[libc] Reduce overall code size by externalising strncmp()

Typical saving is 20-30 bytes in each file using strncmp().

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86/core/x86_string.c

 		return __memcpy_reverse ( dest, src, len );
 	}
 }
+
+/**
+ * Compare strings (up to a specified length)
+ *
+ * @v str1		First string
+ * @v str2		Second string
+ * @v len		Maximum length
+ * @ret diff		Difference
+ */
+int strncmp ( const char *str1, const char *str2, size_t len ) {
+	const void *discard_S;
+	const void *discard_D;
+	size_t discard_c;
+	int diff;
+
+	__asm__ __volatile__ ( "\n1:\n\t"
+			       "dec %2\n\t"
+			       "js 2f\n\t"
+			       "lodsb\n\t"
+			       "scasb\n\t"
+			       "jne 3f\n\t"
+			       "testb %b3, %b3\n\t"
+			       "jnz 1b\n\t"
+			       /* Equal */
+			       "\n2:\n\t"
+			       "xor %3, %3\n\t"
+			       "jmp 4f\n\t"
+			       /* Not equal; CF indicates difference */
+			       "\n3:\n\t"
+			       "sbb %3, %3\n\t"
+			       "orb $1, %b3\n\t"
+			       "\n4:\n\t"
+			       : "=&S" ( discard_S ), "=&D" ( discard_D ),
+				 "=&c" ( discard_c ), "=&a" ( diff )
+			       : "0" ( str1 ), "1" ( str2 ), "2" ( len ) );
+
+	return diff;
+}

src/arch/x86/include/bits/string.h

 }
 
 #define __HAVE_ARCH_STRNCMP
-static inline int strncmp(const char * cs,const char * ct,size_t count)
-{
-register int __res;
-int d0, d1, d2;
-__asm__ __volatile__(
-	"1:\tdecl %3\n\t"
-	"js 2f\n\t"
-	"lodsb\n\t"
-	"scasb\n\t"
-	"jne 3f\n\t"
-	"testb %%al,%%al\n\t"
-	"jne 1b\n"
-	"2:\txorl %%eax,%%eax\n\t"
-	"jmp 4f\n"
-	"3:\tsbbl %%eax,%%eax\n\t"
-	"orb $1,%%al\n"
-	"4:"
-		     :"=a" (__res), "=&S" (d0), "=&D" (d1), "=&c" (d2)
-		     :"1" (cs),"2" (ct),"3" (count));
-return __res;
-}
+
+extern int strncmp ( const char *str1, const char *str2, size_t len );
 
 #define __HAVE_ARCH_STRLEN
 static inline size_t strlen(const char * s)