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kbuild: use -MMD instead of -MD to exclude system headers from dependency

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This omits system headers from the generated header dependency.

System headers are not updated unless you upgrade the compiler. Nor do
they contain CONFIG options, so fixdep does not need to parse them.

Having said that, the effect of this optimization will be quite small
because the kernel code generally does not include system headers
except <stdarg.h>. Host programs include a lot of system headers,
but there are not so many in the kernel tree.

At first, keeping system headers in .*.cmd files might be useful to
detect the compiler update, but there is no guarantee that <stdarg.h>
is included from every file. So, I implemented a more reliable way in
the previous commit.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
This commit is contained in:
Masahiro Yamada 2020-04-23 23:23:53 +09:00
parent 8b59cd81dc
commit 30a7729771
4 changed files with 8 additions and 8 deletions
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2
scripts/Kbuild.include
View File

@ -16,7 +16,7 @@ pound := \#
dot-target = $(dir $@).$(notdir $@)
###
# The temporary file to save gcc -MD generated dependencies must not
# The temporary file to save gcc -MMD generated dependencies must not
# contain a comma
depfile = $(subst $(comma),_,$(dot-target).d)

4
scripts/Makefile.host
View File

@ -88,8 +88,8 @@ _hostcxx_flags += -I $(objtree)/$(obj)
endif
endif
hostc_flags = -Wp,-MD,$(depfile) $(_hostc_flags)
hostcxx_flags = -Wp,-MD,$(depfile) $(_hostcxx_flags)
hostc_flags = -Wp,-MMD,$(depfile) $(_hostc_flags)
hostcxx_flags = -Wp,-MMD,$(depfile) $(_hostcxx_flags)
#####
# Compile programs on the host

8
scripts/Makefile.lib
View File

@ -171,22 +171,22 @@ modkern_aflags = $(if $(part-of-module), \
$(KBUILD_AFLAGS_MODULE) $(AFLAGS_MODULE), \
$(KBUILD_AFLAGS_KERNEL) $(AFLAGS_KERNEL))
c_flags = -Wp,-MD,$(depfile) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) \
c_flags = -Wp,-MMD,$(depfile) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) \
-include $(srctree)/include/linux/compiler_types.h \
$(_c_flags) $(modkern_cflags) \
$(basename_flags) $(modname_flags)
a_flags = -Wp,-MD,$(depfile) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) \
a_flags = -Wp,-MMD,$(depfile) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) \
$(_a_flags) $(modkern_aflags)
cpp_flags = -Wp,-MD,$(depfile) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) \
cpp_flags = -Wp,-MMD,$(depfile) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) \
$(_cpp_flags)
ld_flags = $(KBUILD_LDFLAGS) $(ldflags-y) $(LDFLAGS_$(@F))
DTC_INCLUDE := $(srctree)/scripts/dtc/include-prefixes
dtc_cpp_flags = -Wp,-MD,$(depfile).pre.tmp -nostdinc \
dtc_cpp_flags = -Wp,-MMD,$(depfile).pre.tmp -nostdinc \
$(addprefix -I,$(DTC_INCLUDE)) \
-undef -D__DTS__

2
usr/include/Makefile
View File

@ -8,7 +8,7 @@
# We cannot go as far as adding -Wpedantic since it emits too many warnings.
UAPI_CFLAGS := -std=c90 -Wall -Werror=implicit-function-declaration
override c_flags = $(UAPI_CFLAGS) -Wp,-MD,$(depfile) -I$(objtree)/usr/include
override c_flags = $(UAPI_CFLAGS) -Wp,-MMD,$(depfile) -I$(objtree)/usr/include
# The following are excluded for now because they fail to build.
#