field splitting

[Doug Evans]

Eric Christopher writes:
 > I have an instruction that looks something like this:
 > 
 > [opcode][4-bits][src][12-bits]
 > 
 > Where the 4 and 12-bit fields are really a part of the same 16-bit
 > offset.  I've been unable to think of any way to define the field in
 > such a way that I can make it a 16-bit operand that just happens to
 > contain bits at different spots.
 > 
 > Currently the field is just defined as 2 separate fields.
 > 
 > Any ideas?

This is what multi-ifields are for.

grep for multi-ifield in doc/rtl.texi and *.cpu.