[patch] 2.4: Problems with dead code/data with gcc 3.4

[patch] 2.4: Problems with dead code/data with gcc 3.4

[Maciej W. Rozycki]

Hello,

 After upgrading gcc to 3.4 I've discovered the compiler is now more
serious about discarding dead code or data.  As a result parts of the
kernel that appear unused, such as the ".setup.init" or ".modinfo"
sections, get removed, and the "unused" attribute only masks the problem
by quieting warnings.  However, gcc currently provides another attribute,
"used", that instead of removing warnings, actually tells the compiler
that whatever it's attached to is indeed used, perhaps in a way that
cannot be observed by the compiler.  There's code in init/main.c that
assumes a non-empty ".setup.init" section and failing that leads to
unpredictable behavior (it just hangs for me on a MIPSel/Linux box).

 Here's a patch that makes the kernel work for me for i386/Linux.  It's
based on similar changes that are already present in 2.6 and leaves the
setup as is for gcc versions before 3.3 and using the new attribute for
newer versions, by defining an "__attribute_used__" macro appropriately.

 Marcelo, would consider such a change for past 2.4.25?

 Ralf, for MIPS/Linux the change is insufficient -- due to fragile
constructs in arch/mips/kernel/syscall.c some code is removed, but using
the "used" attribute does not help.  While the code is back again, it's
reordered compared to what's emitted by older compilers and it's enough to
stop it working.  I'm currently investigating a solution, or have you
perhaps looked at the problem already?  It's there for 2.6 as well.

  Maciej

-- 
+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+--------------------------------------------------------------+
+        e-mail: macro@ds2.pg.gda.pl, PGP key available        +

patch-2.4.24-gcc3-0.1
diff -up --recursive --new-file linux-2.4.24.macro/include/linux/compiler.h linux-2.4.24/include/linux/compiler.h
--- linux-2.4.24.macro/include/linux/compiler.h	2001-09-18 14:12:45.000000000 +0000
+++ linux-2.4.24/include/linux/compiler.h	2004-02-12 14:57:06.000000000 +0000
@@ -13,4 +13,10 @@
 #define likely(x)	__builtin_expect((x),1)
 #define unlikely(x)	__builtin_expect((x),0)
 
+#if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 3)
+#define __attribute_used__	__attribute__((__used__))
+#else
+#define __attribute_used__	__attribute__((__unused__))
+#endif
+
 #endif /* __LINUX_COMPILER_H */
diff -up --recursive --new-file linux-2.4.24.macro/include/linux/init.h linux-2.4.24/include/linux/init.h
--- linux-2.4.24.macro/include/linux/init.h	2004-02-12 12:48:26.000000000 +0000
+++ linux-2.4.24/include/linux/init.h	2004-02-12 14:57:06.000000000 +0000
@@ -2,6 +2,7 @@
 #define _LINUX_INIT_H
 
 #include <linux/config.h>
+#include <linux/compiler.h>
 
 /* These macros are used to mark some functions or 
  * initialized data (doesn't apply to uninitialized data)
@@ -67,7 +68,7 @@ extern struct kernel_param __setup_start
 
 #define __setup(str, fn)								\
 	static char __setup_str_##fn[] __initdata = str;				\
-	static struct kernel_param __setup_##fn __attribute__((unused)) __initsetup = { __setup_str_##fn, fn }
+	static struct kernel_param __setup_##fn __attribute_used__ __initsetup = { __setup_str_##fn, fn }
 
 #endif /* __ASSEMBLY__ */
 
@@ -76,12 +77,12 @@ extern struct kernel_param __setup_start
  * or exit time.
  */
 #define __init		__attribute__ ((__section__ (".text.init")))
-#define __exit		__attribute__ ((unused, __section__(".text.exit")))
+#define __exit		__attribute_used__ __attribute__ ((__section__ (".text.exit")))
 #define __initdata	__attribute__ ((__section__ (".data.init")))
-#define __exitdata	__attribute__ ((unused, __section__ (".data.exit")))
-#define __initsetup	__attribute__ ((unused,__section__ (".setup.init")))
-#define __init_call	__attribute__ ((unused,__section__ (".initcall.init")))
-#define __exit_call	__attribute__ ((unused,__section__ (".exitcall.exit")))
+#define __exitdata	__attribute_used__ __attribute__ ((__section__ (".data.exit")))
+#define __initsetup	__attribute_used__ __attribute__ ((__section__ (".setup.init")))
+#define __init_call	__attribute_used__ __attribute__ ((__section__ (".initcall.init")))
+#define __exit_call	__attribute_used__ __attribute__ ((__section__ (".exitcall.exit")))
 
