What does this make rule do? - makefile

Question

I am writing makefile by example of another and have stumbled upon this target rule:
ifeq ($(MAKECMDGOALS),build_executable)
$(firstword $(dependency_files)): $(application_path)/config/gcc/app.mk
#rm -rf $(object_output_path)
-include $(dependency_files)
endif
This is placed between other rules. This makefile runs recursively reinvoking itself with different goals.
All variables pretty much explains themself. Prerequisite app.mk contains just configuration by some variables set which are used in this make file. Dependency files variable constructed like this:
dependency_files := $(object_files:%.o=%.d)
My question is what "common practice" this rule corresponds to, what does it do and why it is like that. If my understanding is correct (please correct me if I am wrong), I do understand that first of all it includes all dependency files if they exist. Then this rule may be ran by "makeflow". What I do not understand is the intention of doing this. Also, what is the trigger of running this rule on that one (basically the random one) dependency file since .d files are generated with GCC -MMD -MP options.
Thanks in advance.
EDIT:
###############################################################################
#
# #brief This file is part of the TouchGFX 4.8.0 evaluation distribution.
#
# #author Draupner Graphics A/S <http://www.touchgfx.com>
#
###############################################################################
#
# #section Copyright
#
# This file is free software and is provided for example purposes. You may
# use, copy, and modify within the terms and conditions of the license
# agreement.
#
# This is licensed software for evaluation use, any use must strictly comply
# with the evaluation license agreement provided with delivery of the
# TouchGFX software.
#
# The evaluation license agreement can be seen on www.touchgfx.com
#
# #section Disclaimer
#
# DISCLAIMER OF WARRANTY/LIMITATION OF REMEDIES: Draupner Graphics A/S has
# no obligation to support this software. Draupner Graphics A/S is providing
# the software "AS IS", with no express or implied warranties of any kind,
# including, but not limited to, any implied warranties of merchantability
# or fitness for any particular purpose or warranties against infringement
# of any proprietary rights of a third party.
#
# Draupner Graphics A/S can not be held liable for any consequential,
# incidental, or special damages, or any other relief, or for any claim by
# any third party, arising from your use of this software.
#
###############################################################################
# Get name of this Makefile
makefile_name := $(lastword $(MAKEFILE_LIST))
# Get path of this Makefile
makefile_path := $(dir $(abspath $(makefile_name)))
# Get path where the Application is
application_path := $(abspath $(makefile_path)/../..)
# Change makefile_name to a relative path
makefile_name := $(patsubst $(application_path)/%,%,$(abspath $(makefile_name)))
# Get relative path to makefile
makefile_path_relative = $(patsubst $(application_path)/%,%,$(makefile_path))
# Get path to bsp
bsp_path := $(patsubst $(application_path)/%,%,$(abspath $(makefile_path_relative)../bsp))
# Get OS path
os_path := $(patsubst $(application_path)/%,%, $(abspath $(makefile_path_relative)../CMSIS-RTOS2))
# Get target path
cmsis_core_path := $(patsubst $(application_path)/%,%, $(abspath $(makefile_path_relative)../CMSIS-COREM))
# Get identification of this system
ifeq ($(OS),Windows_NT)
UNAME := MINGW32_NT-6.2
else
UNAME := $(shell uname -s)
endif
board_name := NONE
platform := cortex_m4f
.PHONY: all clean assets flash intflash
ifneq ($(words $(makefile_path))$(words $(MAKEFILE_LIST)),11)
all: $(filter clean,$(MAKECMDGOALS))
all clean assets flash intflash:
$(error Spaces not allowed in path)
else
all: $(filter clean,$(MAKECMDGOALS))
all clean assets:
