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kbuild: make built-in.a rule robust against too long argument error

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Kbuild runs at the top of objtree instead of changing the working
directory to subdirectories. I think this design is nice overall but
some commands have a scalability issue.

The build command of built-in.a is one of them whose length scales with:

    O(D * N)

Here, D is the length of the directory path (i.e. $(obj)/ prefix),
N is the number of objects in the Makefile, O() is the big O notation.

The deeper directory the Makefile directory is located, the more easily
it will hit the too long argument error.

We can make it better. Trim the $(obj)/ by Make's builtin function, and
restore it by a shell command (sed).

With this, the command length scales with:

    O(D + N)

In-tree modules still have some room to the limit (ARG_MAX=2097152),
but this is more future-proof for big modules in a deep directory.

For example, you can build i915 as builtin (CONFIG_DRM_I915=y) and
compare drivers/gpu/drm/i915/.built-in.a.cmd with/without this commit.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # LLVM-14 (x86-64)
This commit is contained in:
Masahiro Yamada 2022-05-27 19:01:52 +09:00
parent 31cb50b559
commit cd968b97c4
1 changed files with 6 additions and 1 deletions
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7
scripts/Makefile.build
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@ -377,9 +377,14 @@ $(subdir-modorder): $(obj)/%/modules.order: $(obj)/% ;
#
# Rule to compile a set of .o files into one .a file (without symbol table)
#
# To make this rule robust against "Argument list too long" error,
# remove $(obj)/ prefix, and restore it by a shell command.
quiet_cmd_ar_builtin = AR $@
cmd_ar_builtin = rm -f $@; $(AR) cDPrST $@ $(real-prereqs)
cmd_ar_builtin = rm -f $@; \
echo $(patsubst $(obj)/%,%,$(real-prereqs)) | \
sed -E 's:([^ ]+):$(obj)/\1:g' | \
xargs $(AR) cDPrST $@
$(obj)/built-in.a: $(real-obj-y) FORCE
$(call if_changed,ar_builtin)