[PATCH u-boot 16/39] build: use thin archives instead of incremental linking

[Marek Behún]

Currently we use incremental linking (ld -r) to link several object
files from one directory into one built-in.o object file containing the
linked code from that directory (and its subdirectories).

Linux has, some time ago, moved to thin archives instead.

Thin archives are archives (.a) that do not really contain the object
files, only references to them.

Using thin archives instead of incremental linking
- saves disk space
- apparently works better with dead code elimination
- makes things easier for LTO

The third point is the important one for us. With incremental linking
there are several options how to do LTO, and that would unnecessarily
complicate things.

On the other hand, by using thin archives we can make (via the
--whole-archive use flag) the final linking behave as if we passed all
the object files from the archives to the linking program as arguments.

We also need to use the P flag for ar, otherwise final linking may fail.

Signed-off-by: Marek Behún <marek.behun at nic.cz>
---
 Makefile               |  4 ++--
 arch/sandbox/config.mk | 10 +++++++---
 scripts/Makefile.build | 16 ++++++++--------
 scripts/Makefile.spl   |  4 ++--
 4 files changed, 19 insertions(+), 15 deletions(-)

diff --git a/Makefile b/Makefile
index 7b0ba9df9a..6dc47810ea 100644
--- a/Makefile
+++ b/Makefile
@@ -1754,9 +1754,9 @@ ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(ARCH)/Makefile.postlink)
 quiet_cmd_u-boot__ ?= LD      $@
       cmd_u-boot__ ?= $(LD) $(KBUILD_LDFLAGS) $(LDFLAGS_u-boot) -o $@		\
 		-T u-boot.lds $(u-boot-init)					\
-		--start-group							\
+		--start-group --whole-archive					\
 			$(u-boot-main)						\
-		--end-group							\
+		--no-whole-archive --end-group					\
 		$(PLATFORM_LIBS) -Map u-boot.map;				\
 		$(if $(ARCH_POSTLINK), $(MAKE) -f $(ARCH_POSTLINK) $@, true)
 
diff --git a/arch/sandbox/config.mk b/arch/sandbox/config.mk
index 189e9c2b0c..d11b9c63c9 100644
--- a/arch/sandbox/config.mk
+++ b/arch/sandbox/config.mk
@@ -17,13 +17,17 @@ PLATFORM_CPPFLAGS += $(shell $(SDL_CONFIG) --cflags)
 endif
 
 cmd_u-boot__ = $(CC) -o $@ -Wl,-T u-boot.lds $(u-boot-init) \
-	-Wl,--start-group $(u-boot-main) -Wl,--end-group \
+	-Wl,--start-group -Wl,--whole-archive \
+		$(u-boot-main) \
+	-Wl,--no-whole-archive -Wl,--end-group \
 	$(PLATFORM_LIBS) -Wl,-Map -Wl,u-boot.map
 
 cmd_u-boot-spl = (cd $(obj) && $(CC) -o $(SPL_BIN) -Wl,-T u-boot-spl.lds \
 	$(patsubst $(obj)/%,%,$(u-boot-spl-init)) \
-	-Wl,--start-group $(patsubst $(obj)/%,%,$(u-boot-spl-main)) \
-	$(patsubst $(obj)/%,%,$(u-boot-spl-platdata)) -Wl,--end-group \
+	-Wl,--start-group -Wl,--whole-archive \
+		$(patsubst $(obj)/%,%,$(u-boot-spl-main)) \
+		$(patsubst $(obj)/%,%,$(u-boot-spl-platdata)) \
+	-Wl,--no-whole-archive -Wl,--end-group \
 	$(PLATFORM_LIBS) -Wl,-Map -Wl,u-boot-spl.map -Wl,--gc-sections)
 
 CONFIG_ARCH_DEVICE_TREE := sandbox
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 705a886cb9..a9a02d7d33 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -331,12 +331,11 @@ $(sort $(subdir-obj-y)): $(subdir-ym) ;
 # Rule to compile a set of .o files into one .o file
 #
 ifdef builtin-target
-quiet_cmd_link_o_target = LD      $@
+quiet_cmd_link_o_target = AR      $@
 # If the list of objects to link is empty, just create an empty built-in.o
 cmd_link_o_target = $(if $(strip $(obj-y)),\
-		      $(LD) $(ld_flags) -r -o $@ $(filter $(obj-y), $^) \
-		      $(cmd_secanalysis),\
-		      rm -f $@; $(AR) rcs$(KBUILD_ARFLAGS) $@)
+		      rm -f $@; $(AR) cDPrsT $@ $(filter $(obj-y), $^), \
+		      rm -f $@; $(AR) cPrsT$(KBUILD_ARFLAGS) $@)
 
 $(builtin-target): $(obj-y) FORCE
 	$(call if_changed,link_o_target)
@@ -362,7 +361,7 @@ $(modorder-target): $(subdir-ym) FORCE
 #
 ifdef lib-target
 quiet_cmd_link_l_target = AR      $@
-cmd_link_l_target = rm -f $@; $(AR) rcs$(KBUILD_ARFLAGS) $@ $(lib-y)
+cmd_link_l_target = rm -f $@; $(AR) cPrs$(KBUILD_ARFLAGS) $@ $(lib-y)
 
 $(lib-target): $(lib-y) FORCE
 	$(call if_changed,link_l_target)
@@ -382,10 +381,11 @@ $(filter $(addprefix $(obj)/,         \
 $($(subst $(obj)/,,$(@:.o=-objs)))    \
 $($(subst $(obj)/,,$(@:.o=-y)))), $^)
 
-quiet_cmd_link_multi-y = LD      $@
-cmd_link_multi-y = $(LD) $(ld_flags) -r -o $@ $(link_multi_deps) $(cmd_secanalysis)
 
-quiet_cmd_link_multi-m = LD [M]  $@
+quiet_cmd_link_multi-y = AR      $@
+cmd_link_multi-y = rm -f $@; $(AR) cDPrsT $@ $(link_multi_deps)
+
+quiet_cmd_link_multi-m = AR [M]  $@
 cmd_link_multi-m = $(cmd_link_multi-y)
 
 $(multi-used-y): FORCE
diff --git a/scripts/Makefile.spl b/scripts/Makefile.spl
index 3b5f5046c9..889debb93c 100644
--- a/scripts/Makefile.spl
+++ b/scripts/Makefile.spl
@@ -423,10 +423,10 @@ quiet_cmd_u-boot-spl ?= LD      $@
       cmd_u-boot-spl ?= (cd $(obj) && \
 		$(LD) $(KBUILD_LDFLAGS) $(LDFLAGS_$(@F))		\
 		$(patsubst $(obj)/%,%,$(u-boot-spl-init))		\
-		--start-group						\
+		--start-group --whole-archive				\
 			$(patsubst $(obj)/%,%,$(u-boot-spl-main))	\
 			$(patsubst $(obj)/%,%,$(u-boot-spl-platdata))	\
-		--end-group						\
+		--no-whole-archive --end-group				\
 		$(PLATFORM_LIBS) -Map $(SPL_BIN).map -o $(SPL_BIN)	\
 			)
 
-- 
2.26.2