The AVR backend has several critical atomics bugs.
This change invokes libcalls for all atomic operations on AVR.
Now `testdata/atomic.go` compiles and runs correctly.
The target triples have to match mostly to be able to link LLVM modules.
Linking LLVM modules is already possible (the triples already match),
but testing becomes much easier when they match exactly.
For macOS, I picked "macosx10.12.0". That's an old and unsupported
version, but I had to pick _something_. Clang by default uses
"macos10.4.0", which is much older.
Because of a bug in the ARM backend of LLVM, the cmpxchg instruction is
lowered using ldrexd/strexd instructions which don't exist on Cortex-M
cores. This leads to an "undefined instruction" exception at runtime.
Therefore, this patch works around this by lowering directly to a call
to the __sync_val_compare_and_swap_8 function, which is what the backend
should be doing.
For details, see: https://reviews.llvm.org/D95891
To test this patch, you can run the code on a Cortex-M3 or higher
microcontroller, for example:
tinygo flash -target=pca10040 ./testdata/atomic.go
Before this patch, this would trigger an error. With this patch, the
behavior is correct. The error (without this patch) could look like
this:
fatal error: undefined instruction with sp=0x200007cc pc=nil
Ayke van Laethem
Nia Waldvogel