Meeting weekly 2011 11 22

Attending

Brian, Dave, Marijn, Niko, Patrick

Last use analysis

• Dave: concerned it makes significant move-vs-copy decision based on non-trivial flow analysis
• Marijn: important optimization, and should be done anyway, but user-visible since it accepts otherwise rejected program
• Dave: Niko and Patrick have been discussing an idea that makes copyability a structural decision rather than using flow analysis
• Marijn: don't want to have to explicitly copy all over the place
• Niko: wouldn't explicit copy scalars, but generics conservatively requiring explicit copies could become a pain point
• Marijn: about 1/3 of the stdlib generics do copies; could potentially annotate type params as copy to alleviate constant annotations. We could investigate a more explicit model for some types, but this is a very basic analysis and seems just plain helpful
• Dave: ok, we can investigate the combination; I'm a bit concerned about allowing many types to be auto-copy, because whenever you distinguish kinds you complicate generics

(After meeting, Niko and Dave discussed what the last-use analysis was doing and understood better that it's orthogonal to the alternative copy semantics Patrick and Niko have been discussing.)

Outparams

• Patrick: another concern is cost of using outparams everywhere
• Marijn: LLVM optimizes this well; doesn't seem like a real perf problem

Unique vectors and strings

• Marijn: another issue is how to go forward with vectors and strings
• Patrick: Niko and I have been thinking about this lately too
• Marijn: please post your ideas
• Niko: executive summary: change built-in vectors to be: a. fixed-length arrays (Ocaml) or a. double-indirection (Python)
• Niko: leaning towards a) for default, b) in lib
• Marijn: can we make this as efficient? e.g., for realloc?
• Niko: programmer can explicitly contain with unique pointers
• Marijn: what about refs inside growable vecs?
• Niko: disallowed
• Patrick: that depends; if you have refcounted buffers and can't shrink, it can be made to work
• Niko: that's a sort of unpredictable semantics
• Patrick: with mutation, it gets hairy
• Marijn: can check not shrinkable when unique
• Patrick: could have runtime hand you a buffer rather than a ref inside the buffer
• Niko: dynamic-size arrays need to be task-local or unique
• Marijn: at any rate, could become less powerful and efficient than current system if you can't take refs in as many places