strmap: take advantage of FLEXPTR_ALLOC_STR when relevant

By default, we do not use a mempool and strdup_strings is true; in this
case, we can avoid both an extra allocation and an extra free by just
over-allocating for the strmap_entry leaving enough space at the end to
copy the key.  FLEXPTR_ALLOC_STR exists for exactly this purpose, so
make use of it.

Also, adjust the case when we are using a memory pool and strdup_strings
is true to just do one allocation from the memory pool instead of two so
that the strmap_clear() and strmap_remove() code can just avoid freeing
the key in all cases.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strmap.c

@@ -59,11 +59,8 @@ static void strmap_free_entries_(struct strmap *map, int free_values)
 	hashmap_for_each_entry(&map->map, &iter, e, ent) {
 		if (free_values)
 			free(e->value);
-		if (!map->pool) {
-			if (map->strdup_strings)
-				free((char*)e->key);
+		if (!map->pool)
 			free(e);
-		}
 	}
 }
 
@@ -84,16 +81,25 @@ static struct strmap_entry *create_entry(struct strmap *map,
 					 void *data)
 {
 	struct strmap_entry *entry;
-	const char *key = str;
 
-	entry = map->pool ? mem_pool_alloc(map->pool, sizeof(*entry))
-			  : xmalloc(sizeof(*entry));
+	if (map->strdup_strings) {
+		if (!map->pool) {
+			FLEXPTR_ALLOC_STR(entry, key, str);
+		} else {
+			size_t len = st_add(strlen(str), 1); /* include NUL */
+			entry = mem_pool_alloc(map->pool,
+					       st_add(sizeof(*entry), len));
+			memcpy(entry + 1, str, len);
+			entry->key = (void *)(entry + 1);
+		}
+	} else if (!map->pool) {
+		entry = xmalloc(sizeof(*entry));
+	} else {
+		entry = mem_pool_alloc(map->pool, sizeof(*entry));
+	}
 	hashmap_entry_init(&entry->ent, strhash(str));
-
-	if (map->strdup_strings)
-		key = map->pool ? mem_pool_strdup(map->pool, str)
-				: xstrdup(str);
-	entry->key = key;
+	if (!map->strdup_strings)
+		entry->key = str;
 	entry->value = data;
 	return entry;
 }
@@ -139,11 +145,8 @@ void strmap_remove(struct strmap *map, const char *str, int free_value)
 		return;
 	if (free_value)
 		free(ret->value);
-	if (!map->pool) {
-		if (map->strdup_strings)
-			free((char*)ret->key);
+	if (!map->pool)
 		free(ret);
-	}
 }
 
 void strintmap_incr(struct strintmap *map, const char *str, intptr_t amt)

strmap.h

@@ -14,6 +14,7 @@ struct strmap_entry {
 	struct hashmap_entry ent;
 	const char *key;
 	void *value;
+	/* strmap_entry may be allocated extra space to store the key at end */
 };
 
 int cmp_strmap_entry(const void *hashmap_cmp_fn_data,