3 ms·

Ah, right, it was about guaranteed total order on all stores in a single memory location. Re colocation and x86, IIRC the intel memory model has wordings regar

Ah, right, it was about guaranteed total order on all stores in a single memory location. Re colocation and x86, IIRC the intel memory model has wordings regarding read and writes to a a memory location having to be of the same size to take advantage of the memory model guarantees.

moonchild 2y ago
total order on all accesses to a given location—loads from a single location can't be reordered w.r.t. each other either i don't remember seeing any wording relating to mixed-size accesses in the intel manual (not withstanding that the official models are ... ambiguous, to say the least, compared with what 3rd-party researchers have done)
- gpderetta 2y ago
  For a total store order to be meaningful of course it implies that loads are also non visibly reordered. If a store falls in the forest but nobody is around to load it, was it really ordered :)
  - moonchild 2y ago
    Tbf you could say stores happen in order, and loads can happen out of order unless you fence. Personally I don't understand why we need such strong ordering constraints for weakly ordered reads—istm you can go much weaker and maintain sanity.
- gpderetta 2y ago
  > i don't remember seeing any wording relating to mixed-size accesses in the intel manual (not withstanding that the official models are ... ambiguous, to say the least, compared with what 3rd-party researchers have done) I was probably misremembering the details. The manual has to say this regarding #LOCK prefixed operations: "Software should access semaphores (shared memory used for signalling between multiple processors) using identical addresses and operand lengths. For example, if one processor accesses a semaphore using a word access, other processors should not access the semaphore using a byte access" which is already vague enough, but regarding general atomic load and stores I couldn't find anything.

moonchild 2y ago

total order on all accesses to a given location—loads from a single location can't be reordered w.r.t. each other either i don't remember seeing any wording relating to mixed-size accesses in the intel manual (not withstanding that the official models are ... ambiguous, to say the least, compared with what 3rd-party researchers have done)

gpderetta 2y ago
For a total store order to be meaningful of course it implies that loads are also non visibly reordered. If a store falls in the forest but nobody is around to load it, was it really ordered :)
- moonchild 2y ago
  Tbf you could say stores happen in order, and loads can happen out of order unless you fence. Personally I don't understand why we need such strong ordering constraints for weakly ordered reads—istm you can go much weaker and maintain sanity.
gpderetta 2y ago
> i don't remember seeing any wording relating to mixed-size accesses in the intel manual (not withstanding that the official models are ... ambiguous, to say the least, compared with what 3rd-party researchers have done) I was probably misremembering the details. The manual has to say this regarding #LOCK prefixed operations: "Software should access semaphores (shared memory used for signalling between multiple processors) using identical addresses and operand lengths. For example, if one processor accesses a semaphore using a word access, other processors should not access the semaphore using a byte access" which is already vague enough, but regarding general atomic load and stores I couldn't find anything.