Re: [PATCH 1/2] riscv: Add memmove string operation.

[Palmer Dabbelt]

On Mon, 12 Aug 2019 08:04:46 PDT (-0700), Christoph Hellwig wrote:
> On Wed, Aug 07, 2019 at 03:19:14PM +0800, Nick Hu wrote:
> > There are some features which need this string operation for compilation,
> > like KASAN. So the purpose of this porting is for the features like KASAN
> > which cannot be compiled without it.
> >
> > KASAN's string operations would replace the original string operations and
> > call for the architecture defined string operations. Since we don't have
> > this in current kernel, this patch provides the implementation.
> >
> > This porting refers to the 'arch/nds32/lib/memmove.S'.
>
> This looks sensible to me, although my stringop asm is rather rusty,
> so just an ack and not a real review-by:
>
> Acked-by: Christoph Hellwig <hch@xxxxxx>

FWIW, we just write this in C everywhere else and rely on the compiler to 
unroll the loops.  I always prefer C to assembly when possible, so I'd prefer 
if we just adopt the string code from newlib.  We have a RISC-V-specific memcpy 
in there, but just use the generic memmove.

Maybe the best bet here would be to adopt the newlib memcpy/memmove as generic 
Linux functions?  They're both in C so they should be fine, and they both look 
faster than what's in lib/string.c.  Then everyone would benefit and we don't 
need this tricky RISC-V assembly.  Also, from the look of it the newlib code is 
faster because the inner loop is unrolled.