linux-kernel - Re: [PATCH v2 2/2] ARM: Replace calls to __aeabi

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Message-ID: <20151126012859.GX8644@n2100.arm.linux.org.uk>
Date:	Thu, 26 Nov 2015 01:28:59 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Måns Rullgård <mans@...sr.com>
Cc:	Nicolas Pitre <nico@...xnic.net>,
	Stephen Boyd <sboyd@...eaurora.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org, Michal Marek <mmarek@...e.com>,
	linux-kbuild@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
Subject: Re: [PATCH v2 2/2] ARM: Replace calls to __aeabi_{u}idiv with
 udiv/sdiv instructions

On Thu, Nov 26, 2015 at 12:50:08AM +0000, Måns Rullgård wrote:
> If not calling the function saves an I-cache miss, the benefit can be
> substantial.  No, I have no proof of this being a problem, but it's
> something that could happen.

That's a simplistic view of modern CPUs.

As I've already said, modern CPUs which have branch prediction, but
they also have speculative instruction fetching and speculative data
prefetching - which the CPUs which have idiv support will have.

With such features, the branch predictor is able to learn that the
branch will be taken, and because of the speculative instruction
fetching, it can bring the cache line in so that it has the
instructions it needs with minimal or, if working correctly,
without stalling the CPU pipeline.

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