From: Segher Boessenkool > Sent: 13 February 2022 09:16 .... > > > What happens on x86-64? > > > > Trying to do the same in the x86 ipcsum code tended to make the code worse. > > (Although that test is for an odd length fragment and can just be removed.) > > In an ideal world the compiler could choose the optimal code sequences > everywhere. But that won't ever happen, the search space is way too > big. So compilers just use heuristics, not exhaustive search like > superopt does. There is a middle way of course, something with directed > searches, and maybe in a few decades systems will be fast enough. Until > then we will very often see code that is 10% slower and 30% bigger than > necessary. A single insn more than needed isn't so bad :-)
But it can be a lot more than that. > Making things branch-free is very much worth it here though! I tried to find out where 'here' is. I can't get godbolt to generate anything like that object code for a call to csum_shift(). I can't actually get it to issue a rotate (x86 of ppc). I think it is only a single instruction because the compiler has saved 'offset & 1' much earlier instead of doing testing 'offset & 1' just prior to the conditional. It certainly has a nasty habit of doing that pessimisation. So while it helps a specific call site it may be much worse in general. I also suspect that the addc/addze pair could be removed by passing the old checksum into csum_partial. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)