On Thu, Feb 18, 2021 at 11:12:26AM +0100, Richard Biener via Gcc-patches wrote:
> On Thu, Feb 18, 2021 at 12:35 AM Hans-Peter Nilsson via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> >
> > If we're not going to eliminate the clz, it's better for the
> > comparison to use that result than its input, so we don't
> > extend the lifetime of the input. Also, an additional use
> > of the result is more likely cheaper than a compare of the
> > input, in particular considering that the clz may have made
> > available a non-zero condition matching the original use.
>
> Of course the non-zero compare can execute concurrently
> with the clz if we use the input which can result in faster
> operation. It's one of the many cases where local folding
> doesn't tell us enough. One of the usual issues with
> single_use checks is that the IL is not fully DCEd while
> we fold it and thus dead uses can make sth !single_use
> even though it really is single_use.
And on the other side, the IL might not be fully CSEd yet and
so we can see single use, but after SCCVN it wouldn't be single use.
Jakub
