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

[Richard Biener]

On Wed, 25 Nov 2015, Jakub Jelinek wrote:

> On Wed, Nov 25, 2015 at 09:58:15AM +0100, Marc Glisse wrote:
> > I guess it got lost in my text, but if a user writes:
> > 
> > unsigned diff = a - b;
> > if (b > a) { /* overflow */ ... }
> > else { ... }
> > 
> > Your patch will not detect it. It seems that replacing x-y>x with y>x could
> 
> Sorry, already committed the patch (without incremental that hasn't been
> tested anyway).
> It is true that the patch does not detect this, but it is harder that way.
> What if it is
> if (b > a) ...
> // Huge amount of code
> r = a - b;
> ?  Trying to emit the subtraction above the comparison would then very
> likely increase register preassure.  So, I'd really prefer doing x-y>x to y>x
> only for single use.

I think that's ok for now.  For the above case you'd need to do sth
similar to what cse sincos / divmod do, with an added "cost" check
(same basic-block?).

Richard.