add pure & const function attributes suggested by gcc -Wsuggest-attribute

This only covers the ones in the pre-written code.  There are many more
suggested in the generated code, which will require changing the generator
and will thus be handled separately.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Part-of: <https://gitlab.freedesktop.org/xorg/lib/libxcb/-/merge_requests/64>

src/xcb.h

@@ -51,12 +51,32 @@ extern "C" {
 # define __has_attribute(x) 0  /* Compatibility with older compilers. */
 #endif
 
+/*
+ * For the below checks, we currently assume that __GNUC__ indicates
+ * gcc 3.0 (released 2001) or later, as we require support for C99.
+ */
+
+/* Supported in gcc 2.5 and later */
+#if defined(__GNUC__) || __has_attribute(__const__)
+#define XCB_CONST_FUNCTION __attribute__((__const__))
+#else
+#define XCB_CONST_FUNCTION XCB_PURE_FUNCTION
+#endif
+
+/* Supported in gcc 2.7 and later */
 #if defined(__GNUC__) ||  __has_attribute(__packed__)
 #define XCB_PACKED __attribute__((__packed__))
 #else
 #define XCB_PACKED
 #endif
 
+/* Supported in gcc 2.96 and later */
+#if defined(__GNUC__) || __has_attribute(__pure__)
+#define XCB_PURE_FUNCTION __attribute__((__pure__))
+#else
+#define XCB_PURE_FUNCTION
+#endif
+
 /**
  * @defgroup XCB_Core_API XCB Core API
  * @brief Core API of the XCB library.
@@ -469,6 +489,7 @@ void xcb_prefetch_extension_data(xcb_connection_t *c, xcb_extension_t *ext);
  *
  * The result must not be freed.
  */
+XCB_PURE_FUNCTION
 const struct xcb_setup_t *xcb_get_setup(xcb_connection_t *c);
 
 /**
@@ -479,6 +500,7 @@ const struct xcb_setup_t *xcb_get_setup(xcb_connection_t *c);
  * Accessor for the file descriptor that was passed to the
  * xcb_connect_to_fd call that returned @p c.
  */
+XCB_PURE_FUNCTION
 int xcb_get_file_descriptor(xcb_connection_t *c);
 
 /**
@@ -499,6 +521,7 @@ int xcb_get_file_descriptor(xcb_connection_t *c);
  * @return XCB_CONN_CLOSED_PARSE_ERR, error during parsing display string.
  * @return XCB_CONN_CLOSED_INVALID_SCREEN, because the server does not have a screen matching the display.
  */
+XCB_PURE_FUNCTION
 int xcb_connection_has_error(xcb_connection_t *c);
 
 /**

src/xcbext.h

@@ -297,6 +297,7 @@ int xcb_poll_for_reply64(xcb_connection_t *c, uint64_t request, void **reply, xc
  * @param replylen The size of the reply.
  * @return Pointer to the location where received file descriptors are stored.
  */
+XCB_CONST_FUNCTION
 int *xcb_get_reply_fds(xcb_connection_t *c, void *reply, size_t replylen);
 
 
@@ -306,6 +307,7 @@ int *xcb_get_reply_fds(xcb_connection_t *c, void *reply, size_t replylen);
  * @param mask The mask to check
  * @return The number of set bits in the mask
  */
+XCB_CONST_FUNCTION
 int xcb_popcount(uint32_t mask);
 
 /**
@@ -313,6 +315,7 @@ int xcb_popcount(uint32_t mask);
  * @param len The length of the array
  * @return The sum of all entries in the array.
  */
+XCB_PURE_FUNCTION
 int xcb_sumof(uint8_t *list, int len);
 
 #ifdef __cplusplus

src/xcbint.h

@@ -225,6 +225,7 @@ struct xcb_connection_t {
 
 void _xcb_conn_shutdown(xcb_connection_t *c, int err);
 
+XCB_CONST_FUNCTION
 xcb_connection_t *_xcb_conn_ret_error(int err);
 
 int _xcb_conn_wait(xcb_connection_t *c, pthread_cond_t *cond, struct iovec **vector, int *count);