4 ms·

Does anything care about Alpha? The platform hasn't been sold in 20 years.

by loeg 10mo ago

gpderetta 10mo ago
consume is trivial on alpha, it is the same as acquire (always needs a #LoadLoad). It is also the same as acquire (and relaxed) on x86 and SPARC (a plain load, #LoadLoad is always implied). The only place where consume matters is on relaxed but not too relaxed architectures like ARM and POWER, where consume relies on the implicit #LoadLoad of controls and data dependencies.
- bonzini 10mo ago
  Also on alpha there's only store-store and full memory barriers. Acquire is very expensive.
  - gpderetta 10mo ago
    Indeed. On the other hand recently ARM has added explicit load acquires primitives which are relatively cheap, so converting a consume to an acquire is not a big loss (and Linus considered doing it for the kernel a while ago just to avoid having to think too hard about compiler optimizations).
jcranmer 10mo ago
It's a persistent misunderstanding that release-consume is about Alpha. It's not; in fact, Alpha is one of the few architectures where release-consume doesn't help. In a TSO architecture like x86 or SPARC, every "regular" memory load/store is effectively a release/acquire by default. Using release/consume or relaxed provides no extra speedup on these architectures. In weak memory models, you need to add in acquire barriers to get release/acquire architectures. But also, most weak memory models have a basic rule that a data-dependent load has an implicit ordering dependency on the values that computed it (most notably, loading *p has an implicit dependency on p). The goal of release/consume is to be able to avoid having an acquire fence if you have only those dependencies--to promote a hardware data dependency semantic rule to a language-level semantic rule. For Alpha's ultra-weak model, you still need the acquire fence in this mode, it doesn't help Alpha one whit. Unfortunately, for various reasons, no one has been able to work out a language-level semantics for consume that compilers are willing to implement (preserving data dependencies through optimizations is a lot more difficult than it appears), so all compilers have remapped consume to acquire, making it useless.

Does anything care about Alpha? The platform hasn't been sold in 20 years.

by loeg 10mo ago

Does anything care about Alpha? The platform hasn't been sold in 20 years.

consume is trivial on alpha, it is the same as acquire (always needs a #LoadLoad). It is also the same as acquire (and relaxed) on x86 and SPARC (a plain load, #LoadLoad is always implied). The only place where consume matters is on relaxed but not too relaxed architectures like ARM and POWER, where consume relies on the implicit #LoadLoad of controls and data dependencies.

bonzini 10mo ago
Also on alpha there's only store-store and full memory barriers. Acquire is very expensive.
- gpderetta 10mo ago
  Indeed. On the other hand recently ARM has added explicit load acquires primitives which are relatively cheap, so converting a consume to an acquire is not a big loss (and Linus considered doing it for the kernel a while ago just to avoid having to think too hard about compiler optimizations).

jcranmer 10mo ago

It's a persistent misunderstanding that release-consume is about Alpha. It's not; in fact, Alpha is one of the few architectures where release-consume doesn't help. In a TSO architecture like x86 or SPARC, every "regular" memory load/store is effectively a release/acquire by default. Using release/consume or relaxed provides no extra speedup on these architectures. In weak memory models, you need to add in acquire barriers to get release/acquire architectures. But also, most weak memory models have a basic rule that a data-dependent load has an implicit ordering dependency on the values that computed it (most notably, loading *p has an implicit dependency on p). The goal of release/consume is to be able to avoid having an acquire fence if you have only those dependencies--to promote a hardware data dependency semantic rule to a language-level semantic rule. For Alpha's ultra-weak model, you still need the acquire fence in this mode, it doesn't help Alpha one whit. Unfortunately, for various reasons, no one has been able to work out a language-level semantics for consume that compilers are willing to implement (preserving data dependencies through optimizations is a lot more difficult than it appears), so all compilers have remapped consume to acquire, making it useless.