[Bug target/108938] Missing bswap detection

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108938

--- Comment #12 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Hongtao.liu from comment #11)
> (In reply to Jakub Jelinek from comment #9)
> > Though, if more than one replacement operation is emitted, one needs to be
> > careful not to emit more expensive replacement than the original sequence
> > (especially if some subexpressions aren't single use).
> 
> The patch(support swap + bit_and + rotate) doesn't show much impact on
> SPEC2017, is there any other benchmark that I can try to find some
> performance regressiones with bswap + bit_and + rotate?
> So that I'll restrict the patch to only bswp + rotate/shift.

I didn't mean to look at SPEC numbers, I meant count number of statements that
could be DCEd.  The code currently computes n_ops as it walks the statements
that compute the value.  So, the idea would be to have another counter next to
it, initialized
very similarly, except on a statement that !has_single_use it would be reset to
zero (essentially not counting that statement or any of its defining statements
(transitively)).
So it would count solely the statements that could be DCEd.