[ovs-dev] [PATCH] test-atomic: Fix warnings for GCC4.9 and sparse

[Daniele Di Proietto]

There's no reason for the local variable 'old_count' to be atomic. In fact, if
it is atomic it triggers a GCC4.9 warning (Wunused-value)
The global variables 'a' and 'paux' could be static, according to sparse.

Signed-off-by: Daniele Di Proietto <ddiproietto at vmware.com>
---
There's something interesting about the warning: GCC does not complain about
'old_count' being atomic, but about "right-hand operand of comma expression
having no effect" (the comma comes from OVS macro atomic_add_explicit()).

I've isolated a test case to trigger the warning:

/* main.c */
#include <stdatomic.h>
#include <stdint.h>

int main()
{
    _Atomic(uint32_t) a = 0;
#ifdef TEST_ATOMIC_DST
    _Atomic(uint32_t) b = 0;
#else
    uint32_t b = 0;
#endif
    b = atomic_fetch_add_explicit ((&a), (1), (memory_order_seq_cst)),
        (void) 0;

    return b;
}

When compiled with

    gcc main.c -Wall

there's no warning. When compiled with 

    gcc main.c -Wall -DTEST_ATOMIC_DST

GCC complains with

main.c: In function ‘main’:
main.c:13:70: warning: right-hand operand of comma expression has no effect
[-Wunused-value]
     b = atomic_fetch_add_explicit ((&a), (1), (memory_order_seq_cst)),
                                                                      ^
I'm using gcc (Debian 4.9.1-1) 4.9.1
---
 tests/test-atomic.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/tests/test-atomic.c b/tests/test-atomic.c
index ca293ba..31c06fa 100644
--- a/tests/test-atomic.c
+++ b/tests/test-atomic.c
@@ -167,7 +167,7 @@ test_atomic_flag(void)
     ovs_assert(atomic_flag_test_and_set(&flag) == false);
 }
 
-uint32_t a;
+static uint32_t a;
 
 struct atomic_aux {
     atomic_uint32_t count;
@@ -175,7 +175,7 @@ struct atomic_aux {
     ATOMIC(uint32_t *) data;
 };
 
-ATOMIC(struct atomic_aux *) paux = ATOMIC_VAR_INIT(NULL);
+static ATOMIC(struct atomic_aux *) paux = ATOMIC_VAR_INIT(NULL);
 static struct atomic_aux *auxes = NULL;
 
 #define ATOMIC_ITEM_COUNT 1000000
@@ -273,7 +273,7 @@ static void *
 atomic_writer(void *aux_)
 {
     struct atomic_aux *aux = aux_;
-    atomic_uint32_t old_count;
+    uint32_t old_count;
     uint32_t *data;
     size_t i;
 
-- 
2.0.1