From: Bin Meng > Sent: 15 June 2021 14:09 > > On Tue, Jun 15, 2021 at 4:57 PM David Laight <David.Laight@xxxxxxxxxx> wrote: > > ... > > I'm surprised that the C loop: > > > > > + for (; count >= bytes_long; count -= bytes_long) > > > + *d.ulong++ = *s.ulong++; > > > > ends up being faster than the ASM 'read lots' - 'write lots' loop. > > I believe that's because the assembly version has some unaligned > access cases, which end up being trap-n-emulated in the OpenSBI > firmware, and that is a big overhead. Ah, that would make sense since the asm user copy code was broken for misaligned copies. I suspect memcpy() was broken the same way. I'm surprised IP_NET_ALIGN isn't set to 2 to try to avoid all these misaligned copies in the network stack. Although avoiding 8n+4 aligned data is rather harder. Misaligned copies are just best avoided - really even on x86. The 'real fun' is when the access crosses TLB boundaries. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
