py/mkrules: Automatically build mpy-cross if it doesn't exist.

Commit 41739506 removed automatic building
of mpy-cross, which rebuilt it whenever any of its dependent source files
changed.

But needing to build mpy-cross, and not knowing how, is a frequent issue.
This commit aims to help by automatically building mpy-cross only if it
doesn't exist.  For Makefiles it uses an order-only prerequisite, while
for CMake it uses a custom command.

If MICROPY_MPYCROSS (which is what makemanifest.py uses to locate the
mpy-cross executable) is defined in the environment then automatic build
will not be attempted, allowing a way to prevent this auto-build if needed.

Thanks to Trammell Hudson aka @osresearch for the original idea; see #5760.
Signed-off-by: Damien George <damien@micropython.org>

py/mkenv.mk

@@ -55,11 +55,15 @@ AR = $(CROSS_COMPILE)ar
 
 MAKE_MANIFEST = $(PYTHON) $(TOP)/tools/makemanifest.py
 MAKE_FROZEN = $(PYTHON) $(TOP)/tools/make-frozen.py
-MPY_CROSS = $(TOP)/mpy-cross/mpy-cross
 MPY_TOOL = $(PYTHON) $(TOP)/tools/mpy-tool.py
 
 MPY_LIB_DIR = $(TOP)/../micropython-lib
 
+ifeq ($(MICROPY_MPYCROSS),)
+MICROPY_MPYCROSS = $(TOP)/mpy-cross/mpy-cross
+MICROPY_MPYCROSS_DEPENDENCY = $(MICROPY_MPYCROSS)
+endif
+
 all:
 .PHONY: all
 

py/mkrules.cmake

@@ -129,11 +129,27 @@ if(MICROPY_FROZEN_MANIFEST)
         set(MICROPY_LIB_DIR ${MICROPY_DIR}/../micropython-lib)
     endif()
 
+    # If MICROPY_MPYCROSS is not explicitly defined in the environment (which
+    # is what makemanifest.py will use) then create an mpy-cross dependency
+    # to automatically build mpy-cross if needed.
+    set(MICROPY_MPYCROSS $ENV{MICROPY_MPYCROSS})
+    if(NOT MICROPY_MPYCROSS)
+        set(MICROPY_MPYCROSS_DEPENDENCY ${MICROPY_DIR}/mpy-cross/mpy-cross)
+        if(NOT MICROPY_MAKE_EXECUTABLE)
+            set(MICROPY_MAKE_EXECUTABLE make)
+        endif()
+        add_custom_command(
+            OUTPUT ${MICROPY_MPYCROSS_DEPENDENCY}
+            COMMAND ${MICROPY_MAKE_EXECUTABLE} -C ${MICROPY_DIR}/mpy-cross
+        )
+    endif()
+
     add_custom_command(
         OUTPUT ${MICROPY_FROZEN_CONTENT}
         COMMAND ${Python3_EXECUTABLE} ${MICROPY_DIR}/tools/makemanifest.py -o ${MICROPY_FROZEN_CONTENT} -v "MPY_DIR=${MICROPY_DIR}" -v "MPY_LIB_DIR=${MICROPY_LIB_DIR}" -v "PORT_DIR=${MICROPY_PORT_DIR}" -v "BOARD_DIR=${MICROPY_BOARD_DIR}" -b "${CMAKE_BINARY_DIR}" -f${MICROPY_CROSS_FLAGS} ${MICROPY_FROZEN_MANIFEST}
         DEPENDS MICROPY_FORCE_BUILD
             ${MICROPY_QSTRDEFS_GENERATED}
+            ${MICROPY_MPYCROSS_DEPENDENCY}
         VERBATIM
     )
 endif()

py/mkrules.mk

@@ -136,9 +136,15 @@ $(OBJ_DIRS):
 $(HEADER_BUILD):
 	$(MKDIR) -p $@
 
+ifneq ($(MICROPY_MPYCROSS_DEPENDENCY),)
+# to automatically build mpy-cross, if needed
+$(MICROPY_MPYCROSS_DEPENDENCY):
+	$(MAKE) -C $(dir $@)
+endif
+
 ifneq ($(FROZEN_MANIFEST),)
 # to build frozen_content.c from a manifest
-$(BUILD)/frozen_content.c: FORCE $(BUILD)/genhdr/qstrdefs.generated.h
+$(BUILD)/frozen_content.c: FORCE $(BUILD)/genhdr/qstrdefs.generated.h | $(MICROPY_MPYCROSS_DEPENDENCY)
 	$(Q)$(MAKE_MANIFEST) -o $@ -v "MPY_DIR=$(TOP)" -v "MPY_LIB_DIR=$(MPY_LIB_DIR)" -v "PORT_DIR=$(shell pwd)" -v "BOARD_DIR=$(BOARD_DIR)" -b "$(BUILD)" $(if $(MPY_CROSS_FLAGS),-f"$(MPY_CROSS_FLAGS)",) --mpy-tool-flags="$(MPY_TOOL_FLAGS)" $(FROZEN_MANIFEST)
 
 ifneq ($(FROZEN_DIR),)
@@ -164,10 +170,10 @@ FROZEN_MPY_PY_FILES := $(shell find -L $(FROZEN_MPY_DIR) -type f -name '*.py' |
 FROZEN_MPY_MPY_FILES := $(addprefix $(BUILD)/frozen_mpy/,$(FROZEN_MPY_PY_FILES:.py=.mpy))
 
 # to build .mpy files from .py files
-$(BUILD)/frozen_mpy/%.mpy: $(FROZEN_MPY_DIR)/%.py
+$(BUILD)/frozen_mpy/%.mpy: $(FROZEN_MPY_DIR)/%.py | $(MICROPY_MPYCROSS_DEPENDENCY)
 	@$(ECHO) "MPY $<"
 	$(Q)$(MKDIR) -p $(dir $@)
-	$(Q)$(MPY_CROSS) -o $@ -s $(<:$(FROZEN_MPY_DIR)/%=%) $(MPY_CROSS_FLAGS) $<
+	$(Q)$(MICROPY_MPYCROSS) -o $@ -s $(<:$(FROZEN_MPY_DIR)/%=%) $(MPY_CROSS_FLAGS) $<
 
 # to build frozen_mpy.c from all .mpy files
 $(BUILD)/frozen_mpy.c: $(FROZEN_MPY_MPY_FILES) $(BUILD)/genhdr/qstrdefs.generated.h