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From: "zaikin.icc at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/98226] Slow std::countr_one
Date: Fri, 11 Dec 2020 09:17:05 +0000 [thread overview]
Message-ID: <bug-98226-4-gTCWDdWMAk@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
--- Comment #6 from Oleg Zaikin <zaikin.icc at gmail dot com> ---
(In reply to Jonathan Wakely from comment #2)
> Oh, but you didn't enable any optimization at all, so who cares about the
> performance?
Let me give the whole picture. The issue is very close to that from
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97759 where a C-style
implementation was replaced by C++20 std::has_single_bit. We have a complex
project that of course is compiled with all proper optimization flags. 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
}
When we switched from C++17-based g++ to C++20-based g++, the performance of
the whole program decreased by about 7 %. It turned out that the main reason is
the firstzero function.
next prev parent reply other threads:[~2020-12-11 9:17 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 [this message]
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
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
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