MixerAll, Volume: add "pure" attributes

src/MixerAll.hxx

@@ -25,10 +25,13 @@
 #ifndef MPD_MIXER_ALL_HXX
 #define MPD_MIXER_ALL_HXX
 
+#include "Compiler.h"
+
 /**
  * Returns the average volume of all available mixers (range 0..100).
  * Returns -1 if no mixer can be queried.
  */
+gcc_pure
 int
 mixer_all_get_volume(void);
 
@@ -46,6 +49,7 @@ mixer_all_set_volume(unsigned volume);
  * software mixers.  See #software_mixer_plugin.  This function fails
  * if no software mixer is configured.
  */
+gcc_pure
 int
 mixer_all_get_software_volume(void);
 

src/Volume.hxx

@@ -20,12 +20,15 @@
 #ifndef MPD_VOLUME_HXX
 #define MPD_VOLUME_HXX
 
+#include "Compiler.h"
+
 #include <stdio.h>
 
 void volume_init(void);
 
 void volume_finish(void);
 
+gcc_pure
 int volume_level_get(void);
 
 bool volume_level_change(unsigned volume);
@@ -41,6 +44,7 @@ void save_sw_volume_state(FILE *fp);
  * determine whether the state has changed and the state file should
  * be saved.
  */
+gcc_pure
 unsigned
 sw_volume_state_get_hash(void);