linux/scripts/Makefile.build

# SPDX-License-Identifier: GPL-2.0
# ==========================================================================
# Building
# ==========================================================================

src := $(srcroot)/$(obj)

PHONY := $(obj)/
$(obj)/:

# Init all relevant variables used in kbuild files so
# 1) they have correct type
# 2) they do not inherit any value from the environment
obj-y :=
obj-m :=
lib-y :=
lib-m :=
always-y :=
always-m :=
targets :=
subdir-y :=
subdir-m :=
asflags-y  :=
ccflags-y  :=
rustflags-y :=
cppflags-y :=
ldflags-y  :=

subdir-asflags-y :=
subdir-ccflags-y :=

# Read auto.conf if it exists, otherwise ignore
-include $(objtree)/include/config/auto.conf

include $(srctree)/scripts/Kbuild.include
include $(srctree)/scripts/Makefile.compiler
include $(kbuild-file)
include $(srctree)/scripts/Makefile.lib

# flags that take effect in current and sub directories
KBUILD_AFLAGS += $(subdir-asflags-y)
KBUILD_CFLAGS += $(subdir-ccflags-y)
KBUILD_RUSTFLAGS += $(subdir-rustflags-y)

# Figure out what we need to build from the various variables
# ===========================================================================

# When an object is listed to be built compiled-in and modular,
# only build the compiled-in version
obj-m := $(filter-out $(obj-y),$(obj-m))

# Libraries are always collected in one lib file.
# Filter out objects already built-in
lib-y := $(filter-out $(obj-y), $(sort $(lib-y) $(lib-m)))

# Subdirectories we need to descend into
subdir-ym := $(sort $(subdir-y) $(subdir-m) \
			$(patsubst %/,%, $(filter %/, $(obj-y) $(obj-m))))

# Handle objects in subdirs:
# - If we encounter foo/ in $(obj-y), replace it by foo/built-in.a and
#   foo/modules.order
# - If we encounter foo/ in $(obj-m), replace it by foo/modules.order
#
# Generate modules.order to determine modorder. Unfortunately, we don't have
# information about ordering between -y and -m subdirs. Just put -y's first.

ifdef need-modorder
obj-m := $(patsubst %/,%/modules.order, $(filter %/, $(obj-y)) $(obj-m))
else
obj-m := $(filter-out %/, $(obj-m))
endif

ifdef need-builtin
obj-y		:= $(patsubst %/, %/built-in.a, $(obj-y))
else
obj-y		:= $(filter-out %/, $(obj-y))
endif

# Expand $(foo-objs) $(foo-y) etc. by replacing their individuals
suffix-search = $(strip $(foreach s, $3, $($(1:%$(strip $2)=%$s))))
# List composite targets that are constructed by combining other targets
multi-search = $(sort $(foreach m, $1, $(if $(call suffix-search, $m, $2, $3 -), $m)))
# List primitive targets that are compiled from source files
real-search = $(foreach m, $1, $(if $(call suffix-search, $m, $2, $3 -), $(call suffix-search, $m, $2, $3), $m))

# If $(foo-objs), $(foo-y), $(foo-m), or $(foo-) exists, foo.o is a composite object
multi-obj-y := $(call multi-search, $(obj-y), .o, -objs -y)
multi-obj-m := $(call multi-search, $(obj-m), .o, -objs -y -m)
multi-obj-ym := $(multi-obj-y) $(multi-obj-m)

# Replace multi-part objects by their individual parts,
# including built-in.a from subdirectories
real-obj-y := $(call real-search, $(obj-y), .o, -objs -y)
real-obj-m := $(call real-search, $(obj-m), .o, -objs -y -m)

always-y += $(always-m)

# hostprogs-always-y += foo
# ... is a shorthand for
# hostprogs += foo
# always-y  += foo
hostprogs += $(hostprogs-always-y) $(hostprogs-always-m)
always-y += $(hostprogs-always-y) $(hostprogs-always-m)

# userprogs-always-y is likewise.
userprogs += $(userprogs-always-y) $(userprogs-always-m)
always-y += $(userprogs-always-y) $(userprogs-always-m)

