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

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Message-ID: <yw1x4mg9posf.fsf@unicorn.mansr.com>
Date:	Thu, 26 Nov 2015 00:50:08 +0000
From:	Måns Rullgård <mans@...sr.com>
To:	Nicolas Pitre <nico@...xnic.net>
Cc:	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,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	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

Nicolas Pitre <nico@...xnic.net> writes:

> On Thu, 26 Nov 2015, Måns Rullgård wrote:
>
>> Nicolas Pitre <nico@...xnic.net> writes:
>> 
>> > 3) In fact I was wondering if the overhead of the branch and back is 
>> >    really significant compared to the non trivial cost of a idiv 
>> >    instruction and all the complex infrastructure required to patch 
>> >    those branches directly, and consequently if the performance 
>> >    difference is actually worth it versus simply doing (2) alone.
>> 
>> Depending on the operands, the div instruction can take as few as 3
>> cycles on a Cortex-A7.
>
> Even the current software based implementation can produce a result with 
> about 5 simple ALU instructions depending on the operands.
>
> The average cycle count is more important than the easy-way-out case. 
> And then how significant the two branches around it are compared to idiv 
> alone from direct patching of every call to it.

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.

Of course, none of this is going to be as good as letting the compiler
generate div instructions directly.

-- 
Måns Rullgård
mans@...sr.com
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