The lack of DWCAS as a primitive in general is really annoying, C++ aside. RISC-V's -A extension has no form of it, either; you only get XLEN-sized AMO + LL/SC

The lack of DWCAS as a primitive in general is really annoying, C++ aside. RISC-V's -A extension has no form of it, either; you only get XLEN-sized AMO + LL/SC (no 2*XLEN LL/SC either!) It's one of those features that when you want it, you really really really want it, and the substitutions are all pretty bad in comparison.