linux-kernel - Re: [tip:perf/core] perf/x86/intel: Clean-up/reduce PEBS code

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Date:	Mon, 16 Sep 2013 01:17:45 -0500
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Andi Kleen <andi@...stfloor.org>, mingo@...nel.org,
	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/core] perf/x86/intel: Clean-up/reduce PEBS code

On 09/16/2013 01:07 AM, Peter Zijlstra wrote:
> On Fri, Sep 13, 2013 at 11:18:21PM -0700, Andi Kleen wrote:
>> tip-bot for Peter Zijlstra <tipbot@...or.com> writes:
>>> +
>>> +	at  = (struct pebs_record_nhm *)(unsigned long)ds->pebs_buffer_base;
>>> +	top = (struct pebs_record_nhm *)(unsigned long)ds->pebs_index;
>>>
>>>   	ds->pebs_index = ds->pebs_buffer_base;
>>>
>>> +	n = (top - at) / x86_pmu.pebs_record_size;
>>
>>
>> This adds a full slow division to the PEBS hot path.
>>
>> Does not seem like a improvement to me.
>
> There already was an implicit division there, and
> sizeof(pebs_record_hsw) = 176, can it really optimize that constant
> division?
>

gcc can optimize ANY constant division, if nothing else then by 
reciprocal multiplication.

	-hpa


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