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 6cabf20ce66a3bc687755b2df190c5f68032345a..0c3dc983439bc51ee771285a8b63126bec4eee38 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 2045855d0b7598e65a8d8e6f131bce2427c5db0e..c8a4a033dc3e9b2cd5fa71c62862c45d8ed8830f 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 4b799737722c89f670d763bef0146685fb333b96..12f6a331a8f330a4117aa2c57d62e9b42bf9c6c5 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 b568a95d1f62e68b5ec830f68ab42d1878c6dc7f..5a7ee3e5ed869066d479fad6730c933a6bf04762 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.
 #