[compiler] Prevent empty weak function stubs from being removed

Even with the noinline specifier added by commit 1a260f8, gcc may
skip calls to non-inlinable functions that it knows have no side
effects. This caused the get_cached_dhcpack() call in start_dhcp(),
the weak stub of which has no code in its body, to be removed,
preventing cached DHCP from working.

Fix by adding a __keepme macro to compiler.h expanding to asm(""),
as recommended by gcc's info page, and using it in the weak stub
for get_cached_dhcpack().

Reported-by: Aaron Brooks <aaron@brooks1.net>
Tested-by: Aaron Brooks <aaron@brooks1.net>
Signed-off-by: Joshua Oreman <oremanj@rwcr.net>

diff --git a/src/include/compiler.h b/src/include/compiler.h
index 926308b..88aa242 100644 (file)
--- a/src/include/compiler.h
+++ b/src/include/compiler.h
@@ -188,7 +188,15 @@ REQUEST_EXPANDED ( CONFIG_SYMBOL );
  * used).  This results in the non-weak symbol never being used, so explicitly
  * mark the function as noinline to prevent inlining.
  */
-#define __weak         __attribute__ (( weak )) __attribute (( noinline ))
+#define __weak         __attribute__ (( weak, noinline ))
+
+/** Prevent a function from being optimized away without inlining
+ *
+ * Calls to functions with void return type that contain no code in their body
+ * may be removed by gcc's optimizer even when inlining is inhibited. Placing
+ * this macro in the body of the function prevents that from occurring.
+ */
+#define __keepme       asm("");
 
 #endif
 

diff --git a/src/net/udp/dhcp.c b/src/net/udp/dhcp.c
index 07c90e9..4bfcb80 100644 (file)
--- a/src/net/udp/dhcp.c
+++ b/src/net/udp/dhcp.c
@@ -1409,7 +1409,7 @@ static struct sockaddr dhcp_peer = {
 /**
  * Get cached DHCPACK where none exists
  */
-__weak void get_cached_dhcpack ( void ) {}
+__weak void get_cached_dhcpack ( void ) { __keepme }
 
 /**
  * Start DHCP state machine on a network device