 /* For assembly routines */
 #define __INIT		.section	".text.init","ax"
diff -up --recursive --new-file linux-2.4.24.macro/include/linux/module.h linux-2.4.24/include/linux/module.h
--- linux-2.4.24.macro/include/linux/module.h	2004-02-12 13:12:17.000000000 +0000
+++ linux-2.4.24/include/linux/module.h	2004-02-12 15:08:00.000000000 +0000
@@ -8,6 +8,7 @@
 #define _LINUX_MODULE_H
 
 #include <linux/config.h>
+#include <linux/compiler.h>
 #include <linux/spinlock.h>
 #include <linux/list.h>
 
@@ -202,19 +203,22 @@ extern int try_inc_mod_count(struct modu
 
 /* For documentation purposes only.  */
 
-#define MODULE_AUTHOR(name)						   \
-const char __module_author[] __attribute__((section(".modinfo"))) = 	   \
-"author=" name
-
-#define MODULE_DESCRIPTION(desc)					   \
-const char __module_description[] __attribute__((section(".modinfo"))) =   \
-"description=" desc
+#define MODULE_AUTHOR(name)						\
+const char __module_author[]						\
+  __attribute_used__							\
+  __attribute__((section(".modinfo"))) = "author=" name
+
+#define MODULE_DESCRIPTION(desc)					\
+const char __module_description[]					\
+  __attribute_used__							\
+  __attribute__((section(".modinfo"))) = "description=" desc
 
 /* Could potentially be used by kmod...  */
 
-#define MODULE_SUPPORTED_DEVICE(dev)					   \
-const char __module_device[] __attribute__((section(".modinfo"))) = 	   \
-"device=" dev
+#define MODULE_SUPPORTED_DEVICE(dev)					\
+const char __module_device[]						\
+  __attribute_used__							\
+  __attribute__((section(".modinfo"))) = "device=" dev
 
 /* Used to verify parameters given to the module.  The TYPE arg should
    be a string in the following format:
@@ -229,15 +233,17 @@ const char __module_device[] __attribute
 	s	string
 */
 
-#define MODULE_PARM(var,type)			\
-const char __module_parm_##var[]		\
-__attribute__((section(".modinfo"))) =		\
-"parm_" __MODULE_STRING(var) "=" type
-
-#define MODULE_PARM_DESC(var,desc)		\
-const char __module_parm_desc_##var[]		\
-__attribute__((section(".modinfo"))) =		\
-"parm_desc_" __MODULE_STRING(var) "=" desc
+#define MODULE_PARM(var,type)						\
+const char __module_parm_##var[]					\
+  __attribute_used__							\
+  __attribute__((section(".modinfo"))) =				\
+  "parm_" __MODULE_STRING(var) "=" type
+
+#define MODULE_PARM_DESC(var,desc)					\
+const char __module_parm_desc_##var[]					\
+  __attribute_used__							\
+  __attribute__((section(".modinfo"))) =				\
+  "parm_desc_" __MODULE_STRING(var) "=" desc
 
 /*
  * MODULE_DEVICE_TABLE exports information about devices
@@ -283,9 +289,10 @@ static const struct gtype##_id * __modul
  * 3.	So vendors can do likewise based on their own policies
  */
  
-#define MODULE_LICENSE(license) 	\
-static const char __module_license[] __attribute__((section(".modinfo"))) =   \
-"license=" license
+#define MODULE_LICENSE(license) 					\
+static const char __module_license[]					\
+  __attribute_used__							\
+  __attribute__((section(".modinfo"))) = "license=" license
 
 /* Define the module variable, and usage macros.  */
 extern struct module __this_module;
@@ -296,11 +303,11 @@ extern struct module __this_module;
 #define MOD_IN_USE		__MOD_IN_USE(THIS_MODULE)
 
 #include <linux/version.h>
-static const char __module_kernel_version[] __attribute__((section(".modinfo"))) =
-"kernel_version=" UTS_RELEASE;
+static const char __module_kernel_version[] __attribute_used__
+	__attribute__((section(".modinfo"))) = "kernel_version=" UTS_RELEASE;
 #ifdef MODVERSIONS
-static const char __module_using_checksums[] __attribute__((section(".modinfo"))) =
-"using_checksums=1";
+static const char __module_using_checksums[] __attribute_used__
+	__attribute__((section(".modinfo"))) = "using_checksums=1";
 #endif
 
 #else /* MODULE */

Re: [patch] 2.4: Problems with dead code/data with gcc 3.4

[Marcelo Tosatti]

On Thu, 12 Feb 2004, Maciej W. Rozycki wrote:

