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

From: Richard Henderson
Subject: Re: [Qemu-arm] [PATCH 01/23] scripts: Add decodetree.py
Date: Thu, 11 Jan 2018 11:26:52 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2

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.


> 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...


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