linux-kernel - Re: [PATCH] sched: Optimize __calc

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CABk29Nu67D8Wf-xEyaxZ39FsqPB9tpw8cGHPFmZrTqfM-JgYjQ@mail.gmail.com>
Date:   Wed, 3 Mar 2021 14:00:18 -0800
From:   Josh Don <joshdon@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Clement Courbet <courbet@...gle.com>,
        Oleg Rombakh <olegrom@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-toolchains@...r.kernel.org
Subject: Re: [PATCH] sched: Optimize __calc_delta.

On Wed, Mar 3, 2021 at 2:02 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, Mar 02, 2021 at 12:57:37PM -0800, Josh Don wrote:
> > On gcc, the asm versions of `fls` are about the same speed as the
> > builtin. On clang, the versions that use fls (fls,fls64) are more than
> > twice as slow as the builtin. This is because the way the `fls` function
> > is written, clang puts the value in memory:
> > https://godbolt.org/z/EfMbYe. This can be fixed in a separate patch.
>
> Is this because clang gets the asm constraints wrong? ISTR that
> happening before, surely the right thing is to fix clang?

https://bugs.llvm.org/show_bug.cgi?id=49406 filed