[PATCH u-boot v3 16/39] build: use thin archives instead of incremental linking

Marek Behún
2021-03-16T12:25:46Z

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.

We have to use the --whole-archive/--no-whole-archive linking option
instead of --start-group/--end-group, otherwise linking may fail because
of unresolved symbols, or the resulting binary will be unusable.

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

Signed-off-by: Marek Beh?n <marek.behun@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 e668940aba..c9b5fac7a9 100644
--- a/Makefile
+++ b/Makefile
@@ -1749,9 +1749,9 @@ 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 \
+ --whole-archive \
$(u-boot-main) \
- --end-group \
+ --no-whole-archive \
$(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..2149771324 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,--whole-archive \
+ $(u-boot-main) \
+ -Wl,--no-whole-archive \
$(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,--whole-archive \
+ $(patsubst $(obj)/%,%,$(u-boot-spl-main)) \
+ $(patsubst $(obj)/%,%,$(u-boot-spl-platdata)) \
+ -Wl,--no-whole-archive \
$(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..7e59ca54cd 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 $@; $(AR) cDPrsT$(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 $@; $(AR) cDPrsT$(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 $@; $(AR) cDPrsT$(KBUILD_ARFLAGS) $@ $(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 297f98dafd..44ed6d6826 100644
--- a/scripts/Makefile.spl
+++ b/scripts/Makefile.spl
@@ -432,10 +432,10 @@ quiet_cmd_u-boot-spl ?= LD $@
cd $(obj) && \
$(LD) $(KBUILD_LDFLAGS) $(LDFLAGS_$(@F)) \
$(patsubst $(obj)/%,%,$(u-boot-spl-init)) \
- --start-group \
+ --whole-archive \
$(patsubst $(obj)/%,%,$(u-boot-spl-main)) \
$(patsubst $(obj)/%,%,$(u-boot-spl-platdata)) \
- --end-group \
+ --no-whole-archive \
$(PLATFORM_LIBS) -Map $(SPL_BIN).map -o $(SPL_BIN) \
)

--
2.26.2