Link-time dead code elimination for the STM8

Overview

This small code example demonstrates the new capabilities that have been implemented into the STM8 port of GNU ld that allows using --gc-sections and thus remove unused code at link-time. Feel free to comment out or create any symbols so you can see how final memory usage is affected.

Usage

make

This is the expected output from make:

sdcc src/main.c -DSTM8S003 -mstm8 --out-fmt-elf -c --debug --opt-code-size --gas --function-sections --data-sections -Iinc/ -MM > obj/main.d
sdcc src/main.c -DSTM8S003 -mstm8 --out-fmt-elf -c --debug --opt-code-size --gas --function-sections --data-sections -Iinc/ -o obj/main.o
sdcc src/bar.c -DSTM8S003 -mstm8 --out-fmt-elf -c --debug --opt-code-size --gas --function-sections --data-sections -Iinc/ -MM > obj/bar.d
sdcc src/bar.c -DSTM8S003 -mstm8 --out-fmt-elf -c --debug --opt-code-size --gas --function-sections --data-sections -Iinc/ -o obj/bar.o
stm8-ld obj/main.o obj/bar.o -o obj/STM8.elf -T./elf32stm8.x --print-memory-usage --gc-sections -Map obj/map_STM8.map 
Memory region         Used Size  Region Size  %age Used
      interrupts:           4 B        128 B      3.12%
             RAM:           5 B        894 B      0.56%
           STACK:          0 GB        128 B      0.00%
          eeprom:          0 GB        128 B      0.00%
            fuse:          0 GB         11 B      0.00%
            lock:          0 GB         1 KB      0.00%
             ROM:         100 B         8 KB      1.22%
       signature:          0 GB         1 KB      0.00%
 user_signatures:          0 GB         1 KB      0.00%
rm obj/bar.d obj/main.d

On the other hand, it demonstrates how SDCC can now output *.o files by generating GAS-compatible assembly code and calling stm8-as afterwards. This allows support for other tools from the GNU binutils package, such as objdump, size, readelf, etc.

Toolchain download links

GNU as-compatible SDCC

https://github.com/XaviDCR92/sdcc-gas

GNU binutils package modified to support the STM8

https://github.com/XaviDCR92/stm8-binutils-gdb (forked from https://stm8-binutils-gdb.sourceforge.io)

Background

As of SDCC 3.9.0, ELF and DWARF2 support were implemented via the in-house linker, sdld. Another package, stm8-binutils-gdb, provides a port of both GNU binutils and the GNU debugger for the STM8. However, this implementation worked partially. These were some of the issues I faced when using the SDCC toolchain:

Unused code was not being removed at link-time since SDCC did not implement an equivalent for GCC's -ffunction-sections or -fdata-sections.
Unused code was not being removed at link-time since sdld lacked such feature.
stm8-size would not report valid results. Instead, it placed all symbols into the .text section.
stm8-objdump -S would not return any code, even if debugging symbols existed.
stm8-nm output would be cluttered with tons of intermediate and unneeded labels created by the compiler during the compiling and assembling steps.
No compatibility with *.a files created from GNU ar, so no support for closed-source third-party libraries that could be created using the GNU toolchain.

Known issues

An ad-hoc linker script has been provided here since stm8-binutils-gdb currently generates a generic script whose memory limits are not defined by any part.
SDCC's Makefile compiles libc and other libraries in its own object format (*.rel), so the sources must be moved to the project and compiled separately using the GNU toolchain. Otherwise, you will get an undefined reference link-time error when performing some operations such as integer division. This will be solved once these forks are integrated into the main branch.
These forks are new and experimental, so other unexpected issues could occur. In such case, please report them so we can get a robust and stable implementation.

XaviDCR92/stm8-dce-example