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I edited my comment before you posted yours. Making heap allocations super complicated just to avoid the stack is a confusing idea. Generational GCs surely do

I edited my comment before you posted yours. Making heap allocations super complicated just to avoid the stack is a confusing idea. Generational GCs surely do not do a single bump allocate for all local variables. How could the GC possibly know where each object starts and ends if it did it as a single allocation? Instead, it treats them all as individual allocations within an allocation buffer, which means bumping for each one separately. Yes, they will then get copied out if they survive long enough, but that’s not the same thing as avoiding the 10+ instructions per allocation. It’s entirely possible I’m wrong when it comes to Truffle, but at a minimum it seems like you would need arenas for each size class, and then you’d have to bump each arena by the number of local variables of that size class. The stack can do better than that.

chrisseaton 5y ago
When you allocate objects individually it looks like this: object_a = tlab tlab += 8 check tlab limit object_b = tlab tlab += 8 check tlab limit When you allocate as a single allocation it looks like this: object_a = tlab object_b = tlab + 8 tlab += 16 check tlab limit What about that do you see as impossible? > How could the GC possibly know where each object starts and ends if it did it as a single allocation? As above.
- deleted 5y ago
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- coder543 5y ago
  In your example, the objects are all the same size. That would certainly be easy. If you have three local objects that are 8 bytes, 16 bytes, and 32 bytes… if you do a single 48 byte allocation on the TLAB, how can the GC possibly know that there are three distinct objects, when it comes time to collect the garbage? I can think of a few ways to kind of make it work in a single buffer, but they would all require more than the 48 bytes that the objects themselves need. Separate TLAB arenas per size class seem like the best approach, but it would still require three allocations because each object is a different size. I understand you’re some researcher related to Truffle… this is just the first I’m hearing of multiple object allocation being done in a single block with GC expected to do something useful.
  - chrisseaton 5y ago
    > If you have three local objects that are 8 bytes, 16 bytes, and 32 bytes… if you do a single 48 byte allocation on the TLAB, how can the GC possibly know that there are three distinct objects, when it comes time to collect the garbage? Because the objects are self-describing - they have a class which tells you their size. object_a = tlab object_a.class = ClassA object_b = tlab + 8 object_b.class = ClassB object_c = tlab + 16 object_c.class = ClassC tlab += 48 check tlab limit
    - coder543 5y ago
      Ok, so it’s not as simple as bumping the TLAB pointer by 48. Which was my point. You see how that’s multiple times as expensive as stack allocating that many variables? Even something as simple as assigning the class to each object still costs something per object. The stack doesn’t need self describing values because the compiler knows ahead of time exactly what every chunk means. Then the garbage collector has to scan each object’s self description… which way more expensive than stack deallocation, by definition. You’re extremely knowledgeable on all this, so I’m sure that nothing I’m saying is surprising to you. I don’t understand why you seem to be arguing that heap allocating everything is a good thing. It is certainly more expensive than stack allocation, even if it is impressively optimized. Heap allocating as little as necessary is still beneficial.
      - chrisseaton 5y ago
        Every stack allocation scheme I've seen creates a fully reified object though, with a class pointer as normal. You may be confusing with scalar replacement of aggregates, which is a separate concept. https://chrisseaton.com/truffleruby/seeing-escape-analysis/ https://chrisseaton.com/truffleruby/seeing-escape-analysis/
        coder543 5y ago
        Go does not put class pointers on stack variables. Neither does Rust or C. The objects are the objects on the stack. No additional metadata is needed. The only time Go has anything like a class pointer for any object on the stack is in the case of something cast to an interface, because interface objects carry metadata around with them. These days, Go doesn’t even stack allocate all non-escaping local variables… sometimes they will exist only within registers! Even better than the stack.
chrisseaton 5y ago
> at a minimum it seems like you would need arenas for each size class But TLABs are heterogenous by size. Objects of all sizes go in one linear allocation space. So allocating two objects next to each other is the same as allocating them both at the same time.
- coder543 5y ago
  That doesn’t answer how the GC knows where the boundaries of each object are.

chrisseaton 5y ago

When you allocate objects individually it looks like this: object_a = tlab tlab += 8 check tlab limit object_b = tlab tlab += 8 check tlab limit When you allocate as a single allocation it looks like this: object_a = tlab object_b = tlab + 8 tlab += 16 check tlab limit What about that do you see as impossible? > How could the GC possibly know where each object starts and ends if it did it as a single allocation? As above.

deleted 5y ago
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coder543 5y ago
In your example, the objects are all the same size. That would certainly be easy. If you have three local objects that are 8 bytes, 16 bytes, and 32 bytes… if you do a single 48 byte allocation on the TLAB, how can the GC possibly know that there are three distinct objects, when it comes time to collect the garbage? I can think of a few ways to kind of make it work in a single buffer, but they would all require more than the 48 bytes that the objects themselves need. Separate TLAB arenas per size class seem like the best approach, but it would still require three allocations because each object is a different size. I understand you’re some researcher related to Truffle… this is just the first I’m hearing of multiple object allocation being done in a single block with GC expected to do something useful.
- chrisseaton 5y ago
  > If you have three local objects that are 8 bytes, 16 bytes, and 32 bytes… if you do a single 48 byte allocation on the TLAB, how can the GC possibly know that there are three distinct objects, when it comes time to collect the garbage? Because the objects are self-describing - they have a class which tells you their size. object_a = tlab object_a.class = ClassA object_b = tlab + 8 object_b.class = ClassB object_c = tlab + 16 object_c.class = ClassC tlab += 48 check tlab limit
  - coder543 5y ago
    Ok, so it’s not as simple as bumping the TLAB pointer by 48. Which was my point. You see how that’s multiple times as expensive as stack allocating that many variables? Even something as simple as assigning the class to each object still costs something per object. The stack doesn’t need self describing values because the compiler knows ahead of time exactly what every chunk means. Then the garbage collector has to scan each object’s self description… which way more expensive than stack deallocation, by definition. You’re extremely knowledgeable on all this, so I’m sure that nothing I’m saying is surprising to you. I don’t understand why you seem to be arguing that heap allocating everything is a good thing. It is certainly more expensive than stack allocation, even if it is impressively optimized. Heap allocating as little as necessary is still beneficial.
    - chrisseaton 5y ago
      Every stack allocation scheme I've seen creates a fully reified object though, with a class pointer as normal. You may be confusing with scalar replacement of aggregates, which is a separate concept. https://chrisseaton.com/truffleruby/seeing-escape-analysis/ https://chrisseaton.com/truffleruby/seeing-escape-analysis/
      - coder543 5y ago
        Go does not put class pointers on stack variables. Neither does Rust or C. The objects are the objects on the stack. No additional metadata is needed. The only time Go has anything like a class pointer for any object on the stack is in the case of something cast to an interface, because interface objects carry metadata around with them. These days, Go doesn’t even stack allocate all non-escaping local variables… sometimes they will exist only within registers! Even better than the stack.

chrisseaton 5y ago

> at a minimum it seems like you would need arenas for each size class But TLABs are heterogenous by size. Objects of all sizes go in one linear allocation space. So allocating two objects next to each other is the same as allocating them both at the same time.

coder543 5y ago
That doesn’t answer how the GC knows where the boundaries of each object are.