On Sat, Dec 12, 2020 at 01:25:39PM +0100, Marc Glisse wrote:
> On Sat, 12 Dec 2020, Jakub Jelinek via Gcc-patches wrote:
>
> > This patch adds the ~(X - Y) -> ~X + Y simplification requested
> > in the PR (plus also ~(X + C) -> ~X + (-C) for constants C that can
> > be safely negated.
>
> Would it have been wrong to produce ~X - C without caring about negating
> (and then extending it to non-constants)?
Extending it to non-constants is what I wanted to avoid.
For ~(X + Y), because + is commutative, it wouldn't be a canonicalization
as it would pick more-less randomly whether to do ~X + Y or X + ~Y.
> I wonder if this makes
> /* ~(~X - Y) -> X + Y and ~(~X + Y) -> X - Y. */
> useless.
Maybe the former, but not the latter.
Jakub
