Different length variants of the same instruction

Different length variants of the same instruction

[Michael Chapman]

I can't figure out how to get the assembler to work with different
lengths of the same instruction e.g.
    mov r1, #imm4    (16 bit instruction for immediate values 0-15)
and
    mov r1, #imm16  (32 bit instruction for immediate values 0-65535)

If I simply have the following description

(dni mov-rd-imm4 "mov rd,#imm4"
     ()
     "mov $rd,$hash$imm4"
     (+ (f-opcode #x10) rd imm4)
     (set rd imm4)
     ()
)

(dni mov-rd-imm16 "mov rd,#imm16"
     ()
     "mov $rd,$hash$imm16"
     (+ (f-opcode #x60) rd (f-rsvd-rs 0) imm16)
     (set rd imm16)
     ()
)

I.e. the imm4 variant before the imm16 variant then I can write
    mov r1, #0x1234
to get a 32 bit instruction.

However if I try
   mov r1, #(l2-l1)
where l1 and l2 are instruction labels, it always tries to assemble this
with the imm4 variant -
even if the value of the expression is too big.

Why?

I have a similiar problem with conditional branches which come in two
lengths as well.

I have experimented with RELAX and RELAXABLE with no sucess.

Can someone explain things?

Thanks,
Mike Chapman