>  (simplify
>   (cmp (minus:s @0 @1) @0)
>   (if (TYPE_UNSIGNED (TREE_TYPE (@0)))
>    (cmp @1 @0))))
> 
> and a similar one for x<y-x. (I don't think TYPE_UNSIGNED needs protection
> against floats or whatever, but I could be wrong)

floats should be fine here.  But eventually saturating types need to
be excluded?

Note that the :s on the minus has no effect as the result is a
single operation.  If you want to restrict this nevertheless
you need a && single_use (@2) and capture the minus with
(minus@2 @0 @1) instead.

Richard.