Packed pointers

[daurnimator]

What if aligned pointers could be packed? *packed align(4096) foo only needs usize-12 bits.

Such pointers would mostly be used in a packed struct such as:

packed struct {
    ptr: *packed foo,
    needs_lock: bool,
}

assuming that foo has an alignment requirement of e.g. 8.

This lets zig code nicely implement the common pattern of using lower pointer bits as tag bits without lots of error-prone @ptrToInt and masking. An immediate user would be the red-black tree code.

[daurnimator]

An attribute on the pointer type might not be necessary: similar to how e.g. a u7 might be stored in a 8 bit register, may be passed as a 32 bit parameter and takes up 8 bits in a non-packed struct: *align(4096) foo may be passed in a 64bit register and takes up usize bits in a non-packed struct, but only takes up usize-12 bits in a packed struct.

[shawnl]

The current packed structs are C ABI compatible. Everywhere else in Zig extern signifies external ABI compatibility, thus I propose we change the existing packed to extern packed. We can then add this feature as packed on structs (not pointers).

packed struct {
    ptr: *foo, // and alignof(foo) >= 2
    needs_lock: bool,
}

then will take up 32 or 64 bits. You can do fancier things which you couldn't do with the c-compatible packed:

packed struct {
  hi: bool,
  ptr: *foo,
  bi: bool,
  ye: u2,
  ptr: big_struct,
  another_bool: bool,
}

and a allocator algorithm would run and assign the small ones to be packed into the pointers. It could eventually even be smart and pick less-used pointers. You can also do this manually using a sub-struct:

struct {
 performance_critical_pointer: *foo,
 packed struct {
   stuffed_ptr: *hi,
   flag: bool,
 }
}