I found that .data is not correctly initialized with firmware images
that use over 64kB of flash. The problem is that the data in .data
(which is stored in flash, and copied to RAM at reset) is beyond the
64kB limit and must therefore be loaded using the elpm instruction
instead of the lpm instruction.
I encountered this issue while getting testdata/math.go to work for AVR.
The following command mostly works with this patch, while it prints
garbage withtout it:
tinygo run -target=simavr -size=short -scheduler=none ./testdata/math.go
(This also requires a patch to picolibc to work, see
https://github.com/picolibc/picolibc/pull/371)
It still doesn't work entirely with this patch: some of the math
operations have an incorrect result. But at least it's an improvement as
it won't print garbage anymore.
This might sound crazy, but I think it's better to enable the GC by
default to avoid surprises. It costs 1130 bytes of flash and 16 bytes of
RAM (plus heap overhead) so it's not exactly free, but if needed it can
easily be disabled with `-gc=leaking`. On the Uno (32kB flash, 2kB RAM)
that's not massive, on the DigiSpark (8kB flash, 0.5kB RAM) that may be
too much depending on the application.
This increases code size by 1 instruction (2 bytes) because LLVM isn't
yet smart enough to recognize that it doesn't need to clear a register
to use 0: it can just use r1 which is always 0 according to the
convention. It makes initialization a lot easier to read, however.