#cd $(application_path) && $(MAKE) -r -f $(makefile_name) -s $(MFLAGS) _$#_
flash intflash: all
#cd $(application_path) && $(MAKE) -r -f $(makefile_name) -s $(MFLAGS) _$#_
# Directories containing application-specific source and header files.
# Additional components can be added to this list. make will look for
# source files recursively in comp_name/src and setup an include directive
# for comp_name/include.
components := gui target generated/gui_generated
# Location of folder containing bmp/png files.
asset_images_input := assets/images
# Location of folder to search for ttf font files
asset_fonts_input := assets/fonts
# Location of folder where the texts.xlsx is placed
asset_texts_input := assets/texts
build_root_path := build
object_output_path := $(build_root_path)/$(board_name)
binary_output_path := $(build_root_path)/bin
# Location of output folders where autogenerated code from assets is placed
asset_root_path := generated
asset_images_output := $(asset_root_path)/images
asset_fonts_output := $(asset_root_path)/fonts
asset_texts_output := $(asset_root_path)/texts
#include application specific configuration
include $(application_path)/config/gcc/app.mk
os_source_files := $(os_path)/RTX/Config/RTX_Config.c \
$(os_path)/RTX/Source/rtx_lib.c
os_include_paths := $(os_path)/Include \
$(os_path)/RTX/Config \
$(os_path)/RTX/Include
#$(os_path)/RTX/Source \
os_wrapper := $(os_path)/OSWrappers_cmsis.cpp
### END OF USER SECTION. THE FOLLOWING SHOULD NOT BE MODIFIED ###
ifeq ($(UNAME), Linux)
imageconvert_executable := $(touchgfx_path)/framework/tools/imageconvert/build/linux/imageconvert.out
fontconvert_executable := $(touchgfx_path)/framework/tools/fontconvert/build/linux/fontconvert.out
else
imageconvert_executable := $(touchgfx_path)/framework/tools/imageconvert/build/win/imageconvert.out
fontconvert_executable := $(touchgfx_path)/framework/tools/fontconvert/build/win/fontconvert.out
st_link_executable := "$(PROGRAMFILES)\\STMicroelectronics\\STM32 ST-LINK Utility\\ST-LINK Utility\\ST-LINK_CLI.exe"
st_link_external_loader := "$(PROGRAMFILES)\\STMicroelectronics\\STM32 ST-LINK Utility\\ST-LINK Utility\\ExternalLoader\\N25Q128A_STM32469I-DISCO.stldr"
endif
target_executable := target.elf
target_hex := target.hex
########### include $(touchgfx_path)/config/toolchain-arm-none-eabi-gcc.mk #################
# Defines the assembler binary and options. These are optional and only
# of relevance if the component includes source files with an
# extension of .asm.
# Consider adding -Wall to c_compiler_options and cpp_compiler_options
#no_libs := -nostdlib -nodefaultlibs For now, include everything.
assembler := arm-none-eabi-gcc
assembler_options += -g \
-nostartfiles -fno-exceptions\
$(no_libs) -mthumb -mno-thumb-interwork \
-Wall
assembler_options += $(float_options)
c_compiler := arm-none-eabi-gcc
c_compiler_options += -g \
-nostartfiles -mthumb -fno-exceptions \
-mno-thumb-interwork -std=c99 \
$(no_libs) \
-Os -fno-strict-aliasing -fdata-sections -ffunction-sections
c_compiler_options += $(float_options)
cpp_compiler := arm-none-eabi-g++
cpp_compiler_options += -g -mthumb \
-nostartfiles $(no_libs) \
-mno-thumb-interwork -fno-rtti -fno-exceptions \
-Os -fno-strict-aliasing -fdata-sections -ffunction-sections
cpp_compiler_options += $(float_options)
linker := arm-none-eabi-g++
linker_options += -g -Wl,-static -nostartfiles -mthumb $(no_libs) -mno-thumb-interwork \
-fno-exceptions -fno-rtti \
-Os -fno-strict-aliasing -Wl,--gc-sections
objcopy := arm-none-eabi-objcopy
archiver := arm-none-eabi-ar
strip := arm-none-eabi-strip
####################### Additional toolchain configuration for Cortex-M4f targets.##########################
float_abi := hard
float_options := -mfpu=fpv4-sp-d16
ifneq ("$(float_abi)","hard")
float_options += -mfloat-abi=softfp
else
float_options += -mfloat-abi=hard
endif
assembler_options += -mcpu=cortex-m4 -march=armv7e-m -Wno-psabi $(float_options) -DCORE_M4 -D__irq=""
c_compiler_options += -mcpu=cortex-m4 -march=armv7e-m -Wno-psabi $(float_options) -DCORE_M4 -D__irq=""
cpp_compiler_options += -mcpu=cortex-m4 -march=armv7e-m -Wno-psabi $(float_options) -DCORE_M4 -D__irq=""
linker_options += -mcpu=cortex-m4 -march=armv7e-m -Wno-psabi $(float_options)
############################################################################################################
#include everything + specific vendor folders
framework_includes := $(touchgfx_path)/framework/include
#this needs to change when assset include folder changes.
