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From:   Josh Don <joshdon@google.com>
Date:   Wed, 3 Mar 2021 14:00:18 -0800
Message-ID: <CABk29Nu67D8Wf-xEyaxZ39FsqPB9tpw8cGHPFmZrTqfM-JgYjQ@mail.gmail.com>
Subject: Re: [PATCH] sched: Optimize __calc_delta.
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Ingo Molnar <mingo@redhat.com>, Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Clement Courbet <courbet@google.com>,
        Oleg Rombakh <olegrom@google.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        linux-toolchains@vger.kernel.org
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On Wed, Mar 3, 2021 at 2:02 AM Peter Zijlstra <peterz@infradead.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