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This issue is more straight-forward than #22273. ISTM, we just have to copy large values. For example, in ffi_call, we'd iterate over the arguments and alloca and memcpy the large values prior to calling ffi_call_AMD64:
 case FFI_SYSV:
 /* If a single argument takes more than 8 bytes,
 then a copy is passed by reference. */
 for (unsigned i = 0; i < cif->nargs; i++) {
 size_t z = cif->arg_types[i]->size;
 if (z > 8) {
 void *temp = alloca(z);
 memcpy(temp, avalue[i], z);
 avalue[i] = temp;
 }
 }
 /*@-usedef@*/
 return ffi_call_AMD64(ffi_prep_args, &ecif, cif->bytes,
 cif->flags, ecif.rvalue, fn);
 /*@=usedef@*/
 break;