From: "H. Peter Anvin" <hpa@zytor.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Andi Kleen <andi@firstfloor.org>,
mingo@kernel.org, linux-kernel@vger.kernel.org,
tglx@linutronix.de, linux-tip-commits@vger.kernel.org
Subject: Re: [tip:perf/core] perf/x86/intel: Clean-up/reduce PEBS code
Date: Mon, 16 Sep 2013 01:17:45 -0500 [thread overview]
Message-ID: <5236A289.2010007@zytor.com> (raw)
In-Reply-To: <20130916060736.GN21832@twins.programming.kicks-ass.net>
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@zytor.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
next prev parent reply other threads:[~2013-09-16 6:18 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-09-12 18:04 [tip:perf/core] perf/x86/intel: Clean-up/reduce PEBS code tip-bot for Peter Zijlstra
2013-09-14 6:18 ` Andi Kleen
2013-09-16 6:07 ` Peter Zijlstra
2013-09-16 6:17 ` H. Peter Anvin [this message]
2013-09-16 7:24 ` Peter Zijlstra
2013-09-16 16:09 ` Andi Kleen
2013-09-16 16:03 ` Andi Kleen
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