> When you want to embed a type, you need its definition, but you don’t actually need the full definition. You just need the size/alignment.
Aren't there ABI cases where e.g.
struct foo { float X, Y; }
struct { char[8]; }
Yes. For example consider this function to add two 2D points, which accepts and returns all variables entirely in xmm registers: https://gcc.godbolt.org/z/hPGKrh6W4 (surprisingly, gcc generates some fairly odd assembly code here)
Oh, that explains the seemingly useless movs. Thank you! I simply thought that it was a bug, since clang did not emit them.
Yeah this is correct. You don't want to pass these values around "by value" but, you should be able to "embed them" and pass "pointers to them". It's a middle-ground between a completely opaque type which you would also pass around by address, but, with the added benefit that you allocate your own storage for it.
I sort of mentioned this in the blog but this is good clarification.
> if you want to pass a shared_ptr to Zig, you need to pass a pointer to the shared pointer
For lore, I believe this GitHub thread is where I first learned about the how types of the same size/alignment can still have different ABIs :) https://github.com/microsoft/win32metadata/issues/623#issuec...