Re: x86 memcpy performance
From: Denys Vlasenko
Date: Mon Aug 15 2011 - 09:44:34 EST
On Sun, Aug 14, 2011 at 2:40 PM, Borislav Petkov <bp@xxxxxxxxx> 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.
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