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Note: These are tips for developers who are developing MicroPython itself, not working with MicroPython as a tool.
Link-time optimisation (LTO) is a compiler feature which can significantly reduce code size by finding optimisations across multiple compilation units.
Diagnosing LTO Issues
Sometimes an issue may appear with LTO=1 that goes away with LTO=0. There are generally two possible reasons for this:
- Unexposed errors in the source files, that only appear with the more aggressive cross-compilation-unit optimization done by LTO. (For example, a symbol is declared with different types in different source files.)
- Toolchain or binutils assembler/linker bugs. Hopefully not these!
Some tips to dig into LTO errors:
-save-temps
LTO generates a number of enormous intermediate assembly files, which is often the "source file" shown for an LTO-specific error, i.e.
LINK build-NUCLEO_F091RC/firmware.elf
/tmp/cccK8t2U.s: Assembler messages:
/tmp/cccK8t2U.s: Error: unaligned opcodes detected in executable segment
make[1]: *** [/tmp/ccu7PpiF.mk:5: /tmp/ccCeWcJW.ltrans1.ltrans.o] Error 1
However these files are all temporary (in /tmp) and already deleted when GCC exits. Add LDFLAGS += -save-temps to the Makefile in order to place these files in the build directory and keep them. With this option, the files will now have names like build-NUCLEO_F091RC/firmware.elf.ltrans1.ltrans.s and build-NUCLEO_F091RC/firmware.elf.ltrans1.ltrans.o.
If an assembler error is showing, you can probably recreate it by passing the temporary file directly into as - i.e. arm-none-eabi-as build-NUCLEO_F091RC/firmware.elf.ltrans1.ltrans.s. This lets you tweak the generated assembler code and narrow down a root cause.
Maximum Partitions
Depending on how the call graph is partitioned by the LTO whole program analyzer, a single ltrans assembly file might be 200,000+ lines long. Not the best to debug...
Add LDFLAGS += -save-temps -flto-partition=max to the Makefile. Now, GCC will create one ltrans partition per compilation unit (i.e. per source file). This is very inefficient for LTO, but very good for debugging LTO issues - provided the issue still occurs in this configuration. Each file will now have a name like firmware.elf.ltrans1932.ltrans.s and should contain the output from only one compilation unit.
