Explorations in Vale

Vale's hybrid-generational memory is a new memory model that combines all the best parts of existing memory strategies: it's as easy as garbage collection, as deterministic as reference counting, and as fast as borrow checking. 0

Note that hybrid-generational-memory is not implemented yet, it's still just a design.

There are three ingredients to make hybrid-generational memory work:

  • Generational references!
  • Static analysis (sometimes referred to as the "automatic borrow checker") that can eliminate generation checks when it knows an object is alive.
  • Scope tethering, to keep an object from getting freed when a local has a reference into it.

Start with Generational References

Hybrid-generational memory is built upon generational references. Recall:

  • Every heap allocation has a u4812 generation number before the object.
  • Non-owning references contain a raw pointer and a u48 "target generation" number. 3
  • Before dereferencing an object, assert that the target generation number matches the allocation's generation number.
Notes
0

Vale has three release modes:

  • Resilient mode, which is fast and memory safe; it will halt the program when we try to dereference a freed object.
  • Assist mode, for development, to detect potential problems even earlier.
  • Unsafe mode, which turns off all safety.

Resilient mode uses hybrid-generational memory.

1

u48 means a 48-bit unsigned integer.

2

We chose 48 bits, but we could push it as high as 60 bits if we adjusted the below inlining mechanisms. 48 bits is more than enough though.

3

And a u16 offset to know where the generation is relative to the object, see generational references.

Static Analysis: Eliminate Most Liveness Checks

Use static analysis to reduce the number of liveness checks as much as possible. For example:

  • For each dereference, figure out if an in-scope local indirectly owns it. If so, skip the liveness check. For more on this, see HGM Static Analysis, Part 1.
  • Automatically track this information through intermediate stores/loads from struct members, where possible.
  • Automatically track this information through function calls like an automtic borrow checker, where possible.

This static analysis only works when a nearby local holds the owning reference. The scope tethering explained further below will make it work with non-owning locals too.

Add Scope Tethering

The above static analysis only worked when a nearby local holds the owning reference. Now we'll make it work when a nearby local holds a non-owning reference too.

We'll add a u1 "tethered" bit to every allocation, next to the u48 generation number. A local with a non-owning reference can set this bit to 1 to keep its allocation alive. 4 Inside the scope of the local, we can skip all generation checks.

  • When the object is allocated, the tethered bit will be 0.
  • When a local wants to delay the object's destruction, it will:
    • Do a generation check, to see if the object is still alive. If live, load the pointer to the object, otherwise load null. 5
    • Save the old value of the tethered bit. 6
    • Write a 1 to the tethered bit.
  • When the local goes out of scope, it will:
    • Write the old value back to the tethered bit.
  • When the object is deallocated, if the tethered bit is 1, we'll add it to a queue to check later. 7

Not every non-owning local will tether. Static analysis will make a non-owning tether when it's dereferenced several times. Otherwise, it will just allow the generation checks to happen.

4

Someone letting go of the object's owning reference will still call its destructor, regardless of the tethered bit. If the tethered bit is 1, the destructor will not free the object. Instead, the last tethering local will free the object.

5

Loading from null is a memory safe operation: it's guaranteed to correctly seg-fault if we load from it.

6

The old tethered bit will usually be 0, but if another local is tethering the object, it could be 1 already.

7

Specifically, every time we allocate, we check the front of the queue to see if something's tether has expired, and if so, reuse that object. If not, move it to the back of the queue and ask generational malloc instead. Similar to a free-list!

That's basically it! There are some more things we could do to speed it up even more, using virtual memory, regions, or more static analysis, but we'll stop the explanation here.

Minor Extra Details

To address some frequently asked questions:

  • When we move something across thread boundaries, we must recurse through 8 and:
    • Assert each tethered bit is zero; assert that there are no locals pointing at the object.
    • Increment each generation number, effectively cutting off access to the rest of this thread.
  • When a generation number hits the maximum, don't use that generation number anymore.
    • genFree could slice up the allocation into smaller ones that don't include the initial 8b.

8

Similar to how Pony scans all incoming and outgoing objects.

Potential Weaknesses

Some potential weaknesses to explore:

  • Storing the generation number at the top of a <=64b allocation means a liveness check won't incur an extra cache miss since we're about to dereference the object anyway, and the entire object is on one cache line. However, for larger objects, it does incur an extra cache miss. Most objects are small, but programs with an unusually large proportion of medium sized objects not in an array could suffer a small performance hit.
  • Adding the offset to every reference could interfere with optimizations. If so, we'll have to write our own LLVM pass. 9
  • In environments without virtual memory 10, memory fragmentation could be worse, because we can't give pages back to the OS. This is mitigated by regions, where region-calling can guarantee no references pointing into a certain region. 11

9

Presumably, we would make every generational reference have a pointer to the object, and a target generation number, and a pointer to the current generation. The LLVM pass would eliminate the latter.

