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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

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
> These days, Go doesn’t even stack allocate all non-escaping local variables… sometimes they will exist only within registers! Did you read the article I linked? That's what it says - and this isn't stack allocation, it's SRA. Even 'registers' is overly constrained - they're dataflow edges.
- coder543 5y ago
  I was addressing your statement that seemed to say class metadata is included in every language when dealing with stack variables. I trusted your earlier statements that this was definitely the case with Java/Truffle. Misunderstandings on my part are entirely possible. Sorry I haven’t had time to read your article. It’s on my todo list for later.
- coder543 5y ago
  Go isn't just doing SRA, as far as I understood it from your article, though it is certainly doing that too. Go will happily allocate objects on the stack with their full in-memory representation, which does not include any kind of class pointer. Here is a contrived example that creates a 424 byte struct: https://godbolt.org/z/5cKh7xTzq https://godbolt.org/z/5cKh7xTzq As can be seen in the disassembly, the object created in "Process" does not leave the stack until it is copied to the heap in "ProcessOuter" because ProcessOuter is sending the value to a global variable. The on-stack representation is the full representation of that object, as you can also see by the disassembly in ProcessOuter simply copying that directly to the heap. (The way the escape analysis plays into it, the copying to the heap happens on the first line of ProcessOuter, which can be confusing, but it is only being done there because the value is known to escape to the heap later in the function on the second line. It would happily remain on the stack indefinitely if not for that.) It's cool that Graal does SRA, but Go will actually let you do a lot of work entirely on the stack (SRA'd into registers when possible), even crossing function boundaries. In your SRA blog post example, when the Vector is returned from the function, it has to be heap allocated into a "real" object at that point. Go doesn't have to do that heap allocation, and won’t have to collect that garbage later. Most of the time, objects are much smaller than this contrived example, so they will often SRA into registers and avoid the stack entirely… and this applies across function boundaries too, from what I’ve seen, but I haven’t put as much effort into verifying this.

chrisseaton 5y ago

> These days, Go doesn’t even stack allocate all non-escaping local variables… sometimes they will exist only within registers! Did you read the article I linked? That's what it says - and this isn't stack allocation, it's SRA. Even 'registers' is overly constrained - they're dataflow edges.

coder543 5y ago
I was addressing your statement that seemed to say class metadata is included in every language when dealing with stack variables. I trusted your earlier statements that this was definitely the case with Java/Truffle. Misunderstandings on my part are entirely possible. Sorry I haven’t had time to read your article. It’s on my todo list for later.
coder543 5y ago
Go isn't just doing SRA, as far as I understood it from your article, though it is certainly doing that too. Go will happily allocate objects on the stack with their full in-memory representation, which does not include any kind of class pointer. Here is a contrived example that creates a 424 byte struct: https://godbolt.org/z/5cKh7xTzq https://godbolt.org/z/5cKh7xTzq As can be seen in the disassembly, the object created in "Process" does not leave the stack until it is copied to the heap in "ProcessOuter" because ProcessOuter is sending the value to a global variable. The on-stack representation is the full representation of that object, as you can also see by the disassembly in ProcessOuter simply copying that directly to the heap. (The way the escape analysis plays into it, the copying to the heap happens on the first line of ProcessOuter, which can be confusing, but it is only being done there because the value is known to escape to the heap later in the function on the second line. It would happily remain on the stack indefinitely if not for that.) It's cool that Graal does SRA, but Go will actually let you do a lot of work entirely on the stack (SRA'd into registers when possible), even crossing function boundaries. In your SRA blog post example, when the Vector is returned from the function, it has to be heap allocated into a "real" object at that point. Go doesn't have to do that heap allocation, and won’t have to collect that garbage later. Most of the time, objects are much smaller than this contrived example, so they will often SRA into registers and avoid the stack entirely… and this applies across function boundaries too, from what I’ve seen, but I haven’t put as much effort into verifying this.