public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
From: "amonakov at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/98226] Slow std::countr_one
Date: Fri, 11 Dec 2020 11:28:39 +0000 [thread overview]
Message-ID: <bug-98226-4-xm4Pktqsup@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-98226-4@http.gcc.gnu.org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98226
Alexander Monakov <amonakov at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |amonakov at gcc dot gnu.org
--- Comment #10 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
(In reply to Oleg Zaikin from comment #6)
> There
> we have a function firstzero(unsigned x) that returns 2^i, with i the
> position of the first 0 of x, and 0 iff there is no 0. Its implementation is:
> unsigned firstzero(const unsigned x) noexcept {
> #if __cplusplus > 201703L
> return x == unsigned(-1) ? 0 : unsigned(1) << std::countr_one(x);
> #else
> const unsigned y = x+1; return (y ^ x) & y;
> #endif
> }
But why you are trying to use a more complex branchy expression in C++17 mode
when you already have a more efficient expression as a "fallback"?
Note that a cheaper way is available:
return (x+1) & ~x;
(though gcc can optimize '(y ^ x) & y' you have to the same machine code)
next prev parent reply other threads:[~2020-12-11 11:28 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-10 16:51 [Bug libstdc++/98226] New: " zaikin.icc at gmail dot com
2020-12-10 17:31 ` [Bug libstdc++/98226] " redi at gcc dot gnu.org
2020-12-10 17:33 ` redi at gcc dot gnu.org
2020-12-10 18:08 ` pinskia at gcc dot gnu.org
2020-12-10 22:00 ` cvs-commit at gcc dot gnu.org
2020-12-10 22:08 ` redi at gcc dot gnu.org
2020-12-11 9:17 ` zaikin.icc at gmail dot com
2020-12-11 9:18 ` zaikin.icc at gmail dot com
2020-12-11 10:59 ` redi at gcc dot gnu.org
2020-12-11 11:08 ` redi at gcc dot gnu.org
2020-12-11 11:28 ` amonakov at gcc dot gnu.org [this message]
2020-12-11 12:16 ` zaikin.icc at gmail dot com
2020-12-11 12:21 ` zaikin.icc at gmail dot com
2020-12-11 12:33 ` zaikin.icc at gmail dot com
2021-03-29 20:01 ` cvs-commit at gcc dot gnu.org
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=bug-98226-4-xm4Pktqsup@http.gcc.gnu.org/bugzilla/ \
--to=gcc-bugzilla@gcc.gnu.org \
--cc=gcc-bugs@gcc.gnu.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).