4 ms·
Not my area of expertise and it is yours, but if you eliminate a zero instruction before a copy instruction how can you be sure that doesn't affect other thread
by heythere124 9y ago
Not my area of expertise and it is yours, but if you eliminate a zero instruction before a copy instruction how can you be sure that doesn't affect other threads?
var x Int
// Pass x to a thread by reference
x = 0
time.Sleep(1000)
x = 1
- Filligree 9y agoThe answer to that one would be to embed thread-safety in the type system, aka. Rust. For languages with less sophisticated type systems you get a choice between inefficiency (Go), or complicated rules which state that the programmer is wrong for coding that way (C).
- heythere124 9y agoYour "solution" is possible in many languages. It's just to give threads complete ownership of data they use. It doesn't require a special type system. In general I don't think Rust actually adds much abstraction that isn't already in say Python. What it does is enforce tight constraints.
- Filligree 9y agoThe constraints are important. They're what let you code safely, and also give the compiler the ability to optimize better.
- heythere124 9y agoNot sure those optimisations are worth much. And it's only safety by forcing lowest common denominator code and making you justify everything to a dumb compiler. Rust serves a niche, but it is a tight niche IMO.
- pcwalton 9y agoAliasing info is hugely important in optimization. In fact, it's needed to solve this very bug. Aliasing info is, fundamentally, what allows Rust to have memory safety without GC.
- the8472 9y agoHow can you be sure that the other threads ever see it in time? they might be suspended for a whole second because a HDD needs to spin up or something like that. So threads never seeing the value is already a valid outcome, so the compiler might as well always do that.
- heythere124 9y agotime.Sleep(1000) could be replaced with a real syncing mechanism of some kind -- it's only for illustration.
- the8472 9y agoa real syncing mechanism would introduce compiler barriers that prevent such an optimization.
- pcwalton 9y agoThe memory dependence analysis must prove the memory is unaliased, which ensures among other things that no other thread can have a reference to it. Presumably in Go return pointers are guaranteed to be unaliased.