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

Alexander Graf
2017-03-27T07:14:47Z

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@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