How to guarantee 64-bit writes are atomic?

Question

When can 64-bit writes be guaranteed to be atomic, when programming in C on an Intel x86-based platform (in particular, an Intel-based Mac running MacOSX 10.4 using the Inte

Answer 1

On X86, the fastest way to atomically write an aligned 64-bit value is to use FISTP. For unaligned values, you need to use a CAS2 (_InterlockedExchange64). The CAS2 operation is quite slow due to BUSLOCK though so it can often be faster to check alignment and do the FISTP version for aligned addresses. Indeed, this is how the Intel Threaded building Blocks implements Atomic 64-bit writes.