To make <stdatomic.h> work on MIPS (and ARM) using GCC, we need to
provide implementations of the __sync_*() functions. I already added
these functions for 4 and 8 byte types to libcompiler-rt some time ago,
based on top of <machine/atomic.h>.
Unfortunately, <machine/atomic.h> only provides a subset of the features
needed to implement <stdatomic.h>. This means that in some cases we had
to do compare-and-exchange calls in loops, where a simple ll/sc would
suffice.
Also implement these functions for 1 and 2 byte types. MIPS only
provides ll/sc instructions for 4 and 8 byte types, but this is of
course no limitation. We can simply load 4 bytes and use some bitmask
tricks to modify only the bytes affected.
Discussed on: mips, arch
Tested with: QEMU
2e5d5dc936