On Sat, Mar 04, 2017 at 04:56:38PM -0800, h...@zytor.com wrote: > That's what the -march= and -mtune= option do!
How does that even help with a distro kernel built with -mtune=generic ? gcc can't possibly know on what targets is that kernel going to be booted on. So it probably does some universally optimal things, like in the dmi_scan_machine() case: memcpy_fromio(buf, p, 32); turns into: .loc 3 219 0 movl $8, %ecx #, tmp79 movq %rax, %rsi # p, p movq %rsp, %rdi #, tmp77 rep movsl Apparently it thinks it is fine to do 8*4-byte MOVS. But why not 4*8-byte MOVS? That's half the loops. [ It is a whole different story what the machine actually does underneath. It being a half cacheline probably doesn't help and it really does the separate MOVs but then it would be cheaper if it did 4 8-byte ones. ] One thing's for sure - both variants are certainly cheaper than to CALL a memcpy variant. What we probably should try to do, though, is simply patch in the body of REP; MOVSQ or REP; MOVSB into the call sites and only have a call to memcpy_orig() because that last one if fat. I remember we did talk about it at some point but don't remember why we didn't do it. -- Regards/Gruss, Boris. SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg) --