> Hello,
>
>  After upgrading gcc to 3.4 I've discovered the compiler is now more
> serious about discarding dead code or data.  As a result parts of the
> kernel that appear unused, such as the ".setup.init" or ".modinfo"
> sections, get removed, and the "unused" attribute only masks the problem
> by quieting warnings.  However, gcc currently provides another attribute,
> "used", that instead of removing warnings, actually tells the compiler
> that whatever it's attached to is indeed used, perhaps in a way that
> cannot be observed by the compiler.  There's code in init/main.c that
> assumes a non-empty ".setup.init" section and failing that leads to
> unpredictable behavior (it just hangs for me on a MIPSel/Linux box).
>
>  Here's a patch that makes the kernel work for me for i386/Linux.  It's
> based on similar changes that are already present in 2.6 and leaves the
> setup as is for gcc versions before 3.3 and using the new attribute for
> newer versions, by defining an "__attribute_used__" macro appropriately.
>
>  Marcelo, would consider such a change for past 2.4.25?
>
>  Ralf, for MIPS/Linux the change is insufficient -- due to fragile
> constructs in arch/mips/kernel/syscall.c some code is removed, but using
> the "used" attribute does not help.  While the code is back again, it's
> reordered compared to what's emitted by older compilers and it's enough to
> stop it working.  I'm currently investigating a solution, or have you
> perhaps looked at the problem already?  It's there for 2.6 as well.

Hi Maciej,

For me your patch looks alright to be included in 2.4.26-pre.

Please resend me it when .26 gets started.

Thanks!

