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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/108938] Missing bswap detection
Date: Tue, 07 Mar 2023 08:40:18 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108938

--- 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 sequen=
ce
> > (especially if some subexpressions aren't single use).
>=20
> 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 t=
hat
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 rese=
t to
zero (essentially not counting that statement or any of its defining statem=
ents
(transitively)).
So it would count solely the statements that could be DCEd.=