kbuild: remove top-level built-in.a

The symbol table in the final archive is unneeded; the linker does not
require the symbol table after the --whole-archive option. Every object
file in the archive is included in the link anyway.

Pass thin archives from subdirectories directly to the linker, and
remove the final archiving step.

Fix up the document and comments as well.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Nicholas Piggin <npiggin@gmail.com>

Documentation/kbuild/makefiles.txt

@@ -154,13 +154,8 @@ more details, with real examples.
 
 	Kbuild compiles all the $(obj-y) files.  It then calls
 	"$(AR) rcSTP" to merge these files into one built-in.a file.
-	This is a thin archive without a symbol table, which makes it
-	unsuitable as a linker input.
-
-	The scripts/link-vmlinux.sh script later makes an aggregate
-	built-in.a with "${AR} rcsTP", which creates the thin archive
-	with a symbol table and an index, making it a valid input for
-	the final vmlinux link passes.
+	This is a thin archive without a symbol table. It will be later
+	linked into vmlinux by scripts/link-vmlinux.sh
 
 	The order of files in $(obj-y) is significant.  Duplicates in
 	the lists are allowed: the first instance will be linked into

scripts/Makefile.build

@@ -394,14 +394,10 @@ $(obj)/%.asn1.c $(obj)/%.asn1.h: $(src)/%.asn1 $(objtree)/scripts/asn1_compiler
 $(sort $(subdir-obj-y)): $(subdir-ym) ;
 
 #
-# Rule to compile a set of .o files into one .o file
+# Rule to compile a set of .o files into one .a file (without symbol table)
 #
 ifdef builtin-target
 
-# built-in.a archives are made with no symbol table or index which
-# makes them small and fast, but unable to be used by the linker.
-# scripts/link-vmlinux.sh builds an aggregate built-in.a with a symbol
-# table and index.
 quiet_cmd_ar_builtin = AR      $@
       cmd_ar_builtin = rm -f $@; \
                      $(AR) rcSTP$(KBUILD_ARFLAGS) $@ $(filter $(real-obj-y), $^)
@@ -426,7 +422,7 @@ $(modorder-target): $(subdir-ym) FORCE
 	$(Q)(cat /dev/null; $(modorder-cmds)) > $@
 
 #
-# Rule to compile a set of .o files into one .a file
+# Rule to compile a set of .o files into one .a file (with symbol table)
 #
 ifdef lib-target
 quiet_cmd_link_l_target = AR      $@

scripts/link-vmlinux.sh

@@ -44,24 +44,6 @@ info()
 	fi
 }
 
-# Thin archive build here makes a final archive with symbol table and indexes
-# from vmlinux objects INIT and MAIN, which can be used as input to linker.
-# KBUILD_VMLINUX_LIBS archives should already have symbol table and indexes
-# added.
-#
-# Traditional incremental style of link does not require this step
-#
-# built-in.a output file
-#
-archive_builtin()
-{
-	info AR built-in.a
-	rm -f built-in.a;
-	${AR} rcsTP${KBUILD_ARFLAGS} built-in.a			\
-				${KBUILD_VMLINUX_INIT}		\
-				${KBUILD_VMLINUX_MAIN}
-}
-
 # Link of vmlinux.o used for section mismatch analysis
 # ${1} output file
 modpost_link()
@@ -69,7 +51,8 @@ modpost_link()
 	local objects
 
 	objects="--whole-archive				\
-		built-in.a					\
+		${KBUILD_VMLINUX_INIT}				\
+		${KBUILD_VMLINUX_MAIN}				\
 		--no-whole-archive				\
 		--start-group					\
 		${KBUILD_VMLINUX_LIBS}				\
@@ -88,7 +71,8 @@ vmlinux_link()
 
 	if [ "${SRCARCH}" != "um" ]; then
 		objects="--whole-archive			\
-			built-in.a				\
+			${KBUILD_VMLINUX_INIT}			\
+			${KBUILD_VMLINUX_MAIN}			\
 			--no-whole-archive			\
 			--start-group				\
 			${KBUILD_VMLINUX_LIBS}			\
@@ -99,7 +83,8 @@ vmlinux_link()
 			-T ${lds} ${objects}
 	else
 		objects="-Wl,--whole-archive			\
-			built-in.a				\
+			${KBUILD_VMLINUX_INIT}			\
+			${KBUILD_VMLINUX_MAIN}			\
 			-Wl,--no-whole-archive			\
 			-Wl,--start-group			\
 			${KBUILD_VMLINUX_LIBS}			\
@@ -160,7 +145,6 @@ cleanup()
 	rm -f .tmp_System.map
 	rm -f .tmp_kallsyms*
 	rm -f .tmp_vmlinux*
-	rm -f built-in.a
 	rm -f System.map
 	rm -f vmlinux
 	rm -f vmlinux.o
@@ -217,8 +201,6 @@ fi;
 # final build of init/
 ${MAKE} -f "${srctree}/scripts/Makefile.build" obj=init
 
-archive_builtin
-
 #link vmlinux.o
 info LD vmlinux.o
 modpost_link vmlinux.o