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When CONFIG_TRIM_UNUSED_KSYMS is enabled, Kbuild recursively traverses
the directory tree to determine which EXPORT_SYMBOL to trim. If an
EXPORT_SYMBOL turns out to be unused by anyone, Kbuild begins the
second traverse, where some source files are recompiled with their
EXPORT_SYMBOL() tuned into a no-op.
Linus stated negative opinions about this slowness in commits:
- 5cf0fd591f2e ("Kbuild: disable TRIM_UNUSED_KSYMS option")
- a555bdd0c58c ("Kbuild: enable TRIM_UNUSED_KSYMS again, with some guarding")
We can do this better now. The final data structures of EXPORT_SYMBOL
are generated by the modpost stage, so modpost can selectively emit
KSYMTAB entries that are really used by modules.
Commit f73edc8951b2 ("kbuild: unify two modpost invocations") is another
ground-work to do this in a one-pass algorithm. With the list of modules,
modpost sets sym->used if it is used by a module. modpost emits KSYMTAB
only for symbols with sym->used==true.
BTW, Nicolas explained why the trimming was implemented with recursion:
https://lore.kernel.org/all/2o2rpn97-79nq-p7s2-nq5-8p83391473r@syhkavp.arg/
Actually, we never achieved that level of optimization where the chain
reaction of trimming comes into play because:
- CONFIG_LTO_CLANG cannot remove any unused symbols
- CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is enabled only for vmlinux,
but not modules
If deeper trimming is required, we need to revisit this, but I guess
that is unlikely to happen.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
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Commit 0e0345b77ac4 ("kbuild: redo fake deps at include/config/*.h")
simplified the Kconfig/fixdep interaction a lot.
For CONFIG_FOO_BAR_BAZ, Kconfig now touches include/config/FOO_BAR_BAZ
instead of the previous include/config/foo/bar/baz.h .
This commit simplifies the TRIM_UNUSED_KSYMS feature in a similar way:
- delete .h suffix
- delete tolower()
- put everything in 1 directory
For EXPORT_SYMBOL(FOO_BAR_BAZ), scripts/adjust_autoksyms.sh now touches
include/ksym/FOO_BAR_BAZ instead of include/ksym/foo/bar/baz.h .
This is more precise, avoiding possibly unnecessary rebuilds.
EXPORT_SYMBOL(FOO_BAR_BAZ)
EXPORT_SYMBOL(_FOO_BAR_BAZ)
EXPORT_SYMBOL(__FOO_BAR_BAZ)
were previously mapped to the same header, include/ksym/foo/bar/baz.h
but now are handled separately.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
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When CONFIG_TRIM_UNUSED_KSYMS is enabled, I see some warnings like this:
nm: arch/x86/entry/vdso/vdso32/note.o: no symbols
$NM (both GNU nm and llvm-nm) warns when no symbol is found in the
object. Suppress the stderr.
Fangrui Song mentioned binutils>=2.37 `nm -q` can be used to suppress
"no symbols" [1], and llvm-nm>=13.0.0 supports -q as well.
We cannot use it for now, but note it as a TODO.
[1]: https://sourceware.org/bugzilla/show_bug.cgi?id=27408
Fixes: bbda5ec671d3 ("kbuild: simplify dependency generation for CONFIG_TRIM_UNUSED_KSYMS")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
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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>
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Masahiro Yamada
Masahiro Yamada