linux-kernel - Re: [PATCH v2] ARM: Use udiv/sdiv for __aeabi

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Date:	Mon, 11 Nov 2013 17:23:12 -0800
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Matt Sealey <neko@...uhatsu.net>
CC:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>,
	Christopher Covington <cov@...eaurora.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Måns Rullgård <mans@...sr.com>,
	Rob Herring <robherring2@...il.com>
Subject: Re: [PATCH v2] ARM: Use udiv/sdiv for __aeabi_{u}idiv library functions

On 11/08/13 22:46, Matt Sealey wrote:
> On Fri, Nov 8, 2013 at 5:00 PM, Stephen Boyd <sboyd@...eaurora.org> wrote:
>> If we're running on a v7 ARM CPU, detect if the CPU supports the
>> sdiv/udiv instructions and replace the signed and unsigned
>> division library functions with an sdiv/udiv instruction.
>>
>> Running the perf messaging benchmark in pipe mode
>>
>>  $ perf bench sched messaging -p
>>
>> shows a modest improvement on my v7 CPU.
>>
>> before:
>> (5.060 + 5.960 + 5.971 + 5.643 + 6.029 + 5.665 + 6.050 + 5.870 + 6.117 + 5.683) / 10 = 5.805
>>
>> after:
>> (4.884 + 5.549 + 5.749 + 6.001 + 5.460 + 5.103 + 5.956 + 6.112 + 5.468 + 5.093) / 10 = 5.538
>>
>> (5.805 - 5.538) / 5.805 = 4.6%
> Even with the change to the output constraint suggested by Mans, you
> get absolutely identical benchmark results? There's a lot of variance
> in any case..

Yeah sorry I didn't run the testcase again to see if numbers changed
because I assumed one less instruction would be in the noise. I agree
there is a lot of variance so if you have any better
benchmarks/testcases please let me know.

>
> BTW has there been any evaluation of the penalty for the extra
> branching, or the performance hit for the ARMv7-without-division
> cases?

I haven't done any. I'll factor that in for the next round.

-- 
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