do O(1) instead of O(n) atomic operations in register functions

about 1ms faster startup time
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/utils.c

@@ -206,7 +206,8 @@ av_cold void avcodec_register(AVCodec *codec)
     avcodec_init();
     p = &first_avcodec;
     codec->next = NULL;
-    while(avpriv_atomic_ptr_cas((void * volatile *)p, NULL, codec))
+
+    while(*p || avpriv_atomic_ptr_cas((void * volatile *)p, NULL, codec))
         p = &(*p)->next;
 
     if (codec->init_static_data)
@@ -3191,7 +3192,7 @@ void av_register_hwaccel(AVHWAccel *hwaccel)
 {
     AVHWAccel **p = &first_hwaccel;
     hwaccel->next = NULL;
-    while(avpriv_atomic_ptr_cas((void * volatile *)p, NULL, hwaccel))
+    while(*p || avpriv_atomic_ptr_cas((void * volatile *)p, NULL, hwaccel))
         p = &(*p)->next;
 }
 

libavfilter/avfilter.c

@@ -490,7 +490,7 @@ int avfilter_register(AVFilter *filter)
 
     filter->next = NULL;
 
-    while(avpriv_atomic_ptr_cas((void * volatile *)f, NULL, filter))
+    while(*f || avpriv_atomic_ptr_cas((void * volatile *)f, NULL, filter))
         f = &(*f)->next;
 
     return 0;

libavformat/format.c

@@ -54,7 +54,7 @@ void av_register_input_format(AVInputFormat *format)
     AVInputFormat **p = &first_iformat;
 
     format->next = NULL;
-    while(avpriv_atomic_ptr_cas((void * volatile *)p, NULL, format))
+    while(*p || avpriv_atomic_ptr_cas((void * volatile *)p, NULL, format))
         p = &(*p)->next;
 }
 
@@ -63,7 +63,7 @@ void av_register_output_format(AVOutputFormat *format)
     AVOutputFormat **p = &first_oformat;
 
     format->next = NULL;
-    while(avpriv_atomic_ptr_cas((void * volatile *)p, NULL, format))
+    while(*p || avpriv_atomic_ptr_cas((void * volatile *)p, NULL, format))
         p = &(*p)->next;
 }