kbuild: allow archs to select link dead code/data elimination

Introduce LD_DEAD_CODE_DATA_ELIMINATION option for architectures to
select to build with -ffunction-sections, -fdata-sections, and link
with --gc-sections. It requires some work (documented) to ensure all
unreferenced entrypoints are live, and requires toolchain and build
verification, so it is made a per-arch option for now.

On a random powerpc64le build, this yelds a significant size saving,
it boots and runs fine, but there is a lot I haven't tested as yet, so
these savings may be reduced if there are bugs in the link.

    text      data        bss        dec   filename
11169741   1180744    1923176	14273661   vmlinux
10445269   1004127    1919707	13369103   vmlinux.dce

~700K text, ~170K data, 6% removed from kernel image size.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.com>

1 files changed, 13 insertions, 25 deletions

diff --git a/include/linux/init.h b/include/linux/init.h
index 6935d02..e571fec 100644
--- a/include/linux/init.h
+++ b/include/linux/init.h
@@ -150,24 +150,8 @@ extern bool initcall_debug;
 
 #ifndef __ASSEMBLY__
 
-#ifdef CONFIG_LTO
-/* Work around a LTO gcc problem: when there is no reference to a variable
- * in a module it will be moved to the end of the program. This causes
- * reordering of initcalls which the kernel does not like.
- * Add a dummy reference function to avoid this. The function is
- * deleted by the linker.
- */
-#define LTO_REFERENCE_INITCALL(x) \
-	; /* yes this is needed */			\
-	static __used __exit void *reference_##x(void)	\
-	{						\
-		return &x;				\
-	}
-#else
-#define LTO_REFERENCE_INITCALL(x)
-#endif
-
-/* initcalls are now grouped by functionality into separate 
+/*
+ * initcalls are now grouped by functionality into separate
  * subsections. Ordering inside the subsections is determined
  * by link order. 
  * For backwards compatibility, initcall() puts the call in 
@@ -175,12 +159,16 @@ extern bool initcall_debug;
  *
  * The `id' arg to __define_initcall() is needed so that multiple initcalls
  * can point at the same handler without causing duplicate-symbol build errors.
+ *
+ * Initcalls are run by placing pointers in initcall sections that the
+ * kernel iterates at runtime. The linker can do dead code / data elimination
+ * and remove that completely, so the initcall sections have to be marked
+ * as KEEP() in the linker script.
  */
 
 #define __define_initcall(fn, id) \
 	static initcall_t __initcall_##fn##id __used \
-	__attribute__((__section__(".initcall" #id ".init"))) = fn; \
-	LTO_REFERENCE_INITCALL(__initcall_##fn##id)
+	__attribute__((__section__(".initcall" #id ".init"))) = fn;
 
 /*
  * Early initcalls run before initializing SMP.
@@ -216,15 +204,15 @@ extern bool initcall_debug;
 
 #define __initcall(fn) device_initcall(fn)
 
-#define __exitcall(fn) \
+#define __exitcall(fn)						\
 	static exitcall_t __exitcall_##fn __exit_call = fn
 
-#define console_initcall(fn) \
-	static initcall_t __initcall_##fn \
+#define console_initcall(fn)					\
+	static initcall_t __initcall_##fn			\
 	__used __section(.con_initcall.init) = fn
 
-#define security_initcall(fn) \
-	static initcall_t __initcall_##fn \
+#define security_initcall(fn)					\
+	static initcall_t __initcall_##fn			\
 	__used __section(.security_initcall.init) = fn
 
 struct obs_kernel_param {