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Comment by twic | Hacker News Reader

The whole point of jgtrosh's link is that there is a way to hide data behind a function pointer, so Rust could convert any closure to a function pointer. But it requires writable-and-executable memory, so it's a pretty bad idea (in GCC's implementation, that memory is on the stack, which is an extra bad idea, but i don't think it needs to be).

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Definitely.

Technically this can also be done via static code trampolines that are mmap'd as well [1]. That approach has been used on iOS in the past to turn blocks into raw function pointers.

If you have a platform that allows W+X on code (yikes!), you can do [2] as well.

[1] https://github.com/plausiblelabs/plblockimp/blob/master/Sour... [2] https://www.mikeash.com/pyblog/friday-qa-2010-02-12-trampoli...

Anything that doesn't require W+X would need an entire page allocated per closure, wouldn't it?

No, you can of course allocate W+X pages from the OS and put multiple closures in them using a standard userspace memory allocator.

Or if the OS doesn't support W+X allocation at all, then you can have a bunch of tightly packed pregenerated trampolines in the binary.

Right, this is how Objective-C's implementation works, except it keeps around one page of trampolines and remaps that around when necessary to be able to "create" more trampolines on the fly, I believe.

Nope! You'd do something to the effect of:

  clo_code:
  4C8B1501100000  mov r10 [rel clo_code+0x1008]
  FF25F30F0000    jmp [rel clo_code+0x1000]
  0F1F00          nop3
  # one page away...
  struct clo_slot {
    void (*func)(void* _R10,...);
    void* data;
    };

Edit: to use r10 rather than rotating all the argument registers.

For example, every platform that has a virtual machine with JIT compilation support.

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