linux-kernel - Re: x86 memcpy performance

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Date:	Mon, 15 Aug 2011 15:44:10 +0200
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Borislav Petkov <bp@...en8.de>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Ingo Molnar <mingo@...e.hu>,
	melwyn lobo <linux.melwyn@...il.com>,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	borislav.petkov@....com
Subject: Re: x86 memcpy performance

On Sun, Aug 14, 2011 at 2:40 PM, Borislav Petkov <bp@...en8.de> wrote:
>> > +   if (__len >= 512)                                       \
>> > +           __ret = __sse_memcpy((dst), (src), __len);      \
>> > +   else                                                    \
>> > +           __ret = __memcpy((dst), (src), __len);          \
>> > +   __ret;                                                  \
>> > +})
>>
>> Please, no. Do not inline every memcpy invocation.
>> This is pure bloat (comsidering how many memcpy calls there are)
>> and it doesn't even win anything in speed, since there will be
>> a fucntion call either way.
>> Put the __len >= 512 check inside your memcpy instead.
>
> In the __len < 512 case, this would actually cause two function calls,
> actually: once the __sse_memcpy and then the __memcpy one.

You didn't notice the "else".

>> You may do the check if you know that __len is constant:
>> if (__builtin_constant_p(__len) && __len >= 512) ...
>> because in this case gcc will evaluate it at compile-time.
>
> That could justify the bloat at least partially.

There will be no bloat in this case.
-- 
vda
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