all_components := $(components) \
$(asset_fonts_output) \
$(asset_images_output) \
$(asset_texts_output)
#keep framework include and source out of this mess! :)
include_paths := $(library_includes) $(foreach comp, $(all_components), $(comp)/include) $(framework_includes) $(source_bsp_paths)
source_paths = $(foreach comp, $(all_components), $(comp)/src)
# Finds files that matches the specified pattern. The directory list
# is searched recursively. It is safe to invoke this function with an
# empty list of directories.
#
# Param $(1): List of directories to search
# Param $(2): The file pattern to search for
define find
$(foreach dir,$(1),$(foreach d,$(wildcard $(dir)/*),\
$(call find,$(d),$(2))) $(wildcard $(dir)/$(strip $(2))))
endef
unexport find
fontconvert_ttf_lower_files := $(call find, $(asset_fonts_input), *.ttf)
fontconvert_ttf_upper_files := $(call find, $(asset_fonts_input), *.TTF)
fontconvert_otf_lower_files := $(call find, $(asset_fonts_input), *.otf)
fontconvert_otf_upper_files := $(call find, $(asset_fonts_input), *.OTF)
fontconvert_bdf_lower_files := $(call find, $(asset_fonts_input), *.bdf)
fontconvert_bdf_upper_files := $(call find, $(asset_fonts_input), *.BDF)
fontconvert_font_files := $(fontconvert_ttf_lower_files) \
$(fontconvert_ttf_upper_files) \
$(fontconvert_otf_lower_files) \
$(fontconvert_otf_upper_files) \
$(fontconvert_bdf_lower_files) \
$(fontconvert_bdf_upper_files)
source_files := $(call find, $(source_paths),*.cpp)
gcc_source_files := $(touchgfx_path)/framework/config/gcc/stdio.c \
$(touchgfx_path)/framework/config/gcc/stdlib.c \
$(touchgfx_path)/framework/config/gcc/string.c
# bsp files
board_c_files := \
$(bsp_path)/src/main.c \
$(bsp_path)/src/debug.c
board_cpp_files := \
$(bsp_path)/src/leds.cpp \
$(bsp_path)/src/KeySampler.cpp \
$(bsp_path)/src/app_gpio.cpp
# bsp include
board_include_paths := \
$(bsp_path)/inc \
gui/include \
generated/gui_generated/include
# Compiler options
c_compiler_options += -DST -DSTM32F469xx -DUSE_OS_SYSTICK -DUSE_FLOATING_POINT -g -gdwarf-2
cpp_compiler_options += -DST -DSTM32F469xx -DUSE_OS_SYSTICK -DUSE_FLOATING_POINT -g -gdwarf-2
include_paths += $(application_path)/platform/os $(board_include_paths) $(os_include_paths)
c_source_files := $(call find, $(source_paths),*.c) $(os_source_files) $(makefile_path_relative)/isr.c $(board_c_files)
source_files += $(os_wrapper) target/gcc/gccstubs.cpp target/main.cpp \
$(board_cpp_files) \
$(bsp_path)/src/BoardConfiguration.cpp \
$(bsp_path)/src/GPIO.cpp
object_files := $(source_files:$(touchgfx_path)/%.cpp=$(object_output_path)/touchgfx/%.o) $(c_source_files:$(touchgfx_path)/%.c=$(object_output_path)/touchgfx/%.o)
object_files := $(object_files:%.cpp=$(object_output_path)/%.o)
object_files := $(object_files:%.c=$(object_output_path)/%.o)
dependency_files := $(object_files:%.o=%.d)
textconvert_script_path := $(touchgfx_path)/framework/tools/textconvert
textconvert_executable := $(call find, $(textconvert_script_path), *.rb)
text_database := $(asset_texts_input)/texts.xlsx
libraries := touchgfx-float-abi-hard RTX_CM4F
library_include_paths := $(touchgfx_path)/lib/core/$(platform)/gcc
library_include_paths += $(os_path)/RTX/Library/GCC
.PHONY: _all_ _clean_ _assets_ _flash_ _intflash_ generate_assets build_executable
# Force linking each time
.PHONY: $(binary_output_path)/$(target_executable)
_all_: generate_assets
ifeq ($(shell find $(application_path) -wholename "$(application_path)/$(binary_output_path)/extflash.bin" -size +0c | wc -l | xargs echo),1)
_flash_: _extflash_
else
_flash_: _intflash_
endif
_extflash_:
#$(st_link_executable) -c -P $(binary_output_path)/target.hex 0x90000000 -Rst -EL $(st_link_external_loader)
_intflash_:
#$(st_link_executable) -c -P $(binary_output_path)/intflash.hex 0x08000000 -Rst
generate_assets: _assets_
#$(MAKE) -f $(makefile_name) -r -s $(MFLAGS) build_executable
build_executable: $(binary_output_path)/$(target_executable)
$(binary_output_path)/$(target_executable): $(object_files)
#echo Linking $(#)
#mkdir -p $(#D)
#mkdir -p $(object_output_path)
#$(file >$(build_root_path)/objects.tmp) $(foreach F,$(object_files),$(file >>$(build_root_path)/objects.tmp,$F))
#$(linker) \
$(linker_options) -T $(makefile_path_relative)/application.ld -Wl,-Map=$(#D)/application.map $(linker_options_local) \
$(patsubst %,-L%,$(library_include_paths)) \
#$(build_root_path)/objects.tmp -o $# \
-Wl,--start-group $(patsubst %,-l%,$(libraries)) -Wl,--end-group
#rm -f $(build_root_path)/objects.tmp
#echo "Producing additional output formats..."
