[PATCH] string: Use memcpy() within memmove() when we can

[Rasmus Villemoes]

On 25/11/2020 12.26, Patrick Delaunay wrote:
> A common use of memmove() can be handled by memcpy(). Also memcpy()
> includes an optimization for large sizes: it copies a word at a time. So
> we can get a speed-up by calling memcpy() to handle our move in this case.
> 
> Update memmove() to call also memcpy() if the source don't overlap
> the destination (src + count <= dest).
> 
> Signed-off-by: Patrick Delaunay <patrick.delaunay at st.com>
> ---
> This patch allows to save 38ms for Kernel Image extraction (7327624 Bytes)
> from FIT loaded at 0xC2000000 for ARMV7 board STM32MP157C-EV1,
> and with kernel destination = Load Address: 0xc4000000,
> located after the FIT without overlap, compared with
> destination = Load Address: 0xc0008000.
> 
> -> 14,332 us vs 52,239 in bootstage report
> 
> In this case the memmove funtion is called in common/image.c::memmove_wd()
> to handle overlap.
> 
> 
>  lib/string.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/string.c b/lib/string.c
> index ae7835f600..ef8ead976c 100644
> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -567,7 +567,7 @@ void * memmove(void * dest,const void *src,size_t count)
>  {
>  	char *tmp, *s;
>  
> -	if (dest <= src) {
> +	if (dest <= src || (src + count) <= dest) {
>  		memcpy(dest, src, count);

Hm. So the condition you add (src + count <= dest) implies no overlap at
all, so that's ok. So this doesn't really have anything to do with your
patch per se.

The existing condition relies on memcpy doing forward-copying. That's
the case if the implementation from lib/string.c is in use, i.e. if
__HAVE_ARCH_MEMCPY is not defined. And if an arch defines
__HAVE_ARCH_MEMMOVE, this memmove() is not used.

But AFAICT, there's a potential problem for the case where
__HAVE_ARCH_MEMCPY is defined but __HAVE_ARCH_MEMMOVE is not, and e.g.
arch/arm/include/asm/string.h does

#if CONFIG_IS_ENABLED(USE_ARCH_MEMCPY)
#define __HAVE_ARCH_MEMCPY
#endif
#undef __HAVE_ARCH_MEMMOVE

Of course, the arch-specific implementation _may_ also do
forward-copying (I haven't tried to decipher it), but that seems to be a
rather fragile assumption. At the very least, some comments would be in
order.

Rasmus