# Add subdir path

ifneq ($(obj),.)
extra-y		:= $(addprefix $(obj)/, $(extra-y))
always-y	:= $(addprefix $(obj)/, $(always-y))
targets		:= $(addprefix $(obj)/, $(targets))
obj-m		:= $(addprefix $(obj)/, $(obj-m))
lib-y		:= $(addprefix $(obj)/, $(lib-y))
real-obj-y	:= $(addprefix $(obj)/, $(real-obj-y))
real-obj-m	:= $(addprefix $(obj)/, $(real-obj-m))
multi-obj-m	:= $(addprefix $(obj)/, $(multi-obj-m))
subdir-ym	:= $(addprefix $(obj)/, $(subdir-ym))
endif

ifndef obj
$(warning kbuild: Makefile.build is included improperly)
endif

ifeq ($(need-modorder),)
ifneq ($(obj-m),)
$(warning $(patsubst %.o,'%.ko',$(obj-m)) will not be built even though obj-m is specified.)
$(warning You cannot use subdir-y/m to visit a module Makefile. Use obj-y/m instead.)
endif
endif

# ===========================================================================

# subdir-builtin and subdir-modorder may contain duplications. Use $(sort ...)
subdir-builtin := $(sort $(filter %/built-in.a, $(real-obj-y)))
subdir-modorder := $(sort $(filter %/modules.order, $(obj-m)))

targets-for-builtin := $(extra-y)

ifneq ($(strip $(lib-y) $(lib-m) $(lib-)),)
targets-for-builtin += $(obj)/lib.a
endif

ifdef need-builtin
targets-for-builtin += $(obj)/built-in.a
endif

targets-for-modules := $(foreach x, o mod, \
				$(patsubst %.o, %.$x, $(filter %.o, $(obj-m))))

ifdef need-modorder
targets-for-modules += $(obj)/modules.order
endif

targets += $(targets-for-builtin) $(targets-for-modules)

# Linus' kernel sanity checking tool
ifeq ($(KBUILD_CHECKSRC),1)
  quiet_cmd_checksrc       = CHECK   $<
        cmd_checksrc       = $(CHECK) $(CHECKFLAGS) $(c_flags) $<
else ifeq ($(KBUILD_CHECKSRC),2)
  quiet_cmd_force_checksrc = CHECK   $<
        cmd_force_checksrc = $(CHECK) $(CHECKFLAGS) $(c_flags) $<
endif

ifneq ($(KBUILD_EXTRA_WARN),)
  cmd_checkdoc = PYTHONDONTWRITEBYTECODE=1 $(KERNELDOC) -none $(KDOCFLAGS) \
        $(if $(findstring 2, $(KBUILD_EXTRA_WARN)), -Wall) \
        $<
endif

# Compile C sources (.c)
# ---------------------------------------------------------------------------

quiet_cmd_cc_s_c = CC $(quiet_modtag)  $@
      cmd_cc_s_c = $(CC) $(filter-out $(DEBUG_CFLAGS) $(CC_FLAGS_LTO), $(c_flags)) -fverbose-asm -S -o $@ $<

$(obj)/%.s: $(obj)/%.c FORCE
	$(call if_changed_dep,cc_s_c)

quiet_cmd_cpp_i_c = CPP $(quiet_modtag) $@
cmd_cpp_i_c       = $(CPP) $(c_flags) -o $@ $<

$(obj)/%.i: $(obj)/%.c FORCE
	$(call if_changed_dep,cpp_i_c)

getexportsymbols = $(NM) $@ | sed -n 's/.* __export_symbol_\(.*\)/$(1)/p'

gendwarfksyms = $(objtree)/scripts/gendwarfksyms/gendwarfksyms	\
	$(if $(KBUILD_SYMTYPES), --symtypes $(@:.o=.symtypes))	\
	$(if $(KBUILD_GENDWARFKSYMS_STABLE), --stable)

genksyms = $(objtree)/scripts/genksyms/genksyms		\
	$(if $(KBUILD_SYMTYPES), -T $(@:.o=.symtypes))	\
	$(if $(KBUILD_PRESERVE), -p)			\
	$(addprefix -r , $(wildcard $(@:.o=.symref)))

# These mirror gensymtypes_S and co below, keep them in synch.
ifdef CONFIG_GENDWARFKSYMS
cmd_gensymtypes_c = $(if $(skip_gendwarfksyms),,	\
	$(call getexportsymbols,\1) | $(gendwarfksyms) $@)
else
cmd_gensymtypes_c = $(CPP) -D__GENKSYMS__ $(c_flags) $< | $(genksyms)
endif # CONFIG_GENDWARFKSYMS

