ThinLTO results in a small code size reduction, which is nice
(especially on these very small chips). It also brings us one step
closer to using ThinLTO everywhere.
- Use compiler-rt and picolibc instead of avr-libc.
- Use ld.lld instead of avr-ld (or avr-gcc).
This makes it much easier to get started with TinyGo on AVR because
installing these extra tools (gcc-avr, avr-libc) can be a hassle.
It also opens the door for future improvements such as ThinLTO.
There is a code size increase but I think it's worth it in the long run.
The code size increase can hopefully be reduced with improvements to the
LLVM AVR backend and to compiler-rt.
Unfortunately, the .rodata section can't be stored in flash. Instead, an
explicit .progmem section should be used, which is supported in LLVM as
address space 1 but not exposed to normal programs.
Eventually a pass should be written that converts trivial const globals
of which all loads are visible to be in addrspace 1, to get the benefits
of storing those globals directly in ROM.
This convention is followed by most of the avr-gcc toolchain but older
versions of binutils don't mind overlapping program/data spaces.
However, newer versions start complaining about an overlap in address
space:
avr-ld: section .stack VMA [0000000000000100,00000000000002ff] overlaps section .text VMA [0000000000000000,0000000000000225]
This commit moves the data space in the linker script to 0x800000, like
the rest of the toolchain does.
Previously, the RAM was set to start at address 0. This is incorrect: on
AVR, the first few addresses are taken up by memory-mapped I/O. The
reason this didn't lead to problems (yet) was because the stack was
usually big enough to avoid real problems.
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 prevents it from being of type PROGBITS in lld 9, it should always
be NOBITS. It should fix the following error in lld 9:
ROM segments are non-contiguous
