[PATCH RFC] include/linux: Constant folding for kernel core_param

[Iulia Manda]

This patch aims to prove GCC's capability to constant fold kernel command line
arguments, if they are specified at compile time and no argument is passed at
boot time.

At the moment, this patch treats only core_param arguments and exemplified on
initcall_debug. Also, initcall_debug variable is set to a default value,
known at compile time.

Because at some point we need to access the address of var, it can not be const.
This is why we need to add a dummy variable that does not affect the rest of
the code and only provides us with a valid address of var, knowing that in this
case that address will never be used.

Bloat-o-meter output:

add/remove: 1/0 grow/shrink: 0/2 up/down: 1/-95 (-94)
function                                     old     new   delta
__initcall_debug                               -       1      +1
async_run_entry_fn                           178     174      -4
do_one_initcall                              339     248     -91

Signed-off-by: Iulia Manda <iulia.manda21 at gmail.com>
---
 include/linux/init.h        |    4 ++++
 include/linux/moduleparam.h |    8 ++++++++
 init/main.c                 |    4 ++++
 3 files changed, 16 insertions(+)

diff --git a/include/linux/init.h b/include/linux/init.h
index 2df8e8d..cb96cd3 100644
--- a/include/linux/init.h
+++ b/include/linux/init.h
@@ -155,7 +155,11 @@ int __init init_rootfs(void);
 
 extern void (*late_time_init)(void);
 
+#ifdef CONFIG_CMDLINE_OVERRIDE
+extern const bool initcall_debug;
+#else
 extern bool initcall_debug;
+#endif
 
 #endif
   
diff --git a/include/linux/moduleparam.h b/include/linux/moduleparam.h
index 1c9effa..4d42302 100644
--- a/include/linux/moduleparam.h
+++ b/include/linux/moduleparam.h
@@ -310,8 +310,16 @@ static inline void __kernel_param_unlock(void)
 #define core_param(name, var, type, perm)				\
 	param_check_##type(name, &(var));				\
 	__module_param_call("", name, &param_ops_##type, &var, perm, -1, 0)
+
+#ifdef CONFIG_CMDLINE_OVERRIDE
+#define core_default_param(name, var, val, type, perm)			\
+        const type name = val;                                          \
+        type __##var;                                                   \
+        core_param(name, __##var, type, perm);
+#endif
 #endif /* !MODULE */
 
+
 /**
  * module_param_string - a char array parameter
  * @name: the name of the parameter
diff --git a/init/main.c b/init/main.c
index 61b99376..9a9fd10 100644
--- a/init/main.c
+++ b/init/main.c
@@ -698,8 +698,12 @@ static void __init do_ctors(void)
 #endif
 }
 
+#ifdef CONFIG_CMDLINE_OVERRIDE
+core_default_param(initcall_debug, initcall_debug, 0, bool, 0644);
+#else
 bool initcall_debug;
 core_param(initcall_debug, initcall_debug, bool, 0644);
+#endif
 
 #ifdef CONFIG_KALLSYMS
 struct blacklist_entry {
-- 
1.7.10.4