#echo " target.hex - Combined internal+external hex"
#$(objcopy) -O ihex $# $(#D)/target.hex
#echo " intflash.elf - Internal flash, elf debug"
#$(objcopy) --remove-section=ExtFlashSection $# $(#D)/intflash.elf 2>/dev/null
#echo " intflash.hex - Internal flash, hex"
#$(objcopy) -O ihex --remove-section=ExtFlashSection $# $(#D)/intflash.hex
#echo " extflash.bin - External flash, binary"
#$(objcopy) -O binary --only-section=ExtFlashSection $# $(#D)/extflash.bin
$(object_output_path)/touchgfx/%.o: $(touchgfx_path)/%.cpp $(application_path)/config/gcc/app.mk
#echo Compiling $<
#mkdir -p $(#D)
#$(cpp_compiler) \
-MMD -MP $(cpp_compiler_options) $(cpp_compiler_options_local) $(user_cflags) \
$(patsubst %,-I%,$(include_paths)) \
-c $< -o $#
$(object_output_path)/%.o: %.cpp $(application_path)/config/gcc/app.mk
#echo Compiling $<
#mkdir -p $(#D)
#$(cpp_compiler) \
-MMD -MP $(cpp_compiler_options) $(cpp_compiler_options_local) $(user_cflags) \
$(patsubst %,-I%,$(include_paths)) \
-c $< -o $#
$(object_output_path)/%.o: %.c $(application_path)/config/gcc/app.mk
#echo Compiling $<
#mkdir -p $(#D)
#$(c_compiler) \
-MMD -MP $(c_compiler_options) $(c_compiler_options_local) $(user_cflags) \
$(patsubst %,-I%,$(include_paths)) \
-c $< -o $#
ifeq ($(MAKECMDGOALS),build_executable)
$(firstword $(dependency_files)): $(application_path)/config/gcc/app.mk
#rm -rf $(object_output_path)
-include $(dependency_files)
endif
_assets_: BitmapDatabase $(asset_texts_output)/include/texts/TextKeysAndLanguages.hpp
alpha_dither ?= no
dither_algorith ?= 2
remap_identical_texts ?= yes
.PHONY: BitmapDatabase
BitmapDatabase:
#echo Converting images
#$(imageconvert_executable) -dither $(dither_algorithm) -alpha_dither $(alpha_dither) -opaque_image_format $(opaque_image_format) -non_opaque_image_format $(non_opaque_image_format) $(screen_orientation) -r $(asset_images_input) -w $(asset_images_output)
$(asset_texts_output)/include/texts/TextKeysAndLanguages.hpp: $(text_database) $(application_path)/config/gcc/app.mk $(textconvert_executable) $(fontconvert_executable) $(fontconvert_font_files)
#rm -f $(asset_fonts_output)/src/*
#rm -f $(asset_fonts_output)/include/fonts/*
#rm -f $(asset_fonts_output)/UnicodeList*.txt
#rm -f $(asset_fonts_output)/CharSizes*.csv
#mkdir -p $(asset_texts_output)/include/texts
#ruby $(textconvert_script_path)/main.rb $(text_database) $(fontconvert_executable) $(asset_fonts_output) $(asset_texts_output) $(asset_fonts_input) . $(remap_identical_texts) $(text_data_format)
_clean_:
#echo Cleaning
#rm -rf $(build_root_path)
# Do not remove gui_generated
#rm -rf $(asset_images_output)
#rm -rf $(asset_fonts_output)
#rm -rf $(asset_texts_output)
# Create directory to avoid error if it does not exist
#mkdir -p $(asset_root_path)
# Remove assets folder if it is empty (i.e. no gui_generated folder)
#rmdir --ignore-fail-on-non-empty $(asset_root_path)
endif

Answer 1

Changing the app.mk file might make the dependency files obsolete. What this rule does is forcing the re-building of the dependency files by deleting their directory.
For instance the dependencies may change because preprocessor symboles where added or modified. Let's assume that in the provided example the c_compiler_options has been modified to add the -DUSE_FLOATING_POINT option:
c_compiler_options += -DST -DSTM32F469xx -DUSE_OS_SYSTICK -DUSE_FLOATING_POINT -g -gdwarf-2
In a source file this may add or remove include file directives:
#ifdef USE_FLOATING_POINT
#include <some_file.h>
#else
#incude <anotherfile.h>
#endif
Hence the need for running the dependendy evaluation again.