linux-kernel - Re: [PATCH 07/25] hrtimer: Reduce conditional code (hres

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Date:   Mon, 25 Sep 2017 16:28:07 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Anna-Maria Gleixner <anna-maria@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Christoph Hellwig <hch@....org>, keescook@...omium.org,
        John Stultz <john.stultz@...aro.org>
Subject: Re: [PATCH 07/25] hrtimer: Reduce conditional code (hres_active)

On Mon, 25 Sep 2017, Peter Zijlstra wrote:
> On Thu, Aug 31, 2017 at 12:23:36PM -0000, Anna-Maria Gleixner wrote:
> > The hrtimer_cpu_base struct has the CONFIG_HIGH_RES_TIMERS conditional
> > struct member hres_active. All related functions to this member are
> > conditional as well.
> > 
> > There is no functional change, when the hres_active member is unconditional
> > with all related functions and is set to zero during initialization. This
> > makes the code easier to read.
> 
> But you make what was a compile time dead-code-elimination into a
> runtime load-and-branch.
> 
> Unless the compiler is overly clever and is able to deduce from the lack
> of assignments that it must always be 0, is it?

Probably not. So the simple way to solve this is to make the new common

is_highres_active()
{
	return IS_ENABLED(CONFIG_HIGH_RES) ? base->hres_active : 0;
}

Thanks,

	tglx