assign_parms, arguments in BLKmode

[Hendrik Greving]

I am debugging an old backend for a custom machine (sorry this is a
question regarding a very old compiler, but I checked new sources and
they look similar). The problem is the following. The backend does
support passing struct function argument via registers. For a given
function, I see that this is happening, but when accessing member of
the struct, the entire struct is first copied to the stack before
being accessed. This is happening in assign_parms, because the
argument is BLKmode: See "If a BLKmode arrives in registers, copy it
to a stack slot" comment in function.c:asign_parms. What causes it to
be BLKmode?

I debugged a x86 equivalent of the same version, and I see the same
C-code leading to the struct being passed in DImode to assign_parms.
Hence, I guess something in my backend must cause this.

Any hints where I should look? I am seeing that if the parameters are
going in DImode to assign_parms, this is much better for the code
generated later, in some cases unnecessary copies to the stack are
avoided.

Thanks,
Regards,
Hendrik Greving