x86_64: Use byte-wise atomic load for first load in 128-bit RMWs #22
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Based on the code generated for DW RMWs by LLVM.
Currently, the first load is done by cmpxchg16b, which is not efficient because it means that cmpxchg16b is called at least twice. (although it is actually fast enough in some cases because we use a hack that assumes initial values)