Re: [PATCH] Expand mempcpy into memcpy

[OndÅej BÃlka <neleai at seznam dot cz>]

On Wed, Feb 06, 2013 at 01:40:00PM -0500, David Miller wrote:
> From: OndÅej BÃlka <neleai@seznam.cz>
> Date: Wed, 6 Feb 2013 18:40:21 +0100
> 
> > This makes all mempcpy call to call memcpy instead.
> > 
> > There are two reasons, one is that it will simplify maintainance. 
> > Second is that posible performance gains are saving one addition and
> > perhaps spilling one register. Problem is that implementation is quite
> > big - 1105 bytes on x64. Probably cost of instruction/branch cache
> > misses outweigth speedup we gained.
> > 
> > I will move unused inline functions later.
> 
> The overhead of mempcpy vs. memcpy on sparc is exactly 2 instructions
> and 1 extra cycle.
> 
> I think you are making an extremely cpu specific decision on how this
> macro behaves.

I wrote this unclearly. 
"that posible performance gains are saving one addition and perhaps spilling one register."
comment refered to old macro.

Then downside of old macro is that memcpy and mempcpy occupy 2000 bytes
of instruction cache.

With my change with memcpy only 1000 bytes will be occupied which means
fewer cache misses. 

I could avoid that overhead by adding common function like
struct {char *begin,*end} memcpy_be(char *dest,char *src,size_t n)
 when return struct are passed in registers but it
changes ABI and is IMO not worth doing.