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Date: Wed, 3 Mar 2021 11:02:29 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Don <joshdon@...gle.com>
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 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?
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