10

Every mainstream OS has virtual memory, but WASM does not.

11

One day, we could write a compactor for Vale which could also help this, though its probably unnecessary.

Afterword: How might it compare to Rust?

Vale is a high-level language with zero unsafe, so an apples-to-apples comparison would be with a Rust where the only unsafe is in the standard library.

Memory safety is never free, except for the most trivial of programs. Cyclone, Rust, ATS, Fortran, and every other language incurs some overhead to ensure safety. This usually comes in the form of branching, cache misses, and extra memory usage, see Beyond Rust: Innovations in Safety, Speed, and Flexibility for more.

Speed

Rust programs uses borrow references where they can. Where the borrow checker gets in their way, they incur costs:

  • From RefCell:
    • Branching, to check if someone else has locked it.
    • Possible cache missing, if the lock bit isn't in the same cache line as the object or we don't dereference the object at all.
  • From Rc:
    • Incrementing and decrementing costs,
    • Branching, to check if the counter is zero.
    • Possible cache missing if the counter isn't in the same cache line.
    • Possible cache missing if we never dereference the object's members.
  • From generational indices:
    • Bounds checking, to make sure the index is in bounds,
    • Reallocation when we need to add more objects, 12
    • Branching, to check the generation matches.
    • Possible cache missing, if the generation isn't in the same cache line as the object.

12

This is often amortized, but often not, if a generational array is temporary.

Hybrid-generational memory also has some speed costs, depending on how it's using a particular reference:

  • From generational references:
    • Branching, to check the generation matches.
    • Possible cache missing, if the generation isn't in the same cache line as the object.
  • From scope tethering:
    • Cost of loading the generation and setting the tethered bit, 13
    • Possible cache missing if the bit and generation aren't in the same cache line as the object.

Note that not all Rust code uses Rc or RefCell or generational indices. Sometimes, a piece of code's requirements are such that they can use Cell (which is free) or just Vecs (which still have bounds checking).

13

Setting the tethered bit is generally free because loading the generation brought the tethered bit into the cache.

Memory

The borrow checker tends to force Rust code to overuse Vecs (even when Vecs iteration benefits don't apply 14), which incur rather massive memory costs; a Vec will use up to 2x as much memory as the most elements the Vec has ever had.

Hybrid-generational memory doesn't rely on expanding arrays. However, the generational stack does keep ten stacks, each which contains the maximum amount of memory it's ever had.

RefCell, and generational indices use 8b per object, and Rc uses 16. Hybrid-generational memory uses 8b per object. Though once again, it should be noted that not all Rust code uses Rc or RefCell or generational indices.

14

Iterating over Vecs is very fast, if the Vec is large enough for the CPU to pre-fetch the next elements from the cache, which happens after ~50 elements.

Immutability, Arenas, Pools

Rust has its borrow checker and Vale has its region borrow checker. Both allow the language to know when something's immutable, to optimize away redundant loads. Additionally, Vale lets us specify structs and interfaces as deeply immutable, which does the same thing without any borrow checking.

Rust's borrow checker often forces us to use Vecs, and Vale's regions allow us to use pool or arena allocation. Both enable incredibly fast allocation. Vale's regions can apply it to any type, whereas in Rust we need to specifically add and keep track of every Vec, and some Rust allocations can't be put into Vecs. 15

Regions make hybrid-generational memory even faster, because inside a pool or arena region, an object doesn't need a generation at all.

Rust's borrow checker is more difficult 16. Rust also forces the user to use the borrow checker uniformly across the entire program, whereas Vale's region borrow checking is opt-in.

In the end, this could mean:

  • Vale programs could be faster, because it takes so little effort to use pool and arena regions.
  • Rust programs could be faster, because a Vale programmer will stop once they get it working, whereas Rust doesn't allow the program to work until it is also fast.
  • They could be the same; a Vale program could take half the time to write, giving extra time for optimizing the hot path with regions, equalling the Rust program in the end.
15

For example, Rcd objects are put on the heap.

16

Rust's borrow checker enforces mutability xor aliasability, while Vale's region borrow checker doesn't need that restriction.

Conclusion

Hybrid-generational memory looks very promising, and could have Rust-like speed, while being much easier to learn and use.

Of course, this is all just theories until we see some benchmarking. Unfortunately, it's impossible to get a meaningful benchmark yet, firstly because hybrid-generational memory isn't implemented yet, and because Rust has ten years of unrelated optimizations that would confound a meaningful comparison.

Once we implement hybrid-generational memory fully, we'll benchmark it against Rust, and then optimize until it catches up.

Time will tell how hybrid-generational memory compares with the borrow checker!