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

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>

diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index 6cabf20..0c3dc98 100644 (file)
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -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)
 

diff --git a/scripts/Makefile.host b/scripts/Makefile.host
index 2045855..c8a4a03 100644 (file)
--- a/scripts/Makefile.host
+++ b/scripts/Makefile.host
@@ -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

diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 4b79973..12f6a33 100644 (file)
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -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__
 

diff --git a/usr/include/Makefile b/usr/include/Makefile
index b568a95..5a7ee3e 100644 (file)
--- a/usr/include/Makefile
+++ b/usr/include/Makefile
@@ -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.
 #