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kbuild: simplify dependency generation for CONFIG_TRIM_UNUSED_KSYMS

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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>
This commit is contained in:
Masahiro Yamada
2018-11-30 10:05:26 +09:00
parent ee3e46b7ef
commit bbda5ec671
6 changed files with 55 additions and 71 deletions
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22
include/linux/export.h
View File

@@ -92,22 +92,22 @@ struct kernel_symbol {
*/
#define __EXPORT_SYMBOL(sym, sec)
#elif defined(__KSYM_DEPS__)
/*
* 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).
*/
#define __EXPORT_SYMBOL(sym, sec) === __KSYM_##sym ===
#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 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 __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)