The x86-64 ABI does specify for multi-word structs to be returned in registers. If I remember correctly, it's up to 4 registers - rax, rdx and 2 others which I'

The x86-64 ABI does specify for multi-word structs to be returned in registers. If I remember correctly, it's up to 4 registers - rax, rdx and 2 others which I'll need to look up. This only happens if the struct fields on their own would be returned in this way, so an array of 16 chars would not typically be returned in 2 8-byte registers. To use SSE or AVX registers for anything other than float or double scalars, you'll need to use the special vector types. I think x86 lets you return in eax and edx, but only if you're returning a single 64-bit value, not a struct with 2 32-bits. Not 100% on all of this.