Optimize ffs() implementation

- Use gcc builtin
- Define as a static inline function in the .h so no function call overhead is needed
- Keep the function in libroot for backwards compatibility
- Remove a duplicate implementation in the freebsd compatibility layer

gcc2 does not document the builtin, but it is in fact already available
there as well.

Fixes #3281.

Change-Id: I94f8a2548637aa70e85febbfab06f07c1a427005
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2605
Reviewed-by: waddlesplash <[email protected]>

headers/posix/strings.h

@@ -13,7 +13,7 @@
 extern "C" {
 #endif
 
-extern int ffs(int i);
+static inline int ffs(int i) { return __builtin_ffs(i); }
 
 extern int		strcasecmp(const char *string1, const char *string2);
 extern int		strncasecmp(const char *string1, const char *string2,

src/libs/compat/freebsd_network/device.c

@@ -624,23 +624,6 @@ printf(const char *format, ...)
 }
 
 
-#ifndef __clang__
-int
-ffs(int value)
-{
-	int i = 1;
-
-	if (value == 0)
-		return 0;
-
-	for (; !(value & 1); i++)
-		value >>= 1;
-
-	return i;
-}
-#endif
-
-
 int
 resource_int_value(const char *name, int unit, const char *resname,
 	int *result)

src/system/libroot/posix/string/ffs.cpp

@@ -1,24 +1,12 @@
 /*
- * Copyright 2005, Ingo Weinhold, [email protected].
+ * Copyright 2020, Adrien Destugues <[email protected]>.
  * Distributed under the terms of the MIT License.
  */
 
-#include <strings.h>
 
 // find first (least significant) set bit
-int
+extern "C" int
 ffs(int value)
 {
-	if (!value)
-		return 0;
-
-	// ToDo: This can certainly be optimized (e.g. by binary search). Or not
-	// unlikely there's a single assembler instruction...
-	for (int i = 1; i <= (int)sizeof(value) * 8; i++, value >>= 1) {
-		if (value & 1)
-			return i;
-	}
-
-	// never gets here
-	return 0;
+	return __builtin_ffs(value);
 }