GCC M68K Assembler Optimizer

Only useful for the SGDK framework, which builds m68k elf artifacts prior to the final binary rom artifact.

This solution is intended to be executed with gcc plugin feature at PLUGIN_FINISH phase, which is the last opportunity we have to access assembly code before exiting gcc and start the linking phase.

At PLUGIN_FINISH phase we can access the m68k assembly code generated from all the .c units in our project and the SGDK library, including the inline asm blocks. Assembly .s units are excluded from this phase.

The optimization depends heavily on the project you run it over. On some, it saves 1% of CPU per frame (approximately 2 scanlines), which is a sign that the project's hot path might be better rewritten in asm (if not already).

See optimize_lst.py at header section for a list of all peepholes and switches the optimizer provides.

These are four of my projects in which I use the optimizer. The picture outlines how many patterns were applied.

Build gcc and the plugin

First, build gcc m68k toolchain with plugin support enabled. This is straight forward on Linux systems, even on WSL (Windows Subsytem for Linux), but impossible on Windows systems (I couldn't build it yet).
Use script build_gcc.sh (or build_gcc_mingw32.sh WIP).
Add the toolchain into your PATH so SGDK can see it, as well for the next step to build the plugin.

Then, compile the gcc plugin optimizer_plugin.c responsible to execute the python script over the gcc m68k assembly code. See script build_plugin.sh (or build_plugin_mingw32.sh WIP).
This will create optimizer_plugin.so (or optimizer_plugin.dll) file. Move it into SGDK's tools folder, along with the optimize_lst.py file.

Optionally, build SGDK. You can use script build_sgdk.sh.
This step is optional if you have your SGDK already built/installed. Then you can rely on SGDK's makefile.gen will take the new m68k-elf- binaries added to the PATH in previous step.

Execution

• Open SGDK's makefile.gen:
  • Add next line after the inclusion of common.mk:
    PLUGIN_PARAM := -fplugin=$(GDK)/tools/optimizer_plugin.so -fplugin-arg-optimizer_plugin-disable=0 -fplugin-arg-optimizer_plugin-keep-files=0
  • Add $(PLUGIN_PARAM) to the target:
    $(OUT_DIR)/rom.out: $(OUT_DIR)/sega.o $(OUT_DIR)/cmd_ \$(LIBMD)
    Eg:

    $(OUT_DIR)/rom.out: $(OUT_DIR)/sega.o $(OUT_DIR)/cmd_ $(LIBMD)
    	@$(MKDIR) -p $(dir $@)
    	$(CC) $(PLUGIN_PARAM) -m68000 -B$(BIN) -n -T $(GDK)/md.ld -nostdlib $(OUT_DIR)/sega.o @$(OUT_DIR)/cmd_ $(LIBMD) $(LIBGCC) -o $(OUT_DIR)/rom.out -Wl,--gc-sections -flto -flto=auto -ffat-lto-objects
    	@$(RM) $(OUT_DIR)/cmd_
    

• Make sure python is in your PATH.
• Build your project:
  • make -f $GDK/makefile.gen release -j1