On 23/06/14 12:49 AM, Cameron Zwarich wrote: > On Jun 22, 2014, at 9:35 PM, Daniel Micay <danielmi...@gmail.com > <mailto:danielmi...@gmail.com>> wrote: > >> An operation that can unwind isn't pure. It impedes code motion such as >> hoisting operations out of a loop, which is very important for easing >> the performance issues caused by indexing bounds checks. LLVM doesn't >> model the `nounwind` effect on functions simply for fun. > > It gets easier to optimize if you adopt a less precise model of > exceptions. For example, you could pick a model where you preserve > control dependence and externally visible side effects, but allow > reordering in other cases. This does get tricky if destructors > themselves have externally visible side effects that are dependent on > intervening stores that can be elided. > > This probably requires whole-program compilation with some knowledge of > externally visible side effects, or more restrictions placed on > destructors than there are currently. It also is hard to make work with > unsafe code, since unsafe code might require exact placement of > unwinding for memory safety in destructors. > > Cameron
Adding restrictions to destructors sounds like adding an effects system to Rust. I think the trait system will get in the way of an attempt to do that. For example, should a trait like `Eq` use pure methods? If they're not pure, then no implementation can be considered pure in generic code. Anything using that generic code can't be considered pure, and so on.
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