RE: [PATCH] eal/x86: optimize memcpy of small 64-byte blocks

> -----Original Message-----
> From: Morten Brørup <[email protected]>
> Sent: Wednesday, August 5, 2026 6:56 AM
> To: Konstantin Ananyev <[email protected]>; Stephen
> Hemminger <[email protected]>
> Cc: [email protected]; Bruce Richardson <[email protected]>
> Subject: RE: [PATCH] eal/x86: optimize memcpy of small 64-byte blocks
> 
> > From: Konstantin Ananyev [mailto:[email protected]]
> > Sent: Wednesday, 5 August 2026 07.46
> >
> > > > From: Stephen Hemminger [mailto:[email protected]]
> > > > Sent: Tuesday, 4 August 2026 17.52
> > > >
> > > > On Tue,  4 Aug 2026 14:33:04 +0000
> > > > Morten Brørup <[email protected]> wrote:
> > > >
> > > > > +	/* Common way for small copy size of 64-byte blocks.
> > Unlikely, so
> > > > constant size only */
> > > > > +	if (__rte_constant(n) && (n & 63) == 0 && n <=
> > > > RTE_MEMCPY_BLOCK_64_MAX) {
> > > > > +		void *ret = dst;
> > > > > +
> > > >
> > > > Maybe just let compiler decide, it will generate vector
> > instructions in
> > > > most cases.
> > > >
> > > > 	if (__rte_constant(n))
> > > > 		return mempcpy(dst, src, n);
> > >
> > > Maybe in most, but not in all:
> > > https://godbolt.org/z/KvdKqT5rY
> >
> > With '-mavx' or '-mavx512f' it looks like it does for your sample code.
> 
> It also does with -msse4.2 when SZ is reduced to 256 bytes.
> Clang switches to inline when SZ is reduced to 128 bytes.
> 
> It seems the compiler has a threshold for when to inline and when to call the C
> library's memcpy subroutine.
> The threshold depends on both copy size and vector register size.
> And it is compiler dependent.

I think there are compiler options to specify desired threshold values.
Let say for gcc there is  ' -mmemcpy-strategy=strategy'.
For that example in that particular case
-mmemcpy-strategy=vector_loop:512:align,loop:-1:align
generates sse loads/stores.
Might be we can exploit it somehow?
I am not really happy that our home-brewed memcpy code-block keeps growing,
while we keep talking that it would be good to eliminate it completely.




> With rte_memcpy() it is always inline.