# LLVM assembly
# Generate .ll files from .c
quiet_cmd_cc_ll_c = CC $(quiet_modtag)  $@
      cmd_cc_ll_c = $(CC) $(c_flags) -emit-llvm -S -fno-discard-value-names -o $@ $<

$(obj)/%.ll: $(obj)/%.c FORCE
	$(call if_changed_dep,cc_ll_c)

# C (.c) files
# The C file is compiled and updated dependency information is generated.
# (See cmd_cc_o_c + relevant part of rule_cc_o_c)

is-single-obj-m = $(and $(part-of-module),$(filter $@, $(obj-m)),y)

ifdef CONFIG_MODVERSIONS
# When module versioning is enabled the following steps are executed:
# o compile a <file>.o from <file>.c
# o if <file>.o doesn't contain a __export_symbol_*, i.e. does
#   not export symbols, it's done.
# o otherwise, we calculate symbol versions using the good old
#   genksyms on the preprocessed source and dump them into the .cmd file.
# o modpost will extract versions from that file and create *.c files that will
#   be compiled and linked to the kernel and/or modules.

gen_symversions =								\
	if $(NM) $@ 2>/dev/null | grep -q ' __export_symbol_'; then		\
		$(cmd_gensymtypes_$1) >> $(dot-target).cmd;			\
	fi

cmd_gen_symversions_c =	$(call gen_symversions,c)

endif

ifdef CONFIG_FTRACE_MCOUNT_USE_RECORDMCOUNT
# compiler will not generate __mcount_loc use recordmcount or recordmcount.pl
ifdef BUILD_C_RECORDMCOUNT
ifeq ("$(origin RECORDMCOUNT_WARN)", "command line")
  RECORDMCOUNT_FLAGS = -w
endif
# Due to recursion, we must skip empty.o.
# The empty.o file is created in the make process in order to determine
# the target endianness and word size. It is made before all other C
# files, including recordmcount.
sub_cmd_record_mcount =					\
	if [ $(@) != "scripts/mod/empty.o" ]; then	\
		$(objtree)/scripts/recordmcount $(RECORDMCOUNT_FLAGS) "$(@)";	\
	fi;
recordmcount_source := $(srctree)/scripts/recordmcount.c \
		    $(srctree)/scripts/recordmcount.h
else
sub_cmd_record_mcount = perl $(srctree)/scripts/recordmcount.pl "$(ARCH)" \
	"$(if $(CONFIG_CPU_BIG_ENDIAN),big,little)" \
	"$(if $(CONFIG_64BIT),64,32)" \
	"$(OBJDUMP)" "$(OBJCOPY)" "$(CC) $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS)" \
	"$(LD) $(KBUILD_LDFLAGS)" "$(NM)" "$(RM)" "$(MV)" \
	"$(if $(part-of-module),1,0)" "$(@)";
recordmcount_source := $(srctree)/scripts/recordmcount.pl
endif # BUILD_C_RECORDMCOUNT
cmd_record_mcount = $(if $(findstring $(strip $(CC_FLAGS_FTRACE)),$(_c_flags)),	\
	$(sub_cmd_record_mcount))
endif # CONFIG_FTRACE_MCOUNT_USE_RECORDMCOUNT

# 'OBJECT_FILES_NON_STANDARD := y': skip objtool checking for a directory
# 'OBJECT_FILES_NON_STANDARD_foo.o := 'y': skip objtool checking for a file
# 'OBJECT_FILES_NON_STANDARD_foo.o := 'n': override directory skip for a file

is-standard-object = $(if $(filter-out y%, $(OBJECT_FILES_NON_STANDARD_$(target-stem).o)$(OBJECT_FILES_NON_STANDARD)n),$(is-kernel-object))

ifdef CONFIG_OBJTOOL
$(obj)/%.o: private objtool-enabled = $(if $(is-standard-object),$(if $(delay-objtool),$(is-single-obj-m),y))
endif

ifneq ($(findstring 1, $(KBUILD_EXTRA_WARN)),)
cmd_warn_shared_object = $(if $(word 2, $(modname-multi)),$(warning $(kbuild-file): $*.o is added to multiple modules: $(modname-multi)))
endif

# Built-in and composite module parts
$(obj)/%.o: $(obj)/%.c $(recordmcount_source) FORCE
	$(call if_changed_rule,cc_o_c)
	$(call cmd,force_checksrc)

