On 28/06/17 00:53, Alexander Monakov wrote: > On Tue, 27 Jun 2017, Richard Earnshaw wrote: >> Only *seems* to work. The ldxp operation is *NOT* atomic and atomicity >> is only guaranteed if a store back of the original read value completes. >> >> In general this makes ldxp not useful for atomic operations since we >> cannot guarantee that the location is writable. > > But the original question was not about atomic reads, it was about atomic CAS, > so obviously the location is going to written. > > It seems in armv8.1a there's also a separate doubleword cas instruction, CASP? > It appears to be implemented in Binutils: https://sourceware.org/ml/binutils/2014-09/msg00021.html > but not in GCC. Yes. I wonder why ARM did that. Unless the internal implementation is different (no ERG) then it seems just a wrapper for an LL/SC loop, and for me it's hard to imagine that being worth an instruction.
