Yes, GCC can pass it in two registers. On the other hand Microsoft's x64 ABI doesn't: >Structs and unions of size 8, 16, 32, or 64 bits, and __m64 types, are p

Yes, GCC can pass it in two registers. On the other hand Microsoft's x64 ABI doesn't: >Structs and unions of size 8, 16, 32, or 64 bits, and __m64 types, are passed as if they were integers of the same size. Structs or unions of other sizes are passed as a pointer to memory allocated by the caller. https://learn.microsoft.com/en-us/cpp/build/x64-calling-convention?view=msvc-170 https://learn.microsoft.com/en-us/cpp/build/x64-calling-conv...