[UNIKRAFT PATCH v2] lib/ukalloc: make uk_alloc_get_default static inline

[Hugo Lefeuvre <hugo.lefeuvre@xxxxxxxxx>]

Every allocation done through the libc malloc wrapper triggers a function
call to uk_alloc_get_default in order to get a pointer to the default
allocator.  This is not great, because the allocation path is performance
critical, and we should avoid any superflous overhead there.

This patch removes the capability to change the default allocator once it
has been set. This never really worked anyways... the libc wrapper would
forward free() calls to the new allocator, potentially resulting in memory
corruption if the new allocator doesn't handle these foreign pointers
correctly, and memory leaks because these pointers won't be freed anymore.

We make the uk_alloc_head variable global and uk_alloc_get_default static
inline. We rename uk_alloc_head to _uk_alloc_head -- it should preferably
be accessed through uk_alloc_get_default.

Setting aside compiler optimizations, retrieving the default allocator
becomes less expensive: a single memory read instead of a function call +
memory read.

It appears that uk_alloc_set_default was not called anywhere, so this patch
does not require any additional changes in external libraries.

Signed-off-by: Hugo Lefeuvre <hugo.lefeuvre@xxxxxxxxx>
---
 lib/ukalloc/alloc.c            | 45 ++++--------------------------------------
 lib/ukalloc/exportsyms.uk      |  2 +-
 lib/ukalloc/include/uk/alloc.h | 10 +++++++---
 3 files changed, 12 insertions(+), 45 deletions(-)

diff --git a/lib/ukalloc/alloc.c b/lib/ukalloc/alloc.c
index 437bc4b..127bc6f 100644
--- a/lib/ukalloc/alloc.c
+++ b/lib/ukalloc/alloc.c
@@ -57,14 +57,14 @@
        (ALIGN_UP((unsigned long)(size), __PAGE_SIZE) / __PAGE_SIZE)
 #define page_off(x) ((unsigned long)(x) & (__PAGE_SIZE - 1))
 
-static struct uk_alloc *uk_alloc_head;
+struct uk_alloc *_uk_alloc_head;
 
 int uk_alloc_register(struct uk_alloc *a)
 {
-       struct uk_alloc *this = uk_alloc_head;
+       struct uk_alloc *this = _uk_alloc_head;
 
-       if (!uk_alloc_head) {
-               uk_alloc_head = a;
+       if (!_uk_alloc_head) {
+               _uk_alloc_head = a;
                a->next = NULL;
                return 0;
        }
@@ -76,43 +76,6 @@ int uk_alloc_register(struct uk_alloc *a)
        return 0;
 }
 
-struct uk_alloc *uk_alloc_get_default(void)
-{
-       return uk_alloc_head;
-}
-
-int uk_alloc_set_default(struct uk_alloc *a)
-{
-       struct uk_alloc *head, *this, *prev;
-
-       head = uk_alloc_get_default();
-
-       if (a == head)
-               return 0;
-
-       if (!head) {
-               uk_alloc_head = a;
-               return 0;
-       }
-
-       this = head;
-       while (this->next) {
-               prev = this;
-               this = this->next;
-               if (a == this) {
-                       prev->next = this->next;
-                       this->next = head->next;
-                       head = this;
-                       return 0;
-               }
-       }
-
-       /* a is not registered yet. Add in front of the queue. */
-       a->next = head;
-       uk_alloc_head = a;
-       return 0;
-}
-
 struct metadata_ifpages {
        unsigned long   num_pages;
        void            *base;
diff --git a/lib/ukalloc/exportsyms.uk b/lib/ukalloc/exportsyms.uk
index 2357501..2c8a90f 100644
--- a/lib/ukalloc/exportsyms.uk
+++ b/lib/ukalloc/exportsyms.uk
@@ -1,6 +1,5 @@
 uk_alloc_register
 uk_alloc_get_default
-uk_alloc_set_default
 uk_malloc_ifpages
 uk_free_ifpages
 uk_realloc_ifpages
@@ -10,3 +9,4 @@ uk_memalign_compat
 uk_realloc_compat
 uk_palloc_compat
 uk_pfree_compat
+_uk_alloc_head
diff --git a/lib/ukalloc/include/uk/alloc.h b/lib/ukalloc/include/uk/alloc.h
index 8ab41d6..5bfa2f2 100644
--- a/lib/ukalloc/include/uk/alloc.h
+++ b/lib/ukalloc/include/uk/alloc.h
@@ -53,9 +53,6 @@ extern "C" {
 #define uk_zalloc(a, size)  uk_calloc(a, 1, size)
 #define uk_do_zalloc(a, size) uk_do_calloc(a, 1, size)
 
-struct uk_alloc *uk_alloc_get_default(void);
-int uk_alloc_set_default(struct uk_alloc *a);
-
 typedef void* (*uk_alloc_malloc_func_t)
                (struct uk_alloc *a, size_t size);
 typedef void* (*uk_alloc_calloc_func_t)
@@ -103,6 +100,13 @@ struct uk_alloc {
        int8_t priv[];
 };
 
+extern struct uk_alloc *_uk_alloc_head;
+
+static inline struct uk_alloc *uk_alloc_get_default(void)
+{
+       return _uk_alloc_head;
+}
+
 /* wrapper functions */
 static inline void *uk_do_malloc(struct uk_alloc *a, size_t size)
 {
-- 
2.7.4