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Date: Sun, 8 Feb 2015 21:09:27 +0530
Message-ID: <CA+HOOsgiV_CG5tWEC91-h0wcfP5oZJ02ztOCr+=WjM6wHjVufA@mail.gmail.com>
Subject: Re: [PATCH] lib/int_sqrt.c: Optimize square root function
From: Anshul Garg <aksgarg1989@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "anshul.g@samsung.com" <anshul.g@samsung.com>
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Dear Mr. linus,

Thanks for quick replies.

Yes performance numbers are not conclusive enough.
So its better to discard this patch as of now.

I will try to explore more in this area.


Thanks & regards
Anshul Garg


On Fri, Feb 6, 2015 at 1:07 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Thu, Feb 5, 2015 at 10:43 AM, Anshul Garg <aksgarg1989@gmail.com> wrote:
>>
>> NOTE ::
>> I have not used gcc optimizations while compilation.
>> With O2 level optimization proposed solution is taking more time.
>
> The thing is, the kernel is compiled with -O2, so that's what matters.
>
> Also, for very tight loops like this, the major costs tend to be very
> subtle microarchitectural details, particularly branch prediction.
> Which in turn end up sometimes depending on just exactly where the
> branches were placed, and even whether two conditional branches were
> in the same 8-byte aligned region etc things (because the branch
> prediction might be done ignoring the low bits of the EIP etc). So not
> only does the exact microarchitecture matter, things that don't *seem*
> like they should matter can change behavior a lot.
>
> My point is really that the performance numbers are very ambiguous.
> The patch may well help in some situations, but hurt in others.
>
>                       Linus
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