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kbuild: implement CONFIG_TRIM_UNUSED_KSYMS without recursion

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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:

 - 5cf0fd591f ("Kbuild: disable TRIM_UNUSED_KSYMS option")
 - a555bdd0c5 ("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 f73edc8951 ("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>
This commit is contained in:
Masahiro Yamada
2023-06-12 00:50:57 +09:00
parent 700c48b439
commit 5e9e95cc91
11 changed files with 78 additions and 264 deletions
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3
scripts/basic/fixdep.c
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@@ -246,8 +246,7 @@ static void *read_file(const char *filename)
/* Ignore certain dependencies */
static int is_ignored_file(const char *s, int len)
{
return str_ends_with(s, len, "include/generated/autoconf.h") ||
str_ends_with(s, len, "include/generated/autoksyms.h");
return str_ends_with(s, len, "include/generated/autoconf.h");
}
/* Do not parse these files */