kbuild: simplify dependency generation for CONFIG_TRIM_UNUSED_KSYMS

My main motivation of this commit is to clean up scripts/Kbuild.include
and scripts/Makefile.build.

Currently, CONFIG_TRIM_UNUSED_KSYMS works with a tricky gimmick;
possibly exported symbols are detected by letting $(CPP) replace
EXPORT_SYMBOL* with a special string '=== __KSYM_*===', which is
post-processed by sed, and passed to fixdep. The extra preprocessing
is costly, and hacking cmd_and_fixdep is ugly.

I came up with a new way to find exported symbols; insert a dummy
symbol __ksym_marker_* to each potentially exported symbol. Those
dummy symbols are picked up by $(NM), post-processed by sed, then
appended to .*.cmd files. I collected the post-process part to a
new shell script scripts/gen_ksymdeps.sh for readability. The dummy
symbols are put into the .discard.* section so that the linker
script rips them off the final vmlinux or modules.

A nice side-effect is building with CONFIG_TRIM_UNUSED_KSYMS will
be much faster.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Nicolas Pitre <nico@linaro.org>

2 files changed, 17 insertions, 14 deletions

diff --git a/include/asm-generic/export.h b/include/asm-generic/export.h
index 4d73e6e3c66c..294d6ae785d4 100644
--- a/include/asm-generic/export.h
+++ b/include/asm-generic/export.h
@@ -59,16 +59,19 @@ __kcrctab_\name:
 .endm
 #undef __put
 
-#if defined(__KSYM_DEPS__)
-
-#define __EXPORT_SYMBOL(sym, val, sec)  === __KSYM_##sym ===
-
-#elif defined(CONFIG_TRIM_UNUSED_KSYMS)
+#if defined(CONFIG_TRIM_UNUSED_KSYMS)
 
 #include <linux/kconfig.h>
 #include <generated/autoksyms.h>
 
+.macro __ksym_marker sym
+        .section ".discard.ksym","a"
+__ksym_marker_\sym:
+         .previous
+.endm
+
 #define __EXPORT_SYMBOL(sym, val, sec)                          \
+        __ksym_marker sym;                                      \
         __cond_export_sym(sym, val, sec, __is_defined(__KSYM_##sym))
 #define __cond_export_sym(sym, val, sec, conf)                  \
         ___cond_export_sym(sym, val, sec, conf)
diff --git a/include/linux/export.h b/include/linux/export.h
index ce764a5d2ee4..fd8711ed9ac4 100644
--- a/include/linux/export.h
+++ b/include/linux/export.h
@@ -92,22 +92,22 @@ struct kernel_symbol {
  */
 #define __EXPORT_SYMBOL(sym, sec)
 
-#elif defined(__KSYM_DEPS__)
+#elif defined(CONFIG_TRIM_UNUSED_KSYMS)
+
+#include <generated/autoksyms.h>
 
 /*
  * For fine grained build dependencies, we want to tell the build system
  * about each possible exported symbol even if they're not actually exported.
- * We use a string pattern that is unlikely to be valid code that the build
- * system filters out from the preprocessor output (see ksym_dep_filter
- * in scripts/Kbuild.include).
+ * We use a symbol pattern __ksym_marker_<symbol> that the build system filters
+ * from the $(NM) output (see scripts/gen_ksymdeps.sh). These symbols are
+ * discarded in the final link stage.
  */
-#define __EXPORT_SYMBOL(sym, sec)       === __KSYM_##sym ===
-
-#elif defined(CONFIG_TRIM_UNUSED_KSYMS)
-
-#include <generated/autoksyms.h>
+#define __ksym_marker(sym)      \
+        static int __ksym_marker_##sym[0] __section(".discard.ksym") __used
 
 #define __EXPORT_SYMBOL(sym, sec)                               \
+        __ksym_marker(sym);                                     \
         __cond_export_sym(sym, sec, __is_defined(__KSYM_##sym))
 #define __cond_export_sym(sym, sec, conf)                       \
         ___cond_export_sym(sym, sec, conf)