> -----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.
