linux-kernel - [PATCH 2/2] kbuild: use -MMD instead of -MD to exclude system headers from dependency

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Date:   Tue, 21 Apr 2020 21:32:27 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     linux-kbuild@...r.kernel.org
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] kbuild: use -MMD instead of -MD to exclude system headers from dependency

This omits system header files 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 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 host programs.

Theoretically, fixdep does not need to parse *.d files for host programs
because host programs should not contain CONFIG options in the first
place. Kbuild re-uses if_changed_dep for host programs, though I do not
think it is a big deal.

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@...nel.org>
---

 scripts/Kbuild.include | 2 +-
 scripts/Makefile.host  | 4 ++--
 scripts/Makefile.lib   | 8 ++++----
 3 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index 6cabf20ce66a..0c3dc983439b 100644
--- 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 2045855d0b75..c8a4a033dc3e 100644
--- 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 97547108ee7f..a94c1e741df9 100644
--- 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__
 
-- 
2.25.1