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
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