I'm really wondering why this is being considered. A documented property of the form "GCC will use a single instruction to do X when possible" means exactly nothing. In particular, to call such a statement a "guarantee" is seriously misleading.
If Linux needs the single-instruction property for atomicity, and it thinks it can rely on this supposed property, then Linux has a bug. To do atomic operations, you have to use primitives that are guaranteed always to have the necessary atomicity properties. Typically those would be found in asm statements. I suspect it would be valuable to have standardized primitives for atomic actions (semaphores, spinlocks, test-and-set primitives, circular buffers, pick one). But GCC's load/store semantics are not those primitives, with or without a documented "single instruction when possible" property. paul
