string: Provide a slimmed-down memset()

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: NSimon Glass <sjg@chromium.org>
Reviewed-by: NHeiko Stuebner <heiko@sntech.de>

lib/Kconfig

@@ -52,6 +52,15 @@ config LIB_RAND
 	help
 	  This library provides pseudo-random number generator functions.
 
+config SPL_TINY_MEMSET
+	bool "Use a very small memset() in SPL"
+	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 writing a word at a time. But in very
+	  size-constrained envrionments even this may be too big. Enable this
+	  option to reduce code size slightly at the cost of some speed.
+
 source lib/dhry/Kconfig
 
 source lib/rsa/Kconfig

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;
+
+#if !CONFIG_IS_ENABLED(TINY_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 */
 	s8 = (char *)sl;
 	while (count--)
 		*s8++ = c;