# To make this rule robust against "Argument list too long" error,
# ensure to add $(obj)/ prefix by a shell command.
cmd_mod = printf '%s\n' $(call real-search, $*.o, .o, -objs -y -m) | \
	$(AWK) '!x[$$0]++ { print("$(obj)/"$$0) }' > $@

$(obj)/%.mod: FORCE
	$(call if_changed,mod)

quiet_cmd_cc_lst_c = MKLST   $@
      cmd_cc_lst_c = $(CC) $(c_flags) -g -c -o $*.o $< && \
		     $(CONFIG_SHELL) $(srctree)/scripts/makelst $*.o \
				     System.map $(OBJDUMP) > $@

$(obj)/%.lst: $(obj)/%.c FORCE
	$(call if_changed_dep,cc_lst_c)

# Compile Rust sources (.rs)
# ---------------------------------------------------------------------------

# The features in this list are the ones allowed for non-`rust/` code.
#
#   - Stable since Rust 1.81.0: `feature(lint_reasons)`.
#   - Stable since Rust 1.82.0: `feature(asm_const)`, `feature(raw_ref_op)`.
#   - Stable since Rust 1.87.0: `feature(asm_goto)`.
#   - Expected to become stable: `feature(arbitrary_self_types)`.
#
# Please see https://github.com/Rust-for-Linux/linux/issues/2 for details on
# the unstable features in use.
rust_allowed_features := asm_const,asm_goto,arbitrary_self_types,lint_reasons,raw_ref_op

# `--out-dir` is required to avoid temporaries being created by `rustc` in the
# current working directory, which may be not accessible in the out-of-tree
# modules case.
rust_common_cmd = \
	OBJTREE=$(abspath $(objtree)) \
	RUST_MODFILE=$(modfile) $(RUSTC_OR_CLIPPY) $(rust_flags) \
	-Zallow-features=$(rust_allowed_features) \
	-Zcrate-attr=no_std \
	-Zcrate-attr='feature($(rust_allowed_features))' \
	-Zunstable-options --extern pin_init --extern kernel \
	--crate-type rlib -L $(objtree)/rust/ \
	--crate-name $(basename $(notdir $@)) \
	--sysroot=/dev/null \
	--out-dir $(dir $@) --emit=dep-info=$(depfile)

# `--emit=obj`, `--emit=asm` and `--emit=llvm-ir` imply a single codegen unit
# will be used. We explicitly request `-Ccodegen-units=1` in any case, and
# the compiler shows a warning if it is not 1. However, if we ever stop
# requesting it explicitly and we start using some other `--emit` that does not
# imply it (and for which codegen is performed), then we would be out of sync,
# i.e. the outputs we would get for the different single targets (e.g. `.ll`)
# would not match each other.

quiet_cmd_rustc_o_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
      cmd_rustc_o_rs = $(rust_common_cmd) --emit=obj=$@ $< $(cmd_objtool)

define rule_rustc_o_rs
	$(call cmd_and_fixdep,rustc_o_rs)
	$(call cmd,gen_objtooldep)
endef

$(obj)/%.o: $(obj)/%.rs FORCE
	+$(call if_changed_rule,rustc_o_rs)

quiet_cmd_rustc_rsi_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
      cmd_rustc_rsi_rs = \
	$(rust_common_cmd) -Zunpretty=expanded $< >$@; \
	command -v $(RUSTFMT) >/dev/null && $(RUSTFMT) $@

$(obj)/%.rsi: $(obj)/%.rs FORCE
	+$(call if_changed_dep,rustc_rsi_rs)

quiet_cmd_rustc_s_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
      cmd_rustc_s_rs = $(rust_common_cmd) --emit=asm=$@ $<

$(obj)/%.s: $(obj)/%.rs FORCE
	+$(call if_changed_dep,rustc_s_rs)

quiet_cmd_rustc_ll_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
      cmd_rustc_ll_rs = $(rust_common_cmd) --emit=llvm-ir=$@ $<

$(obj)/%.ll: $(obj)/%.rs FORCE
	+$(call if_changed_dep,rustc_ll_rs)

quiet_cmd_rustc_rs_rs_S = RSCPP $(quiet_modtag) $@
      cmd_rustc_rs_rs_S = $(CPP) $(c_flags) -xc -C -P $< | sed '1,/^\/\/ Cut here.$$/d' >$@

$(obj)/%.rs: $(obj)/%.rs.S FORCE
	+$(call if_changed_dep,rustc_rs_rs_S)

