How to force GCC to pass 128bits/256bits struct as function param in xmm/ymm register?

Question

How to force GCC to pass 128bits/256bits struct as function param in xmm/ymm register? ie. if my struct is 256bits wide (UnsignedLongLongStruct below)

(I know if I use intrinsics to make a packed integer, gcc is smart enough to put it into %ymm register, but can I do it with struct ?)

typedef struct {
   unsigned long long ull1;
   unsigned long long ull2;
   unsigned long long ull3;
   unsigned long long ull4;
} UnsignedLongLongStruct;

void func1( UnsignedLongLongStruct unsignedLongLongStruct ) {
....
}

Answer 1

TL;DR: It seems the calling conventions explicitly mention __m256 and friends to be placed in the umm regs.

In X86-64 System V ABI, point 3.2.3, you can check how parameters are passed. My reading is that only __m256 arguments will be turned into one SSE and 3 SSEUP 8-byte chunks, which allows them to be passed in a ymm register.

This will make it so that your argument gets passed in memory, which is what we see in clang, gcc, and icc: Test program on godbolt

In order to pass it as a register, as I read the calling conventions, it seem that you have to pass it as a __m256 (or a variant of it).