DEC Alpha Memory Model
DEC's Alpha is widely regarded as the weakest mainstream hardware memory model. It permits all four load/store reorderings and, distinctively, does not preserve dependent (address- or data-dependent) load ordering without an explicit memory barrier (mb) — behaviour no other mainstream ISA exhibits, and the historical case that motivated Linux's smp_read_barrier_depends()/RCU barrier machinery. Non-multi-copy-atomic.
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
- no Multicopy atomic
Ordering relationships
- Strictly weaker than
- Relaxed Memory Order (RMO) — 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.
- Strictly stronger than
- Cache Coherence — Alpha guarantees per-location (SC-per-location) coherence and enforces cross-location ordering only via mb/wmb fences (Alpha Architecture Reference Manual; Alglave, Maranget & Tautschnig, Herding Cats, TOPLAS 2014, p.4); bare coherence has neither. Separating outcome: MP+mb+mb, forbidden by Alpha, allowed by Coherence. Cited (Alpha has no herd7 cat model), not machine-run.
References
- Richard L. Sites. Alpha Architecture Reference Manual. Digital Press, 1992. doi:10.5555/129616
- Paul E. McKenney, David Howells. Linux Kernel Memory Barriers (Documentation/memory-barriers.txt). Linux kernel documentation, 2021. www.kernel.org/doc/Documentation/memory-barriers.txt