# Compile assembler sources (.S)
# ---------------------------------------------------------------------------

# .S file exports must have their C prototypes defined in asm/asm-prototypes.h
# or a file that it includes, in order to get versioned symbols. We build a
# dummy C file that includes asm-prototypes and the EXPORT_SYMBOL lines from
# the .S file (with trailing ';'), and run genksyms on that, to extract vers.
#
# This is convoluted. The .S file must first be preprocessed to run guards and
# expand names, then the resulting exports must be constructed into plain
# EXPORT_SYMBOL(symbol); to build our dummy C file, and that gets preprocessed
# to make the genksyms input or compiled into an object for gendwarfksyms.
#
# These mirror gensymtypes_c and co above, keep them in synch.
getasmexports =								\
   { echo "\#include <linux/kernel.h>" ;				\
     echo "\#include <linux/string.h>" ;				\
     echo "\#include <asm/asm-prototypes.h>" ;				\
     $(call getexportsymbols,EXPORT_SYMBOL(\1);) ; }

ifdef CONFIG_GENDWARFKSYMS
cmd_gensymtypes_S =							\
	$(getasmexports) |						\
	$(CC) $(c_flags) -c -o $(@:.o=.gendwarfksyms.o) -xc -;		\
	$(call getexportsymbols,\1) |					\
	$(gendwarfksyms) $(@:.o=.gendwarfksyms.o)
else
cmd_gensymtypes_S =							\
	$(getasmexports) |						\
	$(CPP) -D__GENKSYMS__ $(c_flags) -xc - | $(genksyms)
endif # CONFIG_GENDWARFKSYMS

quiet_cmd_cpp_s_S = CPP $(quiet_modtag) $@
cmd_cpp_s_S       = $(CPP) $(a_flags) -o $@ $<

$(obj)/%.s: $(obj)/%.S FORCE
	$(call if_changed_dep,cpp_s_S)

ifdef CONFIG_ASM_MODVERSIONS

# versioning matches the C process described above, with difference that
# we parse asm-prototypes.h C header to get function definitions.

cmd_gen_symversions_S = $(call gen_symversions,S)

endif

$(obj)/%.o: $(obj)/%.S FORCE
	$(call if_changed_rule,as_o_S)

targets += $(filter-out $(subdir-builtin), $(real-obj-y))
targets += $(filter-out $(subdir-modorder), $(real-obj-m))
targets += $(lib-y) $(always-y)

# Linker scripts preprocessor (.lds.S -> .lds)
# ---------------------------------------------------------------------------
quiet_cmd_cpp_lds_S = LDS     $@
      cmd_cpp_lds_S = $(CPP) $(cpp_flags) -P -U$(ARCH) \
	                     -D__ASSEMBLY__ -DLINKER_SCRIPT -o $@ $<

$(obj)/%.lds: $(src)/%.lds.S FORCE
	$(call if_changed_dep,cpp_lds_S)

# ASN.1 grammar
# ---------------------------------------------------------------------------
quiet_cmd_asn1_compiler = ASN.1   $(basename $@).[ch]
      cmd_asn1_compiler = $(objtree)/scripts/asn1_compiler $< \
				$(basename $@).c $(basename $@).h

$(obj)/%.asn1.c $(obj)/%.asn1.h: $(src)/%.asn1 $(objtree)/scripts/asn1_compiler
	$(call cmd,asn1_compiler)

# Build the compiled-in targets
# ---------------------------------------------------------------------------

# To build objects in subdirs, we need to descend into the directories
$(subdir-builtin): $(obj)/%/built-in.a: $(obj)/% ;
$(subdir-modorder): $(obj)/%/modules.order: $(obj)/% ;

#
# Rule to compile a set of .o files into one .a file (without symbol table)
#
# To make this rule robust against "Argument list too long" error,
# remove $(obj)/ prefix, and restore it by a shell command.

quiet_cmd_ar_builtin = AR      $@
      cmd_ar_builtin = rm -f $@; \
	$(if $(real-prereqs), printf "$(obj)/%s " $(patsubst $(obj)/%,%,$(real-prereqs)) | xargs) \
	$(AR) cDPrST $@

$(obj)/built-in.a: $(real-obj-y) FORCE
	$(call if_changed,ar_builtin)

# This is a list of build artifacts from the current Makefile and its
# sub-directories. The timestamp should be updated when any of the member files.

