[U-Boot] [PATCH 2/3] string: Provide a slimmed-down memset()

[Alexander Graf]

On 27/03/2017 01:38, Simon Glass wrote:
> Most of the time the optimised memset() is what we want. For extreme
> situations such as TPL it may be too large. For example on the 'rock'
> board, using a simple loop saves a useful 48 bytes. With gcc 4.9 and
> the rodata bug, this patch is enough to reduce the TPL image below the
> limit.
>
> Signed-off-by: Simon Glass <sjg at chromium.org>
> ---
>
>  lib/Kconfig  | 9 +++++++++
>  lib/string.c | 6 ++++--
>  2 files changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/lib/Kconfig b/lib/Kconfig
> index 65c01573e1..5bf512d8c0 100644
> --- a/lib/Kconfig
> +++ b/lib/Kconfig
> @@ -52,6 +52,15 @@ config LIB_RAND
>  	help
>  	  This library provides pseudo-random number generator functions.
>
> +config FAST_MEMSET
> +	bool "Use an optimised memset()"
> +	default y
> +	help
> +	  The faster memset() is the arch-specific one (if available) enabled
> +	  by CONFIG_USE_ARCH_MEMSET. If that is not enabled, we can still get
> +	  better performance by write a word at a time. Disable this option
> +	  to reduce code size slightly at the cost of some speed.

The comment sounds slightly confused - it took me a few times of reading 
it until I grasped what it was trying to tell me :).

> +
>  source lib/dhry/Kconfig
>
>  source lib/rsa/Kconfig
> diff --git a/lib/string.c b/lib/string.c
> index 67d5f6a421..159493ed17 100644
> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -437,8 +437,10 @@ char *strswab(const char *s)
>  void * memset(void * s,int c,size_t count)
>  {
>  	unsigned long *sl = (unsigned long *) s;
> -	unsigned long cl = 0;
>  	char *s8;
> +
> +#ifdef CONFIG_FAST_MEMSET
> +	unsigned long cl = 0;
>  	int i;
>
>  	/* do it one word at a time (32 bits or 64 bits) while possible */
> @@ -452,7 +454,7 @@ void * memset(void * s,int c,size_t count)
>  			count -= sizeof(*sl);
>  		}
>  	}
> -	/* fill 8 bits at a time */
> +#endif	/* fill 8 bits at a time */

So while this is all neat, a few ideas:

1) Would having memset in a header improve things even more? After all, 
each external function call clobbers registers that you need to 
save/restore...

2) How much would GOLD save you? Have you tried? U-Boot is small enough 
of a code base that global optimizations should be able to give 
significant size savings.


Alex