> patch-2.4.24-gcc3-0.1
> diff -up --recursive --new-file linux-2.4.24.macro/include/linux/compiler.h linux-2.4.24/include/linux/compiler.h
> --- linux-2.4.24.macro/include/linux/compiler.h	2001-09-18 14:12:45.000000000 +0000
> +++ linux-2.4.24/include/linux/compiler.h	2004-02-12 14:57:06.000000000 +0000
> @@ -13,4 +13,10 @@
>  #define likely(x)	__builtin_expect((x),1)
>  #define unlikely(x)	__builtin_expect((x),0)
>
> +#if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 3)
> +#define __attribute_used__	__attribute__((__used__))
> +#else
> +#define __attribute_used__	__attribute__((__unused__))
> +#endif
> +
>  #endif /* __LINUX_COMPILER_H */
> diff -up --recursive --new-file linux-2.4.24.macro/include/linux/init.h linux-2.4.24/include/linux/init.h
> --- linux-2.4.24.macro/include/linux/init.h	2004-02-12 12:48:26.000000000 +0000
> +++ linux-2.4.24/include/linux/init.h	2004-02-12 14:57:06.000000000 +0000
> @@ -2,6 +2,7 @@
>  #define _LINUX_INIT_H
>
>  #include <linux/config.h>
> +#include <linux/compiler.h>
>
>  /* These macros are used to mark some functions or
>   * initialized data (doesn't apply to uninitialized data)
> @@ -67,7 +68,7 @@ extern struct kernel_param __setup_start
>
>  #define __setup(str, fn)								\
>  	static char __setup_str_##fn[] __initdata = str;				\
> -	static struct kernel_param __setup_##fn __attribute__((unused)) __initsetup = { __setup_str_##fn, fn }
> +	static struct kernel_param __setup_##fn __attribute_used__ __initsetup = { __setup_str_##fn, fn }
>
>  #endif /* __ASSEMBLY__ */
>
> @@ -76,12 +77,12 @@ extern struct kernel_param __setup_start
>   * or exit time.
>   */
>  #define __init		__attribute__ ((__section__ (".text.init")))
> -#define __exit		__attribute__ ((unused, __section__(".text.exit")))
> +#define __exit		__attribute_used__ __attribute__ ((__section__ (".text.exit")))
>  #define __initdata	__attribute__ ((__section__ (".data.init")))
> -#define __exitdata	__attribute__ ((unused, __section__ (".data.exit")))
> -#define __initsetup	__attribute__ ((unused,__section__ (".setup.init")))
> -#define __init_call	__attribute__ ((unused,__section__ (".initcall.init")))
> -#define __exit_call	__attribute__ ((unused,__section__ (".exitcall.exit")))
> +#define __exitdata	__attribute_used__ __attribute__ ((__section__ (".data.exit")))
> +#define __initsetup	__attribute_used__ __attribute__ ((__section__ (".setup.init")))
> +#define __init_call	__attribute_used__ __attribute__ ((__section__ (".initcall.init")))
> +#define __exit_call	__attribute_used__ __attribute__ ((__section__ (".exitcall.exit")))
>
>  /* For assembly routines */
>  #define __INIT		.section	".text.init","ax"
> diff -up --recursive --new-file linux-2.4.24.macro/include/linux/module.h linux-2.4.24/include/linux/module.h
> --- linux-2.4.24.macro/include/linux/module.h	2004-02-12 13:12:17.000000000 +0000
> +++ linux-2.4.24/include/linux/module.h	2004-02-12 15:08:00.000000000 +0000
> @@ -8,6 +8,7 @@
>  #define _LINUX_MODULE_H
>
>  #include <linux/config.h>
> +#include <linux/compiler.h>
>  #include <linux/spinlock.h>
>  #include <linux/list.h>
>
> @@ -202,19 +203,22 @@ extern int try_inc_mod_count(struct modu
>
>  /* For documentation purposes only.  */
>
> -#define MODULE_AUTHOR(name)						   \
> -const char __module_author[] __attribute__((section(".modinfo"))) = 	   \
> -"author=" name
> -
> -#define MODULE_DESCRIPTION(desc)					   \
> -const char __module_description[] __attribute__((section(".modinfo"))) =   \
> -"description=" desc
> +#define MODULE_AUTHOR(name)						\
> +const char __module_author[]						\
> +  __attribute_used__							\
> +  __attribute__((section(".modinfo"))) = "author=" name
> +
> +#define MODULE_DESCRIPTION(desc)					\
> +const char __module_description[]					\
> +  __attribute_used__							\
> +  __attribute__((section(".modinfo"))) = "description=" desc
>
>  /* Could potentially be used by kmod...  */
>
> -#define MODULE_SUPPORTED_DEVICE(dev)					   \
> -const char __module_device[] __attribute__((section(".modinfo"))) = 	   \
> -"device=" dev
> +#define MODULE_SUPPORTED_DEVICE(dev)					\
> +const char __module_device[]						\
> +  __attribute_used__							\
> +  __attribute__((section(".modinfo"))) = "device=" dev
>
>  /* Used to verify parameters given to the module.  The TYPE arg should
>     be a string in the following format:
> @@ -229,15 +233,17 @@ const char __module_device[] __attribute
>  	s	string
>  */
>
> -#define MODULE_PARM(var,type)			\
> -const char __module_parm_##var[]		\
> -__attribute__((section(".modinfo"))) =		\
> -"parm_" __MODULE_STRING(var) "=" type
> -
> -#define MODULE_PARM_DESC(var,desc)		\
> -const char __module_parm_desc_##var[]		\
> -__attribute__((section(".modinfo"))) =		\
> -"parm_desc_" __MODULE_STRING(var) "=" desc
> +#define MODULE_PARM(var,type)						\
> +const char __module_parm_##var[]					\
> +  __attribute_used__							\
> +  __attribute__((section(".modinfo"))) =				\
> +  "parm_" __MODULE_STRING(var) "=" type
> +
> +#define MODULE_PARM_DESC(var,desc)					\
> +const char __module_parm_desc_##var[]					\
> +  __attribute_used__							\
> +  __attribute__((section(".modinfo"))) =				\
> +  "parm_desc_" __MODULE_STRING(var) "=" desc
>
>  /*
>   * MODULE_DEVICE_TABLE exports information about devices
> @@ -283,9 +289,10 @@ static const struct gtype##_id * __modul
>   * 3.	So vendors can do likewise based on their own policies
>   */
>
> -#define MODULE_LICENSE(license) 	\
> -static const char __module_license[] __attribute__((section(".modinfo"))) =   \
> -"license=" license
> +#define MODULE_LICENSE(license) 					\
> +static const char __module_license[]					\
> +  __attribute_used__							\
> +  __attribute__((section(".modinfo"))) = "license=" license
>
>  /* Define the module variable, and usage macros.  */
>  extern struct module __this_module;
> @@ -296,11 +303,11 @@ extern struct module __this_module;
>  #define MOD_IN_USE		__MOD_IN_USE(THIS_MODULE)
>
>  #include <linux/version.h>
> -static const char __module_kernel_version[] __attribute__((section(".modinfo"))) =
> -"kernel_version=" UTS_RELEASE;
> +static const char __module_kernel_version[] __attribute_used__
> +	__attribute__((section(".modinfo"))) = "kernel_version=" UTS_RELEASE;
>  #ifdef MODVERSIONS
> -static const char __module_using_checksums[] __attribute__((section(".modinfo"))) =
> -"using_checksums=1";
> +static const char __module_using_checksums[] __attribute_used__
> +	__attribute__((section(".modinfo"))) = "using_checksums=1";
>  #endif
>
>  #else /* MODULE */