cmd_gen_order = { $(foreach m, $(real-prereqs), \
	$(if $(filter %/$(notdir $@), $m), cat $m, echo $m);) :; } \
	> $@

$(obj)/modules.order: $(obj-m) FORCE
	$(call if_changed,gen_order)

#
# Rule to compile a set of .o files into one .a file (with symbol table)
#

$(obj)/lib.a: $(lib-y) FORCE
	$(call if_changed,ar)

quiet_cmd_ld_multi_m = LD [M]  $@
      cmd_ld_multi_m = $(LD) $(ld_flags) -r -o $@ @$< $(cmd_objtool)

define rule_ld_multi_m
	$(call cmd_and_savecmd,ld_multi_m)
	$(call cmd,gen_objtooldep)
endef

$(multi-obj-m): private objtool-enabled := $(delay-objtool)
$(multi-obj-m): private part-of-module := y
$(multi-obj-m): %.o: %.mod FORCE
	$(call if_changed_rule,ld_multi_m)
$(call multi_depend, $(multi-obj-m), .o, -objs -y -m)

# Add intermediate targets:
# When building objects with specific suffix patterns, add intermediate
# targets that the final targets are derived from.
intermediate_targets = $(foreach sfx, $(2), \
				$(patsubst %$(strip $(1)),%$(sfx), \
					$(filter %$(strip $(1)), $(targets))))
# %.asn1.o <- %.asn1.[ch] <- %.asn1
targets += $(call intermediate_targets, .asn1.o, .asn1.c .asn1.h)

# Include additional build rules when necessary
# ---------------------------------------------------------------------------

# $(sort ...) is used here to remove duplicated words and excessive spaces.
hostprogs := $(sort $(hostprogs))
ifneq ($(hostprogs),)
include $(srctree)/scripts/Makefile.host
endif

# $(sort ...) is used here to remove duplicated words and excessive spaces.
userprogs := $(sort $(userprogs))
ifneq ($(userprogs),)
include $(srctree)/scripts/Makefile.userprogs
endif

ifneq ($(need-dtbslist)$(dtb-y)$(dtb-)$(filter %.dtb %.dtb.o %.dtbo.o,$(targets)),)
include $(srctree)/scripts/Makefile.dtbs
endif

# Build
# ---------------------------------------------------------------------------

$(obj)/: $(if $(KBUILD_BUILTIN), $(targets-for-builtin)) \
	 $(if $(KBUILD_MODULES), $(targets-for-modules)) \
	 $(subdir-ym) $(always-y)
	@:

# Single targets
# ---------------------------------------------------------------------------

single-subdirs := $(foreach d, $(subdir-ym), $(if $(filter $d/%, $(MAKECMDGOALS)), $d))
single-subdir-goals := $(filter $(addsuffix /%, $(single-subdirs)), $(MAKECMDGOALS))

$(single-subdir-goals): $(single-subdirs)
	@:

# Descending
# ---------------------------------------------------------------------------

PHONY += $(subdir-ym)
$(subdir-ym):
	$(Q)$(MAKE) $(build)=$@ \
	need-builtin=$(if $(filter $@/built-in.a, $(subdir-builtin)),1) \
	need-modorder=$(if $(filter $@/modules.order, $(subdir-modorder)),1) \
	$(filter $@/%, $(single-subdir-goals))

# Add FORCE to the prerequisites of a target to force it to be always rebuilt.
# ---------------------------------------------------------------------------

PHONY += FORCE

FORCE:

targets += $(filter-out $(single-subdir-goals), $(MAKECMDGOALS))
targets := $(filter-out $(PHONY), $(targets))

# Read all saved command lines and dependencies for the $(targets) we
# may be building above, using $(if_changed{,_dep}). As an
# optimization, we don't need to read them if the target does not
# exist, we will rebuild anyway in that case.

existing-targets := $(wildcard $(sort $(targets)))

-include $(foreach f,$(existing-targets),$(dir $(f)).$(notdir $(f)).cmd)

# Create directories for object files if they do not exist
obj-dirs := $(sort $(patsubst %/,%, $(dir $(targets))))
# If targets exist, their directories apparently exist. Skip mkdir.
existing-dirs := $(sort $(patsubst %/,%, $(dir $(existing-targets))))
obj-dirs := $(strip $(filter-out $(existing-dirs), $(obj-dirs)))
ifneq ($(obj-dirs),)
$(shell mkdir -p $(obj-dirs))
endif

.PHONY: $(PHONY)