Relaxed Memory Order RMO
The weakest SPARC model. Allows all four reorderings (load-load, load-store, store-load, store-store) between independent memory operations.
Properties
Property vector author-extrapolated — set only where the model’s definition pins the cell down (unknown cells are omitted); cells citing a specific source are marked.
- Reordering sound
- yes Store→Load
- yes Store→Store
- yes Load→Load
- yes Load→Store
- Reasoning guarantees
- yes Coherence
- yes No undefined behaviour
- no In-order execution
- yes No out-of-thin-air
- Atomicity guarantees
- yes Multicopy atomic
Ordering relationships
- Strictly weaker than
- Partial Store Order (PSO) — RMO allows all four reordering types including load-load and load-store.
- Strictly stronger than
- Cache Coherence — RMO still enforces per-address coherence; weaker models could violate it.
- DEC Alpha Memory Model — SPARC RMO preserves address/data/control dependencies; DEC Alpha uniquely relaxes dependent-load ordering (Alglave, Maranget & Tautschnig, Herding Cats, TOPLAS 2014, p.18) and is non-multi-copy-atomic. Separating outcome: MP+dmb+addr, forbidden by RMO (address dependency preserved), allowed by Alpha (dependent loads not ordered) — machine-run under herd7 against a portable abstract-alpha.cat (RMO Never 0 3, Alpha Sometimes 1 3). Containment is the litmus-tier monotonicity argument: Alpha relaxes strictly more than RMO (dependencies; store atomicity) and no less on any axis, so Alpha ⊆ RMO, strict by the witness. See litmus/strictly-weaker/RMO-vs-Alpha.
References
- SPARC International. The SPARC Architecture Manual, Version 9. Prentice Hall, 1994. sparc.org/technical-documents/
- Jade Alglave. A Shared Memory Poetics. PhD Thesis, Université Paris 7, 2010. www.cl.cam.ac.uk/~pes20/weakmemory/thesis.pdf
- Sarita V. Adve, Kourosh Gharachorloo. Shared Memory Consistency Models: A Tutorial. IEEE Computer 29(12), 1996. doi:10.1109/2.546611