ifneq ($(O),)
ifeq ($(origin O), command line)
	dummy := $(if $(shell test -d $(O) || echo $(O)),$(error O=$(O) does not exist),)
	ABSOLUTE_O := $(shell cd $(O) ; pwd)
	OUTPUT := $(ABSOLUTE_O)/$(if $(subdir),$(subdir)/)
	COMMAND_O := O=$(ABSOLUTE_O)
ifeq ($(objtree),)
	objtree := $(O)
endif
endif
endif

# check that the output directory actually exists
ifneq ($(OUTPUT),)
OUTDIR := $(shell cd $(OUTPUT) && /bin/pwd)
$(if $(OUTDIR),, $(error output directory "$(OUTPUT)" does not exist))
endif

#
# Include saner warnings here, which can catch bugs:
#
EXTRA_WARNINGS := -Wbad-function-cast
EXTRA_WARNINGS += -Wdeclaration-after-statement
EXTRA_WARNINGS += -Wformat-security
EXTRA_WARNINGS += -Wformat-y2k
EXTRA_WARNINGS += -Winit-self
EXTRA_WARNINGS += -Wmissing-declarations
EXTRA_WARNINGS += -Wmissing-prototypes
EXTRA_WARNINGS += -Wnested-externs
EXTRA_WARNINGS += -Wno-system-headers
EXTRA_WARNINGS += -Wold-style-definition
EXTRA_WARNINGS += -Wpacked
EXTRA_WARNINGS += -Wredundant-decls
EXTRA_WARNINGS += -Wshadow
EXTRA_WARNINGS += -Wstrict-prototypes
EXTRA_WARNINGS += -Wswitch-default
EXTRA_WARNINGS += -Wswitch-enum
EXTRA_WARNINGS += -Wundef
EXTRA_WARNINGS += -Wwrite-strings
EXTRA_WARNINGS += -Wformat

ifneq ($(CC), clang)
EXTRA_WARNINGS += -Wstrict-aliasing=3
endif

# Hack to avoid type-punned warnings on old systems such as RHEL5:
# We should be changing CFLAGS and checking gcc version, but this
# will do for now and keep the above -Wstrict-aliasing=3 in place
# in newer systems.
# Needed for the __raw_cmpxchg in tools/arch/x86/include/asm/cmpxchg.h
ifneq ($(filter 3.%,$(MAKE_VERSION)),)  # make-3
EXTRA_WARNINGS += -fno-strict-aliasing
endif

ifneq ($(findstring $(MAKEFLAGS), w),w)
PRINT_DIR = --no-print-directory
else
NO_SUBDIR = :
endif

ifneq ($(findstring s,$(filter-out --%,$(MAKEFLAGS))),)
  silent=1
endif

#
# Define a callable command for descending to a new directory
#
# Call by doing: $(call descend,directory[,target])
#
descend = \
	+mkdir -p $(OUTPUT)$(1) && \
	$(MAKE) $(COMMAND_O) subdir=$(if $(subdir),$(subdir)/$(1),$(1)) $(PRINT_DIR) -C $(1) $(2)

QUIET_SUBDIR0  = +$(MAKE) $(COMMAND_O) -C # space to separate -C and subdir
QUIET_SUBDIR1  =

ifneq ($(silent),1)
  ifneq ($(V),1)
	QUIET_CC       = @echo '  CC       '$@;
	QUIET_CC_FPIC  = @echo '  CC FPIC  '$@;
	QUIET_AR       = @echo '  AR       '$@;
	QUIET_LINK     = @echo '  LINK     '$@;
	QUIET_MKDIR    = @echo '  MKDIR    '$@;
	QUIET_GEN      = @echo '  GEN      '$@;
	QUIET_SUBDIR0  = +@subdir=
	QUIET_SUBDIR1  = ;$(NO_SUBDIR) \
			  echo '  SUBDIR   '$$subdir; \
			 $(MAKE) $(PRINT_DIR) -C $$subdir
	QUIET_FLEX     = @echo '  FLEX     '$@;
	QUIET_BISON    = @echo '  BISON    '$@;

	descend = \
		+@echo	       '  DESCEND  '$(1); \
		mkdir -p $(OUTPUT)$(1) && \
		$(MAKE) $(COMMAND_O) subdir=$(if $(subdir),$(subdir)/$(1),$(1)) $(PRINT_DIR) -C $(1) $(2)

	QUIET_CLEAN    = @printf '  CLEAN    %s\n' $1;
	QUIET_INSTALL  = @printf '  INSTALL  %s\n' $1;
  endif
endif