On Tue, Sep 19, 2023 at 08:00:12AM +0000, David Laight wrote: > ... > > > So ending up with (something like): > > > end = buff + length; > > > ... > > > while (++ptr < end) { > > > csum += data; > > > carry += csum < data; > > > data = ptr[-1]; > > > } > > > (Although a do-while loop tends to generate better code > > > and gcc will pretty much always make that transformation.) > > > > > > I think that is 4 instructions per word (load, add, cmp+set, add). > > > In principle they could be completely pipelined and all > > > execute (for different loop iterations) in the same clock. > > > (But that is pretty unlikely to happen - even x86 isn't that good.) > > > But taking two clocks is quite plausible. > > > Plus 2 instructions per loop (inc, cmp+jmp). > > > They might execute in parallel, but unrolling once > > > may be required. > > > > > It looks like GCC actually ends up generating 7 total instructions: > > ffffffff808d2acc: 97b6 add a5,a5,a3 > > ffffffff808d2ace: 00d7b533 sltu a0,a5,a3 > > ffffffff808d2ad2: 0721 add a4,a4,8 > > ffffffff808d2ad4: 86be mv a3,a5 > > ffffffff808d2ad6: 962a add a2,a2,a0 > > ffffffff808d2ad8: ff873783 ld a5,-8(a4) > > ffffffff808d2adc: feb768e3 bltu a4,a1,ffffffff808d2acc <do_csum+0x34> > > > > This mv instruction could be avoided if the registers were shuffled > > around, but perhaps this way reduces some dependency chains. > > gcc managed to do 'data += csum' so had add 'csum = data'. > If you unroll once that might go away. > It might then be 10 instructions for 16 bytes. > Although you then need slightly larger alignment code. > > David > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK > Registration No: 1397386 (Wales) > I messed with it a bit and couldn't get the mv to go away. I would expect mv to be very cheap so it should be fine, and I would like to avoid adding too much to the alignment code since it is already large, and I assume that buff will be aligned more often than not. Interestingly, the mv does not appear pre gcc 12, and does not appear on clang. - Charlie