Re: [Qemu-arm] [PATCH 01/23] scripts: Add decodetree.py

[Richard Henderson]

On 01/11/2018 11:21 AM, Peter Maydell wrote:
> On 11 January 2018 at 19:10, Richard Henderson
> <address@hidden> wrote:
>> On 01/11/2018 10:06 AM, Peter Maydell wrote:
>>> On 18 December 2017 at 17:45, Richard Henderson
>>> <address@hidden> wrote:
> 
>>>> +# Pattern examples:
>>>> +#
>>>> +#   addl_r   010000 ..... ..... .... 0000000 ..... @opr
>>>> +#   addl_i   010000 ..... ..... .... 0000000 ..... @opi
>>>> +#
>>>
>>> I think we should insist that a pattern defines all the
>>> bits (either as constant values or as fields that get
>>> passed to the decode function). That will help prevent
>>> accidental under-decoding.
>>
>> Hmm.  What do you suggest then for bits that the cpu does not decode at all?
>> This doesn't happen with ARM (I don't think) but it does happen with HPPA, 
>> and
>> probably others.
> 
> Arm does have undecoded bits (they're in brackets in encoding diagrams),
> but they're UNPREDICTABLE if you don't set them right, so ideally we
> check them all and UNDEF. Our current aarch32 decoder doesn't always
> do this, and it's non-obvious when that happens.
> 
>> I suppose I could either wrap it in a field that the translator ignores, or
>> choose another character besides ".", e.g.
>>
>> mfia            000000 xxxxx 00000 xxx 10100101 t:5
>>
>> where bits [21-25] and bits [13-15] really are ignored by hardware.
> 
> Yes, I'd like to see something so that if you want the translator
> to ignore a bit you have to explicitly mark it as to be ignored.

Ok.

> Something I noticed the doc comment doesn't mention: what's the
> semantics for if the patterns you declare overlap? Is this a
> purely declarative language where you have to make sure an
> insn can only match one pattern (or get an error, presumably),
> or is there an implicit "match starting from the top, so put
> looser patterns last" process?

It *should* error.  But I'm not sure that it does.
It's probably worth adding some unit tests for this...


r~