[LLVMdev] A simpler method to reject undefined encodings

[Tim Northover]

Hi Mihail,

> Here is a practical example: ARM NEON vector instructions may work in
either double word mode (taking d registers) or in quad word mode (taking q
registers). The mode is selected by bit 6 (Q bit). When Q == 1, then quad
word mode is used.
>
> There is an additional constraint: whenever Q == 1, the registers encoded
in the instruction need to be even. Otherwise the encoding is undefined.
>
> This constraint is currently unimplemented and triggers incorrect
behaviour in the MC disassembler. In order to correct this I would have to
create custom decoder methods for a dozen-some instructions which is
wasteful. I would much prefer to be able to define a constraint function
like:

Actually, in this case don't you just have to add another check to
DecodeQPRRegisterClass? It currently completely ignores the
least-significant bit of its field, but should probably check that it's 0.

Or are there instructions that permit that bit to vary without actually
affecting anything? I'm not aware of any, but if so the best solution is
probably to create a second RegisterOperand class to handle this disparity
and migrate some instructions over to it.

Cheers.

Tim.
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