From: Matteo Croce > Sent: 16 June 2021 19:52 > To: Guo Ren <guoren@xxxxxxxxxx> > > On Wed, Jun 16, 2021 at 1:46 PM Guo Ren <guoren@xxxxxxxxxx> wrote: > > > > Hi Matteo, > > > > Have you tried Glibc generic implementation code? > > ref: https://lore.kernel.org/linux-arch/20190629053641.3iBfk9- > I_D29cDp9yJnIdIg7oMtHNZlDmhLQPTumhEc@z/#t > > > > If Glibc codes have the same performance in your hardware, then you > > could give a generic implementation first. Isn't that a byte copy loop - the performance of that ought to be terrible. ... > I had a look, it seems that it's a C unrolled version with the > 'register' keyword. > The same one was already merged in nios2: > https://elixir.bootlin.com/linux/latest/source/arch/nios2/lib/memcpy.c#L68 I know a lot about the nios2 instruction timings. (I've looked at code execution in the fpga's intel 'logic analiser.) It is a very simple 4-clock pipeline cpu with a 2-clock delay before a value read from 'tightly coupled memory' (aka cache) can be used in another instruction. There is also a subtle pipeline stall if a read follows a write to the same memory block because the write is executed one clock later - and would collide with the read. Since it only ever executes one instruction per clock loop unrolling does help - since you never get the loop control 'for free'. OTOH you don't need to use that many registers. But an unrolled loop should approach 2 bytes/clock (32bit cpu). > I copied _wordcopy_fwd_aligned() from Glibc, and I have a very similar > result of the other versions: > > [ 563.359126] Strings selftest: memcpy(src+7, dst+7): 257 Mb/s What clock speed is that running at? It seems very slow for a 64bit cpu (that isn't an fpga soft-cpu). While the small riscv cpu might be similar to the nios2 (and mips for that matter), there are also bigger/faster cpu. I'm sure these can execute multiple instructions/clock and possible even read and write at the same time. Unless they also support significant instruction re-ordering the trivial copy loops are going to be slow on such cpu. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
