Re: [PATCH v2 1/3] lib/string: optimized memcpy

[Matteo Croce]

On Fri, Jul 2, 2021 at 4:37 PM Ben Dooks <ben.dooks@xxxxxxxxxxxxxxx> wrote:
>
> On 02/07/2021 13:31, Matteo Croce wrote:
> > From: Matteo Croce <mcroce@xxxxxxxxxxxxx>
> >
> > Rewrite the generic memcpy() to copy a word at time, without generating
> > unaligned accesses.
> >
> > The procedure is made of three steps:
> > First copy data one byte at time until the destination buffer is aligned
> > to a long boundary.
> > Then copy the data one long at time shifting the current and the next long
> > to compose a long at every cycle.
> > Finally, copy the remainder one byte at time.
> >
> > This is the improvement on RISC-V:
> >
> > original aligned:      75 Mb/s
> > original unaligned:    75 Mb/s
> > new aligned:          114 Mb/s
> > new unaligned:                107 Mb/s
> >
> > and this the binary size increase according to bloat-o-meter:
> >
> > Function     old     new   delta
> > memcpy        36     324    +288
> >
> >
> > Signed-off-by: Matteo Croce <mcroce@xxxxxxxxxxxxx>
> > ---
> >   lib/string.c | 80 ++++++++++++++++++++++++++++++++++++++++++++++++++--
> >   1 file changed, 77 insertions(+), 3 deletions(-)
>
> Doesn't arch/riscv/lib/memcpy.S also exist for an architecture
> optimised version? I would have thought the lib/string.c version
> was not being used?
>
>

Yes, but this series started as C replacement for the assembly one,
which generates unaligned accesses.
Unfortunately the existing RISC-V processors can't handle unaligned
accesses, so they are emulated with a terrible slowdown.
Then, since there wasn't any riscv specific code, it was proposed as
generic code:

Discussion: https://lore.kernel.org/linux-riscv/20210617152754.17960-1-mcroce@xxxxxxxxxxxxxxxxxxx/

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