diff --git a/README.md b/README.md new file mode 100644 index 0000000..3241a7c --- /dev/null +++ b/README.md @@ -0,0 +1,65 @@ +# 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.