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cc-option and as-option are almost the same; both pass the flag to
$(CC). The main difference is the cc-option stops before the assemble
stage (-S option) whereas as-option stops after (-c option).
I chose -S because it is slightly faster, but $(cc-option,-gz=zlib)
returns a wrong result (https://lkml.org/lkml/2020/6/9/1529).
It has been fixed by commit 7b16994437 ("Makefile: Improve compressed
debug info support detection"), but the assembler should always be
invoked for more reliable compiler option tests.
However, you cannot simply replace -S with -c because the following
code in lib/Kconfig.debug would break:
depends on $(cc-option,-gsplit-dwarf)
The combination of -c and -gsplit-dwarf does not accept /dev/null as
output.
$ cat /dev/null | gcc -gsplit-dwarf -S -x c - -o /dev/null
$ echo $?
0
$ cat /dev/null | gcc -gsplit-dwarf -c -x c - -o /dev/null
objcopy: Warning: '/dev/null' is not an ordinary file
$ echo $?
1
$ cat /dev/null | gcc -gsplit-dwarf -c -x c - -o tmp.o
$ echo $?
0
There is another flag that creates an separate file based on the
object file path:
$ cat /dev/null | gcc -ftest-coverage -c -x c - -o /dev/null
<stdin>:1: error: cannot open /dev/null.gcno
So, we cannot use /dev/null to sink the output.
Align the cc-option implementation with scripts/Kbuild.include.
With -c option used in cc-option, as-option is unneeded.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
4d0831e8a0