Class instance alignment

[kinke]

On Saturday, 20 February 2021 at 18:43:53 UTC, Steven 
Schveighoffer wrote:
> Last I checked*, the GC uses pools of 16-byte, 32-byte, 
> 64-byte, etc blocks.

That has changed [to reduce wastage]; the new bin sizes are here 
and include sizes like 176 (11*16): 
https://github.com/dlang/druntime/blob/728f1d9c3b7a37eba4d59ee2637fb924053cba6d/src/core/internal/gc/impl/conservative/gc.d#L1166

> (not sure about stack alignment for scope instances)

This works with LDC at least. E.g., this:

class C
{
     align(64) int[2] data;
}

void foo()
{
     scope c = new C();
}

allocates 72 bytes aligned at a 64-bytes stack boundary. 72 
bytes? :) Yes - vptr, monitor, then 48 padding bytes (for 64-bit 
target...), then 8 `data` bytes with .offsetof of 64. [And 
classes don't need tail padding, as you can't allocate arrays of 
class *instances* directly in the language.]

Structs are generally better suited for alignment purposes, but 
the same GC limitations apply when allocating them on the heap.