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| author | Masahiro Yamada <masahiroy@kernel.org> | 2023-06-12 00:50:57 +0900 |
|---|---|---|
| committer | Masahiro Yamada <masahiroy@kernel.org> | 2023-06-22 21:21:06 +0900 |
| commit | 5e9e95cc9148b82074a5eae283e63bce3f1aacfe (patch) | |
| tree | 72072798944c4a0ffcf2f294b8c264cbd7f60b05 /scripts/Makefile.build | |
| parent | 700c48b439921b67715e25380e0f67e6e490d7b8 (diff) | |
| download | lwn-5e9e95cc9148b82074a5eae283e63bce3f1aacfe.tar.gz lwn-5e9e95cc9148b82074a5eae283e63bce3f1aacfe.zip | |
kbuild: implement CONFIG_TRIM_UNUSED_KSYMS without recursion
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>
Diffstat (limited to 'scripts/Makefile.build')
| -rw-r--r-- | scripts/Makefile.build | 15 |
1 files changed, 1 insertions, 14 deletions
diff --git a/scripts/Makefile.build b/scripts/Makefile.build index 210142c3ff00..4735b958097a 100644 --- a/scripts/Makefile.build +++ b/scripts/Makefile.build @@ -82,7 +82,7 @@ ifdef need-builtin targets-for-builtin += $(obj)/built-in.a endif -targets-for-modules := $(foreach x, o mod $(if $(CONFIG_TRIM_UNUSED_KSYMS), usyms), \ +targets-for-modules := $(foreach x, o mod, \ $(patsubst %.o, %.$x, $(filter %.o, $(obj-m)))) ifdef need-modorder @@ -217,18 +217,12 @@ is-standard-object = $(if $(filter-out y%, $(OBJECT_FILES_NON_STANDARD_$(basetar $(obj)/%.o: objtool-enabled = $(if $(is-standard-object),$(if $(delay-objtool),$(is-single-obj-m),y)) -ifdef CONFIG_TRIM_UNUSED_KSYMS -cmd_gen_ksymdeps = \ - $(CONFIG_SHELL) $(srctree)/scripts/gen_ksymdeps.sh $@ >> $(dot-target).cmd -endif - ifneq ($(findstring 1, $(KBUILD_EXTRA_WARN)),) cmd_warn_shared_object = $(if $(word 2, $(modname-multi)),$(warning $(kbuild-file): $*.o is added to multiple modules: $(modname-multi))) endif define rule_cc_o_c $(call cmd_and_fixdep,cc_o_c) - $(call cmd,gen_ksymdeps) $(call cmd,checksrc) $(call cmd,checkdoc) $(call cmd,gen_objtooldep) @@ -239,7 +233,6 @@ endef define rule_as_o_S $(call cmd_and_fixdep,as_o_S) - $(call cmd,gen_ksymdeps) $(call cmd,gen_objtooldep) $(call cmd,gen_symversions_S) $(call cmd,warn_shared_object) @@ -258,12 +251,6 @@ cmd_mod = printf '%s\n' $(call real-search, $*.o, .o, -objs -y -m) | \ $(obj)/%.mod: FORCE $(call if_changed,mod) -# List module undefined symbols -cmd_undefined_syms = $(NM) $< | sed -n 's/^ *U //p' > $@ - -$(obj)/%.usyms: $(obj)/%.o FORCE - $(call if_changed,undefined_syms) - quiet_cmd_cc_lst_c = MKLST $@ cmd_cc_lst_c = $(CC) $(c_flags) -g -c -o $*.o $< && \ $(CONFIG_SHELL) $(srctree)/scripts/makelst $*.o \ |