libbpf: Extract generic string hashing function for reuse

Calculating a hash of zero-terminated string is a common need when using
hashmap, so extract it for reuse.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200926011357.2366158-5-andriin@fb.com

tools/lib/bpf/btf_dump.c

@@ -90,14 +90,7 @@ struct btf_dump {
 
 static size_t str_hash_fn(const void *key, void *ctx)
 {
-	const char *s = key;
-	size_t h = 0;
-
-	while (*s) {
-		h = h * 31 + *s;
-		s++;
-	}
-	return h;
+	return str_hash(key);
 }
 
 static bool str_equal_fn(const void *a, const void *b, void *ctx)

tools/lib/bpf/hashmap.h

@@ -25,6 +25,18 @@ static inline size_t hash_bits(size_t h, int bits)
 #endif
 }
 
+/* generic C-string hashing function */
+static inline size_t str_hash(const char *s)
+{
+	size_t h = 0;
+
+	while (*s) {
+		h = h * 31 + *s;
+		s++;
+	}
+	return h;
+}
+
 typedef size_t (*hashmap_hash_fn)(const void *key, void *ctx);
 typedef bool (*hashmap_equal_fn)(const void *key1, const void *key2, void *ctx);